Monsoonal and ENSO impacts on particle fluxes and the biological pump in the Indian Ocean

Abstract

Data obtained by sediment trap experiments in the Indian Ocean were evaluated in conjunction with additional information derived mainly from the Joint Global Ocean Flux Study (JGOFS) expeditions in 1994/1995. Our results indicate that wind-driven upwelling, thermohaline mixing, and freshwater inputs are the main physical processes through which the monsoon drives the biogeochemical fluxes in the Indian Ocean. The upwelling system in the Arabian Sea seems to be bottom-up controlled and very sensitive to iron supply affecting system immanent threshold concentrations and to nitrate reduction rates in the mid-water oxygen minimum zones (OMZs). On geologic timescales, nitrate reduction rates were strongly linked to Northern Hemispheric climate variations, whereby the thermohaline mixing and the associated mid-water formation in the northern Arabian Sea could have played a key role. The ratios between organic and inorganic carbon of particles exported from the surface ocean indicate that the biologically mediated CO 2 uptake, referred to as the biological pump, is low in the Arabian Sea during the high productive upwelling period. The biological pump seems to be strongest along the freshwater-influenced continental margins in the eastern Indian Ocean where the climate anomaly El Nino―Southern Oscillation (ENSO) influences the biological pump by its impact on the precipitation rates and the river discharges. Associated physical processes such as the capping effect amplify the ENSO impact on the CO 2 degassing, whereby ENSO increases/decreases the CO 2 degassing into the atmosphere in its negative (El Nino) and positive mode (La Nina).