| Summary: | participant The summer and winter distribution of δ 13 C 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 δ 13 C DIC of -0.017 ‰ yr -1 coherent with the average f CO 2 increase of 2.1 µatm yr -1 observed in this region. The seasonal δ 13 C DIC climatology is thus referenced to the year 2002 by correcting surface δ 13 C DIC data by -0.017 ‰ yr -1 . From 20°S to 60°S surface waters are characterized by higher δ 13 C DIC during summer than during winter. In summer maxima (δ 13 C 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 CO 2 uptake is stronger. As opposed to f CO 2 the seasonal δ 13 C 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 δ 13 C DIC in the photic zone) and deeper mixing in winter (reducing surface δ 13 C DIC ). In the subtropical oligotrophic waters the seasonal δ 13 C DIC signal is ~0.15 ‰ and driven by air-sea CO 2 flux (sink in winter, near equilibrium in summer). This new δ 13 C DIC climatology allows identification of a negative δ 13 C DIC anomaly in the subtropics during summer 2002 associated to an anomalous ocean CO 2 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.
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