ORGANIC-CARBON ISOTOPIC COMPOSITION OF PHYTOPLANKTON AND SEA-SURFACE PCO(2) RECONSTRUCTIONS IN THE SOUTHERN INDIAN-OCEAN DURING THE LAST 50,000 YR

Simultaneous measurements of chlorophyll and carbon content, carboxyla se activities in phytoplankton, isotopic composition of suspended part iculate organic carbon (POC), and pCO(2) in surface sea-water in the s outh Indian Ocean indicate that plankton populations have different ca rbon metabolisms on both sides of the subtropical convergence (STC). N orth of STC, communities are dominated by mixo/heterotrophic organisms while south of the STC, physiological or biomass parameters are diagn ostic of autotrophic phytoplankton. Only phytoplankton delta(13)C valu es, in the southern part, showed a strong correlation with [CO2(aq)] i n surface waters which permitted a relationship between these paramete rs. Using this empirical relationship, we derive, from sedimentary org anic carbon (SOC) delta(13), a reconstruction of the paleo-pCO(2) in s urface water of the Southern Indian Ocean for the last 50 kyr. During the glacial and deglacial periods, the obtained paleo-pCO(2) were high er than those recorded in the Vostok ice core, suggesting that the Sou thern Ocean was a potential source of CO2 for the atmosphere. These re sults are consistent with those of SOC content and indicate that the b iological pump was off during glacial times and unable to lower atmosp heric [CO2(aq)]. Moreover. these data confirm that stratification of t he surface water in summer caused a reduction of gas exchange with the atmosphere during that period. Copyright (C) 1996 Elsevier Science Lt d