Holocene Carbon Dynamics in the Patterned Peatlands of the
Hudson Bay Lowland, Canada: Reducing Landscape-Scale
Uncertainty in a Changing Climate
by
Maara Susanna Packalen
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Department of Geography University of Toronto
© Copyright by Maara Susanna Packalen, 2015
ii
Holocene Carbon Dynamics in the Patterned Peatlands of
the Hudson Bay Lowland, Canada: Reducing Landscape-
Scale Uncertainty in a Changing Climate
Maara Susanna Packalen
Doctor of Philosophy
Department of Geography
University of Toronto
2015
Abstract
Northern peatlands have accumulated ~ 500 Pg of carbon (C) over millennia, and contributed to
a net climate cooling. However, the fate of peatland C pools and related climate-system
feedbacks remain uncertain under scenarios of a changing climate and enhanced anthropogenic
pressure. Here, Holocene C dynamics in the Hudson Bay Lowland, Canada (HBL) are examined
at the landscape scale with respect to glacial isostatic adjustment (GIA), climate, and
ecohydrology. Results confirm that the timing of peat initiation in the HBL is tightly coupled
with GIA, while contemporary climate explains up to half of the spatial distribution of the total C
mass. Temporal patterns in C accumulation rates (CARs) are related to peatland age,
ecohydrology, and possibly paleoclimate, whereby CARs are greatest for younger, minerotrophic
peatlands. Rapid and widespread peatland expansion in the HBL has given rise to a globally
significant C pool, in excess of 30 Pg C and two-thirds of which is of late Holocene age. Yet,
iii
long-term decomposition of previously accrued peat has potentially resulted in some C losses,
especially during the late Holocene when the landscape was occupied by an abundance of
minerotrophic peatlands and climate was characterized by more precipitation and similar-to-
colder temperatures than present. Model deconstruction of HBL C dynamics indicate that 85% of
C losses occurred during the late Holocene, while spatio-temporal scaling of modern methane
(CH4) emissions suggest a potential flux of 1 – 7 Pg CH4 to the late Holocene atmosphere, which
provides evidence of a peatland contribution to the late Holocene CH4 rise recorded in ice cores.
Although HBL peatlands may continue to function as a net C sink, conservative climate
scenarios predict warmer and wetter conditions in the next century – beyond the HBL’s range of
past climate variability, yet within the peatland climate domain – with implications for primary
production and decomposition. Further investigation into controls on spatial-temporal C
dynamics may reduce uncertainty concerning the HBL’s potential to remain a net C sink under
future climate and resource management scenarios, and contribute to our understanding of global
peatland C-climate dynamics.
iv
Acknowledgements
Research Acknowledgements
I would like to express my deepest gratitude to my major supervisor Professor Sarah Finkelstein.
You have been a tremendous mentor for me. I would like to thank you for guiding and
encouraging my research, fostering idea development through countless discussions, and
supporting unique opportunities for field work, collaboration, academic and professional
enrichment, which greatly enhanced my graduate studies at the University of Toronto (U of T).
I would also like to thank my committee members at the U of T, Professors Nathan Basiliko
(now at Laurentian University), Jing Chen, and William Gough, and Dr. James McLaughlin at
the Ontario Ministry of Natural Resources and Forestry (MNRF). Each of you has contributed
meaningful and unique perspectives at various stages of my doctoral research, and I am
enormously grateful. Special thanks to Dr. McLaughlin for establishing a solid foundation to
pursue peatland research in the Far North of Ontario, and specifically across the Hudson Bay
Lowland. This logistically-challenging research would not have been possible, at the scale that it
was conducted, without your tireless efforts to develop compelling research strategies and
establish strong collaborations among government, academia, industry, and First Nation
communities.
Special thanks are also extended to the MNRF, for supporting my request for an educational
leave of absence from my permanent position with the Forest Research and Monitoring Section,
which afforded dedicated time to pursue doctoral studies. And finally, to my external appraiser,
Professor Merritt Turetsky (University of Guelph) – my sincerest appreciation is extended to you
for reviewing my dissertation and providing extremely thoughtful and meaningful feedback.
Your research contributions inspired me early in my career and primed me to undertake this
amazing doctoral research journey; and your ongoing professional contributions and enthusiasm
for research continue to inspire me today. Thank you.
v
Research funding and field support was provided by the MNRF’s Applied Research and
Development Branch (now Science and Research Branch) and Far North Branch, under the
auspices of projects CC-021 and FNIKM 028. Many thanks to MNRF field crews for peat coring
and site surveys (2009 – 2011) and De Beers Canada for logistical support in the vicinity of the
Victor Diamond Mine (2009 – 2012). Many thanks also to MNRF lab personnel for
geochemical analyses. Additional support for field work and radiocarbon dating was provided by
grants (327197-11 and 331284-11) from the Natural Sciences and Engineering Research Council
of Canada (NSERC) and the Ontario Ministry of the Environment and Climate Change
(MOECC) through the Climate Change and Multiple Stressor Research Program at Laurentian
University. Many thanks for field and logistical support from the U of T (Department of
Geography), Laurentian University (Living with Lakes Centre), Queen’s University (PEARL),
Hearst Air Service, and Albert’s Fish Camp during a field campaign near Hawley Lake, ON
(2011). Thanks also for field and logistical support from Western University, McGill University,
Ministry of Environment and Climate Change (MOECC), and De Beers Canada, during a field
campaign near the Victor Diamond Mine (2012). Extensive data syntheses were completed in
support of this research. Thanks are extended to Dr. Arthur Dyke for providing access to the
Canadian basal radiocarbon database; Professor Peter Kuhry for contributing raw peat core data;
and Dr. Dan McKenney for access to gridded climate data.
Graduate stipend support was provided by an NSERC Alexander Graham Bell Canada
Postgraduate Scholarship (CGSD2-426611-2012), Ontario Graduate Scholarship, several
graduate student awards from the Department of Geography at the University of Toronto, and the
Canadian Northern Studies Trust Scholarship from the Association of Canadian Universities for
Northern Studies (2010 – 2014). Field research was further supported by graduate student
research grants from the Society of Wetland Scientists (1) and Aboriginal Affairs and Northern
Development Canada’s Northern Scientific Training Program grants (2).
I am extremely grateful for opportunities to enrich my graduate studies. I spent an invaluable
three months as a visiting scientist at Columbia University, Lamont Doherty Earth Observatory
(LDEO), under the auspices of the NSERC Michael Smith Foreign Study Supplement. This
vi
special research opportunity supported extension of my doctoral research, under the guidance of
Dr. Dorothy Peteet (co-affiliated with NASA-GISS), and included insightful and productive
collaborations with Drs. Liz Corbett, Linda Heusser, and Jonathan Nichols. Thank you to each of
you for welcoming me into your research group. Gratitude is also extended to Professor Zicheng
Yu, at Lehigh University and Professors Tim Moore and Nigel Roulet at McGill University, for
including me in stimulating peatland network meetings and research workshops. Graduate
enrichment opportunities to study multivariate statistics at the University College London (2011)
and to conduct field work in Sweden and across Finland (2012) were further supported by
awards from the University of Toronto’s Centre for Global Change Science (2). Special thanks
to Dr. Jukka Turunen from the Geological Survey of Finland, for mentorship and logistical
support during my 2012 field campaign in Finland.
Personal Acknowledgements
My graduate journey would not have been as successful or fulfilling without the love and support
of my parents, extended family and close friends. They have been my constant foundation, and I
am forever indebted to them for their patience, advice, and interest in my work. Deepest thanks.
Many friends and colleagues at the U of T, within the peatland community, and from around the
world have enriched my graduate experience beyond words. They are too numerous to name;
however, I have deeply enjoyed the conversations, collaborations, and extra-curricular activities.
I sincerely hope our paths will continue to cross, and wish each of them all the best.
vii
Table of Contents
Abstract ........................................................................................................................................... ii
Acknowledgments.......................................................................................................................... iv
Table of Contents .......................................................................................................................... vii
List of Tables ................................................................................................................................. xi
List of Figures ............................................................................................................................... xii
Chapter 1 Introduction .....................................................................................................................1
1.1 Background ..........................................................................................................................1
1.1.1 Holocene carbon dynamics ......................................................................................3
1.1.2 Peat development .....................................................................................................5
1.1.3 Circum-polar carbon accumulation..........................................................................7
1.1.4 The Hudson Bay Lowland, Canada as a model ecosystem .....................................9
1.2 Research objectives ............................................................................................................11
1.2.1 Research questions .................................................................................................12
1.2.2 Hypotheses .............................................................................................................13
1.3 General research approach .................................................................................................13
1.3.1 Detailed peat records..............................................................................................14
1.3.2 Data syntheses ........................................................................................................14
1.4 Thesis structure and publication information ....................................................................15
1.4.1 Chapter 1 ................................................................................................................15
1.4.2 Chapter 2 ................................................................................................................15
1.4.3 Chapter 3 ................................................................................................................16
1.4.4 Chapter 4 ................................................................................................................16
viii
1.4.5 Chapter 5 ................................................................................................................16
Chapter 2 Carbon Storage and Potential Methane Production in the Hudson Bay Lowlands
since Mid-Holocene Peat Initiation ...........................................................................................17
2.1 Abstract ..............................................................................................................................17
2.2 Introduction ........................................................................................................................17
2.3 Methods..............................................................................................................................21
2.3.1 Data synthesis and new peat records .....................................................................21
2.3.2 Holocene peat initiation and carbon dynamics ......................................................23
2.4 Results ................................................................................................................................25
2.4.1 GIA and paleoclimate as controls of HBL peatland dynamics ..............................25
2.4.2 HBL carbon storage and potential Holocene methane emissions ..........................26
2.5 Discussion ..........................................................................................................................28
2.6 Conclusion .........................................................................................................................36
2.7 Acknowledgements ............................................................................................................37
2.8 Author contributions ..........................................................................................................38
2.9 Competing financial interests ............................................................................................38
2.10 Tables .................................................................................................................................39
2.11 Figures................................................................................................................................49
Chapter 3 Quantifying Holocene Variability in Carbon Uptake and Release Since Peat
Initiation in the Hudson Bay Lowlands, Canada ......................................................................52
3.1 Abstract ..............................................................................................................................52
3.2 Introduction ........................................................................................................................53
3.3 Materials and Methods .......................................................................................................58
3.3.1 Study setting...........................................................................................................58
3.3.2 Sample collection and data sources .......................................................................59
ix
3.3.3 Laboratory Analyses ..............................................................................................60
3.4 Results ................................................................................................................................63
3.4.1 Carbon accumulation and peat decay rates ............................................................63
3.4.2 Peatland carbon dynamics......................................................................................65
3.5 Discussion ..........................................................................................................................67
3.5.1 Carbon accumulation patterns in the HBL .............................................................67
3.5.2 Modeled peatland carbon dynamics in the HBL ....................................................70
3.5.3 Net carbon balance and paleoclimate in the HBL .................................................72
3.6 Conclusion .........................................................................................................................75
3.7 Acknowledgements ............................................................................................................76
3.8 Tables .................................................................................................................................78
3.9 Figures................................................................................................................................81
Chapter 4 Climate and Peat Type in Relation to the Spatial Distribution of the Peat Carbon
Mass in the Hudson Bay Lowland, Canada ..............................................................................86
4.1 Abstract: .............................................................................................................................86
4.2 Introduction ........................................................................................................................87
4.3 Study setting.......................................................................................................................92
4.4 Methods..............................................................................................................................93
4.4.1 Peat and bioclimatic data sources ..........................................................................93
4.4.2 Peat physical properties .........................................................................................94
4.5 Results ................................................................................................................................95
4.5.1 Climate and carbon mass spatial relationships across the HBL ............................95
4.5.2 Carbon mass variation among peat types ...............................................................98
4.6 Discussion ..........................................................................................................................99
x
4.7 Conclusions and future implications ................................................................................104
4.8 Acknowledgements ..........................................................................................................106
4.9 Tables ...............................................................................................................................107
4.10 Figures .............................................................................................................................109
Chapter 5 Conclusions and Future Research Directions ..............................................................115
5.1 Summary ..........................................................................................................................115
5.1.1 Peat initiation and carbon storage ........................................................................116
5.1.2 Holocene carbon dynamics ..................................................................................117
5.1.3 Climatic controls of the distribution of the carbon mass .....................................118
5.2 Sources of Uncertainty and Future Research Directions .................................................119
5.2.1 HBL paleohydroclimate and carbon dynamics ....................................................119
5.2.2 Carbon dynamics in permafrost peatlands and peatland pools ............................120
5.2.3 Age-depth modeling and fen decay modeling .....................................................120
5.2.4 Peat carbon dynamics in post-marine environments ...........................................121
5.2.5 Peatland resilience and vulnerability to climatic change .....................................121
References ....................................................................................................................................122
xi
List of Tables
Table 2.10-1 Location, age, and carbon mass of peat cores located in the Hudson Bay Lowlands,
Canada (HBL), sorted by latitude. ................................................................................................ 39
Table 3.8-1 14
C-accelerator mass spectrometry (AMS) dating of peat macrofossil of known
provenance for 10 new sites in the Hudson Bay Lowlands, Canada; sorted by increasing latitude.
....................................................................................................................................................... 78
Table 3.8-2 Summary of deconstruction terms used to describe peatland carbon (C) dynamics in
the Hudson Bay Lowlands, Canada (adapted from Yu, 2011). .................................................... 80
Table 4.9-1 Peat physical properties of 42 sites in the Hudson Bay Lowland (HBL), Canada. . 107
Table 4.9-2 Peatland carbon mass distribution in the Hudson Bay Lowland, Canada. .............. 108
xii
List of Figures
Figure 2.11-1 Land emergence and peatland expansion in the Hudson Bay Lowland, Canada. .. 49
Figure 2.11-2 Holocene peat initiation dynamics in the Hudson Bay Lowlands, Canada. .......... 50
Figure 3.9-1 Physical features and peat study locations in the Hudson Bay Lowlands, Canada. 81
Figure 3.9-2 Peat age, depth, and cumulative carbon (C) mass relationships and modeled
exponential peat decay for the patterned peatlands of the Hudson Bay Lowlands, Canada. ....... 82
Figure 3.9-3 Holocene peat carbon (C) pools and modeled peat C terms (Yu, 2011) for the
Hudson Bay Lowland, Canada, since peat initiation began ~ 8 ky BP......................................... 83
Figure 3.9-4 Holocene peatland area increase, carbon (C) accumulation, and net C balance, since
peat inception for the Hudson Bay Lowlands, Canada. ................................................................ 84
Figure 4.10-1 Physical features, study sites and carbon mass the Hudson Bay Lowland, Canada.
..................................................................................................................................................... 109
Figure 4.10-2 Contemporary climate domain for peatlands of the Hudson Bay Lowland, Canada.
..................................................................................................................................................... 110
Figure 4.10-3 Relationship between peat depth and carbon mass for well described peatlands in
the Hudson Bay Lowland, Canada.............................................................................................. 111
Figure 4.10-4 Peat class distribution relative to peatland continentality in the Hudson Bay
Lowland, Canada. ....................................................................................................................... 112
Figure 4.10-5 Peat carbon mass relative to bioclimate in the Hudson Bay Lowland, Canada. .. 113
Figure 4.10-6 Total carbon mass (kg m-2
) stored in bogs, fens and coastal peatland mesoforms, in
the Hudson Bay Lowland, Canada.............................................................................................. 114
1
Chapter 1 Introduction
1.1 Background
Peat-accumulating wetlands, termed peatlands, accrue carbon (C) rich terrestrial organic deposits
over millennia, as a consequence of complex ecohydroclimatic interactions and under conditions
where net primary productivity exceeds organic matter (OM) decomposition. Peatlands occupy
~ 3% of the global terrestrial extent, yet account for at least one third of the global soil organic C
pool (Gorham, 1991; Hugelius et al., 2014). Although peatlands are globally distributed,
northern high latitude peatlands represent ~ 90% of the global peatland distribution and store an
estimated 250 – 500 Pg C (Loisel et al., 2014; Turunen et al., 2002;Yu, 2011). During the current
interglacial, northern peatlands have sequestered atmospheric C dioxide (CO2) at a mean rate of
0.02 – 0.03 kg C m-2
y-1
, equivalent to 25 to 50% of the atmospheric burden or at least 100 to 200
ppmv CO2 (Frolking and Roulet, 2007; Gorham, 1991). Hence, long-term C storage in peat
deposits has resulted in a net climatic cooling over the Holocene (Frolking et al., 2011). Yet,
complex interactions among internal peatland dynamics and external forcing suggest that the net
C balance of peatlands may vary over seasonal to millennial timescales (Yu, 2012) and may
become increasingly vulnerable to loss under anticipated climate change scenarios.
Studies investigating the timing of peat initiation and lateral expansion reveal periods of
significant methane (CH4) emissions, suggesting variability in the strength of peatlands as C
sinks (MacDonald et al., 2006). Further, syntheses of global peat core records suggest that
climate may be positively correlated with C accumulation at large spatial (degrees latitude)
scales (Beilman et al., 2009) and long temporal (decadal to millennial) scales (Yu, 2012). The
greatest rate of peat initiation in northern peatlands is reported to have occurred during the early
2
Holocene (11.5 – 7 ky), when climate was warmest and driest and favored rapid initiation and
expansion of mostly minerotrophic peatlands (Jones and Yu, 2010; MacDonald et al., 2006).
However, since the early Holocene, peatlands appear to be on a trajectory of reduced C
accumulation rates (Yu, 2011). The extent to which northern peatland initiation and expansion
have influenced polar ice-core inferred Holocene atmospheric CH4 variability continues to fuel
much debate (Reyes and Cooke, 2011), and uncertainty concerning the role of climate in
peatland C dynamics persists. Hence, the mechanisms controlling northern peatland C dynamics
and the role of northern peatlands in the Holocene C cycle are of great interest to the global C
cycle research community.
Global circulation models predict increased hydroclimatic variability in northern high latitudes,
whereby a warmer climate may enhance both vegetation productivity and OM decomposition
depending in part upon concomitant alteration of the net moisture balance. Disproportionate
and/or non-linear changes in the balance between production and decomposition, as a
consequence of hydroclimatic variability and anthropogenic land conversion may have important
implications for both the peatland C sink and C flux potentials. Thus, under future climate and
land-use scenarios, the role and ecological functioning of northern peatlands could be
significantly altered; yet, the magnitude of change and the mechanisms driving peatland C
storage dynamics and climate-system feedbacks remain uncertain.
This dissertation research examines peatland C dynamics at the landscape scale and tracks C
dynamics over the Holocene using complete peat profiles, for the purpose of better constraining
the controls on the net C balance in northern peatlands. Here, the patterned peatlands of the HBL
are considered as a model system to investigate controls on peat initiation, peatland
development, and Holocene C dynamics vis-à-vis post glacial isostatic adjustment, paleoclimate,
3
and ecohydrology, as inferred by peat type. Additionally, the potential contribution of HBL
peatlands to millennial-scale atmospheric CH4 trends is also investigated via model
deconstruction of C sink-source dynamics and peatland development. The objectives of this
research will be approached by coupling new and previously reported peat geochemical evidence
with peat initiation and development chronologies, and model reconstructions of paleoclimate
and paleotopography to track spatio-temporal changes in the peat C pool and elucidate possible
controls on Holocene C dynamics in the HBL. Ultimately, this research is designed to improve
our fundamental understanding of northern peatland C dynamics, and potentially provide a sound
scientific rationale for Canada’s future resource management policies, in the context of a
changing climate.
1.1.1 Holocene carbon dynamics
Although peatlands efficiently sequester CO2 from the atmosphere, some of the fixed CO2 may
also be released as decomposition products in the form of CO2, CH4 and dissolved organic C
(DOC). Wetland ecosystems, including peatlands are important contemporary sources of CH4 to
the atmosphere, releasing an estimated 20 – 45 Tg CH4 y-1
(Mikaloff Fletcher et al., 2004). CH4
is an important greenhouse gas due to its high global warming potential (~ 24 x CO2), abundance
(third, after water vapor and CO2), chemical reactivity in the atmosphere, and sensitivity to
changes in climate (Pickett-Heaps et al., 2011). Although highly variable, peatland C losses in
the form of CH4appear to be greatest from minerotrophic peat forms and/or under
ecohydrological conditions consistent with fen landforms, thermokarsts, and peatland
flarks/pools (Pelletier et al., 2007; Bubier et al., 1993a; Olefeldt et al., 2013).
Polar ice-core records reveal that atmospheric CH4 concentrations rose twice during the
Holocene: first in the early Holocene during a period of continental deglaciation (11 – 8 ky BP)
4
and second in the late Holocene (4 ky BP – present), following a mid-Holocene atmospheric CH4
decline (Brook et al., 2000). Global syntheses of northern peatland initiation dates have
correlated the rapid development of peatlands in the early Holocene to the rise of atmospheric
CH4 during the same period (MacDonald et al., 2006). Peak early Holocene atmospheric CH4
concentrations have been associated with the existence of predominantly warm, wet
minerotrophic fens, recognized CH4 sources in the modern landscape. MacDonald et al. (2006)
proposed that this was related to peak insolation, inferred from Milankovitch cycles, where
warming effects were likely delayed in the northern hemisphere by ice albedo feedbacks during
the retreat of continental ice sheet. By contrast, the mid-Holocene has been associated with a
transition to mainly ombrotrophic Sphagnum bogs, resulting in reduced atmospheric CH4
contributions and moderate CO2 sequestration. However, atmospheric CO2 increased rather
suddenly during the mid-to late Holocene and the oceanic buffering response (carbonate
compensation pump) has been invoked as the most probable mechanism (Yu, 2011).
With declining insolation, below average apparent C accumulation in existing peatlands, and
little evidence of new peat initiation, a connection between the late Holocene atmospheric CH4
rise and peatland initiation and expansion has not been well established. Consequently, many
alternative hypotheses have been advanced to explain the late Holocene atmospheric CH4 rise,
including: (1) early anthropogenic activity, namely rice cultivation and deforestation (Ruddiman
et al., 2008); (2) mid-to-high latitude wetland expansion rather than initiation (Korhola et al.,
2010); and (3) tropical wetland contributions (Yu, 2011; Page et al., 2011). However, an
apparent lack of evidence may stem from: (1) few peat initiation and expansion records from late
emerging, and potentially globally significant peat complexes, such as the HBL (Yu, 2012); (2) a
propensity to sample the deepest part of the peatland, which may result in an inability to
effectively consider the role of peatland expansion in late Holocene C dynamics (Loisel et al.,
5
2014); and (3) the possibility of non-traditional peatland succession dynamics favouring a return
to fen-like conditions due to changes in climate and/or other biophysical processes (van Bellen et
al., 2013).
As a result, evidence in support of a clear connection between northern peatland initiation and
expansion dynamics as a control on polar ice-core inferred Holocene atmospheric CH4 variability
is lacking, and the source of the CH4 rise beginning during the mid-Holocene toward a pre-
industrial maximum remains controversial (Ruddiman et al., 2011). Additional peatland
initiation datasets from other time periods in the Holocene are needed, together with
consideration of whole peat complexes, to understand how peatland development may account
for variation in atmospheric CH4 concentrations, over centennial to millennial timescales.
1.1.2 Peat development
The peat profile is comprised of an acrotelm and a catotelm. The acrotelm by definition lies
above the seasonal water table, typically within 0.5-m of the surface, and experiences the most
variable seasonal water and C dynamics. The poorly- to moderately-decomposed organic matter
that accumulates in the acrotelm is typically of low density resulting in large pore spaces and
oxygenation. Consequently, relatively rapid decomposition occurs in the acrotelm compared with
deeper portions of the peat profile. The portion of the peat profile that lies below the seasonal
water table, termed catotelm, is characterized by slow decomposition and long-term peat
accumulation. Density typically increases with depth along the peat profile, which reduced
oxygen penetration and stimulates the development of anaerobic conditions, while decreasing
hydraulic conductivity results in the decoupling of the accumulating peat column from the
underlying mineral sediments. The boundary region between the aerobic and anaerobic states,
6
occurs at the mean summer water table minimum, and appears to be an extremely active zone for
decomposition processes (Belyea and Baird, 2006).
While seasonally waterlogged conditions are frequently associated with peat accumulation, rates
of accumulation vary as a function of a series of factors, including: vegetation assemblage, OM
quality, biogeochemical interactions, bioclimate (e.g., temperature and moisture balance),
topography, and disturbance (Belyea and Baird, 2006; Turetsky et al., 2005; Waddington et al.,
2015). Peatland hydrology is fundamental to comprehending peat accumulation dynamics and
subsequent peatland development, C accumulation rates, and greenhouse gas exchanges
(Frolking and Roulet, 2007).Vegetation distribution and photosynthetic potential are closely
related to the net water balance, which together influence peat accumulation through effective
net primary production (Sonnentag et al., 2008), and contribute to the development of divergent
peat classes (e.g., bogs and fens) that can be used as an indicator of ecohydrology. Further,
ecohydrologically-driven vegetation distributions are coupled to biogeochemical processes, such
as microbially-driven OM decomposition, through the influence of OM quality, cation exchange
capacity, acidity, and nutrient cycling (Turetsky et al., 2008; Waddington et al., 2015). Coupled
with temperature, decomposition processes produce DOC, CO2 and CH4 during terminal
mineralization, all of which contribute to variability in spatio-temporal C dynamics (Turetsky,
2004). Topography is related to hydrological redistribution through surface and subsurface flow,
resulting in feedbacks to vegetation assemblage, nutrient availability, and peatland development
(Glaser et al., 2004a; Glaser et al., 2004b). However, disturbances such as historical fire patterns
and contemporary land management can result in sudden mass C losses (Turetsky et al., 2011).
Uncertainty exists regarding burn severity and fire regimes across the pan-boreal domain of
northern peatlands, especially as it relates to understanding peatland vulnerability in terms of
critical threshold conditions for changing net water balances.
7
1.1.3 Circum-polar carbon accumulation
Gorham (1991) estimated C accumulation for boreal and subarctic peatlands to be approximately
29 g C m-2
y-1
. These estimates were based on mean peat depth, bulk density and peat accretion
rate of 2.3 m, 112 g dm-3
and 0.5 mm y-1
, respectively. However, peat decays gradually through
time, as described by Clymo et al. (1998). In consideration of a constant decay process, the C
accumulation rate reported in Gorham (1991) was adjusted to 23 g C m-2
y-1
. Turunen et al.
(2002) presented a C accumulation rate 18.5 g C m-2
y-1
for over 1300 dated Finnish boreal and
subarctic peatlands, with a mean peat depth of 1.52 m. Further analysis revealed that raised bogs
accrued C at a greater rate than aapa-mires (also known as patterned peatlands), with reported C
accumulation rates of 26.1 g C m-2
y-1
compared to 17.3 g C m-2
y-1
, respectively. Similarly, bogs
were reported to accrue C at a greater rate than fens (20.8 g C m-2
y-1
compared with 16.9 g C m-
2 y
-1, respectively). The lower mean C accumulation rates reported in this study were attributed
to at least three factors: (1) terrestrialized peatlands have traditionally been over-represented in
the C accumulation literature relative to paludified systems; (2) mean peat depths in Finland
were 1.5 m compared to 2.2 to 2.3 m reported for other northern peatlands; and (3) the mean
bulk density used elsewhere may be too large, potentially resulting in an over-estimation of the
total C pool.
Long-term mean C accumulation rates in the West Siberian peatlands, Russia ranged from 12.1
to 23.7g C m-2
y-1
(Turunen et al., 2001); while, C accumulation rates in this region during the
last 2 ky ranged from 3.8 to 44.1 g C m-2
y-1
(Beilman et al., 2009). In the latter study, C
accumulation was significantly correlated to modern mean annual air temperature, where
maximum rates occurred between -1 and 0 °C. As well, fossil plant composition was
significantly related to C accumulation, where C content was lowest in association with
8
Sphagnum remains (Beilman et al., 2009). C accumulation rates in Canadian peatlands are
variable as well. Vitt et al. (2000) found that continental peatlands in western Canada
accumulated C at a rate of 19.4 g C m-2
y-1
and began accumulating peat around 9 ky BP.
Western Canadian peatlands experienced maximum C accumulation rates during the mid-
Holocene, hindered in the early Holocene by enhanced drying due to maximum insolation and in
the late-Holocene by permafrost development (Vitt et al., 2000). By comparison, peatlands in the
East James Bay Lowland, Quebec accumulated C at a mean rate of 16.2 to 18.5 g C m-2
y-1
,
while recent peat accumulated at a mean rate of 73.6 g C m-2
y-1
(Loisel and Garneau, 2010; van
Bellen et al., 2011a). Similar to the continental peatlands in western Canada, maximum C
accumulation occurred during the mid-Holocene in the East James Bay Lowland, Quebec;
however, in this case it was attributed to rapid peat initiation and lateral expansion beginning
~ 7.5 ky, as a consequence of delayed deglaciation. Further, reduced late-Holocene C
accumulation rates were attributed to neo-glacial cooling and surface drying (van Bellen et al.,
2011b).
Today, there is a growing database of northern peatland C accumulation rates (Loisel et al.,
2014), with a noticeable gap in the HBL and Far East Russia (Yu, 2011). As many studies have
quantified C accumulation using cores sampled from the deepest part of the peatland, uncertainty
regarding the variability of C accumulation within a single peatland has arisen. As a result, this
uncertainty in spatial variability reduces confidence in upscaled total peat C pool estimates.
Thus, quantifying the intra- and inter peatland C accumulation has been identified as a high
priority in future C accumulation quantification (Korhola et al., 2010; van Bellen et al., 2011b).
9
1.1.4 The Hudson Bay Lowland, Canada as a model ecosystem
Development during the current interglacial of a nearly continuous peat cover in the Hudson Bay
Lowland, Canada (HBL) has yielded one of the world’s largest peatland complexes and a C
reservoir of potential global significance. As such, the HBL may play an important role in
moderating global climate and long-term atmospheric C dynamics. Yet, few data are available
from this largely unexplored and undisturbed landscape to support either (1) quantification of the
magnitude of the C reservoir or (2) an understanding the processes that control peatland
development and long-term C dynamics.
The HBL is a poorly drained plain with peatlands covering of an area of 325, 000 km2
(Riley,
2011), second only in size to the Western Siberian Plain (Martini, 2006). This region was
shaped by Pleistocene ice sheets, which resulted in the deposition of till and diamicton, eskers
and moraines, and glacio-marine sediments, overtop of the underlying Paleozoic sedimentary
bedrock (Martini, 2006). Climate in the HBL is a microthermal, strongly influenced by Arctic air
masses and strong winds (Martini, 2006). Modern climate in the HBL is characterized by a mean
annual temperature of -4 °C, with cold winters (January daily mean -15 to -18 °C) and warm
summers (July daily mean 12 to 18 °C). Precipitation patterns are characterized by a moderately
deep snow accumulation (203 to 241 cm) and moderate total precipitation (603 to 660 mm)
(Martini, 2006). Contemporary permafrost features in the HBL range from sporadic occurrences
in the southern regions near James Bay, to discontinuous permafrost in the middle of the HBL
and continuous permafrost in the region immediately south of Hudson Bay, which together
potentially influence < 10% of the region.
Under the weight of the continental ice sheets, subsidence upwards of 200 m gave way to
inundation by glacial seas upon glacial retreat (Peltier, 2004). During the early Holocene (< 7.5
10
ky) the HBL was largely covered by the Tyrrell Sea (Riley, 2011). The post-glacial HBL,
influenced by isostatic rebound and glacial sea emergence, has an average slope of 0.5 m km-1
and contains expansive patterned peatlands with ridge-flark mesoforms, marsh wetlands and tidal
flats alternating with beach ridges near the coast, and inland bog-fen peatland complexes, with
bogs dominating the more inland reaches (Riley, 2011). Average peat depth ranges from less
than 40 cm in coastal areas to greater than four meters at inland bogs (Riley, 2011).
Peatlands in the HBL likely began to initiate in the mid-Holocene as land emerged from the post-
glacial Tyrrell Sea; however, the timing of peat initiation relative to land emergence remains
unconfirmed. C accumulation rates are not well established for the patterned peatlands of the
HBL and few vertically dated peat records exists, thus the magnitude of the C store and
Holocene C dynamics remain unexplored. CH4 contributions from the modern HBL was recently
estimated to be 2.3 Tg y-1
(Pickett-Heaps et al., 2011), equivalent to one tenth of the global
northern peatland annual contribution. Mesoform pattern development in the HBL of Sphagnum
ridges, herbaceous lawns/flarks, and pools may provide a viable source of persistent CH4
emissions over seasonal to millennial timescales, through local redistribution of water and
nutrients and associated ecological feedbacks.
The HBL is an important region for examining the role of peatland initiation and expansion on
the late Holocene CH4 rise due to its delayed post-glacial emergence, yet it has hitherto not been
considered in global-scale syntheses. Given the delayed peat initiation and expansion in the HBL
relative to other northern peatlands and the size of the peat complex, investigation of HBL C
dynamics in terms of potential global climate forcing is a high priority. Yet, the role of the HBL
peat complex in long-term climate dynamics and the fate (and potential vulnerability) of its
11
peatland C pools and related climate system feedbacks remain uncertain under scenarios of a
changing climate and enhanced anthropogenic pressure.
1.2 Research objectives
Uncertainty exists regarding the mechanisms driving northern peatland expansion and pattern
development, the fate of peatland C pools following hydroclimatic change and long-term
peatland-climate feedbacks. To date, an expanding peat inventory has yielding insight into rates
of C accumulation in northern peatlands; however, few studies have deconstructed these rates
into C flux terms (C uptake and C release) (Yu, 2011). Observed patterns in timing of peatland
initiation frequency and possibly lateral expansion have been linked with global atmospheric
CO2 and CH4 fluctuations. An important next step in analyzing peatland C accumulation is to
quantitatively evaluate the role of peatlands in the late-Holocene rise in CO2 and CH4. Further, a
quantitative and mechanistic understanding of these fluxes over the Holocene may inform
projections of the northern peatland C pool response to anticipated hydroclimatic variability and
potential climate system feedbacks.
While climate has been invoked as the dominant control on northern peatland initiation (Jones
and Yu, 2010; MacDonald et al., 2006), post-glacial isostatic adjustment may be an even more
fundamental control of peatland initiation and expansion in the dynamic landscape of the HBL,
where rates of isostatic rebound are among the highest globally. Working under the hypothesis
that peatland initiation, expansion, and pattern development (as a proxy for ecohydrology) are
mechanisms of persistent peatland-derived atmospheric CH4 sources, the overall objective of this
dissertation research is to investigate landscape-scale controls on C dynamics in the HBL and the
potential role of the HBL in contributing to the mid-to-late Holocene rise in atmospheric CH4. In
12
keeping with this objective, this dissertation research seeks to integrate long-term C
accumulation, peatland development, and ecohydrology into a quantitative assessment of C
storage and potential CH4 flux from a mid-to-late Holocene HBL landscape.
1.2.1 Research questions
The guiding question of this dissertation research considers whether (1) the timing of peatland
initiation, (2) the rate of peatland lateral expansion, and (3) the paleoclimate interact at the
landscape scale in the HBL to drive Holocene C dynamics and the observed rise in atmospheric
CH4 during the mid- to late Holocene. To address this overarching research question, each
chapter provides evidence from a range of perspectives (spatio-temporal and local to landscape
scales) with the objective to distinguish between allogenic and autogenic controls on peatland C
dynamics in the HBL. Specific sets of research questions are detailed below.
1. How does the timing of peat initiation in the HBL vary in relation to glacial isostatic
adjustment-driven land emergence and paleoclimate; how much C is sequestered in the
HBL peat complex; what is the potential magnitude of peatland derived CH4 losses
during the period of record; and how do peatland initiation and expansion dynamics
relate to trends in atmospheric CH4 concentrations?
2. What is the apparent rate of C accumulation (CAR) for HBL peatlands; how does CAR
and its modeled C flux terms (C uptake and release) vary through the period of record;
and in relation to paleoclimate, land emergence, and peatland lateral expansion?
3. How does the total C mass vary within the HBL climate domain; and among major peat
types; and is there evidence of both allogenic and autogenic controls on the spatial
distribution of the peat C mass in the HBL?
13
1.2.2 Hypotheses
The hypotheses are: (1) peat initiation is tightly coupled with land emergence, and the principle
mechanism of peat initiation is primary succession or paludification, driven by rapidly emerging
post-glacial sediments; (2) the C mass in the HBL is of global significance, due in part to the size
of the peat complex and the relative lack of disturbance since peat initiation began; (3) C will
accumulate fastest during relatively warm periods due to increased primary productivity and
potentially longer growing periods unless enhanced decomposition is also supported; (4) wet,
cool periods will reduce C accumulation rates due to changes in vegetation assemblages and
potentially greater C losses, including possibly CH4 in relation to C gains; (5) climate shall be
related to the distribution of the C mass, such that warmer climates will support greater C
masses; and (6) long-term C accumulation will occur faster in bog peatlands than in fen peatland
ecosystems, due to variations in hydrothermal properties and decomposition processes, whereby
vegetation assemblage will influence decomposition (e.g., vascular > Sphagnum) of the
accumulated peat and C accumulation rates (e.g., Sphagnum > vascular).
1.3 General research approach
Detailed analyses of the new peat profiles were combined with previously reported peat records
and publically available datasets to examine controls on Holocene C dynamics in the HBL.
Complete peat profiles were collected between 2009 and 2012 to support this dissertation
research. Intensive study sites were established along a hydroclimatic gradient in each of the
following regions: (1) sporadic permafrost zone in the southern HBL, near Kinoje Lake, Ontario
(52 °N, 82 °W); (2) discontinuous permafrost zone in the central HBL near Attawapiskat,
Ontario (53 °N, 84 °W); (3) continuous permafrost zone in the northern HBL near Hawley Lake,
14
Ontario and along the Hudson Bay coast (55.5 °N, 84.5 °W). At the intensive study sites, one to
five transects measuring 500 m were established in both fens and bogs. One to five long cores
(defined as complete peat profiles to mineral contact) were collected every 100 m to capture the
variability between the peat margins and centre to track peatland lateral expansion. Additional
complete peat profiles were sampled across the HBL and along the former Tyrrell Sea margin
(western shore of the ~ 7-ky Hudson Bay coast), and includes sites located in the vicinity of: 54
°N, 88 °W; 51 °N, 87 °W; 51 °N, 86 °W; 51 °N, 80 °W.
1.3.1 Detailed peat records
Detailed peat records were developed using the newly sampled peat cores form the HBL. All
new peat records presented in this dissertation were characterized for peat physico-chemical
properties, including measurements of (1) bulk density; (2) C, nitrogen, calcium, potassium,
phosphorus, and organic matter content; and (3) C radiometric dating of the peat-mineral contact
(termed 14
C basal date). A selection of peat cores also includes vertical C radiometric dating
(14
C), peat humification analysis, and plant macrofossil assessment. A literature review yielded
additional detailed peat records from across the HBL that included at minimum basal-dated peat
records, organic matter content, peat depth, and site location information.
1.3.2 Data syntheses
Publically available datasets were used to support landscape-scaling of controls on peatland
development and long-term C dynamics. These include (1) a basal peat-age database, supported
by a detailed literature review; (2) records of peat depths; (3) pollen-inferred paleoclimate that
reported summer and winter temperature and annual precipitation anomalies relative to the
contemporary climate normals; (4) gridded contemporary climate informed by climate station
15
outputs, interpolated continuously across the landscape; (5) spatially-explicit rates of glacial
isostatic adjustment (one degree latitudinal-longitudinal grid) and paleotopography (0.5 ky
elevation predictions); and (6) ice-core inferred atmospheric CO2 and CH4 concentration for the
period of record examined in the dissertation.
1.4 Thesis structure and publication information
1.4.1 Chapter 1
Chapter 1 provides an introduction to the dissertation, including (1) a brief literature review in
relation to existing knowledge gaps; (2) the context for the research objectives together with
specific research questions and hypotheses; (3) a summary of the methodologies; and (4)
detailed publication information.
1.4.2 Chapter 2
Chapter 2 provides the first estimate of the HBL C pool informed by detailed analyses of peat
records sampled directly from the HBL. It also reports on the coupling between the timing of
peat initiation and land emergence from the post-glacial Tyrrell Sea since peatland development
began in the mid-Holocene, the relationship between peat initiation intensity and paleoclimate,
and estimates potential CH4 efflux as peatlands expanded across the landscape, in the context of
Holocene-scale atmospheric trace gas dynamics. The manuscript was published in Nature
Communications (DOI: 10.1038/ncomms5078). The authors of the manuscript are Maara S.
Packalen (MSP), Sarah A. Finkelstein (SAF), and James W. McLaughlin (JWM). MSP designed
and performed research, analyzed data, and wrote the paper; SAF and JWM designed and
performed research. All authors commented on the manuscript at all stages. The manuscript was
16
subsequently selected and featured in the Permafrost Special Issue in the journal Nature (9 April
2015; www.nature.com/nature/focus/permafrost/).
1.4.3 Chapter 3
Chapter 3 quantifies C accumulation since mid-Holocene peat initiation in relation to
paleoclimatic variation and peat type, and partitions potential C uptake and release as the HBL
peat complex developed. The manuscript was published in The Holocene (DOI:
10.1177/0959683614540728). The authors of the manuscript are Maara S. Packalen (MSP) and
Sarah A. Finkelstein (SAF). MSP designed and performed research, analyzed data, and wrote the
paper; SAF designed and performed research. Both authors commented on the manuscript at all
stages.
1.4.4 Chapter 4
Chapter 4 examines the distribution of the HBL peat C mass in relation spatially explicate
contemporary climate. It further reports on the peat physico-chemical parameters by major peat
type (bog, fen and coastal peatlands). It is in revision in the Journal of Geophysical Research –
Biogeosciences (Manuscript #: 2015JG002938). The authors of the manuscript are Maara S.
Packalen (MSP), Sarah A. Finkelstein (SAF), and James W. McLaughlin (JWM). MSP designed
and performed research, analyzed data, and wrote the paper; SAF and JWM designed and
performed research. All authors commented on the manuscript at all stages.
1.4.5 Chapter 5
Chapter 5 provides an overall summary of the dissertation, outlines important sources of
uncertainty, and identifies future research directions.
17
Chapter 2 Carbon Storage and Potential Methane Production in the Hudson
Bay Lowlands since Mid-Holocene Peat Initiation
2.1 Abstract
Peatlands have influenced Holocene carbon (C) cycling by storing atmospheric C and releasing
methane (CH4). Yet, our understanding of contributions from the world’s second largest
peatland, the Hudson Bay Lowlands, Canada (HBL), to peat-climate-C dynamics is constrained
by the paucity of dated peat records and regional C data. Here, I examine HBL peatland
development in relation to Holocene C dynamics. I show that peat initiation in the HBL is tightly
coupled with glacial isostatic adjustment through most of the record, and occurred within
suitable climatic conditions for peatland development. HBL peatlands initiated most intensively
in the mid-Holocene, when glacial isostatic adjustment was most rapid and climate was cooler
and drier. As the peat mass developed, I estimate that the HBL potentially released 1 – 7 Tg CH4
y-1
during the late Holocene. My results indicate that the HBL currently stores a globally
significant C pool of ~ 30 Pg C and provide support for a peatland-derived CH4 contribution to
the late Holocene atmosphere.
2.2 Introduction
Peatlands are an important component of the global carbon (C) cycle, as long-term atmospheric
C dioxide (CO2) sinks and methane (CH4) sources. Holocene peat accumulation in northern high
latitudes occurs under cool, humid conditions. The storage of atmospheric C in peatlands, as
partially decomposed vegetation, has resulted in a net long-term climatic cooling. Quantifying
the influence of peatland development in the global C-climate system necessitates refined
18
estimates of peatland area extent and development in relation to paleoclimate, peat C
accumulation and long-term potential atmospheric CO2 and CH4 dynamics (Gorham, 1991;
Schuldt et al., 2013). A key uncertainty stems from the paucity of detailed peatland age and C
records from the world’s second largest continuous peatland, the Hudson Bay Lowlands (HBL),
Canada. HBL peatlands are assumed to be a globally significant C pool, and peatland
development in this region is potentially driven by both glacial isostatic adjustment (GIA) and
climate. However, these hypotheses remain untested at the regional scale and have never been
supported by a larger dataset.
Prior to peatland initiation in the HBL, subsidence in excess of 200 m following the final
collapse of the Laurentide ice sheet, resulted in inundation of the region by the post-glacial
Tyrrell Sea (Dyke et al., 2003) until ~ 8.5 ky. Since this time, land has been emerging from the
Hudson and James Bays at GIA rates that are among the fastest globally (Peltier, 2004; Tuittila
et al., 2013;Webber et al., 1970). Today, a nearly continuous peat cover over low relief deposits
of glacio-marine sediments (Martini, 2006) stretches from the shoreline of Hudson and James
Bays inland to the margin of the upland Precambrian Shield. Based upon available land
classification data (Ontario Land Cover Database, 2000), peat accumulating features, dominated
by bogs and fens, occupy up to 90% of the HBL’s 372,000-km2
landscape (Riley, 2011; Ontario
Land Cover Database, 2000; Martini, 2006) or ~ 10% of the northern peatland area (Tarnocai et
al., 2009; Yu, 2012). Permafrost is estimated to occur in ~ 1% of the HBL, (Ontario Land Cover
Database, 2000; Riley, 2011), principally as continuous permafrost along the Hudson Bay coast
(Ontario Land Cover Database, 2000), and as sporadic to discontinuous permafrost in the
southern and central HBL, respectively (Riley, 2011). The HBL’s climate is microthermal,
influenced by Arctic air masses, strong winds, and ice cover on the Hudson Bay. Modern
19
average climate (1971 – 2000) is characterized by a mean annual temperature (MAT) of -2.5 ±
1.8 °C and a mean growing season temperature of 10.8 ± 1.0 °C over a period of 119 – 162 days
(McKenney et al., 2006). Total annual precipitation ranges from 430 – 740 mm with half to two-
thirds of the precipitation occurring during the growing season (McKenney et al., 2006).
Since deglaciation, recent estimates suggest that circum-Arctic peatlands have accrued ~ 500 Pg
C (Tarnocai et al., 2009; Yu, 2011; Gorham, 1991), resulting in the sequestration of even more
atmospheric C dioxide (CO2) than the 100 – 200 ppmv CO2 equivalent reported by Frolking and
Roulet (2007), which was based upon the earlier estimate of the northern peatland C sink of 250
– 450 Pg C. However, northern wetlands and especially minerotrophic peatlands (MacDonald et
al., 2006; Yu et al., 2013), are one of the strongest natural sources of CH4, and release 21 – 43 Tg
CH4 y-1
to the atmosphere (Mikaloff Fletcher et al., 2004; Levine et al., 2011). Atmospheric CH4
concentrations are mostly source-driven (Zurcher et al., 2013; Levine et al., 2011), and peatland
C dynamics can account for part of this variation (Tarnocai et al., 2009; Jones and Yu, 2010;
MacDonald et al., 2006).
Polar ice-core records reveal that atmospheric CH4 concentrations rose twice during the
Holocene: first in the early Holocene during a period of continental deglaciation (11.6 – 8 ky
BP); and second, during the mid- to late Holocene (5 ky BP – present), following a brief decline
in atmospheric CH4 (Brook et al., 2000). Although subject to large uncertainties, inter-
hemispheric CH4 gradients suggest that northern terrestrial wet ecosystems, including peatlands
can explain a portion of the variability in atmospheric CH4 during both the early and late
Holocene (Reyes and Cooke, 2011; MacDonald et al., 2006; Yu, 2011). Examination of the
dynamic coupling between peatland development and Holocene atmospheric CH4 variability may
provide additional insight into long-term peatland-climate-C dynamics.
20
During the early Holocene, high insolation and seasonality coupled with the availability of newly
exposed surfaces following deglaciation, resulted in a tripling of peatland extent and precursor
wetland conditions. Thus, post-glacial wet and reducing conditions together with rapid peatland
expansion during the early Holocene are associated with rising atmospheric CH4 concentrations
(MacDonald et al., 2006; Yu, 2011). However, global syntheses of available northern peatland
initiation dates have shown lower rates of peatland expansion after 9 ky BP, further implying
reduced peatland-derived CH4 contributions to the mid- and late Holocene atmospheres
(MacDonald et al., 2006; Yu, 2011). However, given limited detailed evidence from spatially
extensive and late emerging peatlands such as the HBL, quantification of the potential
contribution of northern peatlands to the late Holocene CH4 rise has not been fully possible (Yu,
2012; Reyes and Cooke, 2011). While northern peatlands could contribute in part to the rise in
CH4 in the late Holocene, potential alternative terrestrial sources may include early
anthropogenic activity, such as rice cultivation, biomass burning and deforestation (Ruddiman,
2003), lateral expansion of existing high latitude peatlands, permafrost dynamics (Tarnocai et al.,
2009), and/or contributions from tropical wet ecosystems (Yu, 2011; Zurcher et al., 2013).
Here, I report on the history of peat development in the HBL, using a synthesis of peat ages and
detailed peat C records. With these new data, I examine the relationship between climatic and
geophysical controls relative to peatland initiation and expansion in the HBL and evaluate the
possible connection between HBL peatland development and atmospheric C dynamics, during
the Holocene. This new evidence supports the hypothesis of potential peatland-derived CH4
emissions during the late Holocene, and fills a key gap in determining the potential contribution
of northern peatlands to late Holocene atmospheric CH4 concentrations. While climate has been
invoked as the dominant control on northern peatland initiation (Jones and Yu, 2010; MacDonald
et al., 2006; Yu et al., 2013), GIA appears to be an even more fundamental control of peatland
21
initiation and expansion in the dynamic landscape of the HBL (Peltier, 2004; Glaser et al.,
2004a; Glaser et al., 2004b). I estimate that HBL peatlands have accumulated a globally
significant peat C pool of ~ 30 Pg C over the Holocene. During the most intense period of peat
initiation between 4 – 7 ky cal BP, thin peat profiles over marine sediments may limit the
potential for CH4 emission, due to the possible influence of marine pore waters driving sulfate
reduction (Liikanen et al., 2009; Koebsch et al., 2013) and the prevailing cold, dry climate
inferred from a landscape scale pollen reconstruction (northern Quebec) (Viau and Gajewski,
2009). However, by 3 ky cal BP peatlands with a mean thickness of one meter occupied 80% of
the HBL’s current extent, and were developing under climatic conditions similar to today (Viau
and Gajewski, 2009), supporting a potential CH4 emission of 1 – 7 Tg CH4 y-1
. Acknowledging
the complexity of atmospheric CH4dynamics, a potential late Holocene CH4 emission of this
magnitude from the HBL cannot alone explain the late Holocene rise in global atmospheric CH4
concentrations. Still, my new estimates provide evidence that peatland development in the HBL
may constitute a potential CH4 contribution to late Holocene atmosphere that should be
accounted for alongside other prospective anthropogenic and natural terrestrial CH4 sources in
global C budgets.
2.3 Methods
2.3.1 Data synthesis and new peat records
I compiled 100 records of peat initiation (Table 2.10-1) from peat profiles located within the
physiographic region of the HBL (Figure 2.11-1), spatially defined according to the North
American Environmental Atlas (2009). The records were compiled for the purpose of examining
the temporal pattern of peatland initiation (Figure 2.11-2) in relation to summer and winter
22
insolation at 60 °N (Berger and Loutre, 1991), variability in ice-core inferred Holocene
atmospheric CH4 (Brook et al., 2000) and CO2 (Monnin et al., 2004), post-glacial land
emergence (Peltier, 2004), and pollen-reconstructed Holocene temperature and precipitation for
the northern Quebec, Canada ecoregion (Viau and Gajewski, 2009). The northern Quebec
ecoregion was selected for the inferred paleoclimate of the HBL due to its proximal location east
of Hudson Bay, within a similar bio-climatic setting to the HBL, and having a similar
paleoclimatic reconstruction to that produced from a pollen record of a lake core retrieved in the
northeastern HBL (McAndrews and Campbell, 1993; McAndrews et al., 1982). Land emergence,
area expansion, and the 5.5-ky and 3-ky shorelines were extracted from the 0.5-ky resolution
paleotopography dataset (Peltier, 2002; Peltier, 2004). Peatland area extent and peat thickness
were quantified to estimate the potential CH4 contribution from the HBL to the mid- and late
Holocene atmosphere, using previously reported snow-free CH4 emissions for the HBL (Pickett-
Heaps et al., 2011). The reported range used here was informed by both static chamber and
airborne CH4 flux measurements.
A literature search and a radiocarbon database were used to prepare the synthesis of all available
basal dated peat records from the HBL (Gorham et al., 2012; Gorham et al., 2007) (Table2.10-1
and original references listed therein), yielding 67 peat records meeting the following three
criteria: location (geographic coordinates) within the physiographic region of the HBL;
availability of 14
C basal peat ages obtained from the peat-mineral interface; and basal peat
material sampled from an ecosystem currently classified as a peatland. To supplement the series
of 67 records, field sampling across the Ontario portion of the HBL in 2009 – 2011 resulted in
my contribution of 33 new peat records. Basal dates for the 33 newly sampled peat cores were
obtained using accelerator mass spectroscopy (AMS) 14
C dating of plant remains of known
23
provenance and/or root-free bulk peat (n < 8) at either the Keck-CCAMS facility (Irvine, USA),
the UGAMS facility (Athens, USA) or Beta Analytic, Inc. (Miami, USA). In the absence of
time-weighted (vertically-dated) peat accumulation rates, paleo-peat thickness was estimated
using linear accumulation following peat inception (n = 100); however, I acknowledge that the
rate of peat accumulation may vary through time in the HBL.
2.3.2 Holocene peat initiation and carbon dynamics
All dates compiled from previous studies, together with the newly dated samples described
above, were calibrated using IntCal09 calibration curve (Reimer et al., 2009) in the clam package
for R (Blaauw, 2010). All calibrated most probable (median) ages are expressed as calendar
years before present (1000 y cal BP = 1 ky cal BP), where BP is equal to AD 1950. While a
histogram approach can be problematic when using calibrated radiocarbon dates spanning a
probability distribution of > 200 years, I selected a bin size equal to the mean 2σ age uncertainty
range of the calibrated radiocarbon dates considered in this study. Each date is represented by a
single occurrence in the histogram, and is plotted at the location on the x-axis of the median
calibrated initiation age (Gorham et al., 2012). Similar to other studies, I repeated the initiation
age binning process using several different numerical approaches and bin sizes (100 – 500 y) and
found that the major trends appeared to be insensitive to length of time used (Reyes and Cooke,
2011). Assessment of the lag between peat initiation and land emergence in the HBL was
accomplished by spatially relating each peat age in my record with the corresponding time of
land emergence. Using the geographic coordinates for each peat record, the corresponding time
of land emergence for that location was extracted from the paleotopography dataset (Peltier,
2002; Peltier, 2004).
24
Quantification of the total C pool was based upon bulk density, loss-on-ignition (LOI) and C
content measurements, using standard methods (Yu, 2012; Chambers et al., 2010/11), on 33 peat
cores retrieved during the 2009 – 2011 field seasons, together with a re-interpretation of seven
author-contributed raw datasets from previously published studies (Bunbury et al., 2012; Kuhry,
2008; Kettles et al., 2000; Kuhry, 1998; O'Reilly et al., 2014). Direct measurements of C content
using homogenized subsamples spanning 4 – 10 cm of core depth were completed using an
Elementar Variomax CN analyzer (Elementar Analysensysteme GmbH Donaustraße 7 63452
Hanau Germany). The summed product of incremental bulk density (g cm-3
) and peat depth (cm)
resulted in the areal dry peat mass (g cm-2
) term. Similar to other studies, areal dry peat mass for
the peat profiles examined here was strongly related to basal peat age (Zoltai, 1991; Turunen et
al., 2002). Here, I expressed areal dry peat mass as areal C mass (kg m-2
), as it was similarly
correlated with peat age. Linear regression analysis of C mass and basal peat age revealed a
significant relationship (F = 41.52; R = 0.75, p < 0.001, n = 36) described by equation 2.3.2.1:
C mass (kg m-2
) = 27.4 – [0.013 * basal peat age (y cal BP)]
(2.3.2.1)
Equation 2.3.2.1 was used to estimate areal C mass for the remaining 62 peat cores that lacked
reported peat physical properties. The total C pool for the HBL was quantified by multiplying the
mean C mass (n = 100) by the total peatland area. C pools for the inland and coastal areas were
also calculated separately using mean C masses for these respective regions. The total C pool
range was expressed in terms of the 2σ age uncertainty. Finally, the mean post-initiation C
accumulation rate for each sample was calculated by dividing the total peat C pool by the age of
the sample and expressed as g C m-2
y-1
.
25
2.4 Results
2.4.1 GIA and paleoclimate as controls of HBL peatland dynamics
Today, land is emerging at a rate of 5 – 12 mm y-1
in the HBL (Peltier, 2004), with maximum
isostatic uplift rates centered near the north-west and south-east reaches of the landscape (Figure
2.11-1). My synthesis of 33 new and 67 previously reported peat ages (Table 2.10-1) documents
the spatio-temporal variability in peat initiation and expansion in the HBL, following retreat of
the Laurentide ice sheet. The mean and standard deviation depth of peat is 203 ± 89 cm and the
median basal age is 5 ky cal BP for this series of peat records, while the oldest basal date
returned was 8.2 ky cal BP recovered from the margin of inundation by the post-glacial Tyrrell
Sea. It provides an oldest minimum peat age for the peatlands of the HBL.
Spatially associating basal peat age inventories with post-glacial land emergence histories
reveals that peat initiation in the HBL was tightly coupled with GIA (Figure2.11-2). The
theoretical rate of GIA decays exponentially with time, hence the fastest rates of isostatic uplift
occur immediately following deglaciation (Peltier, 2004). Likewise, new peatland initiation in
the HBL also appears to decline through time, with the greatest rates of peat initiation in the
HBL recorded during the period 4 – 7.5 ky cal BP, referred to here as the mid-Holocene (Figure
2.11-2). A small bias toward sampling older, deeper peatlands may contribute to the apparent
reduction in the rate of peat initiation events later in the Holocene. However, I expect that a
gridded sampling strategy across the entire HBL would reveal a comparable temporal pattern due
to increasingly smaller additions of new land with time, as a result of long-term exponentially
declining rates of GIA in the HBL. Thus a potential sampling bias toward deeper peatlands does
not undermine my conclusions.
26
Available paleoclimate data (Viau and Gajewski, 2009; McAndrews et al., 1982) for the entire
period of Holocene peatland development in the HBL indicate that climatic conditions were
continuously suitable for peatland development (Yu, 2012). However, the coldest winters and
summers together with the period receiving the least precipitation occurred between 4 – 7 ky cal
BP, synchronous with both the highest rates of peat initiation and emergence of new land
available for paludification. Declining rates of peat initiation up to and during the period of
Neoglacial cooling (~ 2 ky cal BP), were associated with cooler temperatures and more
precipitation based upon regional pollen-based paleoclimatic reconstructions (Figure 2.11-2c)
(Viau and Gajewski, 2009).Contemporary climate in the HBL likely varies spatially, due to the
seasonal influence of the Hudson Bay. However, few paleoclimatic records are available for the
HBL, restricting examination of regional climate variation on peatland initiation and C dynamics
during the Holocene.
2.4.2 HBL carbon storage and potential Holocene methane emissions
The physiographic region of the HBL is composed of two main ecoregions, which differ in terms
of spatial extent, peat age and depth, and C storage capacity. The more recently emerged coastal
HBL ecoregion accounts for 62,370 km2, while the inland HBL and James Bay Lowland (JBL)
ecoregions together account for 309,350 km2. Coastal HBL peatlands included in this study are
occupied by thin peat deposits, with a median peat depth of 58 cm and a mean peat age of 1900 ±
1830 y cal BP (n = 13). By contrast, the inland HBL peatlands studied here have a median peat
depth of 218 cm and a mean peat age of 5220 ± 1450 y cal BP (n = 86).
I estimated the total C pool for the HBL peatland complex and for each of the two principal
ecoregions, based upon peat age and C content. My results reveal a mean areal C mass of 89 ±
27
27 kg C m-2
(n = 100) for the HBL that accumulated at a mean rate of 18.5 ± 5.7g C m-2
y-1
and
has a mean C density of 0.05 ± 0.01 g cm-3
. Multiplication of the mean total peat C mass by the
total peatland surface area of 334,800 km2 results in an HBL C pool of approximately 30 Pg C
(29 – 31 Pg C). Permafrost occupies a relatively small fraction of the HBL, and although my
record proportionally includes permafrost features, possible variation in C mass from permafrost-
impacted peatlands could not be adequately assessed here due to sample limitations. However, I
considered variation in areal C mass between the inland and coastal ecoregions. I found that the
median C mass was significantly greater (U = 107, p < 0.001) inland (99 kg m-2
) than near the
coast (40 kg m-2
).Taking this difference into consideration yields an area-weighted total C pool
of ~ 27.6 Pg C for the inland HBL/JBL ecoregions and ~ 2.2 Pg C for the coastal ecoregion.
Modern seasonal CH4 fluxes range from 1 – 50 g CH4 m-2
y-1
for peatlands in the HBL (Bubier et
al., 1993b; Klinger et al., 1994; Moore et al., 1994; Roulet et al., 1994), with scaled-up areal
emissions ranging from 5 – 20 g CH4 m-2
y-1
for the HBL region (Pickett-Heaps et al., 2011;
Christensen et al., 1996), for the HBL extent defined here. Although peatland CH4 fluxes are
highly uncertain, I use the latter, more conservative regional CH4 flux range to estimate the
potential CH4 contributions from the HBL to the mid- and late Holocene atmosphere. Given a
5.5-ky cal BP HBL landscape, I find that peat could cover 50% of the modern HBL extent, with
a mean peat depth of 33 cm, and could potentially emit 0.7 – 3.7 Tg CH4 y-1
. Climate at this time
was colder and drier than today (Figure 2.11-2c) (Viau and Gajewski, 2009), and CH4 emissions
may have stemmed from the terrestrial-aquatic interface. However, areas with thin peat in
contact with marine sediments, as evidenced by the occurrence of salt marsh vegetation remains
near the peat-mineral interface (O'Reilly et al., 2014), may conversely experience reduced
potential CH4 flux, due in part to interactions with marine pore and surface waters (Koebsch et
28
al., 2013; Liikanen et al., 2009). By contrast, a 3-ky cal BP HBL peatland landscape could
occupy approximately 80% of the modern peatland surface area, with a mean peat thickness of
one meter. The accrual of a thick peat mass could limit the influence of the marine sediments
upon late Holocene peatland dynamics. Accordingly, a 3-ky cal BP peatland extent, operating
under climatic conditions similar today (as indicated by the regional paleoclimate record), could
support a potential CH4 flux of 1.2 – 6.8 Tg CH4 y-1
to the late Holocene atmosphere. Given a
unit conversion factor 2.78 Tg CH4 ppb-1
and an 8.4-y CH4 lifetime (Zurcher et al., 2013; Levine
et al., 2011), the potential CH4 emissions from the HBL during the late Holocene could account
for an increase of 4 – 21 ppb. I used a landscape scale estimate of snow-free CH4 flux for the
geographic extent of the HBL, according to the spatial extent described here. Estimates from
static chamber measurements are subject to large seasonal to inter-annual variability, and if
considered alone may predict reduced potential CH4 emissions at the local scale (Klinger et al.,
1994; Roulet et al., 1994) or enhanced potential CH4 flux up to six times greater than these
regional scale emission rates (Bubier et al., 1993a; Smith et al., 2004).
2.5 Discussion
My synthesis confirms that peat initiation in the HBL began several millennia later than many
northern peatland regions, including the extensive peatlands of western Canada and Siberia. Two
periods of intensified peat initiation are evident during the mid-Holocene. The first occurred
between 5.6 – 7 ky cal BP, as new land emerged from the Tyrrell Sea approximately four times
faster than the modern rate of isostatic uplift (Glaser et al., 2004b). Following a ~ 0.6 ky
apparent gap in peat initiation, a second phase of intensified peat initiation occurred between 4.2
– 5 ky cal BP (Figure 2.11-2), when isostatic uplift was approximately three times faster than
today. While the gap between the two most intense periods of initiation in the HBL may be due
29
to a lack of available peat records in this dataset, other studies have attributed similar peat
initiation gaps to the occurrence of large scale drainage events. In the HBL, regional drainage
events resulting from outbursts of dammed water features or uplift-mediated changes in stream
flow paths have been documented (Sjörs, 1959; Glaser et al., 2004a).
Half of the peatlands examined here initiated synchronously with land emergence from the post-
glacial Tyrrell Sea (Figure 2.11-2b). However, comparison of cumulative peat initiation and
post-glacial land emergence curves reveal a divergence after 5.8 ky cal BP, suggesting a time lag
between emergence of available land and establishment of peat-accumulating features on the
parcels of newly emerged land. This apparent temporal lag in peat initiation relative to the
emergence of the corresponding parcel of land appears to have continued until present. Syntheses
of North American peat initiation records have reported lags between land emergence and peat
initiation of up to 4 ky throughout the Holocene, which have been attributed in part to the slow
migration of peatland plant propagules to the newly emerging surface (Gorham et al., 2012).
However in the HBL, peat-forming vegetation appears to have rapidly established on the newly
emerging land between 5.6 –7 ky cal BP, thus the slow migration of peatland plant propagules
does not adequately account for the time lag between land emergence and delayed peat initiation
on emerged landscapes evident since 5 ky cal BP (Glaser et al., 2004b; Glaser et al., 2004a).
Temporal delays in peatland initiation may also be attributed in part to local inundation of the
emerging land. The structure of peatland vegetation communities and water table position are
tightly coupled, and together control vertical peat growth, C dynamics, and the subsequent
redistribution of water within the peatland (Beilman et al., 2009; Belyea and Baird, 2006; Sjörs,
1959). The hydrogeologic setting in the HBL is influenced by regional (e.g., 102 km
2) variability
in GIA, and may account for the time lags in peat initiation in my record (Gorham et al., 2012;
30
Glaser et al., 2004a). Glaser et al. (2004a) hypothesize that faster rates of isostatic uplift in
downstream reaches compared to corresponding headwaters can reduce slope, impede drainage,
re-direct river channels, and contribute to anoxic conditions and peatland pattern development in
peatlands of the southern HBL (Glaser et al., 2004a; Glaser et al., 2004b). My results suggest
that similar processes occur across the HBL, whereby the main environmental factor driving the
initiation and development of peatlands is the sustained occurrence of waterlogged soils (Glaser
et al., 2004a; Glaser et al., 2004b; Gorham, 1991). In addition, pond development behind beach
ridges during progressive shoreline retreat or winter ice scour in emerging high energy
coastlines, and forest formation on beach ridge prior to peat initiation, as a consequence of
regional isostatic uplift dynamics, are further evidence of the HBL’s dynamic landscape (Riley,
2011; Glaser et al., 2004b; Glaser et al., 2004a; Martini, 2006). Thus, the ecological response to
hydrogeologic variability can delay paludification of available land and/or terrestrialization of
lakes and ponds.
Previous global peatland syntheses indicate that the period of most intense northern peatland
initiation has been driven by optimal climatic conditions, including high insolation and
seasonality, during the early Holocene (Jones and Yu, 2010; MacDonald et al., 2006; Yu et al.,
2010). By contrast, comparisons with available paleoclimate reconstructions reveal an opposite
trend in the HBL, such that the most intense period of peatland initiation in the HBL occurred
under less favourable conditions. A regional (northern Quebec, Canada) pollen-derived
temperature and precipitation reconstruction (Viau and Gajewski, 2009) indicates small climatic
anomalies relative to modern climate in the HBL during the Holocene, with the warmest period
occurring prior to land emergence from the Tyrrell Sea. Consequently, all peat initiation and
expansion in the HBL progressed under conditions of decreasing summer insolation, increasing
31
winter insolation (Figure 2.11-2), and declining seasonality as inferred from summer-winter
insolation differences. In contrast to the warm climate and high summer insolation that supported
rapid peat initiation in other northern peatlands during the early Holocene (Yu, 2012;
MacDonald et al., 2006), maximum rates of peat initiation in the HBL occurred when the climate
was coldest and driest (Figure 2.11-2). While it is possible that peat initiation and expansion may
have occurred more rapidly in the HBL under optimal climatic conditions, as it did elsewhere in
the early Holocene (MacDonald et al., 2006), the tight coupling between land emergence and
peat initiation under cooler/drier mid-Holocene climatic conditions does not appear to have
substantively limited the timing or rate of peat initiation. Since 3.5 ky cal BP, peat initiation in
the HBL remained relatively low, consistent with declining rates of Neoglacial peat initiation for
northern peatlands (Jones and Yu, 2010; MacDonald et al., 2006). However, declining rates of
peat initiation in the HBL occurred under comparatively warmer and wetter conditions relative to
the mid-Holocene HBL climate (Viau and Gajewski, 2009), and appear to lag land emergence.
Examination of North American pollen records of peatland indicators reveals a climatic optimum
for peatland extent characterized by a MAT between -2 and 6 °C and mean total annual
precipitation (MAP) between 630 and 1300 mm (Gajewski et al., 2001). Further evidence from
the West Siberian Lowlands indicates that maximum peatland extent and C pools occur in
association with a MAT of 0 °C, which may be a potential climatic optimum for maximizing the
difference between net primary production and decomposition (Beilman et al., 2009; Sheng et
al., 2004; Swanson et al., 2000). Modern climate across most of the HBL falls within these
boundary conditions. Thus, in spite of some documented paleoclimatic variability in the HBL
(McAndrews et al., 1982; Bunbury et al., 2012; O'Reilly et al., 2014), peatland development has
proceeded within climatic boundary conditions for northern peat initiation and expansion for
32
much of the peat record. Further, on-going rapid land emergence, which results in a lowering of
the regional hydraulic gradient, reduced drainage and sustained waterlogged soil conditions, may
mask the impact of climatic variation on peatland dynamics in the HBL. Hence, the weaker
relationship between climate and peat initiation and expansion in the HBL may be accounted for
by a limited ability to consider paleoclimatic variation spatially, and a lack of evidence in
support of wide temporal variation in climate at the landscape scale in the HBL.
Anticipated high latitude warming may increase peatland primary productivity and C storage;
however, enhanced hydroclimatic variability and intensified land use may augment peatland C
decomposition and CO2/CH4 emissions, and result in a positive feedback to climatic warming.
Future climate scenarios for the HBL indicate a 2 – 5 °C temperature increase over the next
century, with a concomitant increase in precipitation of 10 – 15% (McKenney et al., 2011). The
implications of these projections may be that warmer, wetter conditions and a longer growing
season in the HBL may support continued peatland development and sustained C sequestration
via enhanced primary productivity (Charman et al., 2013). However, results from a multi-year
investigation concerning water balance thresholds in the northern HBL provide confounding
evidence, whereby the predicted precipitation increases over the next century in the HBL may
not be sufficient to offset the growing season water deficit potential brought about by increasing
temperatures and enhanced evaporation (Rouse, 1998). Therefore, disproportionate future
temperature increases relative to concomitant precipitation increases may result in reduced
peatland C storage capacity in the HBL, with C losses potentially exacerbated by increased fire
risk associated with net drier conditions (Yu, 2012) or enhanced anthropogenic activity, such as
infrastructure development (Kuhry, 2008). Hence, further examination of the spatio-temporal
33
relationship between paleoclimate and C dynamics is warranted, and may reveal a dynamic
coupling between the two processes, as it has elsewhere (Charman et al., 2013).
Based upon new detailed records of peat age and areal C mass, I estimate that the HBL currently
stores ~ 30 Pg C, equivalent to approximately 20% of the North American peatland C pool
(Gorham et al., 2012) or approximately 6% of the northern peatland C pool (Yu, 2012). While
my synthesis includes C data for a range of forested and non-forested bog and fen features, I
recognize that the C mass contribution from small bog/fen pools, permafrost features, and
densely forested peatlands may be under- or over represented in my C pool estimate, and this
requires additional investigation. Although refined estimates of land cover and C mass are
needed in the HBL, my evidence provides the most comprehensive synoptic assessment of the
modern HBL C pool and demonstrates that the HBL represents a globally significant C sink,
underscoring its importance in regional and global C assessments. Still, three decades of peatland
CH4 emission measurements have shown that modern HBL peatlands can be persistent sources
of CH4 to the atmosphere (Klinger et al., 1994; Moore et al., 1994; Pickett-Heaps et al., 2011;
Roulet et al., 1994), at a range of spatio-temporal scales.
Peatlands release CH4 to the atmosphere by ebullition, diffusion, and plant- mediated transport,
however, potential flux estimates are subject to both spatial and temporal uncertainty, due in part
to probable non-linear feedbacks among potential controls over peatland CH4 flux (Belyea and
Baird, 2006; Yu, 2011). Complex patterns and large ranges (at least 1 – 50 g CH4 m-2
y-1
) in
contemporary CH4 emissions in the HBL have been variously explained by a combination of
moderate water table position, gradients in growing seasonal temperatures, and heterogeneities in
water chemistry and vegetative cover (Bubier et al., 1993b; Klinger et al., 1994; Moore et al.,
1994; Roulet et al., 1994). Consequently, important sources of CH4 emissions in the HBL have
34
been reported from wet minerotrophic peatlands and those undergoing ombrotrophication,
wetlands at the aquatic-terrestrial interface, and young coastal fens (Bubier, 1995; Bubier et al.,
1993b; Klinger et al., 1994; Moore et al., 1994; Whiting, 1994). However, recent evidence
suggests that CH4 fluxes from shallow coastal systems may be suppressed by sulphur-rich marine
sediments in brackish environments, depending in part upon the vegetation community present
(Koebsch et al., 2013; Liikanen et al., 2009). Thus, the potential for sulphur-mediated CH4
suppression is recognized here in relation to young or early emerging peatlands. Similarly,
landscape-scale assessments using aircraft and surface measurements, together with a chemical
transport model indicate that the inland HBL may release peatland-derived CH4 at twice the rate
of the HBL’s coastal regions (Pickett-Heaps et al., 2011; Christensen et al., 1996). Accordingly,
modern CH4 emissions from the coastal region of the HBL reportedly range between 5 and 10 g
CH4 m-2
y-1
, while the CH4 emission from the inland peatlands of the HBL range between 15 and
20 g CH4 m-2
y-1
. Owing to the unique geographic setting of this region in a zone of rapid
isostatic uplift and the fact that emerging peatlands pass through a salt marsh or marine-
influenced phase, this modern evidence supports my hypothesis that maximum CH4 flux from
HBL peatlands is related to the development of the peat mass rather than simply peat initiation
(Glaser et al., 2004a; O'Reilly et al., 2014).
Although estimates of CH4 flux from peatlands to the atmosphere can be highly dynamic, the
possible connection between HBL peatland development and potential CH4 emissions can be
evaluated using comparisons between HBL initiation histories and inferred potential CH4 fluxes
(Figure 2.11-2). Peatland development, rather than simply initiation, has been proposed to
explain the relationship between CH4 flux and peat development in the early Holocene (Yu et al.,
2013; MacDonald et al., 2006; Smith et al., 2004; Jones and Yu, 2010). In the HBL, I find that
35
during the most intense period of peat initiation, ~ 5.5 ky cal BP, the landscape could potentially
support CH4 emissions of 0.7 – 3.7 Tg CH4 y-1
from minerotrophic peatlands and pre-cursor
marsh conditions, the occurrence of which is evidenced by reconstruction of past environments
following examination of testate amoebae, plant macrofossils and pollen (Bunbury et al., 2012;
O'Reilly et al., 2014; Glaser et al., 2004a; Glaser et al., 2004b). However, colder, drier conditions
during the mid-Holocene together with a possible biogeochemical connection between shallow
peat and the underlying marine sediments likely limited the CH4 flux to the lower end of the
range.
Temporal lags in peat initiation relative to land availability observed after 5 ky cal BP, coupled
with expansion and patterning of peatlands with C rich pools could further contribute to
enhanced peat-derived CH4 from the HBL to the late Holocene atmosphere, owing to
decomposition of both new and old peat (Korhola et al., 2010; Pelletier et al., 2007; Yu, 2011).
Therefore, following several millennia of peatland expansion, a 3-ky cal BP HBL landscape
could have potentially contributed 1.2 – 6.8 Tg CH4 y-1
to the late Holocene atmosphere, from
the widespread minerotrophic to ombrotrophic peatlands (Glaser et al., 2004a; Klinger et al.,
1994; Riley, 2011) that were functioning under climatic condition similar to today. My late
Holocene estimates of CH4 emissions are comparable to those reported using an Earth system
modeling approach (Kleinen et al., 2012; Schuldt et al., 2013; Zurcher et al., 2013). Using a
modeling approach, the HBL was predicted to release ~ 5 Tg CH4 y-1
, equivalent to ~ 10% of
boreal CH4 emissions (Pickett-Heaps et al., 2011; Schuldt et al., 2013). Further, modeled CH4
emissions for the broader Hudson Bay region, which includes the HBL as defined here and the
adjacent upland region on the Precambrian Shield, suggest a mean snow-free emission of 1.6 Tg
CH4 y-1
for the period 6 ky cal BP to pre-industrial levels (Kleinen et al., 2012; Schuldt et al.,
36
2013; Zurcher et al., 2013). Additionally, model evidence used to examine the mid-Holocene
decline in CH4 emissions associated with the 8.2 ky cal BP event, which further confirmed a
slight increase in CH4 emissions from the newly emerging HBL peatlands, at a time when
reduced CH4 emissions from northern peatlands were predicted in response to prevailing
cooler/drier climatic conditions (Zurcher et al., 2013). Considered alone, the potential CH4 fluxes
from the HBL are small on a global scale. While acknowledging the contributions from other
biospheric CH4 sources to late Holocene atmospheric CH4 dynamics, including tropical wet
ecosystems and early anthropogenic activity, my estimates of potential CH4 flux do convey the
role of the HBL in CH4 accounting. I suggest that a contribution from this large region, together
with global peatlands, to atmospheric CH4 dynamics cannot be ignored.
2.6 Conclusion
In conclusion, my findings confirm that the most intense period of peat initiation in the HBL
occurred during the mid-Holocene, when the climate was colder and received less precipitation
than today (Figure 2.11-2c) (Viau and Gajewski, 2009) and insolation was decreasing relative to
the early Holocene (Figure 2.11-2a) (Berger and Loutre, 1991). While climatic conditions in the
HBL remained suitable for peat initiation through the mid- to late Holocene, evidence from my
study reveals a tight coupling between rapid land emergence from the Tyrrell Sea and peat
initiation, as a consequence of GIA. The rate of isostatic uplift varies spatially across the HBL
resulting in dynamic hydrogeologic conditions, and suggests that the mid-Holocene delay in peat
initiation relative to apparent land emergence may be attributed to localized inundation of the
land surface. Based upon new detailed records of peat C density, I estimate that the HBL
currently stores ~ 30 Pg C, which is equivalent to ~ 20% of the North American (Gorham et al.,
2012) or ~ 6 % of the northern peatland C pool (Yu, 2012). The highest rate of peat initiation in
37
the HBL occurred prior to the late-Holocene rise in atmospheric CH4 inferred from ice-cores.
Using modern regional estimates of snow-free CH4 flux from the HBL, scaled to a mid- and late
Holocene HBL landscape, I further estimate a potential late Holocene CH4 flux of 1 – 7 Tg CH4
y-1
. This value represents a small but important potential CH4 emission, when compared to the 21
– 43 Tg CH4 yr-1
released by modern northern wetlands (Mikaloff Fletcher et al., 2004).
Increased drought frequency and severity are anticipated at many high latitude peatlands
(Gorham, 1991; Jones and Yu, 2010). However, future climate scenarios predict that the HBL
may experience a warmer and wetter climate over the next century (McKenney et al., 2011),
permitting the region to continue to function as a long-term C sink. Nevertheless,
disproportionate hydroclimatic change and intensified anthropogenic pressure could decouple the
peatland water balance in the HBL, reduce peatland C storage capacity, and result in a positive
feedback to climatic warming. Improved quantification of modern peatland C stores and regional
controls on Holocene peatland C dynamics in the HBL will reduce the uncertainty surrounding
the sensitivity of northern peatland C pools under future climate scenarios.
2.7 Acknowledgements
Research funding and field support was provided by the Ontario Ministry of Natural Resources’
Applied Research and Development Branch and Far North Branch, under the auspices of projects
CC-021 and FNIKM 028 to JWM. Additional support for field work and radiocarbon dating was
provided by grants (327197-11 and 331284-11) from the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Ontario Ministry of the Environment through the
Climate Change and Multiple Stressor Research Program at Laurentian University to SAF.
Graduate stipend support to MSP was provided in the form of an NSERC Alexander Graham
Bell Canada Postgraduate Scholarship (CGSD2-426611-2012), an Ontario Graduate Scholarship,
38
and an Association of Canadian Universities for Northern Studies, Canadian Northern Studies
Trust Scholarship. Field research grants from the Society of Wetland Scientists and Aboriginal
Affairs and Northern Development Canada’s Northern Scientific Training Program were also
provided to MSP. We thank A. Dyke for providing access to the Canadian basal radiocarbon
database and P. Kuhry for contributing raw peat core data. We also thank three anonymous
reviewers for their thoughtful comments to previous draft manuscripts.
2.8 Author contributions
M.S.P. designed and performed research, analyzed data, and wrote the paper; S.A.F and J.W.M.
designed and performed research. All authors commented on the manuscript at all stages.
2.9 Competing financial interests
The authors declare no competing financial interests.
39
2.10 Tables
Table 2.10-1 Location, age, and carbon (C)-mass of peat cores located in the Hudson Bay Lowlands, Canada (HBL), sorted by latitude.
All conventional 14
C dates were re-calibrated with the IntCal09 calibration curve (Reimer et al., 2009). New HBL basal peat dates (n=33)
contributed by the authors, appear in bold. Note: *Basal peat depth not identified in reference. **C content determined via loss-on-
ignition, at a level of 48% C. ***Areal C mass determined by direct measurement of complete peat profiles (n=38), including samples
(N) 10, 12, 40-44, 52-62, 64-66, 68-71, 83-86, 89-92, 101, 102, 106, 128. If peat physical data were not available (n=62), total C mass
was estimated using a regression model (see methods).
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y
BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
3 Squirrel R 50.07 -83.88 173 278 GSC-6321
basal
woody
peat
3010 ± 60 3210 3010-
3360 68 (1)
7 Harricana R 50.70 -79.33 59 276 Qu-498 wood base
of peat 4200 ± 120 4720
4420-
5210 88 (2)
8 Birthday R 50.70 -79.33 59 218 Y-1164 basal peat 4110 ± 100 4640 4320-
4860 87 (3)
9 Birthday R 50.72 -79.33 50 200 GSC-1493 basal peat 3920 ± 130 4350 3980-
4810 83 (4)
10 KR3A** 50.74 -84.60 123 271 UGAMS-
12713
wood
fragments 6020 ± 30 6860
6760-
6950 126
Current
Study
12 PL101 50.94 -80.33 45 264 UGAMS-
12711
Sphagnum
remains 2840 ± 30 2950
2870-
3060 109
Current
Study
13 Old Man bog 51.03 -84.53 97 420 XXX-06b basal peat 5980 ± 100 6830 6570-
7160 115 (5)
14 Oldman bog-8502 51.07 -84.50 99 445 Beta-42381 basal peat 5920 ± 90 6750 6500-
6960 114 (6)
15 Jaab L site 51.15 -83.05 108 353 WAT-2571 basal peat 5560 ± 110 6360 6030-
6640 109 (7)
17 Sesi M. SWF-
8506A 51.23 -83.03 102 149 Beta-54594 basal peat 5200 ± 60 5970
5760-
6180 104 (6)
40
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
30 Albany Forks 51.42 -84.80 162 450 GSC-885 basal peat 5820 ± 150 6640 6300-
6980 112 (10)
31 Coastal fen site 51.47 -80.62 9 81 WAT-2541 basal peat 1090 ± 70 1010 800-
1180 40 (7)
32 Carling L site 51.50 -81.67 68 258 WAT-2543 basal peat 4110 ± 70 4640 4440-
4830 87 (7)
33 Interior fen site 51.51 -81.88 72 125 WAT-2542 basal peat 1960 ± 70 1910 1730-
2110 52 (7)
34 Kinosheo L, 300 51.55 -81.83 68 98 WAT-2549 basal peat 2580 ± 80 2630 2370-
2850 61 (7)
35 Kinosheo L, 100 51.55 -81.83 68 98 WAT-2554 basal peat 2870 ± 80 3010 2790-
3240 66 (7)
36 Kinosheo L, 200 51.55 -81.83 68 138 WAT-2556 basal peat 3450 ± 70 3720 3490-
3900 75 (7)
37 Kinosheo L, 300 51.55 -81.83 68 218 WAT-2557 basal peat 3910 ± 70 4340 4100-
4520 83 (7)
38 Kinosheo L, 390 51.55 -81.83 68 265 WAT-2572 basal peat 4110 ± 80 4640 4440-
4830 87 (7)
39 Kinosheo L, 490 51.55 -81.83 68 253 WAT-2567 basal peat 3160 ± 70 3390 3220-
3560 71 (7)
40 KJ101 51.55 -81.82 68 297 UGAMS-
12719
bryophyte
remains 4100 ± 25 4610
4520-
4810 130
Current
Study
41 Kinosheo L
Bog** 51.55 -81.81 68 265 TO-4318
wood near
basal 4000 ± 80 4480
4240-
4810 85 (11)
41
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
30 Albany Forks 51.42 -84.80 162 450 GSC-885 basal peat 5820 ± 150 6640 6300-
6980 112 (10)
31 Coastal fen site 51.47 -80.62 9 81 WAT-2541 basal peat 1090 ± 70 1010 800-
1180 40 (7)
32 Carling L site 51.50 -81.67 68 258 WAT-2543 basal peat 4110 ± 70 4640 4440-
4830 87 (7)
33 Interior fen site 51.51 -81.88 72 125 WAT-2542 basal peat 1960 ± 70 1910 1730-
2110 52 (7)
34 Kinosheo L, 300 51.55 -81.83 68 98 WAT-2549 basal peat 2580 ± 80 2630 2370-
2850 61 (7)
35 Kinosheo L, 100 51.55 -81.83 68 98 WAT-2554 basal peat 2870 ± 80 3010 2790-
3240 66 (7)
36 Kinosheo L, 200 51.55 -81.83 68 138 WAT-2556 basal peat 3450 ± 70 3720 3490-
3900 75 (7)
37 Kinosheo L, 300 51.55 -81.83 68 218 WAT-2557 basal peat 3910 ± 70 4340 4100-
4520 83 (7)
38 Kinosheo L, 390 51.55 -81.83 68 265 WAT-2572 basal peat 4110 ± 80 4640 4440-
4830 87 (7)
39 Kinosheo L, 490 51.55 -81.83 68 253 WAT-2567 basal peat 3160 ± 70 3390 3220-
3560 71 (7)
40 KJ101 51.55 -81.82 68 297 UGAMS-
12719
bryophyte
remains 4100 ± 25 4610
4520-
4810 130
Current
Study
41 Kinosheo L
Bog** 51.55 -81.81 68 265 TO-4318
wood near
basal 4000 ± 80 4480
4240-
4810 85 (11)
42
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y
BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
42 KJ4-3 51.59 -81.78 66 131 UGAMS-
12717
wood
fragments 3630 ± 25 3940
3870-
4070 71
Current
Study
43 KJ3-3 51.59 -81.79 65 176 UCIAMS-
97825
wood +
herb
stems
4170 ± 20 4720 4620-
4830 80
Current
Study
44 KJ2-3 51.59 -81.76 65 246 UCIAMS-
97824
wood +
herb
stems
4130 ± 25 4860 4840-
4960 99
Current
Study
45 Sesi M SFen-
8505A 51.62 -82.28 69 166 Beta-53064 basal peat 5370 ± 80 6150
5950-
6300 106 (6)
46 BelecL bog-
8507A 51.62 -82.28 69 236 Beta-54598 basal peat 3960 ± 60 4420
4180-
4780 84 (6)
47 Belec L Int
9210A 51.62 -82.28 69 98 Beta-66733 basal peat 4010 ± 80 4500
4240-
4810 85 (6)
48 Belec L-fen-
9210B 51.62 -82.28 69 109 Beta-66735 basal peat 3840 ± 70 4250
4000-
4430 82 (6)
49 Wabassie bog-
9214 51.72 -83.63 75 209 Beta-64925 basal peat 4500 ± 90 5140
4870-
5450 93 (6)
50 Blackbear-8508A 51.72 -83.23 62 223 Beta-45905 basal peat 3730 ± 50 4080 3930-
4240 80 (6)
51 Albany R 52.25 -81.58 3 96 WAT-1504 basal peat 1350 ± 70 1270 1090-
1390 44 (1)
52 D001 52.67 -84.02 92 231 Beta-310877 bulk peat 5240 ± 30 5980 5920-
6180 102
Current
Study
53 D002 52.71 -84.00 91 219 Beta-310878 bulk peat 5080 ± 30 5810 5750-
5910 83
Current
Study
43
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
54 D003 52.76 -84.14 92 223 Beta-310879 bulk peat 5100 ± 30 5810 5750-
5920 80
Current
Study
55 D200 52.62 -84.12 118 240 Beta-310880 bulk peat 5420 ± 30 6240 6190-
6290 84
Current
Study
56 D201 52.72 -84.25 104 201 Beta-310881 bulk peat 4400 ± 30 4960 4870-
5210 106
Current
Study
57 D202 52.71 -84.32 105 215 Beta-310882 bulk peat 5060 ± 30 5820 5740-
5900 99
Current
Study
58 D204 52.54 -84.58 143 129 Beta-310883 bulk peat 2900 ± 30 3740 2950-
3160 64
Current
Study
59 D206 52.65 -84.58 142 257 Beta-310884 bulk peat 6300 ± 30 7220 7160-
7290 105
Current
Study
60 VM4-5 52.70 -84.18 103 286 UGAMS-
11673
wood
fragment 5530 ± 25 6320
6290-
6400 108
Current
Study
61 VM4-3 52.71 -84.18 103 304 Beta-281004 conifer
fragments 5890 ± 40 6710
6570-
6580 122 (12)
62 VM4-1 52.71 -84.19 105 311 UGAMS-
12718
needles;
wood 5550 ± 25 6340
6300-
6400 126
Current
Study
63 MS15 R2 52.71 -84.17 102 254 UCIAMS-
94010
wood
fragments 5745 ± 20 5640
6480-
6630 100
Current
Study
64 VM3-3 52.71 -84.17 102 242 Beta-281777 conifer
needles 5640 ± 40 6420
6320-
6490 104 (13)
65 VM3-2 52.71 -84.17 102 262 UGAMS-
12716
wood
fragments 5620 ± 25 6400
6310-
6450 127
Current
Study
44
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
66 VM3-5 52.71 -84.17 102 232 UGAMS-
11672
wood
fragments 5750 ± 25 6550
6480-
6640 139
Current
Study
67 TS01 52.72 -83.94 86 218 UCIAMS-
94011 Seeds 5140 ± 20 5910
5770-
5940 103
Current
Study
68 VM1-3 52.72 -83.94 86 210 UGAMS-
12715
herbaceous
remains 4390 ± 20 4940
4870-
5040 98
Current
Study
69 VM2-1 52.72 -83.94 86 211 UGAMS-
12710
Sphagnum
remains 4470 ± 25 5180
4980-
5290 85
Current
Study
70 VM2-3 52.72 -83.94 86 207 UGAMS-
11426 Wood 4100 ± 30 4610
4450-
4810 90
Current
Study
71 VM2-5 52.72 -83.94 86 180 UGAMS-
11674
wood
fragments 4980 ± 25 5700
5640-
5850 84
Current
Study
72 IB wetland 52.73 -77.77 88 188 Beta-254946 basal peat 4920 ± 40 5650 5590-
5730 100 (14)
73 OFL wetland 52.78 -77.51 102 207 Beta-251799 basal peat 4970 ± 40 5700 5600-
5880 100 (14)
74 BF 52.73 -78.50 50 72 Beta-251816
Picea
needles,
conifer
bark
110 ± 40 120 (-4)-270 29 (14)
78 Attawapiskat 53.12 -85.42 117 198 GSC-31 basal peat 5670 ± 110 6470 6280-
6730 110 (15)
79 Attawapiskat R 53.13 -85.30 108 128 GrN-1925 basal peat 4940 ± 80 5690 5490-
5900 100 (16)
80 ML201 54.24 -87.74 143 235 UGAMS-
12712
Sphagnum
remains 7350 ± 30 8160
8040-
8290 108
Current
Study
45
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C Age
± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
81 Hawley L 54.57 -84.67 137 292 GSC(24)7 basal peat 5580 ± 150 6380 6000-
6710 109 (17)
82 Palsa bog 54.57 -84.63 113 89 BGS-6 basal peat 1897 ± 63 1840 1700-
1990 51 (18)
83 HL02 54.61 -84.60 85 230 UGAMS-
11422
Sphagnum
remains 4020 ± 25 4480
4420-
4570 92
Current
Study
84 HL03 54.68 -84.60 99 269 UGAMS-
11423
Sphagnum
remains 2700 ± 25 3550
3290-
3880 122
Current
Study
85 HL04A(West) 54.75 -84.53 89 93 UGAMS-
11424
wood
fragments 4960 ± 30 5685
5610-
5740 73
Current
Study
86 HL04B(East) 54.75 -84.50 97 105 UGAMS-
11251 wood 4400 ± 25 4960
4870-
5040 73
Current
Study
87 C Henrietta
Maria 55.00 -82.33 2 122 GSC-231 basal peat 1210 ± 130 1130
800-
1360 42 (17)
88 Shagamu R
027a* 55.05 -86.75 126 -- BGS-702 basal peat 4020 ± 100 4510
4240-
4830 85 (19)
89 PB3-5 55.23 -84.33 10 44 UGAMS-
11421
Sphagnum
stems 610 ± 20 600 550-650 22
Current
Study
90 PB-SF-5 55.23 -84.30 11 24 UGAMS-
11670
wood
fragments 1340 ± 20 1280
1190-
1300 22
Current
Study
91 PB2-5 55.23 -84.33 10 45 UGAMS-
11420
Sphagnum
stems F14C=1.01 ± 0.003 Modern -- 23
Current
Study
92 PB1-5 55.24 -84.32 9 42 UGAMS-
12714
Sphagnum
stems 610 ± 25 600 550-650 21
Current
Study
46
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
93 Shagamu R
208a* 55.35 -86.70 105 -- BGS-708
basal
peat 5600 ± 140 6400
6020-
6730 109 (19)
94 Shagamu R
62Ca* 55.60 -86.65 32 -- BGS-822
basal
peat 2350 ± 100 2420
2150-
2710 58 (19)
95 Shagamu R
62Ba* 55.65 -86.60 23 -- BGS-709
basal
peat 2060 ± 100 2040
1820-
2310 54 (19)
96 Shagamu R
62Aa* 55.70 -86.51 4 -- BGS-820
basal
peat 800 ± 100 740 560-930 37 (19)
97 Charlebois 56.67 -94.08 117 240 GSC-2760 basal
peat 6280 ± 80 7200
6990-
7420 120 (20)
99 ThibaudeauStn 57.08 -94.16 122 155 GSC-5213 basal
peat 6240 ± 80 7150
6940-
7410 119 (18)
100 Silcox 57.17 -94.24 142 73 GSC-5245 basal
peat 3120 ± 60 3340
3170-
3470 70 (21)
101 Herchmer Bog** 57.38 -94.20 106 167 AECV-
1715C
basal
peat 5970 ± 90 6810
6560-
7150 148 (22)
102 Herchmer Fen** 57.38 -94.20 106 203 AEVC-1714 bulk peat 5580 ± 80 6373 6210-
6550 105 (23)
103 Lost
Moose/O'Day 57.57 -94.32 113 130 GSC-5221
basal
peat 4270 ± 70 4840
4580-
5040 89 (21)
106 McClintock
Bog** 57.80 -94.21 79 163
AECV-
1718C wood 5810 ± 90 6610
6410-
6840 143 (22)
108 Fletcher L 58.17 -93.83 49 324 GSC-3988 basal
peat 3400 ± 60 3650
3480-
3830 74 (24)
47
Table 2.10-1, continued
N Site Name Lat
(dd)
Long
(dd)
Elevation
(m asl)
Basal
Depth
(cm)
Radiometric
Lab
Number
Material
Dated
Conventional 14C
Age ± error (y BP)
Calibrated Age
(median, y cal
BP)
2σ Age
Range
Carbon
Mass (kg
m-2)***
Footnotes
123 Shagamu R
027b* 55.05 -86.75 126 -- BGS-800
basal
peat 5050 ± 100 5800
5600-
5990 102 (19)
124 Shagamu R
208b* 55.35 -86.70 105 -- BGS-801
basal
peat 5270 ± 100 6060
5760-
6290 105 (19)
125 Shagamu R
62Cb* 55.60 -86.65 32 -- BGS-710
basal
peat 2250 ± 110 2250
1950-
2700 56 (19)
126 Shagamu R
62Bb* 55.65 -86.60 23 -- BGS-821
basal
peat 1725 ± 100 1640
1410-
1870 48 (19)
127 Shagamu R
62Ab* 55.70 -86.51 4 -- BGS-701
basal
peat 700 ± 100 660 520-900 36 (19)
128 McLintock Fen** 57.80 -94.20 81 148 AECV-
1717C
basal
peat 4060 ± 100 4570
4290-
4840 68 (23)
Footnotes
1 Gorham, E., Lehman, C., Dyke, A., Clymo, D. & Janssens, J. Long-term carbon sequestration in North American peatlands. Quat. Sci. Rev.58, 77-82,
doi:10.1016/j.quascirev.2012.09.018 (2012)
2 Dionne, J. C. Radiocarbon dates on peat and tree remains from the James Bay area, subarctic Quebec. Can. J. Forest Res.9, 125-129 (1979).
3 Stuiver, M., Deevey, E. S. & Rouse, I. Yale naturalradiocarbonmeasurements VIII. Radiocarbon 5, 312-341 (1963).
4 Skinner, R. Quaternary stratigraphy of the Moose River basin, Ontario. (Geologic Survey of Canada, Bulletin 225, 1973).
5 Hansen, B. C. Conifer stomate analysis as a paleoecological tool: An example from the Hudson Bay Lowlands. Can. J. Bot. 73, 244-252 (1995).
6 Glaser, P. H., Siegel, D. I., Reeve, A. S., Janssens, J. A. &Janecky, D. R. Tectonic drivers for vegetation patterning and landscape evolution in the Albany River
region of the Hudson Bay Lowlands. J. Ecol.92, 1054-U1052, doi:10.1111/j.0022-0477.2004.00930.x (2004).
7 Klinger, L. F., Zimmerman, P. R., Greenberg, J. P., Heidt, L. E. & Guenther, A. B. Carbon trace gas fluxes along a successional gradient in the Hudson Bay
Lowland. J. Geophys. Res.-Atmos.99, 1469-1494, doi:10.1029/93jd00312 (1994).
8 McNeely, R. Geological Survey of Canada Radiocarbon Dates XXXIII. (Geological Survey of Canada, Current Research 2001, 2002) .
9 Dionne, J. C. Formes et phenomenesperiglaciaires en Jamesie, Quebec subarctique. Geogr. Phys. Quat. 32,187 (1978).
10 Craig, B. G. Late-glacial and postglacial history of the Hudson Bay region. (Earth Science Symposium on Hudson Bay, Geological Survey of Canada, Paper 68-
53, 1969).
11 Kettles, I. M., Garneau, M. & Jette, H. Macrofossil, pollen, and geochemical records of peatlands in the Kinosheo Lake and Detour Lake areas, northern Ontario.
(Geological Survey of Canada, Bulletin 545, 2000).
12 Bunbury, J., Finkelstein, S. A. & Bollmann, J. Holocene hydro-climatic change and effects on carbon accumulation inferred from a peat bog in the Attawapiskat
River watershed, Hudson Bay Lowlands, Canada. Quat. Res.78, 275-284, doi:10.1016/j.yqres.2012.05.013 (2012).
48
13 O'Reilly, B. C., Finkelstein, S. & Bunbury, J. Pollen-derived paleovegetation reconstruction and long-term carbon accumulation at a fen site in the Attawapiskat
River watershed, Hudson Bay Lowlands, Canada. Arctic, Antarctic and Alpine Research (2014).
14 Pendea, I.F., Costopoulos, A., Nielsen, C., & Chmura, G.L. A new shoreline displacement model for the last 7 ka from eastern James Bay, Canada. Quaternary
Res. 73, 474-484 (2010).
15 Dyck, W. & Fyles, J. G. Geological Survey of Canada radiocarbon dates I and II (Geological Survey of Canada, Paper 63-21, 1963).
16 Terasmae, J. & Hughes, O.L. Glacial retreat in the North Bay Area, Ontario. Science 131, 1444-1446 (1960).
17 Blake, W., Dyck, W. & Fyles, J. G. Geological Survey of Canada Radiocarbon Dates IV. (Geological Survey of Canada, Paper 65-4, 1965).
18 Railton, J. B. & Sparling, I. H. Preliminary studies on the ecology of palsa mounds in northern Ontario. Can. J. Bot. 51, 1037-1044 (1973).
19 Protz, R., Ross, G.J., Martini, I. P. & Terasmae, J. Rate of podzolic soil formation near Hudson Bay, Ontario. Can. J. Soil Sci. 64, 31-49 (1984).
20 Klassen, R. W. Surficial geology of north-central Manitoba. (Geological Survey of Canada, Memoir 419, 1986).
21 Dredge, L. A. & Mott, R. J. Holocene pollen records and peatland development, northeastern Manitoba. Geogr. Phys. Quat.7, 7-19 (2003) .
22 Kuhry, P. Palsa and peat plateau development in the Hudson Bay Lowlands, Canada: timing, pathways and causes. Boreas 37, 316-327, doi:10.1111/j.1502-
3885.2007.00022.x (2008).
23 Kuhry, P. Late Holocene permafrost dynamics in two subarctic peatlands of the Hudson Bay Lowlands (Manitoba, Canada). Eurasian Soil Sci.31, 529-534
(1998).
24 Blake, W. Geological Survey of Canada radiocarbon dates XXV. (Geological Survey of Canada, Paper 85-7, 1986).
25 Reimer, P. J. et al.INTCAL09 and MARINE09 radiocarbon age calibration curves, 0-50,000 years cal BP. Radiocarbon 51, 1111-1150 (2009).
49
49
2.11 Figures
Figure 2.11-1 Land emergence and peatland expansion in the Hudson Bay Lowland, Canada.
The physiographic region of the Hudson Bay Lowlands, Canada (HBL) includes the coastal
Hudson Bay Lowland (Coastal) and the inland HBL and James Bay Lowland (Inland HBL/JBL)
ecoregions (http://www.cec.org/naatlas/). Contours (grey gradient) represent modern isostatic
uplift rates (mm y-1
) (Peltier, 2004), while brown shading represents modern surface elevation
(m, above sea level) generated using 1:250K Canadian digital elevation data
(http://www.geobase.ca/geobase/en/data/cded/description.html). The 5.5-ky (blue dashed) and
3.0-ky (red dashed) paleo-coasts presented here were extracted from the paleotopography
database (Peltier, 2002; 2004). Peatland study locations are colour-coded by basal 14
C ages and
grouped according to trends in peat initiation frequency since mid-Holocene peat initiation
(Table 2.10-1).
50
50
Figure 2.11-2 Holocene peat initiation dynamics in the Hudson Bay Lowlands, Canada.
51
51
(a) Winter (blue dashed) and summer (red solid) insolation during last 8.5 ky at 60 °N (Berger
and Loutre, 1991). (b) Frequency of post-glacial peatland initiation (red histogram) in the
Hudson Bay Lowlands, Canada inferred from basal radiocarbon dates (n = 100, mean 2σ age
range bins), cumulative peatland initiation (%; n = 100; black solid), and corresponding
cumulative land emergence (%, n = 100; blue dashed). Here, the timing of land emergence,
driven by the exponential decay in glacial isostatic adjustment, is spatially related to each peat
record included in this study. Using the geographic coordinates for each peat record, the
corresponding time of land emergence was extracted from a paleotopography dataset (Peltier,
2002; Peltier, 2004). The cumulative land emergence and peat initiation curves presented here
are directly related and can be compared for the purpose of examining lags between the timing of
land emergence and the timing of peat initiation at specific peatland sites (refer to Figure 2.11-1
for total surface area estimates through time using the paleo-coasts). (c) Reconstruction of
January (black) and July (red) temperature and annual precipitation (blue bars) anomalies from
the North American Pollen Database (northern Quebec pollen region) (Viau and Gajewski,
2009); and (d) GISP2 (Greenland) ice core record of atmospheric methane (CH4) concentration
(Brook et al., 2000) (red) and Antarctic ice core record of atmospheric carbon dioxide (CO2)
concentrations (Monnin et al., 2004) (blue) during the Holocene.
52
Chapter 3 Quantifying Holocene Variability in Carbon Uptake and Release
Since Peat Initiation in the Hudson Bay Lowlands, Canada
3.1 Abstract
Northern peatlands are a globally significant carbon (C) reservoir, yet also function as dynamic
methane (CH4) sources to the atmosphere. The fate of peatland C stores and related climate
system feedbacks remains uncertain under scenarios of a changing climate and enhanced
anthropogenic pressure. Here, we present a synthesis of Holocene peatland C dynamics for the
Hudson Bay Lowlands, Canada (HBL) in relation to past atmospheric CH4 trends, glacial
isostatic adjustment and paleoclimate. We report that peatland age and trophic status (e.g., fen-
bog stage of peatland succession), together with paleoclimate, contribute to explaining some of
the temporal variation in C accumulation rates (CAR) in the HBL. Our results show that
younger, minerotrophic peatlands accumulate C faster, and although detailed paleoclimate data
are not available, the results suggest the possibility of higher CAR in association with warmer
Holocene climates. Peat initiation rates and CAR were greatest during the mid-Holocene;
however, our model results suggest that two-thirds of the HBL C pool is stored in peat of late
Holocene age, owing to long-term peatland expansion and development. Whereas the HBL has
been a net C sink since mid-Holocene peat initiation, the HBL also appears to have been a
modest C source, with 85% of the losses occurring during the late Holocene as a consequence of
the gradual decay of previously accrued peat. Late Holocene peat decay, under wetter climatic
conditions, and from a landscape occupied by an abundance of minerotrophic peatlands,
indicates that the HBL may have been a natural terrestrial source of CH4 to the late Holocene
atmosphere. While the peatlands of the HBL may continue to function as a globally significant C
53
53
store, ongoing C losses from the HBL may have important implications for the global C budget
and climate system.
Key words: net carbon balance, methane, paleoclimate, glacial isostatic adjustment, mire, peat
decay, carbon accumulation
3.2 Introduction
Northern peatlands are important repositories of atmospheric carbon (C), storing at least 500 Pg
C or about one third of the global C pool (Yu, 2011), within approximately 3% of the terrestrial
surface area (Clymo et al., 1998). Although global syntheses confirm the importance of peatlands
as net C sinks in the global C cycle and as cooling agents in the global climate system (Frolking
et al., 2011; MacDonald et al., 2006; Yu, 2011), peatlands also function as important C sources,
principally in the form of carbon dioxide (CO2), methane (CH4), and dissolved organic C (DOC).
Anticipated high-latitude changes in temperature and the net moisture balance may enhance both
peatland net primary productivity (NPP) and decomposition. Recent evidence using peat records
covering the last millennium suggests that NPP may be more important than peat decay in
determining the rate of C sequestration (Yu, 2012; Charman et al., 2013). Further, peat initiation
and long-term development appear to be sensitive to climate; however, the relationship between
paleoclimate and temporal variability in the rate of peat C accumulation that is chronologically-
controlled using multiple vertical peat dates (referred to here as C accumulation rate, or CAR),
remains poorly constrained (Charman et al., 2013). As a result, improving our understanding of
mechanisms that could augment peat C decomposition and CO2/CH4 emissions, which together
could result in a positive feedback to climatic warming, continues to be a high priority (Jones
and Yu, 2010). Assessments of long-term peatland-climate-C dynamics provide an important
54
54
perspective for informing our understanding of the future trajectory of peatland C accumulation,
under conditions of a changing climate and enhanced anthropogenic pressure.
Long-term peatland development is influenced by the interaction of autogenic and allogenic
processes and associated feedbacks. As a consequence of an imbalance between productivity and
decay under generally waterlogged conditions, peat C accumulates over millennia. Examination
of long-term peatland-climate-C cycle dynamics relies upon assessments of the synchronicity
between regional to global syntheses of peat initiation and expansion, trophic status, and CAR in
the context of inferred paleoclimate and long-term atmospheric trace gas variation. Results from
these approaches provide evidence that the majority of northern peatland initiation occurred in
the early Holocene, under conditions of maximum summer insolation and temperature
seasonality (Yu et al., 2010), synchronous with a rapid rise in ice-core inferred, atmospheric CH4
concentrations (MacDonald et al., 2006). Further, the development of CH4 emitting pre-peatland
marsh conditions coupled with rapid peatland initiation and expansion of minerotrophic systems
(both sources of CH4) has been linked to the early Holocene atmospheric CH4 rise (Jones and
Yu, 2010; Smith et al., 2004; Yu et al., 2013).
Late Holocene polar ice-core records reveal a second, more gradual rise of atmospheric CH4
concentrations, following a mid-Holocene reduction (Brook et al., 2000). However, more
uncertainty exists regarding the contribution of northern peatlands to this rise in atmospheric
CH4 during the middle and late Holocene. Apparent declines in the rate of northern peatland
expansion, coupled with potential ombrotrophication (fen to bog transition) of existing peatlands,
may contribute to reductions in peatland-derived CH4 emissions (Yu, 2011). However, recent
evidence from the extensive peatland region of northeastern Canada, document delayed
ombrotrophication until the latter half of the late Holocene (Holmquist and MacDonald, 2014)
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and a late Holocene, possibly climate-driven, return of minerotrophic conditions (van Bellen et
al., 2013). Both scenarios would favor potential late Holocene, peatland-derived CH4 emissions.
However, late Holocene permafrost establishment (Kuhry, 1998; Kuhry, 2008; Lamarre et al.,
2012) may contribute to local reductions in potential CH4 emissions, which may be remobilized
as established permafrost degrades under climate warming or disturbance scenarios (Kuhry et al.,
2010).
Alternative hypotheses assert that possible terrestrial CH4-sources during the mid- to late
Holocene CH4 rise may be related to early anthropogenic activity, such as rice cultivation and
deforestation (Ruddiman, 2003); tropical wetland contributions (Brook et al., 2000; Yu et al.,
2010); and/or lateral expansion of existing high-latitude peatlands through the mid-to late
Holocene (Korhola et al., 2010). Until recently (Holmquist and MacDonald, 2014; Packalen et
al., 2014), potential late Holocene CH4 emission estimates from northern peatlands have been
limited by a lack of spatially explicit evidence of mid- to late Holocene peatland initiation and
expansion dynamics from major peatland regions, such as the Hudson Bay Lowlands, Canada
(HBL). Chapter 2 details new evidence of late Holocene peatland initiation and expansion, so in
consideration of this new evidence, and building upon the hypothesis of potential CH4
contributions related to peatland expansion (Korhola et al., 2010), together with the possibility of
sustained minero- to weakly ombrotrophic conditions in important northern peatland regions, we
suggest that further analyses may indicate some CH4 contributions from northern peatlands to the
late Holocene CH4 rise (Behl, 2011; Packalen et al., 2014; Yu, 2012).
The timing of peat initiation in the HBL appears to be principally controlled by glacial isostatic
adjustment (GIA) following retreat of the Laurentide ice sheet and land emergence from the
post-glacial Tyrrell Sea, as presented in Chapter 2 (Glaser et al., 2004a; Packalen et al., 2014).
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As well, evidence of peat initiation periodicity (Chapter 2) reveals that the most rapid and intense
period of peat initiation in the HBL occurred during the mid-Holocene, in advance of the late
Holocene CH4 rise (Packalen et al., 2014). Although peatland succession trajectories in the HBL
can be diverse (Klinger et al., 1994; Sjörs, 1959), newly initiated peatlands begin as nutrient-
rich, minerotrophic systems that are dominated by herbaceous plant communities and more
easily decomposable plant litter. Following millennia of succession, peatlands may transition to
more nutrient-poor, ombrotrophic systems dominated by mossy vegetation producing litter that is
more difficult to decompose. The trajectory between these two endpoints contributes to variation
in long-term peatland C cycling.
While evidence suggests that climate is an important control on northern peatland initiation and
C accumulation dynamics, the link between regional climate in the HBL and variation in the rate
of C accumulation is not well documented (Gorham et al., 2012). Delayed glacial retreat in
northeastern Canada likely deferred the Holocene Thermal Maximum (HTM) until the mid-
Holocene, as orbitally-driven insolation and seasonal differences in temperature were declining
(Renssen et al., 2009). Millennial-scale climate variation in the vicinity of the HBL is
approximated by a synthesis of pollen-inferred temperature and precipitation records obtained
from northern Quebec, Canada (Viau and Gajewski, 2009). Although northern Quebec and the
HBL are not identical biophysical or physiographic environments, both share similar latitudes
and are influenced by the Hudson Bay. These pollen-based temperature reconstructions indicate
little temperature change since initiation of HBL peatlands began ~ 8 ky BP. Reconstructed
Holocene temperature fluctuations are on the order of ± 1 °C relative to modern climate (1961 –
1990 climate normal), with mostly cooler temperatures during the mid- and late Holocene, and
evidence of similar summer temperatures and a trend toward warmer winters relative to today
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between 4.5 and 2 ky BP. Reconstructed precipitation changes show a shift at about 3 ky from
somewhat reduced precipitation in the earlier part of the record to increased precipitation in the
late Holocene (Viau and Gajewski, 2009).
Here, I present variability in Holocene C balance in relation to the key potential drivers of that
variability, including major temporal trends in paleoclimate, ice-core inferred trace gas variation,
and frequency of regional peat initiation events in the HBL. Working under the assumption that
both allogenic and autogenic processes account for variation in the rate of C sequestration in the
HBL, I quantify CAR and estimate long-term decay in the HBL, at 0.5-ky intervals to model
changes in the net C balance (NCB) through time. Using my analysis of CAR in the HBL,
together with the synthesis presented in Chapter 2 of all available peat initiation records for the
HBL (Packalen et al., 2014, and references therein), I model net C uptake and release terms at
0.5-ky intervals following the method of Yu (2011). I then compare the resulting patterns in C
uptake and release in the HBL with mid- to late Holocene, pollen-inferred paleoclimate. I
hypothesize that reduced precipitation associated with the HTM may enhance CAR, while wetter
conditions associated with Neoglacial climate (post-3 ky BP) may suppress CAR. As a
consequence of generally cooler, wetter conditions in the HBL during the mid-to late Holocene, I
further suggest that such conditions may have supported long-term CH4 production, through
sustained minerotrophic to weakly ombrotrophic peatland conditions, a growing peat mass as
described in Chapter 2 (Packalen et al., 2014), and the potential for greater release of the
products of anaerobic decomposition. Accordingly, I examine peatland development and C
dynamics in the HBL in relation to mid-to late Holocene atmospheric CH4 trends, and evaluate
the potential for HBL peatlands to function as a late-Holocene terrestrial source of CH4.
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3.3 Materials and Methods
3.3.1 Study setting
The HBL is the second largest continuous peatland region globally (Riley, 2011), and is located
between 50° and 60° N, and 76° and 100° W (Figure 3.9-1). The northern and eastern boundaries
of the HBL stretch to the margins of Hudson and James Bays, while the southern and western
margins of the HBL follow the contour of the Canadian Shield. For the purposes of this study,
the area occupied by the HBL is defined according to the Hudson Plains Terrestrial Ecoregion in
the North American Environmental Atlas (http://www.cec.org/naatlas/). Using this definition, the
HBL has a surface area of 372,000 km2, of which up to 90% is classified as peatland (Ontario
Land Cover Database, 2000). The HBL landscape was shaped by the Laurentide Ice Sheet (LIS).
Evidence of past glacial activity is present in the form of continuous and low-relief deposits of
till and diamicton, eskers and moraines, and glacio-marine sediments that were deposited overtop
of Paleozoic sedimentary bedrock (Martini, 2006). Deglaciation dynamics are described in
Chapter 2, such that high rates of GIA and subsequent flooding by the Tyrrell Sea, delayed land
emergence and the onset of peat initiation until ~ 8 ky BP (Packalen et al., 2014). Further, the
final collapse of the LIS in the vicinity of the HBL also delayed the onset of the HTM for
northeastern Canada until the mid-Holocene (Renssen et al., 2009).
Climate in the HBL is a microthermal, strongly influenced by Arctic air masses and strong
winds. Modern average climate (1971–2000) is characterized by a mean and standard deviation
(s.d.) annual temperature of -2.5 ± 1.8 °C and total annual precipitation ranging from 429 to 743
mm. The growing season in the HBL stretches over a period of 119 – 162 days, and is
characterized by a mean ± s.d. growing season temperature of 10.8 ± 1.0 °C, and summer
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precipitation, as rain, equivalent to half to two-thirds of the total annual precipitation (McKenney
et al., 2006). Occasional sporadic to discontinuous permafrost features occur in the HBL
peatlands, together with a narrow stretch of continuous permafrost limited to the northernmost
reaches of the region, near the Hudson Bay coast (Riley, 2011).
3.3.2 Sample collection and data sources
During the 2009 – 2011 field seasons, complete peat profiles were collected from representative
peatlands distributed across the HBL. To minimize peat compaction during sampling, surface
peat (0–80 cm) was collected using a Wardenaar- or Jeglum-type box corer (10 x 10 x 80 cm),
while the subsequent sampling to mineral contact was carried out using a Russian pattern side-
cutting peat sampler (5 x 50 cm). Recovered core segments were wrapped in plastic and
aluminum foil and stored in polyvinylchloride pipe at -10 °C until laboratory analysis.
For the purpose of examining C flux histories using the ‘Super Peatland’ approach described by
Yu (2011), CARs were quantified using measurements of bulk density, C content, and vertical
radiocarbon dating collected from 17 complete peat profiles. Ten of these records are new
contributions (14
C- accelerator mass spectrometry (AMS)), while the remaining seven records
were obtained from previously published literature and include both 14
C-AMS and conventional
dates (Bunbury et al., 2012; Kettles et al., 2000; Kuhry, 1998; Kuhry, 2008; O'Reilly et al.,
2014). Peatland area increase in the HBL was estimated in Chapter 2 using a previously
published synthesis of peat initiation history for the HBL, which comprises 100 new and
previously reported peat basal ages (Packalen et al., 2014, and references therein). Data obtained
from previously published literature were selected on the basis of three criteria: (1) their
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specified location within peatlands of the HBL ecoregion, (2) the availability of radiocarbon
dates at the basal peat-mineral interface, and (3) the availability of total peat depth.
3.3.3 Laboratory Analyses
3.3.3.1 Peat physical properties
Bulk density, loss-on-ignition (LOI), and/or C content were obtained using standard methods
(Yu, 2012). Briefly, contiguous samples of known volume (5 – 10 cm3) were cut from fresh
material at 2- to 4-cm intervals and oven-dried to constant mass. Direct measurement of C
content was completed using an Elementar Variomax CN analyzer (Elementar Analysensysteme
GmbH Donaustraße 7 63452 Hanau Germany). Data from the seven previously reported peat
records were re-interpreted using author-contributed raw datasets of bulk density, LOI and
directly measured C content (Bunbury et al., 2012; O'Reilly et al., 2014) or C content at a rate of
50% LOI , where measured C content was not available (Kettles et al., 2000; Kuhry, 1998;
Kuhry, 2008).
Bulk density (g cm-3
) was calculated by dividing the dry peat mass (g) by the fresh peat volume
(cm3), while C density (g cm
-3) was calculated by multiplying the bulk density of each peat
increment by the corresponding C concentration. Areal dry peat mass (g cm-2
) was determined as
a function of peat depth and cumulative dry mass was determined on an area basis as averages
weighted by layer thickness. Here, I consider the rate of C accumulation using several
conventions commonly reported in the literature, including: (1) the long-term apparent rate of C
accumulation (LORCA), representing average C accumulation since peat inception; (2) CAR,
which uses vertical chronological control to track temporal patterns in the rate of C
accumulation; and (3) NCB, the peat decay adjusted product of peatland area and CAR
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(discussed in detail below), which models the net C sequestration by peatlands through time and
is potentially the most appropriate term to use when comparing historical peatland C dynamics
and the global C cycle (Yu, 2012).
3.3.3.2 Radiocarbon dating
New radiocarbon dates were obtained using identified above-ground plant macrofossils sampled
from a 1-cm peat segment from 10 cores (Table 3.8-1). 14
C-AMS dating of these samples was
completed at either the Keck-CCAMS facility (Irvine, USA), the UGAMS facility (Athens,
USA) or at Beta Analytic, Inc. (Miami, USA). All dates compiled from previous studies, which
include 14
C-AMS dated peat (Bunbury et al., 2012; O'Reilly et al., 2014) and both 14
C-AMS and
radiometric dates (Kettles et al., 2000; Kuhry, 1998; Kuhry, 2008), in addition to the newly dated
samples, were calibrated using IntCal09 (Reimer et al., 2009) in the clam package for R
(Blaauw, 2010). All calibrated ages are expressed as calendar years before present (y cal BP),
where present is AD 1950. For the 100 sites included in the peat initiation synthesis detailed in
Chapter 2 (Packalen et al., 2014) the timing of reported and recalibrated radiocarbon (14
C, AMS
and conventional) dates obtained from the peat- mineral interface is understood to correspond
with primary peatland initiation in the HBL following LIS retreat and land emergence from the
post-glacial Tyrrell Sea.
3.3.3.3 Peat decay
To estimate the true rate of peat C accumulation, the long-term rate of C decay in the peat profile
must be quantified. Other studies where this model has been applied have concluded that the
constant decay function provides the simplest best fit to the data (Yu, 2011). Accordingly, the
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exponential bog decay model (equation 3.3.3.3.1) was applied to the subset of 17 HBL peat
records (Clymo et al., 1998). This model states that:
M = (p/a) *(1-e-at
), (3.3.3.3.1)
where M is the cumulative peat C mass (g C cm-2
); p is the annual peat addition to the catotelm
and determines the slope of M versus peat age curve; a is the peat decay constant that determines
the curvature of M versus peat age; and t is time. The subset of new and/or synthesized age-depth
models was fitted using equation 3.3.3.3.1, such that depth was represented as cumulative peat C
mass (g cm-2
). Alternative and potentially more ecologically meaningful peat decay rules, such
as linear and non-linear (quadratic) decay through time were also explored (Clymo et al., 1998).
Although they appeared to impact estimates of total C losses, they were not found to not improve
curve fitting and were not used in subsequent analyses.
3.3.3.4 Modeling of carbon dynamics
Peatland C dynamics since mid-Holocene peat initiation for the HBL was quantified following
the methods of Yu (2011), using mean CAR from the 17 peat profiles described above, binned at
0.5-ky intervals. Table 3.8-2 provides a summary of the terms used to describe peatland C
dynamics in the HBL; however additional detail is presented in Yu (2011). Briefly, net C balance
(NCB) refers to the difference between net C uptake (NCU) at a given time interval and net C
release (NCR) by the whole peatland, due to decay, prior to the given time interval. To quantify
NCB, mean time-weighted CAR (n = 17) were determined for 0.5-ky peat cohorts. The net C
pool (NCP) was then quantified for each peat cohort by multiplying the mean CAR at each 0.5-
ky bin by the corresponding total peatland area at that time. Peatland area at each time interval
was estimated as a relative proportion of cumulative peat initiation, assuming a modern HBL
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extent of 372,000 km2, with 90% occupied by peatlands. NCU for each peat cohort was
calculated as the total amount of C at each time interval, accounting for previous peat losses due
to decomposition using the exponential decay function calculated above (NCP*1/e-at
). Finally,
NCR was quantified as the total amount of C released by the entire peatland, including
previously accumulated peat, prior to the relative time period. Cumulative C pool, uptake,
release, and balance were also calculated as cumulative sums of the respective net C terms to
track the total C terms through time in the HBL.
3.4 Results
3.4.1 Carbon accumulation and peat decay rates
Vertical radiocarbon data for ten new peat profiles are presented in Table 3.8-1 and were
combined with an additional seven vertically-dated peat profiles drawn from the published
literature (Bunbury et al., 2012; Kettles et al., 2000; Kuhry, 2008; Kuhry, 1998; O'Reilly et al.,
2014). With the exception of two coastal peatland records from the northwest HBL, a region of
permafrost influence, most records considered here are located in the inland peatland region and
include bog and fen peatlands. Shallow coastal peatlands were not sufficiently well constrained
in terms of their chronologies for inclusion in the present analysis. With that in mind, the total
peat depth of the 17 vertically-dated peat records ranged from 131 to 311 cm, while basal peat
ages ranged between 3940 and 6810 y cal BP.
C content per unit dry peat varied along the peat profile from 12 to 54%, with the lowest values
being recorded near the mineral contact. The mean and standard error (s.e.) peat C content (n =
885 from 17 cores) was 48 ± 0.2% and the mean ± s.e. bulk density (n = 885 from 17 cores) was
97.4 ± 2.3 g dm-3
. Using only the basal age of the peat deposit for the subset of peat cores used in
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this study (n = 17), we found that median LORCA ranged between 11.2 and 28.3 g C m-2
y-1
(median: 18.3 g C m-2
y-1
). Consideration of temporal variability in peat accumulation rates
yielded CAR estimates that varied over two orders of magnitude among intervals, and ranged
from 3.5 to 310 g C m-2
y-1
. Incorporating vertical radiocarbon peat dates results in a Holocene
CAR of 25 ± 0.9 (s.e.) g C m-2
y-1
(median: 18.5 g C m-2
y-1
). Upon closer inspection of the peat
C accumulation records, temporal variability in CAR appears to be related to periodic increases
in age-depth model slopes through time, coupled with greater C content in fen peatland records,
which has been observed elsewhere (Holmquist and MacDonald, 2014; Loisel et al., 2014).
Peat accumulation in the HBL varied among the 17 sites examined in this study, with concave,
convex, and linear patterns of peat age versus peat accumulation observed (n = 80 from 17
cores). Considering all sites together, we found a linear relationship (Figure 3.9-2A) between
peat depth and cumulative peat OM mass (R2 = 0.74, p < 0.0001, n = 80), as well as between
peat depth and cumulative peat C mass (R2 = 0.93, p < 0.0001, n = 80). Fitting the C mass data
with a conservative decay model (equation 3.3.3.1) that assumes constant decay through time, to
the relationship between peat age and cumulative C mass for all HBL sites together (n = 80 from
17 cores) yielded a modeled peat C addition rate (PCAR) (Yu, 2011) of 21 g C m-2
y-1
and a
decay constant, a = 0.0000562 y-1
for the HBL (Figure 3.9-2B). Application of alternative peat
decay rules (Clymo et al., 1998), such as linear and non-linear (quadratic) decay through time
increased the decay coefficients to 0.000067 y-1
and 0.000082 y-1
, respectively; however, these
were not found to be statistically significant in this study. Thus, for the purpose of this study, the
simplest decay term was selected; however, further investigation is warranted into ecologically
meaningful representations of decay in peatlands of differing trophic states, the results of which
may better constrain total C mass estimates in the HBL.
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3.4.2 Peatland carbon dynamics
Net C balance (Figure 3.9-3A) is a spatially constrained snapshot of net C sequestration by
representative peatlands in the HBL using the most comprehensive regional record of peat
initiation and expansion currently available. The sum of modeled NCB (CCB) must be equal to
the sum of the measured NCP (CCP) (Yu, 2011); however, the trajectory in arriving at the total
cumulative C term for each differs since peat initiation, owing to the accounting of delayed peat
decay in the NCB term. According to the assumptions and limitations described above, the
modeled potential NCU by HBL peatlands during the mid- to late Holocene ranged from 0.1 –
6.5 Pg C per 0.5-ky interval, with a cumulative C uptake (CCU) since peat initiation of 45 Pg C;
assuming constant peat decay since peat inception and across the growing peat landscape
revealed a potential loss (CCR) of 7.6 Pg C (Figure 3.9-3B). The difference between NCU and
NCR produces an NCB ranging from 0.1 – 5 Pg C, which summed results in a modeled total
HBL C pool of 37 Pg C. Consideration of alternative decay rules, such as non-linear peat decay,
which may be more ecologically meaningful, resulted in the largest decay coefficient for the
HBL (a = 0.000082 y-1
, presented above), and nearly doubles the total potential peat C loss to
~ 15 Pg C. Application of the non-linear decay coefficient to NCU reduces the modeled NCB for
the HBL to ~ 30 Pg C, more consistent with the previous estimate reported in Chapter 2 using
LORCA and peatland expansion dynamics to estimate the magnitude of the HBL C pool
(Packalen et al., 2014). The outcome of curve fitting using non-constant decay terms yielded
non-significant co-variates, thus such results are presented here for qualitative purposes alone to
highlight the need for an improved understanding of peat decay dynamics in the HBL.
The peatlands of the HBL have been a persistent C sink since they began to develop during the
mid-Holocene (Figure 3.9-4). During the past 7.5 ky of peat initiation and expansion in the HBL,
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mean CAR (0.5-ky bins) ranged from 14 to 38 g C m-2
y-1
(Figure 3.9-4D). Late Holocene CAR
in the HBL remained relatively stable, below the mean CAR (25 g C m-2
y-1
). By contrast, mid-
Holocene CAR was highly variable in the HBL, and peaked (~ 35 g C m-2
y-1
) twice during this
period: first 6 – 6.5 ky BP, and again 4 – 4.5 ky BP, following a 1.5 ky period of below average
peat CAR. The variation in CAR appears to track patterns in peatland initiation, such that
maximum CAR occurred synchronously with peat initiation maxima (Figure 3.9-4E). CAR also
peaks in the most recent past (0.5 ky cal BP), likely related to under-decomposed acrotelm peat.
Regional pollen-inferred climate records reveal relatively cold and dry conditions versus
contemporary climate and a trend toward warmer and wetter conditions between 7.5 and 4.5 ky
BP, when peat CAR was greatest and most variable. Mid-Holocene CAR maxima also appear to
be preceded by warmer summers and wetter conditions (Figure 3.9-4C), which may suggest that
periodic enhanced peatland productivity may contribute to CAR variability. By contrast,
relatively similar to slightly cooler conditions compared to modern climate coupled with
anomalously wet conditions were associated with the lowest and least variable CAR, though
some minor variation is noted.
We calculated a modeled peat C sequestration rate for the HBL of ~ 7 Tg C y-1
and a mean NCB
of ~ 4.4 Tg y-1
. Instantaneous C accumulation rates generated from the NCB term account for
both C uptake and C release over the period of peatland development (Yu, 2012). In the HBL,
the modeled instantaneous C accumulation rates ranged between 14 and 42 g C m-2
y-1
, and were
greatest during the mid-Holocene, coincident with the period of rapid land emergence from the
Tyrrell Sea and associated intense peat initiation. A delay in peat decay is apparent in the HBL,
such that most of the modeled C was released during the late Holocene. Accordingly, our results
indicate that ~ 6.4 Pg C was released between 0 – 4 ky BP, compared to a modeled mid-
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Holocene (4 – 7.5 ky) release of ~ 1.2 Pg C, for a total of 7.6 Pg C since the time of peat
initiation in the HBL.
3.5 Discussion
3.5.1 Carbon accumulation patterns in the HBL
Peat accumulation in the HBL has occurred on a topographically favorable landscape since the
mid-Holocene (Chapter 2), against a backdrop of decreasing summer insolation and insolation
seasonality, and within climatic boundary conditions suitable for peatland development (Viau
and Gajewski, 2009; Beilman et al., 2009; Charman et al., 2013; Packalen et al., 2014). GIA-
driven land emergence in the HBL, which decays exponentially through time, is the fundamental
control on the timing of peat initiation in this region as presented in Chapter 2 (Packalen et al.,
2014). LORCA is a widely reported metric for comparing mean C accumulation among peatland
regions (Tolonen and Turunen, 1996). The results presented in Chapter 2, estimated using a
synthesis of 100 basal peat ages, reveal a mean ± s.d. LORCA equal 18.5 ± 5.7 g C m-2
y-1
for
the HBL (Packalen et al., 2014). This rate is within the range of LORCA found for the subset of
peat cores used in the present study (median 18.3 g C m-2
y-1
; n = 17).
LORCA in the HBL is comparable to estimates reported for similarly aged peatlands, such as the
Eastmain region, Quebec, Canada, where inundation by the Tyrrell Sea delayed peat initiation
until the mid-Holocene, and C accumulated at 16.2 to 18.5 g C m-2
y-1
(van Bellen et al., 2011a).
Further, the HBL LORCA is also similar to the rates reported for older peatland regions that
began to initiate in the early Holocene, such as the West Siberian peatlands where the rates range
from 12.1 to 23.7g C m-2
y-1
(Turunen et al., 2001), undrained Finnish peatlands with a rate of
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18.5 g m-2
y-1
(Turunen et al., 2002), and the continental peatlands of western Canada with a rate
of 19.4 g C m-2
y-1
(Vitt et al., 2000).
The rate of peat C accumulation following peat inception is related to both autogenic and
allogenic factors, and likely varies through time. However, LORCA as it was originally defined
(Tolonen and Turunen, 1996) may not adequately account for peat mass losses through time due
to fire and/or erosion, temporal gaps in C accumulation due to permafrost accretion or enhanced
decomposition, and other disturbances (Clymo et al., 1998; Yu, 2012; Tarnocai et al., 2012).
Consequently, LORCA does not permit a complete assessment of temporal patterns in C
accumulation in relation to changes in peat burial rates related to peat type and
ombrotrophication, growing season length, and/or other potential environmental controls (Loisel
et al., 2014).
As an alternative we also consider CAR, a metric that captures the apparent temporal variability
in the rate of C accumulation, through vertical chronological control using multiple radiocarbon
dates along the peat column. Using the subset of well-dated cores from the HBL (n = 17), our
results reveal a Holocene CAR of ~ 25 g C m-2
y-1
(median: 18.5 g C m-2
y-1
) for the HBL, which
is similar to the recently reported northern CAR of 22.9 ± 2 (s.e.) g C m-2
y-1
(Loisel et al., 2014);
and to values from nearby subarctic Quebec, Canada (~ 24 g C m-2
y-1
) (Lamarre et al., 2012)
and neighboring boreal shield peatland region of northwestern Ontario, Canada (~ 24 g C m-2
y-1
)
(Holmquist and MacDonald, 2014).
Temporal variability in CAR may be related in part to the vegetation community composition
and hence relative decomposability of the plant material through time. Previously, bog peat
dominated by Sphagnum species, was shown to be less C rich than peat associated with more
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minerotrophic conditions (Beilman et al., 2009). Further, periods of increased CAR identified for
the northern peatland region (Loisel et al., 2014), were attributed to changes in plant composition
and C density through time, rather than increasing peat bulk density due to compaction of older
peat cohorts. In the HBL, plant macrofossil and pollen-based peat vegetation reconstructions
provide evidence that minerotrophic conditions following peat inception can persist for millennia
(Bunbury et al., 2012; Holmquist and MacDonald, 2014; O'Reilly et al., 2014; Sjörs, 1959; van
Bellen et al., 2013). Thus, for the 17 peatlands examined here, C rich inputs from young,
minerotrophic peatland vegetation, coupled with periods of rapid rates of peat accumulation, may
contribute to explaining apparent CAR fluctuations in this record.
Peat initiation frequency in the HBL was most intense during the mid-Holocene (Chapter 2)
followed by a reduction in the rate of peat initiation through the late Holocene (Packalen et al.,
2014). This peat initiation pattern of generally declining rates though time is associated with the
GIA-controlled reduction in new land available for occupation by new peatlands. Comparing
these trends with HBL CAR reveals that the mid-Holocene periods of intensified peat initiation
in the HBL were accompanied by elevated CAR of ~ 35 g C m-2
y-1
, with maxima around 4 and
6 ky cal BP (Figure 3.9-4). As peat initiation rates declined through the late Holocene, partly in
response to exponentially declining rates of glacial isostatic adjustment, late Holocene CAR was
weakly variable and remained below the long-term mean CAR.
By the onset of the late Holocene (4 ky BP), ~ 75% of the HBL peatlands had initiated, and some
of these peatlands were on a trajectory toward lower C density, Sphagnum-dominated, weakly
ombrotrophic conditions. Previous evidence, inferred using testate amoebae and plant
macrofossils, indicates that many modern bogs in the vicinity of the Hudson Bay transitioned
from the fen stage in the last millennium or during latter half of the late Holocene (Holmquist
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and MacDonald, 2014; Lamarre et al., 2012). Today, ~ 40% of the HBL remains classified as fen
peatlands (Ontario Land Cover Database, second edition, 2000). Thus, lower rates of peat
initiation in the HBL during the late Holocene, and the corresponding trend of lower CAR during
this period may be more strongly influenced by peat succession dynamics occurring in existing
HBL peatlands, than by C inputs from newly emerging peatlands.
3.5.2 Modeled peatland carbon dynamics in the HBL
Using all available peatland records for the HBL with vertical chronological control, which
included minerotrophic to ombrotrophic peatlands in both the presence and absence of
permafrost, we assess the temporal variation in Holocene C uptake and release using the ‘Super
Peatland’ approach (Yu, 2011). As part of this process, peat C decay was estimated assuming
constant decay and resulted in a slightly concave decay model of age versus depth. Though
frequently considered, constant decay may be an oversimplified assumption, as individual
peatlands or peat types may exhibit very different decay patterns or may not be well fitted to the
decay model described by Clymo et al. (1998). In addition to this limitation, young, shallow
coastal peatlands are not well represented in this study, owing to the challenges of dating the
relatively young peat deposits. Thus we also consider alternative decay assumptions
qualitatively, as model outputs yielded non-significant terms in the present study, and sample
limitations precluded additional investigation.
Nevertheless, using a constant peat decay term (Clymo et al., 1998) summation of the NCB
model output term, resulted in a total contemporary C mass equal to 37 Pg C (Figure 3.9-4).
Under conditions of constant peat decay, this modeled total C mass estimate is about 20% greater
than a previous estimate of 30 Pg C presented in Chapter 2, and obtained using patterns of peat
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71
initiation and expansion, land emergence, and measured C mass (Packalen et al., 2014). Still,
peat decay is not well understood for HBL peatlands, especially regarding the relative role of fen
versus bog peatlands. Consideration of alternative decay rules in the calculation of NCB, such as
non-linear peat decay, nearly doubles the total C release term (NCR), and results in a reduced
HBL C mass estimate of 30 Pg C, consistent with the previous estimate presented in Chapter 2
(Packalen et al., 2014). However, we note here that the non-constant decay terms were not
significant in this study, and thus are used only for qualitative purposes to demonstrate the effect
of enhanced peat decay, at the landscape scale, on the total C pool in the HBL. Our results
confirm that a better understanding of peat decay, especially concerning the influence of fen
versus bog peat cover, is needed to fully explore Holocene C dynamics.
Although the absolute values for HBL C pools reported here are uncertain, temporal patterns in
Holocene C dynamics may be more meaningful and suggest that a total of 7.6 Pg C may been
lost from the total peatland C mass since peat inception began in the HBL during the mid-
Holocene. Maximum modeled C sequestration and release was greatest during the late Holocene,
as the peat complex expanded across the rapidly emerging low-relief landscape. Additional
decomposition of C flux terms reveals a mid-Holocene (4 – 7.5 ky BP) net HBL C sequestration
equal to 14 Pg C, which occurred at a mean rate of 4 Tg C y-1
, and included a small C release of
1.2 Pg C. During the late Holocene, new peatland initiation coupled with rapid peat accretion in
existing peatlands resulted in an additional C sequestration of 23 Pg C, at a mean rate of 6 Tg C
y-1
, and includes a larger potential C release of 6.4 Pg C. The larger late Holocene C release
accounts for the gradual decay in the catotelm of previously deposited peat cohorts, which is
expected (Yu, 2011); however, the cumulative effect of long-term decay is smaller in the HBL,
due in part to the relatively young age of the HBL peat complex.
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3.5.3 Net carbon balance and paleoclimate in the HBL
The timing of peat initiation events in the HBL is primarily driven by uplift; however,
paleoclimate may have contributed to facilitating initiation events, and perhaps even more so, in
influencing the rates of Holocene C accumulation and NCB in the millennia following initiation.
As a consequence of the delayed retreat of the LIS near the HBL, the timing of the HTM in
northeastern Canada is thought to have been deferred until ~ 6 – 7 ky BP (Renssen et al., 2009).
The delay of the HTM in the HBL until the mid-Holocene corresponds to the earliest period of
intensified peat initiation and a period of peak C accumulation in the HBL. Coupled climate
model simulations and pollen reconstructions for northeast Canada suggest a possible warming
of ~ 1 °C vs. pre-industrial temperatures, which may account for higher C sequestration rates via
enhanced primary production during this time. However, regional paleoclimate reconstructions
appear to confound this relationship.
Comparisons using apparent CAR among Canadian peatlands regions in the vicinity of the HBL
reveal that maximum CAR occurred during the mid-Holocene, while the lowest CAR occurred in
the late Holocene (van Bellen et al., 2011a; Vitt et al., 2000), and early Holocene for the
continental peatlands of western Canada. Lower CAR in both the western Canadian peatlands
and the Eastmain region, Quebec, during the late Holocene was attributed to a cooling climate
and drier peatland conditions related to height-induced surface drying and/or permafrost
accretion (van Bellen et al., 2011a; Vitt et al., 2000), while low early Holocene CAR was
attributed to dry peatland conditions (Vitt et al., 2000). However, pollen-inferred climate
reconstruction for northern Quebec, Canada (Viau and Gajewski, 2009) that are consistent with
an independent paleoclimatic record from the HBL (McAndrews et al., 1982), imply lower than
present temperature and precipitation during the mid-Holocene (until 4.5 ky BP). Thus, in the
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73
HBL, the negative precipitation anomaly (i.e., reduced precipitation compared with the
contemporary climate normal) corresponded to higher CAR during the mid-Holocene. Given the
uncertainty in mid-Holocene climate in the HBL, the relationship between CAR and climate is
interpreted cautiously here and warrants further investigation.
Regional paleoclimate reconstructions for the late Holocene near the HBL reveal climatic
conditions similar to today, with the exception of the last 1.5 ky when increased precipitation and
reduced temperatures associated with Neoglacial cooling are recorded. During this period, CAR
appears to increase slightly as precipitation increases, and reaches a late Holocene peak CAR
around 1.2 ky BP followed by a small decrease in CAR around 0.8 ky BP, around the Medieval
Climate Anomaly (MCA). Similarly, a decline in CAR during the MCA is also apparent in the
northwestern Ontario peatlands, adjacent to the HBL (Holmquist and MacDonald, 2014); while a
slight increase is noted at the time of the Little Ice Age (LIA). Evidence recorded in Quebec,
Canada peatlands to the east of Hudson Bay document a wet shift during the LIA and a return of
minerotrophic conditions, which were attributed to a cooler, wetter climate (van Bellen et al.,
2013). However, our conclusions in this regard are highly limited by the resolution of the time
series used for CAR in the present study (0.5-ky bins) compared with the century scale of the
MCA and LIA climatic events. This limitation highlights the need for further high resolution
examinations of these time periods to better constrain the relationship between climate and CAR
in the HBL.
Contemporary surface measurements have revealed that C losses from peatlands are tightly
coupled with vegetation composition, trophic status and hydroclimatic conditions (Bubier, 1995;
Moore and Dalva, 1993) and can occur via aerobic decomposition in the acrotelm and by
anaerobic decomposition in the catotelm (Clymo et al., 1998; Gorham, 1991). Further rapid
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initiation and expansion of minerotrophic fens in the early Holocene have been proposed to have
contributed to peak early Holocene CH4 concentrations (MacDonald et al., 2006; Smith et al.,
2004). Further, sources of early Holocene CH4 emissions have more recently been expanded to
include pre-peatland wetlands such as marshes and wet fens as facilitators of biosphere CH4
transport to the atmosphere (Yu et al., 2013). In the HBL, surface and airborne measurements of
contemporary snow-free CH4 flux, interpreted using a chemical transport model (Pickett-Heaps
et al., 2011) suggest that the largest rates of net CH4 flux occur in association with the older,
inland peatland region rather than the younger coastal region. Accordingly, Packalen et al.
(2014) recently hypothesized that the largest atmospheric contributions of peat-derived CH4 from
the HBL may have occurred during the late Holocene, and suggested that a 3-ky BP peat mass
could potentially release 1 – 7 Tg CH4 y-1
to the late Holocene atmosphere (also presented in
Chapter 2).
Here, we examined patterns in Holocene NCB of HBL peatlands, for comparison with ice-core
inferred atmospheric trace gas variation (Figure 3.9-4). Similar climate-CAR patterns are also
apparent in northwestern Ontario peatlands, adjacent to the HBL (Holmquist and MacDonald,
2014). Assuming a constant decay term, our model estimations predict a late Holocene C release
of 6.4 Pg C, equivalent to a mean rate of 1.6 Tg C y-1
(range: 1 – 2.5 Tg C y-1
) from the more
developed HBL peatland landscape. Alternative non-linear decay patterns, though not significant
in this study, but may be more ecologically realistic, suggest even larger C losses from the HBL
C mass, such that total late Holocene C losses increase to ~ 10.2 Pg C, corresponding to a mean
rate of ~ 2.6 Tg C y-1
(range: 1.5 – 4.1 Tg C y-1
) during the late Holocene and from across the
HBL. Assuming a portion of these C losses occur as CH4, then this model evidence, irrespective
of decay assumption is comparable to previous estimates of late Holocene CH4 production
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potential from the HBL. Therefore, our analysis of modeled C dynamics provides an additional
line of evidence of natural terrestrial C contributions from the HBL to the late Holocene
atmosphere, particularly in the form of CH4 flux, as a consequence of the minerotrophic to
weakly ombrotrophic peat patterning that characterizes the HBL landscape.
The trajectory of NCB in the HBL differs from other important peatland regions globally, such
that C sequestration was greatest during the late Holocene, rather than at a minimum. While
evidence of peat succession is present in the HBL, as peatlands shift from minerotrophic to
ombrotrophic systems, our data show that the relationship between cumulative peat mass and
peat depth in the HBL remains linear. Assuming that the relationship between peat mass and
depth is related in part to trophic succession in the HBL, and given that there is little evidence to
suggest that mass peat losses are occurring as a consequence of drying and oxidation of peat, the
peatlands of the HBL may continue to accumulate peat (and sequester C) for some time to come.
Our findings convey the importance of the HBL in global C accounting, and suggest that
significant C losses from the HBL may have important implications for the global C budget and
climate system.
3.6 Conclusion
Here, we have examined trends in the timing and magnitude of variation among CAR and net C
uptake, release, and balance since peat inception began in the HBL. Previous findings (Chapter
2) have confirmed the fundamental control of GIA over the timing of peat initiation in the HBL
and that peatland development has occurred within the context of a weakly varying climate
(Packalen et al., 2014; Glaser et al., 2004a; Glaser et al., 2004b). Our findings do not reveal a
tight coupling between climate and CAR in the HBL; however they do suggest a potential link
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76
with climate that warrants further investigation. Further, we report that CAR may be more
strongly related to peatland succession dynamics in the HBL. Accordingly, we find that CAR is
related to the intensity of peat initiation, the latter of which determines the proportion of young
peatlands with high values for CAR on the rapidly emerging HBL landscape. Our data show that
while the HBL has been a persistent C sink for millennia, more than two-thirds of the total C
mass accrued during the late Holocene, when CAR remained below the long-term mean. Most of
the HBL peatlands initiated during the mid-Holocene, in advance of the late Holocene
atmospheric CH4 rise. Moreover, our findings suggest that most of the potential C lost from the
HBL occurred during the late Holocene, likely owing to decay of previously deposited peat.
Modern evidence suggests the largest CH4 emissions in the HBL occur in association with older,
inland patterned peatlands (Pickett-Heaps et al., 2011). Given that sustained minerotrophic to
weakly ombrotrophic peat patterning typifies the HBL landscape, the persistent C release during
the late Holocene may provide a first line of evidence of natural terrestrial C contributions from
the HBL to the late Holocene atmosphere, particularly in the form of CH4.
3.7 Acknowledgements
Sincere thanks to Jim McLaughlin for research funding and field support, provided by the
Ontario Ministry of Natural Resources’ Applied Research and Development Branch and Far
North Branch, under the auspices of projects CC-021 and FNIKM 028. Additional support for
field work and radiocarbon dating was provided by grants from the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the Ontario Ministry of the Environment
through the Climate Change and Multiple Stressor Research Program at Laurentian University to
SAF. Support in the form of an NSERC Alexander Graham Bell Canada Postgraduate
Scholarship and an Association of Canadian Universities for Northern Studies, Canadian
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77
Northern Studies Trust Scholarship, and field research grants from the Society of Wetland
Scientists and Aboriginal Affairs and Northern Development Canada’s Northern Scientific
Training Program were provided to MSP. We also thank A. Dyke for providing access to the
Canadian basal radiocarbon database, P. Kuhry for contributing raw peat core data, and three
reviewers (D. Beilman, Z. Yu, and one anonymous) for comments on an earlier draft of this
manuscript.
78
3.8 Tables
Table 3.8-114
C-accelerator mass spectrometry (AMS) dating of peat macrofossil of known provenance for 10 new sites in the Hudson
Bay Lowlands, Canada; sorted by increasing latitude. Basal peat dates first reported in Packalen et al., 2014.
Site Lat
(d.d.)
Long
(d.d.)
Elevation
(m)
Peatland
Class.
Peat
Depth
(cm)
Lab Number
Dated
Depth
(cm)
Material
Dated
δ13C
(‰
PDB)
14C-AMS (y BP)
Median
Age (y cal
BP)*
2σ Age
Range (y cal
BP)*
KJ4-3 51.59 -81.78 66 Weakly
Ombrotrophic 131 UGAMS-11256 45-46
wood fragments
-26.4 130 ± 30 128 (-3) - 280
UGAMS-11255 80-81
wood
fragments -25.7 1560 ± 20 1468 1400-1520
UGAMS-12717 129-131
wood
fragments -27.4 3630 ± 25 3940 3870-4070
KJ3-3 51.59 -81.79 65 Weakly
Ombrotrophic 176 UGAMS-11663 40-41
Sphagnum remains
-27.0 F14C=1.024 ± 0.003 -5.50 (-4.4) - (-6.6)
UCIAMS-
97825 175-176
wood + herb
stems 4170 ± 20 4720 4620-4830
KJ2-3 51.59 -81.76 65 Minerotrophic 246 UGAMS-11252 51-52 twig -23.8 1720 ± 20 1629 1560-1700
UGAMS-11253 130-131
Sphagnum
stems -28.5 2960 ± 25 3137 3000-3240
UGAMS-11254 177-178 wood -28.8 3250 ± 25 3466 3400-3560
UCIAMS-
97824 245-246
wood + herb stems
4130 ± 25 4860 4840-4960
VM4-5 52.70 -84.18 103 Weakly
Ombrotrophic 286 UGAMS-11668 53-54 wood -28.2 1390 ± 20 1302 1290-1340
UGAMS-11267 156-157
wood
fragment -26.7 3670 ± 25 4008 3910-4090
UGAMS-11673 285-286
wood
fragment -26.9 5530 ± 25 6320 6290-6400
VM4-1 52.71 -84.19 105 Weakly
Ombrotrophic 311 UGAMS-11265 46-47 wood -27.7 160 ± 20 186 (-2) - 280
UGAMS-11665 95-96 wood -26.8 1620 ± 20 1519 1420-1560
UGAMS-11666 143-144 wood -27.0 2470 ± 25 2571 2370-2710
UGAMS-11266 204-205 wood -28.5 3920 ± 25 4359 4260-4420
UGAMS-11667 253-254 wood -28.1 4860 ± 25 5600 5490-5650
UGAMS-12718 310-311 needles;
wood -27.6 5550 ± 25 6340 6300-6400
79
Table 3.8-1, continued
Site Lat
(d.d.)
Long
(d.d.)
Elevation
(m)
Peatland
Class.
Peat
Depth
(cm)
Lab Number
Dated
Depth
(cm)
Material Dated
δ13C
(‰
PDB)
14C-AMS (y BP) Median Age
(y cal BP)*
2σ Age Range
(y cal BP)*
VM3-2 52.71 -84.17 102 Minerotrophic 262 UGAMS-
11262 87-88 wood -25.4 2530 ± 20 2625 2500-2740
UGAMS-
11263 122-123 wood -28.7 3640 ± 25 3951 3880-4080
UGAMS-
11264 167-168 wood -28.6 3790 ± 25 4174 4090-4240
UGAMS-
11664 206-207 wood fragments -27.9 5070 ± 25 5816 5750-5900
UGAMS-
12716 261-262 wood fragments -27.5 5620 ± 25 6400 6310-6450
VM1-3 52.72 -83.94 86 Weakly
Ombrotrophic 210
UGAMS-
11257 72-73 wood -27.1 2060 ± 25 2028 1950-2110
UGAMS-
12715 209-210
herbaceous remains
-25.4 4390 ± 20 4940 4870-5040
VM2-5 52.72 -83.94 86 Minerotrophic 180 UGAMS-
11258 41-42 twig -25.6 140 ± 20 142 (-3) - 280
UGAMS-
11259 94-95 wood -28.9 2150 ± 20 2143 2060-2300
UGAMS-
11261 127-128 wood -29.3 3890 ± 25 4336 4250-4410
UGAMS-
11674 179-180 wood fragments -29.0 4980 ± 25 5700 5640-5850
HL02 54.61 -84.60 85 Weakly
Ombrotrophic 230
UGAMS-
11249 40-41 wood -26.4 1730 ± 25 1644 1570-1710
UGAMS-
11675 78-79 wood fragment -26.3 2100 ± 20 2073 2000-2130
UGAMS-
11250 109-110 wood -29.4 2450 ± 25 2504 2360-2700
UGAMS-
11676 163-164 twigs -26.1 3150 ± 30 3379 3270-3450
UGAMS-
11422 229-230 Sphagnum remains -26.0 4020 ± 25 4480 4420-4570
HL03 54.68 -84.60 99 Minerotrophic 269 UGAMS-
12720 70-71 Sphagnum remains -27.5 1880 ± 25 1832 1740-1880
UGAMS-
12721 127-128 Sphagnum remains -27.8 2560 ± 25 2723 2520-2750
UGAMS-
12722 190-191 Sphagnum remains -28.7 2920 ± 25 3067 2970-3200
UGAMS-
11423 268-269 Sphagnum remains -26.5 2700 ± 25 3550 3290-3880
* calibrated with IntCal09 calibration curve (Reimer et al., 2009).
80
Table 3.8-2 Summary of deconstruction terms used to describe peatland carbon (C) dynamics in the Hudson Bay Lowlands, Canada
(adapted from Yu, 2011).
Term*
NCP
NCU
NCR
NCB
CCP
CCU
CCR
*All terms reported in Pg C.
Cumulative C pool (sum of NCP) observed from present day peat cores and estimated for the entire
peatland through time.
Cumulative C uptake (sum of NCU) modeled for the entire peatland through time.
Cumulative C released (sum of NCR) modeled for the entire peatland through time.
Net C updake modeled for the entire peatland at 0.5 ky intervals, as the sum of observed NCP and
predicted C released according to the exponential peat decay model (Eq. 3.3.3.3.1), for each peat cohort.
Explanation
Net C pool observed from present day peat cores and derived from the product of C accumulation and
corresponding peatland extent, at 0.5 ky intervals.
Net C release modeled for the entire peatland at 0.5 ky intervals, including previously accumulated peat,
and derived from the exponential peat decay model (Eq. 3.3.3.3.1).
Net C balance modeled as the difference between NCU and NCR.
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3.9 Figures
Figure 3.9-1 Physical features and peat study locations in the Hudson Bay Lowlands, Canada.
The region includes the coastal Hudson Bay Lowland (HBL) and inland HBL and James Bay
Lowland (JBL) ecoregions, where the boundary between the two regions is indicated by a thin
black line (www.cec.org/naatlas/). Numbered contours (thick black lines) represent modern
isostatic uplift rates (mm y-1
) (Peltier, 2004), while grey shading represents modern surface
elevation generated using 1:250K Canadian digital elevation data (www.geobase.ca). Available
basal dated peat core locations for estimating peatland expansion (n = 100; black dots) (Packalen
et al., 2014; and references therein) and vertically dated peat study locations for carbon analysis
(n = 17, white triangles) are indicated.
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Figure 3.9-2 Peat age, depth, and cumulative carbon (C) mass relationships and modeled
exponential peat decay for the patterned peatlands of the Hudson Bay Lowlands, Canada.
A. linear relationships among median peat age (n = 80 from 17 cores; dots) and 2σ age ranges
(error bars) are presented for 17 vertically-dated peat cores sampled from the HBL. Peat ages
were estimated following calibration of 14
C ages using the IntCal09 curve (Reimer et al., 2009);
and B. cumulative C mass was fit with an exponential peat decay model (Clymo et al., 1998).
The best-fit peat C addition rate (PCAR) and long-term peat decay parameter (a) for the HBL are
presented.
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83
Figure 3.9-3 Holocene peat carbon (C) pools and modeled peat C terms (Yu, 2011) for the
Hudson Bay Lowlands, Canada, since peat initiation began ~ 8 ky BP.
A. peat age versus measured (n = 17 cores) net peat C pool (NCP) and modeled (n = 100 cores)
net C uptake (NCU), release (NCR) and balance (NCB), where NCB = NCU – NCR.; and B.
peat age versus cumulative measured peat C pool (CCP) and modeled cumulative peat C uptake
(CCU) and release (CCR) following long-term constant peat decay, are presented at 0.5-ky
intervals.
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84
Figure 3.9-4 Holocene peatland area increase, carbon (C) accumulation, and net C balance, since
peat inception for the Hudson Bay Lowland, Canada.
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85
Data are presented in relation to regional paleoclimate, global insolation trends, and atmospheric
methane (CH4) and carbon dioxide (CO2) variability. A. winter and summer insolation during
last 7.5 ky at 60 °N (Berger and Loutre, 1991); B. GISP2 (Greenland) ice core record of
atmospheric CH4 concentration (Brook et al., 2000) and Antarctic ice core record of atmospheric
CO2 concentrations (Monnin et al., 2004); C. reconstruction of winter and summer temperature
and annual precipitation anomalies inferred using a synthesis of the North American Pollen
Database for the northern Quebec, Canada pollen region (Viau and Gajewski, 2009); D. mean (±
standard error) time-weighted C accumulation rates (CAR, n = 17) ; and modeled (Yu, 2011) net
carbon uptake and balance, at 0.5-ky intervals (n = 100); and E. frequency of post-glacial
peatland initiation in the HBL, inferred from basal radiocarbon dates (n = 100, 0.5-ky bins),
cumulative peatland initiation (%; n = 100), and peatland surface area increase (%, n = 100)
(Packalen et al., 2014).
86
Chapter 4 Climate and Peat Type in Relation to the Spatial Distribution of
the Peat Carbon Mass in the Hudson Bay Lowland, Canada
4.1 Abstract:
Northern peatlands store more than 500 Gt of carbon (C); however, controls on the spatial
distribution of the stored C (termed C mass, kg m-2
) may differ regionally, owing to the complex
interaction among climate, ecosystem processes, and geophysical controls. As a globally
significant C pool, elucidation of controls on the spatial distribution of the peat C mass in the
Hudson Bay Lowland, Canada (HBL) is of particular importance. Although peat age in the HBL
is closely related to timing of land emergence and peat depth, considerable variation in the total
C accumulated among sites of similar peat age suggests that other factors may explain trends in
the distribution of the peat C mass. Here, we present detailed peat lithologies for bog, fen and
coastal mesoforms, and consider climate as a control on the spatial distribution of the peat C
mass (n = 364) across the HBL. We find that temperature, precipitation, and potential
evapotranspiration each explain up to half of the variation in the peat C mass, such that regions
in the HBL characterized by warmer and wetter conditions tend to support larger peat C masses.
Furthermore, we show that the widespread bog-fen patterning across the HBL is related to the
observed spatial variability in C mass, suggesting that small scale topographic and
ecohydrological controls are potentially important determinants of C mass accretion. Our
findings support the hypothesis that both climate and ecohydrological factors are important
drivers of peat C mass accretion, alongside geophysical controls on the timing of peat initiation
in the HBL.
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Keywords or Index terms: peatland, mean annual air temperature (MAAT), mean annual
precipitation (MAP), potential evapotranspiration (PET), moisture balance, continentality
Key Points:
• Carbon-climate-peat linkages are presented for the Hudson Bay Lowland, Canada
• Climate may explain up to half of the spatial distribution of the peat C mass
• Wide C mass variation within local climate space suggests autogenic controls
4.2 Introduction
Northern peatlands occupying landscapes at latitudes north of 45°N, develop within a broad
climatic domain, where mean annual air temperatures (MAAT) may range between -20 and
15°C, and mean annual precipitation (MAP) may range from < 100 to 3000 mm (Yu, 2012). Yet,
northern high latitude peatlands occupy a unique biogeographic niche where net primary
production (NPP) exceeds organic matter decomposition under sustained waterlogged
conditions. Currently, northern peatlands are estimated to store 250 – 550 Pg carbon (C), most of
which has accumulated during the present interglacial (Gorham, 1991; Loisel et al., 2014;
Turunen et al., 2002;Yu et al., 2010). Some of the main source of uncertainty in global peatland
C mass estimates stem from the need for region specific values of peat depth, dry peat mass, and
C density (Yu, 2012).
Northern high latitudes are expected to experience the most intense climatic change (McGuire et
al., 2009), making cold climate peatlands especially vulnerable to changes in net C storage by
potentially affecting rates of NPP and peat decomposition, enhancing disturbance, and/or
unlocking large peat C pools from thawing permafrost. Further, climate-induced changes to C
dynamics in northern peatlands may result in vastly different C flux scenarios depending upon
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88
peat type (Loisel et al., 2014) and climatic conditions. While the role of climate and peat type as
controls over the amount of C stored as peat remains poorly understood, contemporary evidence
suggests that recent accelerated climatic change is impacting the net C stored in northern
peatlands (Gorham et al., 2012). Under conditions of natural and anthropogenic climatic change,
NPP may be enhanced, suppressed, or offset by decomposition, altering the capacity of peatlands
to function as long-term C reservoirs. Thus, alteration of peatland C dynamics may have
important implications for the nature of future climate. As a result, anticipating the future
trajectory of peatland C storage potential necessitates improved data on spatio-temporal controls
on peatland C dynamics to support inclusion of these ecosystems in Earth and climate system
models and C budget accounting.
In the present study, we quantify the roles of climate and peat type on the spatial distribution of
the peat C mass in the Hudson Bay Lowland, Canada (HBL). Today, the HBL represents the
second largest peatland region in the world. Within a nearly continuous peat cover patterned with
bog- and fen-like features, the HBL maintains a globally significant C reservoir in excess of 30
Pg C, as quantified in Chapter 2 and 3 (Packalen and Finkelstein, 2014; Packalen et al., 2014)
and that C mass has accumulated since mid-Holocene peat initiation. While climate has
supported long-term C storage in the HBL, it remains unclear what factors control the size and
spatial variation of the peat C mass in this region. Modern gridded climate data reveal a series of
climate gradients, controlled by latitudinal position and proximity to the Hudson and James Bays
where annual sea-ice formation is a major driver of regional climate (Gough et al., 2004). These
gradients may explain patterns in the distribution of the total peat C mass across the HBL.
Although long-term paleoclimate records for the HBL are few, it is apparent from data presented
in Chapters 2 and 3 that paleoclimate in the HBL exhibits relatively small departures from
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89
contemporary climate normals over the past eight millennia (Packalen and Finkelstein, 2014;
Viau and Gajewski, 2009), and provide evidence that climate variability through space likely
remained relatively consistent through time.
Glacial isostatic adjustment (GIA), especially during the mid-Holocene, is coupled to the rate
and history of peat initiation in the HBL. Consequently, peat depth and C mass are related in part
to the timing of land emergence. That being said, wide scatter within given time intervals suggest
that other factors contribute to patterns in peatland development and C accretion in the HBL.
Moreover, peatlands initiated in many regions of the HBL under cooler- and wetter-than-present
climatic conditions (Glaser et al., 2004a; Glaser et al., 2004b; Packalen et al., 2014).
Nonetheless, empirical and model evidence presented in Chapter 3 reveal that Holocene-scale
variation in apparent C accumulation rates in the HBL appears to be related to peat type and to a
lesser extent to climate (Packalen and Finkelstein, 2014). Accordingly, maximum C
accumulation rates occurred during the mid-Holocene when young, early emerging fen peatlands
dominated, while maximum total C mass development occurred during the late Holocene as the
spatial extent of the accumulating peat mass expanded (Packalen and Finkelstein, 2014).
Notwithstanding the role of GIA, maximum and minimum temperature and precipitation have
been previously reported to be important controls on the development of the circum-Arctic
peatland C mass, together with bioclimatic parameters, such as potential evapotranspiration
(PET), net moisture balance (PET/MAP), growing degree days (GDD), and photosynthetically
active radiation (PAR) (Charman et al., 2013; Eppinga et al., 2009; Holmquist and MacDonald,
2014; Loisel et al., 2012). MAAT, GDD and PAR have been invoked as controls on NPP for
both vascular and non-vascular vegetation, whereby rates of peat accumulation rise in response
to increasing MAAT, GDD, and PAR (Beilman et al., 2009; Charman et al., 2013). Further,
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increasing PAR and growing season length, in the absence of moisture stress, lead to increased
rates of C accumulation and a potential negative feedback to climatic warming (Loisel et al.,
2012). These trends are supported by Holocene-scale peat-paleoclimate records that reveal
maximal peat C accretion during the warm early Holocene, when orbitally induced summer
insolation peaked (Jones and Yu, 2010; MacDonald et al., 2006).
While climate may be an important control over the spatial variability in C mass at larger spatial
scales (e.g., degree latitude), peat type as determined by local ecohydrological processes, trophic
status, and vegetation type may also contribute to the spatial variation of the C mass at local to
regional spatial scales. Further, non-linear responses by peatlands to autogenic controls, such as
vegetation decomposition, and allogenic controls, such as internal (e.g., polar pattern,
ENSO)/external (e.g., PAR) forcing (Belyea and Baird, 2006) may disrupt the theoretical fen-to-
bog peat succession trajectory. Still, maintenance of sustained waterlogged conditions is one of
the principal mechanisms responsible for peat accumulation, and climate-driven changes may be
reflected in peatland vegetation patterning and ecohydrology (Frolking et al., 2011; Jones and
Yu, 2010). In the HBL, precipitation, PET, and local drainage patterns are anticipated to
contribute to the maintenance of differentially waterlogged conditions to support peatland
development characterized by bog-fen patterning (Sjörs, 1959). However, changing trophic
status and associated increases in bulk density with depth disconnect the surface peat over time
from groundwater and nutrient supplies via progressively decreasing hydraulic conductivity. As
a result, we expect that the younger, emerging landscapes near the coast will support more fen
peatlands, while more inland reaches likely support a shift toward bog peatlands. As the
landscape continues to emerge, effective moisture in near-surface peat, together with peat
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temperature may become even more important predictors of peatland capacity to continue to
accumulate peat and sequester C (Beilman et al., 2009; Loisel et al., 2012).
Given the HBL’s proximity to the Hudson and James Bays, the moisture balance (PET/MAP)
and/or the relative continentality may be important explanatory factors for the distribution of the
peat C mass in the HBL. PET-dominated regions (PET/MAP ratios approaching 1) are
characterized by enhanced nutrient use efficiency among vegetation to support hummock-hollow
patterning; however, drainage-dominated peatlands have moisture balances typically much less
than one, and are associated with enhanced peat accumulation and ridge-flark (akin to bog- and
fen-like features) pattern development (Eppinga et al., 2009). Greater temperature seasonality
combined with PET-dominated climates reflect a drier, more continental climate and the
implications of this on the total C mass remain uncertain under future climatic conditions.
Temperature seasonality in relation to PET/MAP can thus be used as an indicator of the relative
continentality of climate, and previous results reveal faster Sphagnum growth rates in drier, more
continental climates characterized by both greater seasonality and PET/MAP ratios (Loisel et al.,
2012). Although Sphagnum-dominated peat may be less C dense (Beilman et al., 2009; Loisel et
al., 2014), potentially deeper peat may contribute to a larger total C mass that is more resistant to
decomposition. Future C dynamics are likely to be more sensitive to sustained changes to the net
moisture balance rather than short-term hydrologic variability (Dise, 2009), as rising
temperatures potentially enhance evapotranspiration and support enhanced disturbance.
Consequently, temperature and continentality will be evaluated in the HBL for the whole
peatland mass, and in relation to the spatial distribution of peat type across the landscape.
The objectives of this study are to elucidate important factors that explain the spatial distribution
of the peat C mass in the HBL. I hypothesize that both climatic and ecohydrological controls, as
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indicated by peat type, are important in explaining the spatial pattern in the distribution of the
peat C mass in the HBL. I further hypothesize that increasing temperature, GDD and PAR would
be positively correlated with the total peat C mass, while precipitation may be positively
correlated with the peat C mass except in permafrost-impacted peatlands where both positive and
negative relationships have been reported (Charman et al., 2013; Holmquist et al., 2014; Loisel et
al., 2012). I expect a gradient in the relative continentality of the HBL from the coast inland,
with implication on the distributions of the net moisture balance. Moreover, as the distribution of
the peat C mass may also be related to the peat type, we distinguish among bog, fen and coastal
peatland mesoforms in terms of both peat C characteristics and in relation to the major
bioclimatic parameters presented here (Loisel et al., 2014). To this end, I hypothesize that fen
peat will be shallower with greater C density than Sphagnum-dominated bog peat, owing in part
to differences in the relative decomposability of the vegetation inputs to the accumulating peat
mass.
4.3 Study setting
I define the HBL here using the inland and coastal HBL/James Bay Lowland (JBL) ecoregions
described in the North American Environmental Atlas (http://www.cec.org/naatlas/).
Accordingly, the HBL stretches along the shoreline of Hudson and James Bays and inland
toward the north-western margin of the Precambrian Shield (Figure 4.10-1). Under the influence
of the Hudson and James Bays, the HBL has a microthermal climate, strongly influenced by
Arctic air masses and strong winds (Figure 4.10-2). During the early Holocene (until ~ 8.5 ky
BP), the HBL was largely covered by the Tyrrell Sea, providing a maximum age for HBL
peatlands that is several millennia younger than the extensive peatlands of north-western North
America and Siberia. In the HBL, a rapidly rebounding landscape following the final collapse of
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the Laurentide Ice Sheet (LIS) during the mid-Holocene, gave way to widespread peat initiation
as documented in Chapter 2 (Packalen et al., 2014) over low-relief deposits of till and diamicton,
eskers and moraines, and glacio-marine sediments overtop of Paleozoic sedimentary bedrock
(Martini, 2006). Approximately 8-ky hence, a nearly continuous peat cover, with a mean slope of
0.5 m km-1
, has developed across the HBL’s ~ 372,000 km2 landscape (an updated spatial extent
presented in Chapter 2) and accounts for nearly 10% of the arctic and subarctic peatland area
(Packalen et al., 2014; Tarnocai et al., 2009). Nearly 90% of the HBL is classified as peatland,
characterized by alternating bog and fen features inland that becomes more fen-dominated in the
coastal ecoregion. Further, the HBL includes the southernmost extent of non-alpine permafrost
globally, which affects ~ 1% of the peatlands, principally as a narrow stretch of continuous
permafrost along the Hudson Bay coast, and elsewhere as sporadic to discontinuous permafrost
(Riley, 2011). As well, coastal deltas, fen water tracks, networks of low order streams, small
pools, and shallow lakes can be observed throughout the region, as was observed during several
field campaigns and described in Chapters 2 and 3 (Glaser et al., 2004a; Glaser et al., 2004b;
Packalen and Finkelstein, 2014; Packalen et al., 2014; Sjörs, 1959). Consequently, little
infrastructure development has occurred in the HBL, resulting in a large geographic region
characterized by extreme remoteness, limited accessibility, and little evidence of anthropogenic
disturbance.
4.4 Methods
4.4.1 Peat and bioclimatic data sources
Here, I use 42 detailed peat C records summarizing new and previously reported peat properties
(Table 4.9-1, and references therein), together with a larger synthesis of 364 peat depths drawn
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from previously published literature (Riley, 2011), to scale C mass across the HBL (Figure 4.10-
1). Modern climate data (Figure 4.10-2) were used to examine potential climatic controls on the
spatial distribution of total C pools across the HBL. Although subject to some uncertainty, these
publicly available data are one of the most comprehensive spatial estimates currently available.
Briefly, these contemporary climate data are generated from the ANUSPLINE climate model,
which interpolates verified North American meteorological station data using thin-plate
smoothing splines to generate a continuous climate estimate (McKenney et al., 2006).
Bioclimatic variables considered in this analysis, as possible controls on the spatial distribution
of the C mass in the HBL, include: MAAT, MAP, PET, and over the growing season, as well as,
growing season length, timing, and GDD above 5 °C (GDD5). A strong correlation between
GDD and PAR (inferred from latitude and cloudiness) has previously been demonstrated
(Charman et al., 2013), owing to the shared growing season length among the two variables, thus
only the former is presented here in relation to the spatial distribution of the HBL’s C mass.
Temperature seasonality (difference between summer and winter temperatures) in relation to the
net moisture balance (PET/MAP) was used to evaluate the relative continentality of climate
(Loisel et al., 2012). Correlations among individual climatic variables and HBL peat C masses
for all peatlands together (n = 364), and among major peat types, including bogs (n = 195), fens
(n = 121) and coastal/permafrost (n = 48) were evaluated using Pearson product moment
correlation analysis, at a significance of p ≤ 0.05 (Table 4.9-2). Further, regression analysis of
the most ecologically significant climate-peat C relationships is presented.
4.4.2 Peat physical properties
Quantification of the peatland C mass was based upon detailed analysis of 42 peat cores for bulk
density, loss-on-ignition (LOI) and C content measurements (Table 4.9-1), using standard
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methods described in Chapters 2 and 3 (Packalen and Finkelstein, 2014; Packalen et al., 2014).
Of these, 11 are re-interpreted from previously published studies (Bunbury et al., 2012;
Holmquist et al., 2014; Kettles et al., 2000; Kuhry, 1998; Kuhry; O'Reilly et al., 2014; van
Bellen et al., 2011a), while new peat physical characteristics for 31 previously described peat
cores (Chapters 2 and 3) are presented (Packalen and Finkelstein, 2014; Packalen et al., 2014).
Local C mass (kg m-2
) was calculated for all samples, by summing the product of incremental C
density and increment length. Areal C mass was linearly related to peat depth (Figure 4.10-3),
and this relationship was used to infer peat C mass for peatland sites with known peat depth (n =
322). The relationship (R2 = 0.81, p < 0.001) is described by equation 4.4.2.1:
C mass = 0.4 * peat depth + 17.1 (4.2.2.1)
Differences in peat physical features among peatland types were compared using a t-test or
analysis of variance (ANOVA). For analyses that failed the normality test, ANOVA on ranks
was completed, followed by multiple comparisons using Dunn’s method. Significance was
determined at p ≤ 0.05.
4.5 Results
4.5.1 Climate and carbon mass spatial relationships across the HBL
4.5.1.1 Climate Space
The physiographic region of the HBL, as defined here, stretches across nearly 10 degrees of
latitude, from ~ 50 – 60 °N. Modern average climate (1971 – 2000) is characterized by mean
annual temperatures ranging between -7 and 0 °C (mean ± standard deviation: -2.5 ± 1.8 °C),
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such that warmest conditions occur in the southern HBL, while the coldest conditions follow the
Hudson Bay coastline in a northwesterly direction (Figure 4.10-2a). The growing season for
most vegetation types in the HBL begins in mid-May [earlier for Sphagnum spp. as growth may
take place as soon as temperatures exceed freezing during the snow-free period (Charman et al.,
2013)], and extends for a period of 120 to 160 days with a mean growing season temperature of
10.8 ± 1.0 °C. Total annual precipitation ranges from 430 to 740 mm with half to two-thirds of
the precipitation occurring during the growing season. The southern HBL receives the most
precipitation and the northwestern HBL receives the least, with moderate amount of precipitation
recorded along the Hudson Bay coast and the northeastern HBL (Figure 4.10-2a). Mean
temperature seasonality is 49 ± 1.4 °C and mean PET/MAP ranges between 0.4 and 0.7.
Temperature seasonality and PET/MAP for the peatlands considered here are linearly related (R
= 0.93, p < 0.001). Accordingly, a greater moisture balance (lower PET/MAP ratio) was
associated with lower temperature seasonality, reflecting a more maritime influence on the
peatland ecosystems developing in the coastal regions of the HBL, compared to the more PET-
dominated, more continental climate further inland (Figure 4.10-4).
4.5.1.2 Spatial distribution of peatlands within the HBL climate domain
The distribution of the peat C mass in the HBL for the 364 peatland sites considered here is
presented in Figure 4.10-1, and our results reveal that the peat C mass ranges between 20 and
180 kg C m-2
. Consideration of all peatlands together revealed significant positive correlations
between the total C mass and total precipitation and mean temperature, albeit with wide variation
(Figure 4.10-5a, b; Table 4.9-2). GDD5, growing season length, and PET were also significantly
and positively correlated with the distribution of the total peat C mass (Figure 4.10-5, Table 4.9-
2). A gradient of low to high C mass is apparent that increases with peat depth, from the coasts
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of the Hudson and James Bay inland, reflecting the role of GIA in the timing of peat initiation.
However, no relationship between peat age (n=101) and either latitude (R2 = 0.02, p = 0.134) or
longitude (R2 = 0.02, p = 0.214) is apparent, providing evidence for a lack of connection between
C mass and climate that is driven by the timing of peat initiation across the HBL (Packalen et al.,
2014). Further, substantial local variability is also apparent, confirming our hypothesis that other
controls may contribute to explaining the distribution of the peat C mass in the HBL.
Both the length and the timing of the start of the growing season (GS) are similar for fens and
bogs in the HBL, while the GS for the HBL’s coastal peatlands begins ~ 20 days later and last
for 132 days (median), compared with 156 and 157 GS days (medians) for inland fens and bogs,
respectively. The median temperature during the GS does not differ among fens and bogs (~ 13
°C) in the HBL; however, the coastal peatlands appear to be developing under cooler GS
conditions (10 °C), with reduced seasonality. Moreover, the GGD5 for inland bogs (990) and
fens (960) are nearly double the GDD5 for peatlands developing in the coastal (530) HBL.
From a surface wetness standpoint, our analysis of climatic patterns in the HBL reveals that bogs
receive significantly (Kruskal-Wallis ANOVA on Ranks at p < 0.05) more GS precipitation
(median: 422 mm) than either fens (406 mm, Dunn’s method: Q = 2.9) or coastal peatlands (357
mm; Q = 10.2). Further, PET is lower for the coastal peatlands (211 mm), than for either bogs
(359 mm; Q = 10.4) or fens (349 mm; Q = 8.4), resulting in a greater net moisture balance
(PET/MAP) for coastal peatlands (0.42) than for bogs (0.55, Q = 8.5) and fens (0.55, Q = 7.7) in
the HBL. Increased moisture availability near the coast may enhance permafrost accretion
particularly when coupled with significantly lower winter precipitation (median: 62 mm), which
provides less surface insulation from the significantly (p < 0.05) colder winter temperatures
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(median: -22.4 °C, Q = 7.3 vs. fen and Q = 9.5 vs. bog) near the coast relative to the inland
extent of the HBL (~ -19 °C).
4.5.2 Carbon mass variation among peat types
4.5.2.1 Peatland geography
The dataset includes a variety of peatland types and comparative statistics were used to evaluate
differences among main peatland classifications used in this study. The mean peat depth of 207 ±
93 cm (n = 364) varied among peatland types across the HBL. Kruskal-Wallis ANOVA on
ranks, followed by Dunn’s test revealed that the deepest peat was significantly associated with
bog mesoforms (235 cm, n = 195, Q = 5 vs. fens and Q = 7.5 vs. coastal), followed by fen
mesoforms (180 cm, n = 121, Q = 3.9 vs. coastal), and then coastal peatlands (100 cm, n = 48).
4.5.2.2 Peat carbon content among peat types
Mean (± standard deviation (s.d.)) peat characteristics for a selection of peat cores examined in
detail here are presented in Table 4.9-1. C density for the subset of peatlands examined in this
study was 48 (14) g dm-3
, and was found to differ significantly among the major peatland
classifications considered here. Kruskal-Wallis ANOVA on ranks indicated that bogs had the
lowest median C density (45 g dm-3
, n = 795 from 20 complete peat cores), while coastal
peatlands had the greatest median C density (60 g dm-3
, n = 38, from 7 complete peat cores). Fen
features patterned with bogs in the interior reaches of the HBL were found to have an
intermediate median C density of 49 g dm-3
(n = 603, from 15 complete peat cores). Mean total C
mass among all peatland samples considered in the HBL was 92 ± 35 kg m-2
, and did not differ
significantly among bog (106 kg m-2
, n = 20) and fen (98 kg m-2
, n = 15) features (Figure 4.10-
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6). However, coastal mesoforms had a significantly (t = 4.4 vs. bogs and t = 3.2 vs. fens, p <
0.05) lower median C mass (53 kg m-2
, n = 7).
4.6 Discussion
As one of the largest continuous peatland regions globally (Chapter 2), the HBL currently stores
a C pool of sufficient magnitude to potentially influence global climate via positive and negative
feedbacks associated with changes in peat C dynamics (Packalen et al., 2014). As a result,
understanding drivers of peat-C dynamics in the HBL is of high priority. While climate has been
shown elsewhere to be a major factor in explaining the spatial patterning of the peat C mass
across major northern peatland regions (Beilman et al., 2009; Charman et al., 2013), these
relationships have not been well described for the HBL. Here, we consider the relationship
between gridded modern bioclimatic parameters and the spatial distribution of the total C mass
across the physiographic region of the HBL. Our results confirm that the total C mass is
positively correlated with several climate parameters for the full extent of the HBL. Spatial
climate patterns approximate a latitudinal gradient which skews toward the northwest around the
Hudson Bay. Under the influence of the thermal properties of the Hudson Bay, somewhat sharp
transitions in annual and seasonal temperature trends are apparent just south of the Hudson Bay
coast (Figure 4.10-2a). By contrast, precipitation patterns appear to be influenced by proximity to
both the Hudson and James Bays in addition to elevation (Figure 4.10-2b). Accordingly,
precipitation maxima occur in the southern HBL, and decreases north toward the higher
elevation inland extents.
We find that the warmest annual temperatures and most precipitation in the HBL occur in the
southern portions, where the peat C mass tends to be greatest (Figure 4.10-1). The southwestern
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extent of the HBL is the most continental, experiencing the greatest seasonality and largest
moisture losses by PET. Although the distribution of the peat C mass is related in part to the
timing of peat initiation (Chapter 2), such that earlier initiation dates tend to be located inland
and perpendicular to the coasts of the Hudson and James Bays (Packalen et al., 2014), similarly
aged peatlands located across the latitudinal gradient in the HBL do not store equivalent C
masses. Moreover, our analysis of the relationship between peat age versus latitude and
longitude reveal that no correlation exists for peatlands in the HBL, suggesting that the
relationships we report here between C mass and important climate variables is not controlled
by the timing of peat initiation. Accordingly, our results confirm that similarly aged peat deposits
in the southern HBL store more C than the more northern counterparts, providing additional
evidence of a climatic control on the distribution of the peat C mass in the HBL.
Conversely, the coldest and driest climatic conditions occur toward the northwest HBL, where
the continuous permafrost region lies. Within the coastal region of the HBL, a large range of peat
C masses are apparent (Figure 4.10-1), with some of the largest stores located in the most
northwestern reaches of the HBL. Previous evidence indicates that permafrost developed in the
HBL during the late Holocene (Kuhry, 1998; Kuhry, 2008), which suggests the influence of
permafrost in either (1) promoting the long-term storage of mid-Holocene age peat C (e.g., in the
northwest HBL) or (2) preventing the accrual of newer peat C, such as in the northeast HBL,
where land has only recently emerged from the Hudson Bay. Moreover, a study of late Holocene
peat depths found that permafrost occurrence significantly reduced vertical peat growth during
the late Holocene, and hence rates of C accumulation, during the past 2 ky (Holmquist et al.,
2014).
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Several potential climatic controls of ecological importance to the accrual of the peat C mass
were considered in this study. A longer growing season, with temperatures above 5 °C (GDD5)
and a warmer mean GS temperature may influence both net primary production and/or
decomposition, resulting in potential non-linear responses in peatland C dynamics. Given
adequate moisture for peatland development, the total C mass increases with temperature,
GDD5, and growing season length in the HBL, and supports the hypothesis that the net C stored
in peatlands is influenced by NPP and vertical peat growth. These results are consistent with
peat-climate relationships reported for circum-Arctic peatlands. Beilman et al. (2009) found
positive correlations between temperature and vertical peat growth for the last 2 ky in the West
Siberian Lowlands, and found that rates of C accumulation were significantly correlated with
contemporary MAAT, where maximum rates occurred between -1 and 0 °C. Similarly,
Holmquist et al. (2014) reported positive correlations among vertical peat growth and MAAT,
GDD, precipitation for peatlands location on the Boreal Shield and Hudson Plains of northern
Ontario. Finally, Charman et al. (2013) found a positive correlation between the rate of C
accumulation and GDD for the last 1 ky in circum-Arctic peatlands. However, warmer winters,
with enhanced snow cover are thought to potentially enhance peat C losses due to respiration
(Jones and Yu, 2010), and this potential effect needs further consideration in the HBL.
Our analysis of the relationship among bioclimatic factors and the distribution of the peat C mass
in the non-coastal extent of the HBL reveal significantly positive correlations among the
distribution of the peat C mass in the HBL and each of the parameters considered. Although
significant correlations among bioclimatic parameters and the distribution of the C mass in the
HBL are apparent, the spread of the data is large, suggesting an important contribution of local-
scale ecohydrological processes in the accretion of peat C spatially, a result that was also found
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in Chapter 3 to be important temporally in an analysis of Holocene trends (Packalen and
Finkelstein, 2014). As the peatlands of the HBL are patterned with ridge-flark features, our
synthesis considers the distribution of C mass among bog- and fen-like features, as a proxy for
ecohydrological variation (Figure 4.10-3). Our results reveal that C density was lowest in
association with bog peatlands, which are characterized by poorly-decomposable bryophyte
remains. These findings are consistent with previous investigations into the role of vegetation
type on C mass development, which found that fossil plant composition was significantly related
to the rate of C accumulation due to the low C content of Sphagnum remains (Beilman et al.,
2009; Loisel et al., 2014).
Mean total C mass for all HBL peatlands (91 ± 30 kg m-2
) did not appear to vary significantly
between inland patterned bog and fen features; however both stored significantly more C than
coastal fen peatlands. Compared to circum-Arctic peatlands, the C content of HBL peatlands is
comparable to those reported for Canadian peatlands, but lower than those reported for Finnish,
Swedish, and Siberian peatlands (Gorham et al., 2012). The bulk density of HBL bogs appear to
be within the range of those reported for other northern peatlands. However, fen bulk densities in
the HBL appear to be greater than Finnish fens (Turunen et al., 2002), and consistent with
western Canadian shrub and open fens (Vitt et al., 2000). The fens considered in the present
study include a range of peatland classifications, including open, shrub/treed, intermediate
succession, and poor fens. This variability appears to be reflected in the mean and standard
deviation of the fen peat characteristics. Considered together, the mean C and bulk density for
HBL peatlands appears to be consistent with the estimates originally reported by Gorham (1991)
for North American peatlands and greater than that reported by Turunen et al. (2002) for Finnish
peatlands.
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Though bogs and fens are common mesoforms throughout the HBL, the bogs considered here
tended to occur at higher elevations (median = 97 m) than fens (median = 79.5; p = 0.003). This
finding is related in part to the proximity of the peatland mesoform to the coast, such that fens
tended to occur ~ 10 km closer to the coast than did bogs (p = 0.01). Fen peatlands also tended to
be located further north than bog peatlands (median latitude 52.7 °N versus 51.6 °N,
respectively; p = 0.002). While peatlands tend to begin as fens and the younger peatlands are
located in proximity to emerging shorelines, the occurrence of fen peatlands further north is not
significantly related to peat age in the HBL. Rather the tendency of fens to occur further north
than bogs may be partially explained by maritime nature of the climate related to the influence of
the Hudson Bay, supported by the relationship between seasonality and PET/MAP. By contrast,
bogs were associated with increased GGD5 (median = 962) likely due to the more southerly
position in the HBL, which was significantly different from fens (median = 843). GDD5 was
also significantly correlated with peat depth (R = 0.4, p < 0.001), supporting the hypothesis of
increased NPP in association with warmer annual conditions.
Long-term records indicate that historical peat accumulation was maximal in circum-Arctic
peatlands during the early Holocene thermal maximum, when plant production was high due to
warm summers and respiration was low due to cold winters (Yu et al., 2010). Such conditions
are consistent with more continental climates. In assessing the relative continentality of the HBL,
we show that the greatest C mass occurred in the region of the HBL experiencing the largest
seasonality. Moisture, as inferred from PET/MAP in the HBL ranged between 0.39 and 0.66,
which is similar to the range observed at Degerö Stormyr, Sweden and lower than the more
continental peatlands of Vasyugan Bog, Siberia (Eppinga et al., 2009). Previous studies have
shown that as the PET/MAP ratio approaches 1, the C storage capacity of a peatland may
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decline, and favour a nutrient accumulation-driven mechanism of peat patterning (Eppinga et al.,
2009). Under these conditions, peatland hummock development is favoured, as nutrients such as
nitrogen and phosphorus accumulate and stimulate vegetation growth. Alternatively, when the
PET/MAP ratio is much less than 1, such as in the HBL, drainage-dominated peatland
development favoring a peat accumulating mechanism results in patterning characterized by
ridge-flark features and nutrients accumulating in sparsely vegetated hollows (Eppinga et al.,
2009). Wetter, warmer future climate conditions in the HBL may continue to favour reduced
PET/MAP ratios, supporting enhanced C storage capacity.
4.7 Conclusions and future implications
Although peat depth and age are closely related to timing of land emergence, wide scatter within
given time intervals and across latitudes suggests other factors control peat development.
Climate is shown here to be an important factor in explaining landscape-scale spatial patterns in
the distribution of the peat C mass in the HBL. The spatial patterns in the distribution of the peat
C mass are best explained by temperature, especially GS temperature and precipitation gradients.
Further, variables related to GS conditions (e.g., PAR, GDD5, growing season length and
seasonality) significantly explain distribution of C mass in the HBL. Although all peat classes
appear to be sensitive to temperature and precipitation trends in the HBL, fens may be more
sensitive to growing season conditions, while bogs may be more sensitive to the net moisture
balance. Climatic conditions also appear to explain major trends in the geographic distribution of
the peat C mass within a given peat type (bog, fen, coastal) in the HBL. Yet, the widespread bog-
fen patterning across HBL suggests that small scale topographic and ecohydrological controls are
also critical determinants of C mass development, even when local climatic conditions remain
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constant. Our results suggest that ecohydrological factors, together with climate are both
important determinants of the distribution of the peat C mass in the HBL.
While conservative climate estimates suggest warmer and wetter conditions within the range of
past climate variability, the simultaneous occurrence of warmer/wetter conditions is not
documented in the HBL’s Holocene paleoclimatic record. Increased primary productivity is
anticipated in the HBL as a consequence of a predicted warmer climate over the next century,
lower PET/MAP ratios, and in association with longer growing seasons. However, warmer
winters may enhance decomposition and net C losses, as it has elsewhere (Jones and Yu, 2010)
and a net reduction in surface moisture conditions may further enhance mass C loss as a
consequence of disturbance (e.g., fire). Nevertheless, potential climate feedbacks under future
climate scenarios related to augmented CO2 and CH4 emissions, enhanced microbially-driven
decomposition and thermokarst formation from thawing permafrost may confound these
relationships in the HBL. Today, the largest peat C masses in the HBL appear to occur in
association with more continental climates. Yet, analysis of contemporary patterns in PET/MAP
reveal that the peatlands in the HBL occur in association with both maritime- and continental-
type climates, suggesting that both net moisture balance and precipitation are important controls
on peat accumulation. Although maintenance of moist surface conditions supports herbaceous
productivity and the accretion of more C dense peat, Sphagnum productivity is greater under
more continental climatic conditions. The result is a larger C mass, due in part to the
accumulation of decay-resistant vegetation remains. If a more continental climate is coupled with
a net moisture balance approaching one, then reduced C storage capacity is anticipated. By
contrast, continued peat accumulation may be anticipated under warmer/wetter climatic
conditions, if peatlands in the HBL respond in a manner similar to the boreal peatlands located to
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the south. Consequently, consideration and elucidation of a moisture balance threshold for
sustained C sequestration in the HBL is warranted, in anticipation of future climate scenarios.
4.8 Acknowledgements
Research funding and field support was provided by the Ontario Ministry of Natural Resources
and Forestry’s Science and Research Branch and Far North Branch, under the auspices of
projects CC-021 and FNIKM 028 to JWM. Additional support for field work and radiocarbon
dating was provided by grants from the Natural Sciences and Engineering Research Council of
Canada (NSERC) and the Ontario Ministry of the Environment through the Climate Change and
Multiple Stressor Research Program at Laurentian University to SAF. Additional support in the
form of an NSERC Alexander Graham Bell Canada Postgraduate Scholarship, Ontario Graduate
Scholarship, and an Association of Canadian Universities for Northern Studies, Canadian
Northern Studies Trust Scholarship, and field research grants from the Society of Wetland
Scientists and Aboriginal Affairs and Northern Development Canada’s Northern Scientific
Training Program were provided to MSP. We thank A. Dyke for providing access to the
Canadian basal radiocarbon database and P. Kuhry for contributing raw peat core data. Gridded
climate data were provided by D. McKenney and P. Papadopol, Canadian Forest Service, Great
Lakes Forestry Centre, Sault Ste. Marie, Canada. We also thank J. Harden and an anonymous
reviewer for thoughtful comments on an earlier draft manuscript. All data included in this study,
unless publically available/otherwise stated, may be obtained from the corresponding author.
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4.9 Tables
Table 4.9-1 Peat physical properties of 42 sites in the Hudson Bay Lowland (HBL), Canada.
Included here are new details for 31 previously reported cores (Packalen et al., 2014; Packalen
and Finkelstein, 2014) and a summary of previously reported peat data, as referenced herein.
S iteElevation
(m asl)
Basal Depth
(cm)
Mean Bulk
Density (g cm-3
)
Mean
[Carbon]
Mean Carbon
Density (g cm-3
)
Areal Carbon
Mass (kg m-2
)
Bog mesoforms (n=795)
KR3A* 123 271 0.096 0.486 0.047 126
PL101 45 264 0.084 0.493 0.039 102
KJ101 68 297 0.087 0.499 0.044 130
Kinosheo L*1
68 265 0.074 0.475 0.035 93
KJ4-3 66 131 0.108 0.500 0.055 72
KJ3-3 65 176 0.093 0.490 0.046 81
D001 92 231 0.077 0.518 0.040 102
D003 92 223 0.104 0.531 0.055 80
D200 118 240 0.099 0.526 0.052 84
D206 142 257 0.073 0.490 0.036 105
VM4-5 103 286 0.076 0.497 0.038 109
VM4-32
103 304 0.087 0.462 0.040 123
VM4-1 105 311 0.081 0.503 0.041 126
VM1-3 86 210 0.096 0.485 0.046 97
ML201 143 235 0.096 0.481 0.045 106
HL02 85 230 0.079 0.506 0.040 92
JBL76
150 330 0.080 0.470 0.038 129
JBL46
108 176 0.086 0.480 0.041 72
LLC7
305 250 -- -- 0.043 106
Herchmer Palsa Bog*5
106 167 0.235 0.378 0.089 192
Mean (n=20) 109 243 0.095 0.488 0.045 106
s.d. 54 52 0.035 0.032 0.012 27
KJ2-3 65 246 0.081 0.492 0.040 99
D002 91 219 0.099 0.527 0.052 83
D201 104 201 0.073 0.489 0.036 106
D202 105 215 0.077 0.471 0.036 99
D204 143 129 0.076 0.493 0.037 64
VM3-33
102 242 0.094 0.459 0.043 105
VM3-2 102 262 0.095 0.509 0.048 127
VM3-5 102 232 0.121 0.496 0.060 139
VM2-1 86 211 0.083 0.487 0.040 85
VM2-3 86 207 0.087 0.496 0.043 89
VM2-5 86 180 0.092 0.510 0.047 85
HL03 99 269 0.088 0.517 0.045 122
HL04A(West) 89 93 0.167 0.471 0.079 73
HL04B(East) 97 105 0.140 0.494 0.069 72
Herchmer Fen*4
106 203 0.120 0.429 0.051 119
Mean (n=15) 98 201 0.100 0.489 0.049 98
s.d. 17 54 0.027 0.024 0.012 22
JBL5 peat plateau6
143 141 0.135 0.465 0.063 78
McClintockFen*4
81 148 0.097 0.472 0.046 68
McClintock plateau*5
79 163 0.234 0.376 0.088 143
PB3-5 10 44 0.104 0.481 0.050 22
PB-SF-5 11 24 0.246 0.377 0.075 18
PB2-5 10 45 0.108 0.480 0.052 23
PB1-5 9 42 0.102 0.483 0.049 21
Mean (n=7) 49 87 0.147 0.448 0.060 53
s.d. 53 61 0.065 0.049 0.016 47
HBL peatlands
Mean (n=42) 94 196 0.104 0.471 0.048 92
s.d. 48 77 0.042 0.076 0.014 35
Fen mesoforms (n=603)
Coastal mesoforms (n=38)
Data obtained from previously published work: 1Kettles et al., 2000;
2Bunbury et al., 2012;
3O'Reilly et al., 2014;
4Kuhry et al.,
1998; 5Kuhry, 2008;
6Holmquist et al., 2014;
7van Bellen et al., 2011a. *Carbon determined from LOI data at a level of 50% C.
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Table 4.9-2 Peatland carbon mass distribution in the Hudson Bay Lowland, Canada.
Correlation matrix (correlation coefficient, p-value and sample size) of the main annual (a) and
growing season (b) climatic controls on the spatial variation of total carbon mass (kg m-2
) in the
Hudson Bay Lowlands, Canada (HBL). Data are presented for the complete peatland complex
and by broad peatland classification, including bogs, fens, and the coastal peatland region. Bold
font indicates significant correlation at p<0.05.
a.
Latitude
(°)
Elevation
(m, asl)
Mean Annual
Temperature
(MAAT, °C)
Mean Annual
Precipitation
(MAP, mm)
Temperature
Seasonality
(Annual
Range)
Potential
Evapo-
transpiration
(PET)
PET/MAP
All
peatlands -0.428 0.459 0.440 0.486 0.437 0.471 0.312
1.30E-17 2.48E-20 1.06E-18 5.06E-23 2.24E-18 1.60E-21 1.12E-09
364 364 364 364 364 364 364
Bog
peatlands -0.285 0.302 0.276 0.359 0.201 0.301 0.085
5.52E-05 1.78E-05 9.48E-05 2.60E-07 4.84E-03 1.96E-05 2.34E-01
195 195 195 195 195 195 195
Coastal
peatlands 0.320 0.629 -0.127 -0.256 0.506 0.081 0.287
2.64E-02 1.65E-06 3.88E-01 7.92E-02 2.45E-04 5.84E-01 4.82E-02
48 48 48 48 48 48 48
Fen
peatlands -0.366 0.366 0.370 0.468 0.311 0.395 0.180
3.69E-05 3.69E-05 3.02E-05 6.41E-08 5.14E-04 7.54E-06 4.87E-02
121 121 121 121 121 121 121
b.
GS Mean
Temperature
(GST, °C)
GS Mean
Precipitation
(GSP, mm)
GGD 5°CGS Length
(days)
Timing of
GS initiationGS PET
GS
PET/GSP
All
peatlands 0.496 0.459 0.488 0.432 -0.469 0.470 0.394
5.61E-24 2.15E-20 3.33E-23 5.75E-18 2.83E-21 2.21E-21 6.11E-15
364 364 364 364 364 364 364
Bog
peatlands 0.317 0.332 0.312 0.245 -0.270 0.298 0.184
6.31E-06 2.08E-06 9.04E-06 5.44E-04 1.37E-04 2.32E-05 9.97E-03
195 195 195 195 195 195 195
Coastal
peatlands 0.357 -0.288 0.237 -0.057 -0.206 0.082 0.298
1.28E-02 4.71E-02 1.04E-01 7.01E-01 1.59E-01 5.57E-01 3.99E-02
48 48 48 48 48 48 48
Fen
peatlands 0.406 0.427 0.409 0.361 -0.374 0.390 0.290
3.74E-06 1.03E-06 3.12E-06 4.75E-05 2.41E-05 9.62E-06 1.23E-03
121 121 121 121 121 121 121
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4.10 Figures
Figure 4.10-1Physical features, study sites and carbon mass the Hudson Bay Lowland, Canada.
The region includes the coastal Hudson Bay Lowland (HBL) and inland HBL and James Bay
Lowland (JBL) ecoregions (www.cec.org/naatlas/). Contours represent modern isostatic uplift
rates (Peltier, 2004), while shading represents modern surface elevation (0 – 400 m, above sea
level) generated using 1:250K Canadian digital elevation data (www.geobase.ca). Peat depth
locations are scaled according to the carbon (C)-mass at the site. Yellow triangles indicate site
locations with detailed peat physical features.
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a.
b.
Figure 4.10-2 Contemporary climate domain for peatlands of the Hudson Bay Lowland, Canada.
Mean annual air temperature (MAAT) and mean annual precipitation (MAP) for the period
1971-2000 are presented.
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Figure 4.10-3 Relationship between peat depth and carbon (C) mass for well described peatlands
in the Hudson Bay Lowland, Canada.
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Figure 4.10-4 Peat class distribution relative to peatland continentality in the Hudson Bay
Lowland, Canada.
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Figure 4.10-5 Peat carbon mass relative to bioclimate in the Hudson Bay Lowland, Canada.
Data are presented for unique peatland mesoforms considered together as well as for bogs, fens,
and coastal peatland classes. Carbon mass (n = 364) vs. mean annual air temperature (MAAT),
mean annual precipitation (MAP), growing season (GS) temperature and precipitation, growing
degree days above 5 °C (GDD5) and GS potential evapotranspiration (PET) are presented. All
relationships are significant at p < 0.05 (see Table 4.9-2 for correlation coefficients).
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Figure 4.10-6 Total carbon-mass (kg m-2
) stored in bogs, fens and coastal peatland mesoforms,
in the Hudson Bay Lowland, Canada. Median, 25th
and 75th
percentiles, and outliers of peat C
mass are presented for all available peat cores (n = 42) with detailed peat C analyses. Differences
among C mass stores in bog, fen, and coastal classes indicated by letters at a significance of p <
0.05.
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Chapter 5 Conclusions and Future Research Directions
5.1 Summary
Based upon an understanding of controls on peatland trace gas (CO2 and CH4) flux combined
with evidence of widespread peatland initiation in the early Holocene, compelling evidence was
advanced that peatlands can contribute to variations in atmospheric CO2 and CH4 concentrations
(MacDonald et al., 2006). However, questions associated with the precise timing of peatland
initiation versus the timing of atmospheric CO2 and CH4 changes have stimulated controversy
concerning the connection between peatland ecosystem dynamics and the long-term trajectories
of atmospheric trace gases (Reyes and Cooke, 2011). Moreover, a lack of evidence for a
connection between peatland development and late Holocene trace gas dynamics, stemming from
few records from late emerging peatlands, such as the HBL, and few data on the lateral
expansion of peatlands through the late Holocene has perpetuated this debate. Climate-driven
changes in peatland C dynamics are suggested and long-term, externally-forced climatic changes
are potentially documented in the peat archive, such as cyclical climate variations associated
with millennial- to centennial-scale solar forcing (Viau and Gajewski, 2009). Synchronous
evidence of climate variation in multiple records (e.g., peatland, marine sediment, and ice core)
increases confidence that perceived climate variations inferred from peat archives are indeed
tracking allogenic processes (external changes).However strong mechanistic evidence of
peatland induced climate perturbations is lacking, though it has been hypothesized that peatland
expansion and pattern development may be viable mechanisms to substantiate the peatland-
driven climate variability hypothesis (Korhola et al., 2010).
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Peatlands may also record the effects of autogenic factors; that is, internal changes and feedbacks
brought about during peatland development. Consequently, the peat archive can track changes in
ecohydrology against the backdrop of relatively stable climate conditions (Charman et al., 2009).
However, peatland vegetation and hydrology may respond to seasonal temperature fluctuations
through internal feedback mechanisms, and such internal feedbacks may contribute to longer
term departures from stable climate conditions or non-linear responses to local ecohydrological
variability. Alternatively, changes recorded in the peat archive may be the consequence of
externally forced changes in stable climate conditions. Identifying drivers of C dynamics in the
HBL peat record necessitates distinguishing between autogenic factors related to peatland
development, trends in orbital forcing, and regional climatic variation influenced by the HBL’s
proximity to a large marine system with seasonal sea ice dynamics. Thus to contribute to the
debate, disentangling the role of autogenic and allogenic factors in peatland pattern development
is essential. Short-term empirical studies examining patterns in nutrients, hydrology and
hydrochemistry can inform our understanding of short-term ecological processes (Eppinga et al.,
2009). However, predicting ecological responses to environmental change benefits from long-
term systematic observation of ecohydroclimatic dynamics, which can be achieved through a
combination of experimentation, mechanistic and ecological modeling, and examination of the
paleoenvironmental record (Jackson et al., 2009).
5.1.1 Peat initiation and carbon storage
In Chapter 2, the first estimate of the total C sink of HBL peatlands together with a synoptic
assessment of peat initiation frequency is presented. Based upon new detailed records of peat C
density, the HBL is estimated to store ~ 30 Pg C (Packalen et al., 2014), equivalent to ~ 20% of
the North American (Gorham et al., 2012) or ~ 6% of the northern peatland C pool (Yu, 2011).
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Study results also confirm a tight coupling between peat initiation frequency and land emergence
from the post glacial Tyrrell Sea, driven by the rate of glacial isostatic rebound. While climatic
conditions in the HBL remained suitable for peat initiation through the period of record, the most
intense period of peat initiation in the HBL occurred during the mid-Holocene, when the climate
was colder and received less precipitation than today and insolation was decreasing relative to
the early Holocene. Accordingly, the highest rate of peat initiation in the HBL occurred prior to
the late-Holocene rise in atmospheric CH4concentration, providing additional evidence of a peat
contribution to atmospheric trace gas dynamics. The late Holocene CH4 contribution from the
HBL potentially ranged from 1 – 7 Tg CH4 y-1
(Packalen et al., 2014), and represents a small but
important potential CH4 emission, when compared to the 21 – 43 Tg CH4 yr-1
released by
modern northern wetlands (Mikaloff Fletcher et al., 2004). Improved quantification of modern
peatland C stores and regional controls on Holocene peatland C dynamics in the HBL will
reduce the uncertainty surrounding the sensitivity of northern peatland C pools under future
climate scenarios.
5.1.2 Holocene carbon dynamics
Chapter 3 examines temporal C dynamics in the HBL at the landscape scale and partitions net C
uptake, release, and balance since peat inception. Rather than climate variation, C accumulation
in the HBL appears to be more strongly related to peatland succession dynamics (Packalen and
Finkelstein, 2014). Accordingly, CAR is most related to the intensity of peat initiation, the latter
of which determines the proportion of young peatlands on the rapidly emerging HBL landscape
that characteristically have high C accumulation rates. However, few temporally resolved
paleoclimate records are available in the HBL, thus the potential link between C accumulation
and climate warrants further investigation. Our data show that while the HBL has been a
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persistent C sink for millennia, more than two-thirds of the total C mass accrued during the late
Holocene, when CAR remained below the long-term mean. Further, model evidence presented
here suggest that most of the potential C lost from the HBL occurred during the late Holocene,
likely owing to decay of previously deposited peat. Given that sustained minerotrophic to weakly
ombrotrophic peat patterning typifies the HBL landscape, the persistent C release during the late
Holocene may provide an additional line of evidence of natural terrestrial C contributions from
the HBL to the late Holocene atmosphere, particularly in the form of CH4 (Packalen et al., 2014;
Packalen and Finkelstein, 2014).
5.1.3 Climatic controls of the distribution of the carbon mass
Recognizing a limited temporal relationship between climate and peat C dynamics in the HBL
Chapter 4 (Packalen et al., in revision), examines spatially explicate climate factors in relation to
the distribution of the total C mass. Although peat depth and age are closely related to timing of
land emergence, wide scatter within given time intervals suggests other factors control peat
development. Climate is shown here to be an important factor in explaining major spatial
patterns in the distribution of the peat C mass in the HBL, which are best explained by
temperature, especially growing season temperature, and then precipitation gradients. Climatic
conditions also appear to explain major trends in the geographic distribution of the peat C mass
within a given peat type (bog, fen, coastal) in the HBL. Yet, the widespread bog-fen patterning
across HBL suggests that small scale topographic and ecohydrological controls are also critical
determinants of C mass development, even when local climatic conditions remain constant.
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5.2 Sources of Uncertainty and Future Research Directions
While conservative climate estimates suggest warmer and wetter conditions within the range of
past climate variability, the simultaneous occurrence of warmer/wetter conditions is not
documented in the HBL’s Holocene paleoclimatic record. Increased primary productivity is
anticipated in the HBL as a consequence of a predicted warmer climate over the next century,
lower PET/MAP ratios, and in association with longer growing seasons. However, warmer
winters may enhance decomposition and net C losses, as it has elsewhere (Jones and Yu, 2010)
and a net reduction in surface moisture conditions may further enhance mass C loss as a
consequence of disturbance (e.g., fire) (Turetsky et al., 2011).Consequently, consideration and
elucidation of a moisture balance threshold for sustained C sequestration in the HBL is
warranted, in anticipation of future climate scenarios.
5.2.1 HBL paleohydroclimate and carbon dynamics
Today, the largest peat C masses in the HBL appear to occur in association with more
continental climates. Yet analysis of contemporary patterns in PET/MAP reveal that the
peatlands in the HBL occur in association with both maritime- and continental-type climates,
suggesting that both net moisture balance and precipitation are important controls on peat
accumulation. Although maintenance of moist surface conditions supports herbaceous
productivity and the accretion of more C dense peat, Sphagnum productivity is greater under
more continental climatic conditions. The result is a larger C mass, due in part to the
accumulation of decay-resistant vegetation remains (Turetsky et al., 2008). Few paleoclimatic
records are available for HBL peatlands to fully explore the relationship between long-term
spatio-temporal climatic variation and peat C dynamics. Consequently, it is difficult to
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disentangle the relative roles of climate and local autogenic processes in peat C dynamics in the
HBL.
5.2.2 Carbon dynamics in permafrost peatlands and peatland pools
The HBL is patterned with peat-pool complexes, some of which are or may have been influenced
by permafrost. Currently, few data are available to accurately access the spatial extend of either
small pools and permafrost features on the HBL landscape. Further, few records are available
that detail the C dynamics of these features in the HBL. Consequently, the C store estimate
presented here does not account for these uncertainties. Further, potential climate feedbacks
under future climate scenarios related to augmented CO2 and CH4 emissions from pools and
thermokarst ponds, and enhanced microbially-driven decomposition associated with pools and
thawing permafrost features may confound our understanding of C dynamics in the HBL and its
associated C sink potential.
5.2.3 Age-depth modeling and fen decay modeling
Temporally, C dynamics in the HBL were assessed here using a limited set of vertically dated
peat records. Although the data presented here are in agreement with findings reported from
circum-Arctic peatlands, additional well-dated peat records, together with higher resolution
dating may help to disentangle the role of autogenic and allogenic controls on peat C dynamics.
Further, the decay models used here are conservative estimates of long-term peat decay, assume
constant decay through time as a peatland proceeds through typical successional patterns.
Capturing more ecologically meaningful decay rates through time may improve temporal
resolution of C sink-source dynamics in peatlands in the HBL and elsewhere.
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5.2.4 Peat carbon dynamics in post-marine environments
Peat initiation in the HBL occurred over marine sediments, which may have limited early trace
gas emissions (e.g., CH4), due to peat-sediment interactions with marine sulfur species. The
estimates presented here make this assumption and thus provide an estimate for late Holocene
peatlands that have developed sufficient peat depth to reasonably disconnect the potential
sulphur-mediated suppression of peat CH4 emissions. However, few data are available to address
this potential mechanism for either the HBL or for circum-Arctic peatlands that initiated in
former marine environments. This mechanism may have important implications for both the
assessment of long-term peat-climate-C dynamics, as well as, anticipated trace gas emissions
from peatlands subject to the influence of sea level rise.
5.2.5 Peatland resilience and vulnerability to climatic change
Peatlands are anticipated to be subject to enhanced natural and anthropogenic disturbance as a
consequence of a changing climate, intensified land-use and expanding resources management.
However, the vulnerability of peatlands to these changes remains unclear. While peatlands
appear to control the local hydrology as it relates to supporting vegetation productivity, and
historical peatland resilience in the context of changing climate is documented, more rapid,
abrupt or widespread interference with peatland ecosystem functioning may compromise the
future resilience of peatlands. The role of hydrology, fire, and depth of peat vulnerability all
remain outstanding priorities for future research and approaches to ascertaining peatland
vulnerability to climate and/or anthropogenic change may include identification of critical
threshold conditions or probability of change under various climate and land-use scenarios.
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