SUMMER AND WINTER VARIABILITY OF δ¹³C_DIC IN SURFACE WATERS OF SOUTH INDIAN OCEAN

Abstract : The summer and winter distribution of δ¹³C_DIC in the South-West Indian Ocean is analyzed from surface measurements obtained in 1998-2005 (ten OISO cruises). Based on this dataset, we estimate a decrease in δ¹³C_DIC of -0.017 ‰ yr^-1 coherent with the average fCO₂ increase of 2.1 µatm yr ^-1 observed in this region. The seasonal δ¹³C_DIC climatology is thus referenced to the year 2002 by correcting surface δ¹³C_DIC data by -0.017 ‰ yr^-1. From 20°S to 60°S surface waters are characterized by higher δ¹³C_DIC during summer than during winter. In summer maxima (δ¹³C_DIC >2‰) are observed in the sub-Antarctic frontal zone where biological activity is enhanced. In winter minima (<1‰) are found in the subtropical region when the ocean CO₂ uptake is stronger. As opposed to fCO₂ the seasonal δ¹³C_DIC signal is larger in the region 35°S-40°S, with a mean amplitude of ~0.3 ‰; this is attributed to a stronger biological activity in summer (increasing δ¹³C_DIC in the photic zone) and deeper mixing in winter (reducing surface δ¹³C_DIC). In the subtropical oligotrophic waters the seasonal δ¹³C_DIC signal is ~0.15 ‰ and driven by air-sea CO₂ flux (sink in winter, near equilibrium in summer). This new δ¹³C_DIC climatology allows identification of a negative δ¹³C_DIC anomaly in the subtropics during summer 2002 associated to an anomalous ocean CO₂ sink controlled by sea surface cooling. It is our hope that these results will help to evaluate the oceanic uptake of anthropogenic carbon in this region and to constrain and validate atmospheric inversions and ocean carbon models.