Modelling the effect of increasing pCO 2 on pelagic aragonite production and dissolution

1. 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

2. CO32- CO32-satc/a Ocean Acidification CO2 + H2O + CO32- <-> 2 HCO3- Pteropod: Limacina helicina (AWI) Calcite/aragonite saturation state: Ωc/a≈ Ω>1: supersaturation, Ω<1: undersaturation Calcification (Ω>1) / dissolution (Ω<1): Ca2+ + CO32- <-> 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, Δ[CO32-]a (μmol/kg)

3. 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 aragonitein the marine biogeochemical model PISCES PO43- 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) 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)

5. CaCO3 dependency on saturation state Ω 1. Calcification, Ω>1: 2. Dissolution, 0<Ω<1: (PIC/POC)max=0.8, Kmax=0.4 based on experiments with E. huxleyi (Delille et al., 2005; Zondervan et al., 2002) k=10.9 day-1, n=1 derived from sediment trap data (Gehlen et al., 1999; 2006; Dittert et al., 2005) PIC = particulate inorganic carbon POC = particulate organic carbon Gehlen et al., 2007

6. Average modelled aragonite production: 2.4 mgC/m2/d Production (mgC/m2/d) averaged over depth Aragonite production

7. 1.31 0.79 1.27 0.87 0.5 - 1.6 0.56 0.63 0.6 The CaCO3 budget (all fluxes are in PgC/yr) CAL ARAG Literature gross CaCO3 productionnet CaCO3 production *1) *2) CAL: calcite only *5) ARAG: calcite +aragonite CaCO3export flux *3) 100 m *4) CaCO3dissolution 0.55 0.5 ± 0.2 0.48 *4) lower boundary flux 0.31 0.32 0.3 *1) Lee (2001), *2) Berelson et al. (2007), *4) Sarmiento et al. (2002), *4)Feely et al. (2004), *5) Gehlen et al. (2007)

8. CaCO3 dissolution PISCES: both calcite and aragonite PISCES: calcite only 0.18 0.32 CaCO3 dissolution (PgC/yr) 2 km (Gehlen et al., 2007) 0.30 0.23 *  60% of pelagic diss. depth < 2000 m  58% of pelagic diss. depth < 2000 m CaCO3 dissolution (μmolCkg-1y-1)  38% of pelagic diss. depth < 2000 m Including aragonite in the PISCES model improves the vertical distribution of CaCO3 dissolution * Feely et al. (2004)

9. Experimental setup: Transient experiments Run 1: • Increasing pCO2over 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 +

10. Changes in surface ocean Ω with increasing pCO2 Year 1860 Ωcalcite (0-100m) Ωaragonite (0-100m) Year 2100 Ωa=1 Ωc=1

11. Total CaCO3 - 21% Total CaCO3 Calcite - 24% - 16% Calcite - 12% Aragonite Aragonite - 29% - 45% Total CaCO3 - 9% (Rel. to net prod.: +9%) Calcite - 8% (Rel. to net prod.: +6%) Aragonite - 11% (Rel. to net prod.: +15%) Changes in calcification, export and dissolution

12. 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!

13. 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

15. The calcium carbonate (CaCO3) system [CO2 ] + H2O + [CO32-] <-> 2 [HCO3- ] Aragonite production Calcite production Ocean acidification Aragonite dissolution Calcite dissolution