Summary of Abstract Submission

Abstract Submission No. IO50-06-0005PresentationOral

EXPORT VERSUS RECYCLING, THE RESPONSE OF THE ORGANIC CARBON PUMP TO CHANGES IN MONSOON STRENGTH

Rixen, T.*1, Gaye, B.2, Ramaswamy, V.3

1 Leibniz Center for Tropical Marine Ecology, Germany
2 Institute for Geology, University of Hamburg, Germany
3 Geology Division, National Institute of Oceanography, Germany

ABSTRACT :

The monsoon-driven upwelling in the Arabian Sea act as important valve in the exchange of carbon between the ocean and the atmosphere. This exchange is strongly influenced by the organic carbon pump. Although it responds to global change the extent and the direction of change are still unpredictable. Sediment trap experiments have shown differences in the functioning of the organic carbon pump (OCP). A phytoplankton dominated OCP operates in the western Arabian Sea whereas microbial processes and an associated enhanced export of dissolved organic matter (DOM) via vertical mixing becomes more important in the eastern Arabian Sea. The microbial loop is fed by excess phosphate that is produced by nitrate reduction in the mid-water oxygen minimum zone and becomes accessible for cyanobacteria after nitrate is consumed by phytoplankton within the upwelled water. Dust inputs lower the DOM export into the relative shallow water-depths by favouring the formation of fast sinking particles. Compared to phytoplankton-driven export rates these fluxes are low and enhanced C/N ratios of the exported organic matter indicates a more efficient recycling of nutrients at shallower water depth. This implies that the microbial-driven OCP operates more in a recycling mode that keeps nutrients closer to the euphotic zone whereas the phytoplankton-driven OCP lowers the recycling of nutrients by exporting them into greater water-depth. Paleorecords show that a weakening of the monsoon was associated with increasing mid-water nitrate reduction rates suggesting that a weaker monsoon favours the recycling mode of the OCP