Seasonal cycles in biogenic production and export in Northern Bay of Bengal sediment traps

Sediment traps in the northern and north central Bay of Bengal are characterized by highly seasonal fluxes and unusually high efficiency of organic carbon export. However, the mechanism for high export production remains under debate. To evaluate the relationships between production in the photic zone and export processes responsible for the fluxes into the traps, over an annual cycle we examine a series of indicators of production regime set in surface waters. These indicators include communities of planktic foraminifera and coccolithophores, stable isotopic chemistry of foraminifera and coccoliths, and the Sr/Ca ratios in coccoliths. Coccolith and foraminiferal assemblages confirm that the Bay of Bengal is a region of high productivity. Coccolithophore communities are dominated to an unusually high degree (90%) by the lower photic zone dweller Florisphaera profunda, a species adapted to high-nutrient and low-light conditions typical of stratified waters like those induced by the strong halocline in the Bay of Bengal. Cyclonic eddy pumping and strong winds during the southwest monsoon (SWM) increase the relative abundance of upwelling indicator species like foraminifera Globigerina bulloides and the upper photic coccolithophores Globigerina oceanica and Emiliania huxleyi. However, while upwelling and eddy pumping do coincide with high opal and coccolith export, in both traps peak organic carbon export precedes the onset of eddy pumping and upwelling indicators. These data suggest an alternate mode of production in the Bay of Bengal, which is not driven by upwelling but rather high production deeper in the water column, probably by taxa adapted to lower light levels. In both traps, the pulses of organic carbon export coincide with elevated fluxes of planktonic foraminifera, which likely reflect increased primary production. Consequently, while major export pulses of organic carbon coincide with pulses of lithogenic export, the pulses of organic carbon export are not likely to reflect simple scavenging of suspended organic carbon by lithogenic pulses. Rather, they reflect increased organic carbon production. Nonetheless, some seasonal changes increase in coccolith export efficiency during the SWM are suggested by coccolith Sr/Ca ratios. In the northern trap, coccolith carbonate export is decoupled from the main pulse of organic carbon export. Consequently, ballasting of organic carbon aggregates by coccolith carbonate does not play a major role in the seasonal cycle of organic carbon export from the photic zone to shallow (800-m) traps.