Modelling the effect of increasing pCO2 onpelagic aragonite production and dissolution

1. Laboratoire des Sciences du Climat et de l'Environnement (LSCE), France2. Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland

Reidun Gangstø1,2

3rd CARBOOCEAN annual meeting Bremen 2007

Marion Gehlen1, Birgit Schneider1, Laurent Bopp1, Fortunat Joos2 and Olivier Aumont (LOCEAN)

Ocean Acidification

The surface water in the Southern Ocean may be undersaturated with respect to ARAGONITE within this century (Orr et al., 2005)

Pteropod: Limacina helicina (AWI)

Aragonite saturation state, Δ[CO32-]a (μmol/kg)

CO2 + H2O + CO32- <-> 2 HCO3

-

Calcification (Ω>1) / dissolution (Ω<1):

Ca2+ + CO32- <-> CaCO3

CO32-

CO32-

satc/a

Calcite/aragonite saturation state:

Ωc/a ≈

Ω>1: supersaturation, Ω<1: undersaturation

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?

Implementing aragonite in the marine biogeochemical model PISCES

aragonite

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)

calcite

Aumont and Bopp (2006), Gehlen et al. (2007)

PO43-

Diatoms

Micro-zoo

P.O.M

D.O.M

Si

IronNano-phyto

Meso-zoo

NO3-

NH4+

Small Ones Big Ones

CaCO3

CaCO3 dependency on saturation state Ω

nDISS Ω)(1k=R

(PIC/POC)max=0.8, Kmax=0.4

based on experiments with E. huxleyi (Delille et al., 2005; Zondervan et al., 2002)

1. Calcification, Ω>1: 2. Dissolution, 0<Ω<1:

k=10.9 day-1, n=1

derived from sediment trap data (Gehlen et al., 1999; 2006; Dittert et al., 2005)

1)(ΩK

1)(Ω )

POC

PIC ( =

POC

PIC

maxmax

Gehlen et al., 2007PIC = particulate inorganic carbonPOC = particulate organic carbon

Aragonite production

Production (mgC/m2/d) averaged over depth

Average modelled aragonite production:2.4 mgC/m2/d

The CaCO3 budget

*1) Lee (2001), *2) Berelson et al. (2007), *4) Sarmiento et al. (2002),

*4) Feely et al. (2004), *5) Gehlen et al. (2007)

gross CaCO3 production

net CaCO3 production

*1) *2)

Literature(all fluxes are in PgC/yr)

1.27 0.87 0.5 - 1.6

1.31 0.79

0.32 0.30.31lower boundary flux *4)

CAL ARAG

*4)

100 mCaCO3 export flux

0.63 0.60.56*3)

CaCO3 dissolution 0.550.48 0.5 ± 0.2

CAL: calcite only *5)

ARAG: calcite +aragonite

CaCO3 dissolution

* Feely et al. (2004)

60% of pelagic diss.depth < 2000 m

58% of pelagic diss.depth < 2000 m

38% of pelagic diss.depth < 2000 m

0.23

0.32

2 km

0.18

0.30

Including aragonite in the PISCES model improves

the vertical distribution of CaCO3 dissolution

PISCES: calcite only

PISCES: both calcite and aragonite

CaCO3 dissolution (μmolCkg-1y-1)

(Gehlen et al., 2007)

CaCO3 dissolution(PgC/yr)

*

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

Year2000

HistoricalpCO2

A2-scenario

Controlrun

Changes in surface ocean Ω with increasing pCO2

Year 1860

Ωcalcite (0-100m) Ωaragonite (0-100m)

Ωa=1Ωc=1 Year 2100

- 29%

- 16%

- 21%Total CaCO3

Calcite

Aragonite

- 45%

- 12%

- 24%Total CaCO3

Calcite

Aragonite

Total CaCO3

Calcite

Aragonite- 11% (Rel. to net prod.: +15%)

- 8% (Rel. to net prod.: +6%)

- 9% (Rel. to net prod.: +9%)

Changes in calcification, export and dissolution

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!

Conclusions 2: Transient Experiments

• Under an A2 scenario:

- Ω 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 > 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@climate.unibe.ch

The calcium carbonate (CaCO3) system

Ocean acidification

Calciteproduction

Calcitedissolution

Aragoniteproduction

Aragonitedissolution

[CO2 ] + H2O + [CO32-] <-> 2 [HCO3

- ]