Organic carbon isotopic composition of phytoplankton and sea-surface pCO2 reconstructions in the Southern Indian Ocean during the last 50,000 yr

Simultaneous measurements of chlorophyll and carbon content, carboxylase activities in phytoplankton, isotopic composition of suspended particulate organic carbon (POC), and pCO2 in surface sea-water in the south Indian Ocean indicate that plankton populations have different carbon metabolisms on both sides of the subtropical convergence (STC). North of STC, communities are dominated by mixo/heterotrophic organisms while south of the STC, physiological or biomass parameters are diagnostic of autotrophic phytoplankton. Only phytoplankton δ13C values, in the southern part, showed a strong correlation with [CO2(aq)] in surface waters which permitted a relationship between these parameters. Using this empirical relationship, we derive, from sedimentary organic carbon (SOC) δ13C, a reconstruction of the paleo-pCO2 in surface water of the Southern Indian Ocean for the last 50 kyr. During the glacial and deglacial periods, the obtained paleo-pCO2 were higher than those recorded in the Vostok ice core, suggesting that the Southern Ocean was a potential source of CO2 for the atmosphere. These results are consistent with those of SOC content and indicate that the biological pump was off during glacial times and unable to lower atmospheric [CO2(aq)]. Moreover, these data confirm that stratification of the surface water in summer caused a reduction of gas exchange with the atmosphere during that period. Copyright