SUMMER AND WINTER VARIABILITY OF δ13CDIC IN SURFACE WATERS OF SOUTH INDIAN OCEAN
Résumé
The summer and winter distribution of δ13CDIC 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 δ13CDIC of -0.017 ‰ yr-1 coherent with the average fCO2 increase of 2.1 µatm yr -1 observed in this region. The seasonal δ13CDIC climatology is thus referenced to the year 2002 by correcting surface δ13CDIC data by -0.017 ‰ yr-1. From 20°S to 60°S surface waters are characterized by higher δ13CDIC during summer than during winter. In summer maxima (δ13CDIC >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 δ13CDIC 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 δ13CDIC in the photic zone) and deeper mixing in winter (reducing surface δ13CDIC). In the subtropical oligotrophic waters the seasonal δ13CDIC signal is ~0.15 ‰ and driven by air-sea CO2 flux (sink in winter, near equilibrium in summer). This new δ13CDIC climatology allows identification of a negative δ13CDIC 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.