Modelling the effect of increasing pCO 2 on pelagic aragonite production and dissolution 1. Laboratoire des Sciences du Climat et de l'Environnement (LSCE), France 2. Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland Reidun Gangstø 1,2 3rd CARBOOCEAN annual meeting Bremen 2007 Marion Gehlen 1, Birgit Schneider 1, Laurent Bopp 1, Fortunat Joos 2 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[CO 3 2- ] a (μmol/kg) CO 2 + H 2 O + CO HCO 3 - Calcification (Ω>1) / dissolution (Ω<1): Ca 2+ + CO 3 2- CaCO 3 CO 3 2- CO 3 2- sat c/a Calcite/aragonite saturation state: Ω c/a ≈ Ω>1: supersaturation, Ω<1: undersaturation

Questions: What role does aragonite play in the total CaCO 3 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 CaCO 3 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 CaCO 3 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) PO 4 3- Diatoms Micro-zoo P.O.M D.O.M Si Iron Nano-phyto Meso-zoo NO 3 - NH 4 + Small OnesBig Ones CaCO 3

CaCO 3 dependency on saturation state Ω (PIC/POC) max =0.8, K max =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) Gehlen et al., 2007 PIC = particulate inorganic carbon POC = particulate organic carbon

Aragonite production Production (mgC/m 2 /d) averaged over depth Average modelled aragonite production: 2.4 mgC/m 2 /d

The CaCO 3 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 CaCO 3 production net CaCO 3 production * 1) * 2) Literature (all fluxes are in PgC/yr) lower boundary flux * 4) CAL ARAG * 4) 100 m CaCO 3 export flux * 3) CaCO 3 dissolution ± 0.2 CAL: calcite only *5) ARAG: calcite +aragonite

CaCO 3 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 km Including aragonite in the PISCES model improves the vertical distribution of CaCO 3 dissolution PISCES: calcite only PISCES: both calcite and aragonite CaCO 3 dissolution (μmolCkg -1 y -1 ) (Gehlen et al., 2007) CaCO 3 dissolution (PgC/yr) *

Experimental setup: Transient experiments + Run 1: Increasing pCO 2 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 CO 2 -forcing Year 2000 Historical pCO 2 A2- scenario Control run

Changes in surface ocean Ω with increasing pCO 2 Year 1860 Ω calcite (0-100m) Ω aragonite (0-100m) Ω a =1 Ω c =1 Year 2100

- 29% - 16% - 21% Total CaCO 3 Calcite Aragonite - 45% - 12% - 24% Total CaCO 3 Calcite Aragonite Total CaCO 3 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 CaCO 3 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 CaCO 3 production and export is > 20% - Pelagic CaCO 3 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 –

The calcium carbonate (CaCO 3 ) system Ocean acidification Calcite production Calcite dissolution Aragonite production Aragonite dissolution [CO 2 ] + H 2 O + [CO 3 2- ] 2 [HCO 3 - ]