Foraminiferal production and monsoonal upwelling in the Arabian Sea: evidence from sediment traps

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Abstract
Planktonic foraminifera collected in sediment traps in the Arabian Sea during 1986 and 1987 responded to the southern Asian monsoon with changes in productivity, relative abundance of species and isotopic shell chemistry. Most species of foraminifera increased in flux shortly after the advent of the southwest monsoon. G. bulloides increased its production rate by three orders of magnitude. The isotopic chemistry of G. ruber recorded the increase in monsoon upwelling by increasing its δ18O values by about 1‰, accurately reflecting the average 4°C sea surface temperature decrease associated with the upwelling. The mean value of δ18O for G. ruber was greater in the western Arabian Sea than in the central or eastern basins because upwelling in that region cools surface water. The carbon isotopic composition of G. ruber does not have a clear temporal or geographical relationship to upwelling. While its δ13C values decreased in the western Arabian Sea during the upwelling event, the mean δ13C values remained higher in the western than in the eastern and central Arabian Sea. This longitudinal gradient is opposite to that expected from the geographical gradient of upwelling: the region with the most intense upwelling should have lower δ13C values in surface waters because of the upwelling of low-δ13C water to the surface.