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Achievements and challenges in Southern Ocean CO2 research
Dorothee Bakker, Mario Hoppema, Marta Alvarez, Leticia Barbero, Nina Bednarsek, Richard Bellerby, Jacqueline Boutin, Melissa Chierici, Bruno Delille, Judith Hauck, Oliver Huhn, Elisabeth Jones, Andrew Lenton, Nicolas Metzl, Claire Lo Monaco, Benjamin Pfeil, Aida Riós, Henk Zemmelink, ....
Funded by EU CarboOcean and / or national funding bodies
Achievements and Challenges in:
The Southern Ocean CO2 sink# Deep carbon inventories# Air-sea CO2 fluxes# Evolution of the Southern Ocean CO2 sink
Process studies # Upwelling, subduction, mixing# Iron supply# Sea ice# Ocean acidification
Southern Ocean: part of the Meridional Overturning Circulation
(Open University)
North Atlantic Southern Ocean
Exchange of heat, elements and momentum between the deep ocean and the atmosphere.
1.1) Inventory of anthropogenic carbon
(Hanawa and Talley, 2001; Sabine et al., 2004)
Antarctic Intermediate Water (AAIW)
Subantarctic Mode Water (SAMW)
• 20 Pg C or 5% of anthropogenic carbon in SAMW and AAIW in 1994.• Anthropogenic carbon in AABW (Antarctic Bottom Water)?
Cant (mol m-2)
How much Cant in AAIW and AABW?
(Lo Monaco et al., JGR, 2005)
Differences in Cant (µmol/kg) from the C0 and C* methods along 30°E
CDW
AAIW
AABW
• High Cant at the surface, especially north of 60°S and at the shelf where no sea-ice hampers the gas-exchange.
• Low Cant in deep and bottom water - close to the error of the method.
(Hauck, Hoppema et al., in preparation)
Accumulation of Cant in the Weddell Sea between 1992 and 2008
Cant accumulated in the Weddell Sea
(1992 – 2008)-
CT2008 fitted as a
function of θ, S, O2 and p
CT1992 fitted as a
function of θ, S , O2 and p
= Extended Multiple Linear Regression (eMLR)
Cant2008-1992 along 0°W (µmol/kg)
1.2) Air-sea CO2 fluxes
of surface water pCO2
Poor seasonal coverage in surface water fCO2 (Takahashi et al., 2009)
PalmerPolarsternOISO
S.O. CO2 sink (Pg C /yr)Global oceans (1990s, 2000-05)0 2.2±0.5
Surface pCO2
SAZ (STF-SAF,~40-50°S) 2, 4 0.8-1.1PZ (SAF-PF)2 <0.1South of 50°S 0.063-0.44 Atmospheric + ocean modelsSouth of ~45°S5 0.3-0.6
Pg = 1015 g (0 – Denman et al., 2007; 2- Metzl et al., 1999; Boutin et al., 2008; 3 - Takahashi et al., 2009; 5 – Baker et al., 2006; Gruber et al., 2003 at ICDC7; 4 -McNeil et al., 2007)
(Takahashi et al., 2009)
• The ’circumpolar sink zone’ in the Subantarctic Zone (SAZ).• High pCO2 at ice edge
(Takahashi et al., 2009)
Monitoring fCO2 with CARIOCA drifters
• Ocean CO2 sinks of 0.8 Pg C / yr in the SAZ and <0.1 Pg C / yr in the PZ from CARIOCA data since 2001 (Boutin et al., L&O, 2008).
• Assess the effect of SAMW formation on fCO2 in the South Pacific Ocean from CARIOCA and shipboard data (Barbero et al., in preparation, 2009).
• Future: Quantify the effect of mesoscale activity on fCO2 and DIC from CARIOCA and satellite data.
SAF
PF
6
sourcenkfCO2(water -air) (µatm)
oceanic sink
Estimating NCP with CARIOCA drifters
Strong diurnal cycle allows estimation of net community production (NCP) from CARIOCA data.
Future: Provide estimates of NCP from the diurnal cycle in fCO2 and DIC for all CARIOCA drifters.
CARIOCA - 29/11/06 to 8/12/06
365
370
375
380
385
390
395
400
405
410
29/11 30/11 1/12 2/12 3/12 4/12 5/12 6/12 7/12 8/12
local time
fCO
2 (m
icro
atm
)
2110
2112
2114
2116
2118
2120
2122
2124
DIC
(m
icro
mo
l/kg
)
fCO2
fCO2_atm
DICSunset
~ <Net community production>~ 0. 3 mol/kg/day
9 days (Nov-Dec 2006) in the Polar Zone; high fluorescence
~Gross Community Production-Respiration
fCO
2 (
atm
)
DIC
(m
ol/
kg)
(Boutin, Merlivat et al., in revision, GRL, 2008)
Atmospheric CO2 dataand an ocean modelsuggest a reduction in the efficiency of the Southern OceanCO2 sink since 1980.
Changes have been ascribed to an increase in wind speed.
1.3) Evolution of the Southern Ocean CO2 sink
Sea-air CO2 flux anomaly (Pg C/yr)
Le Quéré et al., Science, 2007
More upta-ke
Less upta-ke
+ pulse model
Model, constant winds
Model, observed winds
Trend atmosphere: + 1.7 µatm/yrTrend ocean: + 2.1 µatm/yr
Decrease of ocean sink? -0.4 µatm/yr
OISO Cruises
trend = + 2,11 (0.07) µatm/yr
250
270
290
310
330
350
370
390
410
430
450
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
Year
fCO
2s (µ
atm
)
All 1°x1° average data in the South Indian (20°S-69°S, 30°E-90°E)
(Metzl, 2009, DSR SOCOVV, in press)All data 1991-2008 in SOCAT and CDIAC
A decrease of the S.O. sink?
Decadal changes of natural and anthropogenic carbon
Anthropogenic carbon at 500m in the late 1990’s
(Lo Monaco et al., sub., 2008)
Anthropogenic Carbon change (µmol/kg)
DIC < 0, Cant > 0Decrease in ”natural” carbon
Total Carbon change (µmol/kg)
Section around 70E: Comparing 2000-1985
What drives the observed variability of the carbon cycle in the Southern Ocean ?
(Lenton et al., sub., 2008)
Oceanic CO2 sink - ongoing
Falkland Islands
South Georgia
SCOTIA SEA
Cruises in Scotia Sea on JCR since 2006 (Poster Jones et al.)
• Data synthesis in CARINA and SOCAT (Surface Ocean CO2 Atlas)
• NEW surface pCO2 VOS on RRS James Clark Ross (Hardman-Mountford, Jones, et al.) and FS Polarstern (Hoppema, Neil et al.)
• Hydrographic sections with carbon and tracers
SOCAT version 1, d.d. 21/11/2008, @Benjamin Pfeil
(http://www.ioccp.org/)
2) Process studies on interactions between physics, biology and the CO2 sink
Diffusivity (log (m2/s))Scotia Sea (Naveira-Garabato et al., 200x)
• Enhanced mixing and upwelling over steep topography, iron supply, and occurrence of blooms. • Mesoscale dynamics and eddies;• Entrainment of CDW below ice;• Preconditioning of CO2 before subduction;• Sea ice dynamics.
(Naveira-Garabato et al., 2007; Solokov and Rintoul, 2007; Blain et al., 2007; Bakker et al., 2007, 2008; Boutin et al., 2008)
Marine productivity and sea ice
NASA SeaWiFS project, DAAC/GSFC, ©ORBIMAGE.
Winter Summer
SGeorgia
Crozet
Kerguelen
0
50
100
150
200
250
300
350
400
450
500
0 0.1 0.2 0.3 0.4 0.5 0.6
DFe (nmol L-1)
Dep
th (m
)
A3
C11
C11
A03
Natural iron fertilisation at the Kerguelen Plateau,
(Blain et al., 2007; Jouandet et al., 2008)
KEOPS/OISO-12, January 2005
fCO2 (µatm)
Natural iron fertilisation at Crozet
8 November – 8 December 2004
fCO2(w-a) (µatm)
SAF
Crozet Plateau
Upstream (South): Little effect of marine biota on surface water fCO2.
Downstream (North): Large phytoplankton blooms lower fCO2 by 70 µatm.
Chlorophyll (mg/m3)
6040 5045 55
14 - 18 November 2004
(Bakker et al., 2007)
1 M-P. Jouandet et al., Deep-Sea Res. II, 2008, 55, 856 2 D. Bakker et al., Deep-Sea Res. II, 2007, 54, 2174
SGeorgia Crozet
Kerguelen
NA
SA
Sea
WiF
S p
roje
ct,
DA
AC
/GS
FC
, O
RB
IMA
GE
Island blooms vs HNLC
bloom stations
HNLC stations
12
Blooms are 2-3 times as productive as HNLC waters and are large CO2 sinks.
(Jones et al., see poster)
Entrainment creates high fCO2 and DIC below sea ice in the Weddell Gyre
Below sea ice: fCO2(w-a) 0 to +40 µatm in December 2002.
Upward movement of Warm Deep Water in the Weddell Gyre creates high fCO2 and DIC below the winter ice. The ice prevents outgassing of CO2 (Bakker et al., 2008, Biogeosciences).
Dissolved inorganic carbon (µmol/kg), 17-23°E
WDW
Rapid reduction of surface water fCO2 during and upon ice melt
Brown ice, 17-20/12/02
Below ice: fCO2(w-a) 0 to 40 µatmUpon melt: fCO2(w-a) -50 to 0 µatm
Biological carbon uptake rapidly creates a CO2 sink during and upon ice melt. The importance of ice-related
08-10/12/2004
20/12/2004
0°W Surface fCO2 decrease during ice melt
17/12/2004
(%)
Sea ice cover
CaCO3 processes is not clear. The Weddell Gyre may be an annual CO2 sink. (Bakker et al., 2008)
This supports the role of Antarctic sea ice on glacial-interglacial CO2 variations (Stephens and Keeling, 2000).
(Bakker et al., 2008,Biogeosciences)
Role of ikaite in sea ice
Ikaite CaCO3.6H2O in sea ice (Dieckman et al., 2008)
Ikaite precipitates along brine channels during ice formation, thus increasing fCO2. Ikaite dissolves during/upon ice melt, thus reducing fCO2.
Turbulent CO2 fluxes (g CO2 m-2 d-1) by eddy correlation in December 2004.
Total carbon uptake by the multi-year ice zone of the western Weddell Sea could have been 6.6 Tg C y-1 in December 2004 (Zemmelink, 2005).
Day of the year
335 340 345 350 355 360 365
CO
2 f
lux
(g
m-2
d-1
)
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
Sink
Source
CO2 uptake by multi-year sea ice in the western Weddell Sea
-8°C > T° -8°C < T° < -5°C T° < -5°C
CO2(g)
CO2(aq)
Brine sinking entrain produced CO2 below the pycnocline while
CaCO3 remain trapped within sea ice
CaCO3 + H2O + CO2
CO2(g)
2 HCO3- + Ca2
2+
CaCO3 + H2O + CO2
2 HCO3- + Ca2
2+
CO2 uptake by biology
at both top and bottom of sea ice
Semiletov et al. 2004, 2007Zemmelink et al. 2006
Zemmelink et al. 2006
Delille et al. 2007
Rysgaard et al. 2007
Papadimitriou et al. 2004Dieckmann et al.2008
Graphics by Bruno Delille
Sea ice: Ikaite and biological carbon uptake
CO2(g)
CaCO3 + H2O + CO2
2 HCO3- + Ca2
2+
1) Measurement of air-ice CO2 fluxes by micro-meterological
methods
2) Sea ice processes should be addressed by ice-coring and
related analysis
3) Impact of precipitation of CaCO3 to the water column can be addressed by TA profiles and
specific reanalysis of TA/DIC profiles
Slide by Bruno Delille
Future studies of the role of sea ice in CO2 chemistry
Effect of ocean acidification on the CO2 sink?• A more acid ocean reduces the carbonate concentration and calcification.
• Models predict that the Southern Ocean will become undersaturated for aragonite by 2050 in the IS92a scenario (Orr et al., 2005).
• The importance of calcifying organisms for the Southern Ocean carbon cycle is poorly known.
Abundance of the pteropod Limacina helicina in the Scotia Sea (Nina Bednarsek et al., 2008; poster)
AchievementsSignificant progress has been made on quantifying Southern Ocean CO2 uptake in the CarboOcean era. New topics have emerged, notably the evolution of the Southern Ocean CO2 sink and the role of sea ice.
Challenges:I) Quantify the evolution of the Southern Oceanic CO2 sink• Sustained observations of surface fCO2, deep carbon transport and atmospheric CO2
• Identify the best method(s) for quantification of anthropogenic carbon• Quantify Cant in Antarctic Bottom and Intermediate Water
II) Assess the processes driving (changes in) oceanic CO2 uptake:• Iron supply, • Sea ice,• Entrainment, mixing, subduction, pre-conditioning,• Marine productivity,• Ocean acidification.
Conclusions
(Lenton et al., sub., 2008)Model: IPSL-LOOP-CM4
DpCO2 --
CO2uptake --
(more wind)
Less uptake
Higher pCO2w
Evolution of the oceanic CO2 sink with / without an O3 hole
in a coupled carbon climate model
Day of the year335 340 345 350 355
pCO
2 (p
pmv)
364
366
368
370
372
374
376
378
Atmosphere0.45 m0.35 m0.25 m0.15 m
Day of the year340 345 350 355 360
A B
CO2 concentrations (ppmv) • in the atmosphere at 0.85 m from the ice and • in snow, as a function of distance from the ice surface. (Zemmelink)
Vertical CO2 gradients in snow on top of sea ice in the western Weddell Sea
Over slush Over solid ice
} In snow