Reconsidérer les flux continent-océan
Catherine Jeandel& Eric Oelkers (2014)
CNRS, LEGOS, Observatoire Midi-Pyrénées, Toulouse UniversityCNRS, GET, Observatoire Midi-Pyrénées, Toulouse University
Seminaire LEGOS, 7 novembre 2013
Thank you to: K. Tachikawa, F. Lacan, B. Peucker-Ehrenbrink, M. Jones, C. Pearce, M. Grenier
Weathering Processes….
Chemical Weathering
Ocean
Dissolved load
Mechanical Weathering or
Denudation
Particulate Transport
Global mass fluxes to the ocean
0
5
10
15
20
25
Dissolved flux(Gaillardet et al.,
2003)
Bedload Flux(Walling, 2006)
SuspendedMaterial Flux
(Syvitiski et al.,2003)
Ma
ss
flu
x t
o t
he
oc
ea
ns
(G
T/y
r) Solid flux = 30 times the dissolved one= 50 times the atmospheric one
eNd oceanic distribution follows the general circulation(in the modern ocean as in the sediment)But its concentration doubles only
Along the general oceanic circulation
-15
Lacan et al., Chem. Geol., 2012
Nd paradox
Field Observations (1)
4
CHUR144
143SAMPLE
144
143
Nd 101
Nd
Nd
Nd
Nd
-3.8
Nd concentration isonly doubling
Field Observations (2)Imbalanced Nd oceanic budget
Dust + river inputs : not sufficient
Missing term: river solid load and/or sediments deposited on the marginsLacan&Jeandel, 2001;Tachikawa et al, 2003
Field Observations (4)
« BOUNDARY EXCHANGE » - quantifiable using Nd IC- invisible with concentrations only
FR FA FBE
BE is THE major source term (>95% of the total : 1.1 1010 g(Nd)/an).
Continental inputs & margins: major for Nd(only 3% dissolution)
And for the other chemical elements?
Lacan &J eandel, 2001, 2005Van der Flierdt et al, 2004Carter et al, 2012Rickli et al, 2009, 2011Grasse et al, 2012Wilson et al, 2012Grenier et al, 2013…/…
Arsouze et al, 2009
Strontium
• Ocean 87Sr/86Sr is homogenous at 0.70916
• The primary controls were accepted as dissolved riverine input (87Sr/86Sr ≈ 0.7136) and hydrothermal exchange (87Sr/86Sr ≈ 0.7029)
• However, the unradiogenic flux is a factor of 3 too low to balance the inputs
• Therefore, part of the story is missing (as for Nd oceanic budget)
Field Observations (5)
Strontium Imbalance: hypotheses
• Current riverine fluxes may be elevated due to a recent glaciation (Vance et al., 2009)
• Subsurface weathering of volcanic islands are not included (Allègre et al., 2010)
• Volcanic particulate riverine material could also contribute unradiogenic Sr
Field Observations (6)
Field evidence of particulate dissolution in the ocean: Borgarfjordur Estuary (1)
0
2000
4000
6000
8000
10000
12000
0 2 4 6 8 10 12 14
Distance, km
[Cl]
pp
m
0.704
0.705
0.706
0.707
0.708
0.709
0.71
0 2 4 6 8 10 12 14
Distance, km
Sr8
7/8
6 e
stu
ary
wa
ter
Estuary water as a function of salinity Sr87/Sr86 as a function of salinity
Field Observations (8)
Field evidence of particulate dissolution in the ocean: Borgarfjordur Estuary (2)
-0.0001
0
0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0 5 10 15
Distance, km
Sr8
7/86
( fl
uid
mix
ing
mo
del
- e
stu
ary
wat
er)
Sr87/Sr86 in estuary water as a function of river water fraction
Sr87/Sr86 attributable to particulate dissolution
0.704
0.705
0.706
0.707
0.708
0.709
0.71
0 0.2 0.4 0.6 0.8 1
Fraction river content of estuary water
Sr8
7/8
6 e
stu
ary
wa
ter
50% of Sr from dissolution of particulates
Field Observations (9)
As for Nd, dissolution of lithogenic material is occuring at the land/ocean interface
Silica budget (Mediterranean Sea) 106 Moles/y
Durrieu de Madron et al, 2009
51,000-127,000
288,000- 424,000
Siout = 2 to 8 x (Si in)Total ext. inputs = 25,000-126,000
Si missing = 35,000-348,000
Total dissolved river input = 24,300-118,700
Total Atmospheric input =709-7,350
Field Observations (11)
Source of the Si missing?Range required: 3.5 1010 à 3.4*1011 moles/y
Total river solid discharge = 0.73 109 T/y (Ludwig et al , 2003)
Dissolution of only 1% of this flux
8.3 1010 mol/y SiO2
Same result considering
Release from 1% of the first 10 cm of sediment deposited on the margins (consistent with Tréguer and de la Rocha, 2012)
Field Observations (12)
Fe Particulate/Dissolved flux
Particulate transport dominatesthe flux of most metals to the ocean(Oelkers et al, 2011)
The release in seawater of even a tiny fraction (~1%) of this material deposited on the shelf/margins mayimpact oceanic isotopic & element budgets (Jeandel et al, 2011, Jeandel&Oelkers, 2014)
Fe
Nd
Si
Sr
Field Observations (14)First Intermediate conclusion
Th
Particulate/dissolved flux
Questions
• Processus: Desorption or Dissolution?
• Which phase (Fe-Mn coating) or mineral (primary or secondary)?
• Kinetic?
Batch experiments(Jones et al, 2012a,b; Pearce et al, 2013)
Sediment samples were mixed with open ocean seawater (water of southern origin, eNd = -9.6)
• Marine top core from Kerguelen plateau (station C1, eNd = -1.4)
• Estuarine sediment from SW Iceland (eNd = +7.2)
• Riverine bedload Sediment from SW Iceland (eNd = +7.5)
30 l SW1g/l4 monthsSr, REE, Si aliquots taken weekly
Nd IC aliquots taken every 2 weeks
Batch reactors
-9.6
-1.4 to +7.5
Experiments (1)
Jeandel et al; Traces and tracers, Liège, 2011
Hours
-10
-5
0
5
10
0 1000 2000 3000 4000
Duration (Hours)
N
dExp A
Exp B
Exp C
1 month
Initial Sw value = -9.6
-1.4 Kerguelen
Iceland Riv. 7.2
Hours
Iceland Est.
ISOTOPENd results (Pearce et al, 2013)
0
10
20
30
0 1000 2000 3000 4000
Duration (Hours)
Nd
(pm
ol/k
g)
Exp A
Exp B
Exp C
CONCENTRATION
IC: Nd rapid release (0.2 to 2% of Nd contained in the KER basalt)
Cc: Nd scavenging after release: secondary phases =
REE phosphatemineral rhabdophane (REE(PO4)nH2O) ?
Experiments (4)
Silica release?
0 1000 2000 3000 4000 5000 6000 70000.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
BR1a (Riverine) - 5 °C BR13a (Estuarine) - 5 °C BR1a (Riverine) - 25 °C BR13a (Estuarine) - 25 °C
Time (hrs)
[Si]
(p
pm
)
SW Initial concentration = 0.3 ppm
1 month
5°CSi(OH)4 x 10
25°CSi(OH)4 x 20
Experiments (6)
Second Intermediate conclusion
* Particulate material is chemically reactive, olivine & plagioclase being most susceptible to alteration
* Dissolution of lithogenic material followed by precipitation of secondary phases, is strongly suspected
* Trace elements (Nd, Sr, Mn, Ba, Ni ) & major ones (Si, Mg) are implied
* Kinetic is rapid (weekly to monthly scale)
Jeandel et al; Traces and tracers, Liège, 2011
Other elements that are released(similar experiments not shown here)
10 100 1000 100000.1
1.0
10.0
100.0
1000.0
10000.0
SiLiBMgKCaMnNiRbSrBa
Rela
tive
con
cent
ratio
n in
sea
wat
er
Si
Ba
Ni
Mn
Conclusions & Consequences (1)
Conclusions
Weathering of the particulate load (suspended, bedload, margin sediments)can significantly affect the oceanic budget of isotopes & elements
This processus -previously unconsidered- might help to balancethe marine Sr, Nd, Si cycles and likely Th, Ba, Mg…Ni, Cu, Zn, Mn: Cameron, Little, Vance (all published in 2014)
The weathered fraction includes a significant lithogenic component (feldspath and olivine being among the most sensitive)
Kinetic of this process is rapid (within a month)
Followed by secondary phase precipitation: might affect more the isotopic than the concentration budgets
Conclusions & Consequences (2)
Consequences
Marine Sr cycle: differences in the reactivity of basalticand crustal particles suggest that solid load weatheringmight help account for part of the « missing Sr flux »to the ocean. Impact on the geological reconstitutions?
Marine Nd, REE cycles: consistent with the « Boundary Exchange » hypothesis. Impact on its use as paleo-tracer?
Marine Th isotopes: solid load weathering likely perturbates the coastal 232Th/230Th. Impact on its use as particle dynamic tracer?
Conclusions & Consequences (3)
Consequences
Marine Si concentrations and isotopes: This additional flux increases the Si input term, diminishes the Si residence time by 35 to 50% (at steady-state; Jeandel et al, 2011; Tréguer & de la Rocha, 2012)
Impact on the biological pumpImpact on the CO2 modern cycleImpact on the climatic models
Conclusions & Consequences (4)
(many) Remaining Questions
Other elements?Clearly, one should reconsider the land to ocean fluxes of all the elements, mostly those of the essential marine micro-nutrient
Particle speciation effect?Working on this will allow the best modelling of the impact ofsuch release on a global scale, taking into account the geochemical particularities of watersheds and margins…