The CaCO3 budget

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Modelling the effect of increasing pCO2
on pelagic aragonite production and dissolution
Reidun Gangstø1,2
1. Laboratoire des Sciences du Climat et de
l'Environnement (LSCE), France 2. Climate and
Environmental Physics, Physics Institute,
University of Bern, Switzerland
Marion Gehlen1, Birgit Schneider1, Laurent Bopp1,
Fortunat Joos2 and Olivier Aumont (LOCEAN)
3rd CARBOOCEAN annual meeting Bremen 2007
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Ocean Acidification
CO2 H2O CO32- lt-gt 2 HCO3-
Pteropod Limacina helicina (AWI)
Calcite/aragonite saturation state
Oc/a
Ogt1 supersaturation, Olt1 undersaturation
Calcification (Ogt1) / dissolution (Olt1) Ca2
CO32- lt-gt CaCO3
The surface water in the Southern Ocean may be
undersaturated with respect to ARAGONITE within
this century (Orr et al., 2005)
Aragonite saturation state, ?CO32-a (µmol/kg)
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• Questions
• What role does aragonite play in the total CaCO3
  budget?
• How much will future changes in saturation state
  affect the pelagic production and dissolution of
  aragonite?

4
Implementing aragonite in the marine
biogeochemical model PISCES

• CaCO3 production
• calcifying plankton is not
• included as a distinct
• functional type
• calcification is assigned to
• 1. nanophytoplankton calcite
• 2. mesozooplankton aragonite
• aragonite 1/3 of total CaCO3
• Literature 10-50
• e.g. Berner (1977), Berger (1978), Berner
• Honjo (1981), Betzer et al. (1984), Fabry
• (1989, 1990), Fabry and Deuser (1991),
• Fischer et al. (1996)

PO43-
Diatoms
NH4
Si
Nano-phyto
NO3-
Iron
calcite
Micro-zoo
D.O.M
Meso-zoo
aragonite
P.O.M
CaCO3
Small Ones
Big Ones
Aumont and Bopp (2006), Gehlen et al. (2007)
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CaCO3 dependency on saturation state O
1. Calcification, Ogt1 2.
Dissolution, 0ltOlt1
(PIC/POC)max0.8, Kmax0.4 based on experiments
with E. huxleyi (Delille et al., 2005
Zondervan et al., 2002)
k10.9 day-1, n1 derived from sediment trap
data (Gehlen et al., 1999 2006 Dittert et
al., 2005)
Gehlen et al., 2007
PIC particulate inorganic carbon POC
particulate organic carbon
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Aragonite production
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The CaCO3 budget
(all fluxes are in PgC/yr)
Literature
CAL
ARAG
gross CaCO3 productionnet CaCO3 production
1) 2)
CAL calcite only 5) ARAG calcite aragonite
CaCO3 export flux
3)
100 m
4)
CaCO3 dissolution
0.55
0.5 0.2
0.48
4)
lower boundary flux
0.32
0.3
0.31
1) Lee (2001), 2) Berelson et al. (2007), 4)
Sarmiento et al. (2002), 4) Feely et al. (2004),
5) Gehlen et al. (2007)
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CaCO3 dissolution
PISCES both calcite and aragonite
PISCES calcite only
0.32
0.18
CaCO3 dissolution (PgC/yr)
2 km
(Gehlen et al., 2007)
0.23
0.30

? 60 of pelagic diss. depth lt 2000 m
? 58 of pelagic diss. depth lt 2000 m
CaCO3 dissolution (µmolCkg-1y-1)
? 38 of pelagic diss. depth lt 2000 m
Including aragonite in the PISCES model improves
the vertical distribution of CaCO3 dissolution
Feely et al. (2004)
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Experimental setup Transient experiments

• Run 1
• Increasing pCO2 over 240 years
• from 1860 to 2100 (historical
• development and A2 scenario)
• calcification and dissolution
• dependent on saturation state
• no climate change
• offline simulation (NEMO/PISCES)
• Run 2
• control run without additional
• CO2-forcing

A2- scenario
Year 2000
Historical pCO2
Control run

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Changes in surface ocean O with increasing pCO2
Year 1860
Ocalcite (0-100m)
Oaragonite (0-100m)
Year 2100
Oa1
Oc1
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Changes in calcification, export and dissolution
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Conclusions 1 Initial state

• With aragonite implemented in the PISCES model
• - The modelled aragonite production correspond
  quite well to available literature estimates
• - Total CaCO3 production, export and
  dissolution fit observations
• - The implementation of aragonite to PISCES
  improves the vertical distribution of pelagic
  dissolution
• - The dissolution of aragonite potentially
  contributes significantly to shallow water
  dissolution
• - The role of aragonite in the global carbonate
  budget needs to be assessed
• - More data is needed!

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Conclusions 2 Transient Experiments

• Under an A2 scenario
• - O in the surface water strongly decreases
• - Aragonite production is reduced by almost
  1/3, export by almost 1/2
• - The reduction in total CaCO3 production and
  export is gt 20
• - Pelagic CaCO3 dissolution slightly decreases
  due to less available material, but increases
  relative to the production
• - The response of pteropods to changes in
  carbonate chemistry needs to be investigated
• Future project
• - Further analyses of scenarios including
  climate change
• - Sensitivity studies with the recently coupled
  Bern3D-PISCES model

3rd CARBOOCEAN annual meeting Bremen 2007
gangsto_at_climate.unibe.ch
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The calcium carbonate (CaCO3) system
CO2 H2O CO32- lt-gt 2 HCO3-
Calcite production
Aragonite production
Ocean acidification
Aragonite dissolution
Calcite dissolution