Deciphering isotopic signals of monsoon-induced upwelling in foraminifera from the Western Arabian sea sediment trap

Abstract

Isotopic analyses have been made on four species of foraminifera collected with a PARFLUX Mark VI sediment trap in the Arabian Sea during the 1986 and 1987 monsoons. Two of the species are non-spinose thermocline dwellers (Neogloboquadrina dutertrei, Globorotalia menardii) and two are spinose surface dwellers (Globigerinoides sacculifer, Globigerina bulloides). Individual tests were measured for shape, and analyzed for carbon and oxygen isotopes to document how each species responded to phases of the upwelling system, and to determine the utility of these planktonic foraminifera as upwelling indicators. The oxygen isotopic composition of the two thermocline dwellers remained fairly constant during the seasonal monsoon cycle, showing little to no effect from upwelling whereas the surface dwellers exhibited a wider range of δ18O values. The δ13C signal shows a similar pattern, with the thermocline dwellers showing less variation than the surface dwellers. Results are compared to the empirical model of Kroon and Ganssen (1989) that describes a faunal succession of planktonic foraminiferal species based on isotopic composition and timing of optimum growth conditions. The succession of species based on fluxes was inconclusive and although three of the species showed the same isotopic patterns as described in the model, G. bulloides was not enriched in δ13C, as predicted. The covariance of δ13C and δ180 in this particular species suggests that G. bulloides is isotopically unreliable for reconstruction of monsoon-induced upwelling in the Arabian Sea.