Seasonality and interannual variability of particle fluxes to the deep Bay of Bengal: influence of riverine input and oceanographic processes

Abstract

Abstract Results from sediment trap experiments conducted from October 1987 until November 1997 in the deep northern, central and southern Bay of Bengal, northern Indian Ocean, revealed significant monsoon-induced seasonal and interannual variations in total and component fluxes. Distinct regional differences occurred related to the variable effect of terrestrial input and the development of relatively small-scaled oceanographic processes. The enormous fluvial input to the Bay during SWM is reflected in the high average content of lithogenic matter between 39% at the northernmost and 12.6% at the most distal sampling site in the southern Bay of Bengal. Generally, the riverine input supports a relative increase in the fluxes of biogenic opal. At the stations nearest to the continent (NBBT-N; CBBT) terrestrial inputs determine seasonal and interannual variabilities. Lateral supply of lithogenic matter from the shelf regions lead to distinct increase of fluxes with depth. High-flux events of >300 mg m −2 d −1 consisting of >50% lithogenics occur in the central Bay of Bengal during NEM and in the northern Bay of Bengal during SWM in response to resuspension of shelf sediments and severe floods. When riverine input by Ganges/Brahmaputra and the offshore propagation of the river plume during SWM is reduced, the advection from an upwelling area along the east coast of India during NE–SW intermonsoon becomes the most prominent control mechanism for both biogenic and lithogenic fluxes at NBBT-N. In the central Bay of Bengal (CBBT) fluxes are characterized by a bimodal pattern. High carbonate-dominated fluxes during SWM and NEM occur mainly due to wind-induced nutrient supply, whereas high, opal-rich fluxes during SW–NE intermonsoon are controlled by both upwelling as well as supply of riverine and shelf-derived matter. Wind forcing is the dominant factor controlling particle fluxes at the most distal stations (NBBT-S; SBBT), leading to distinct maxima during SWM with high abundance of biogenic components. Here, interannual variations of particle fluxes are associated with changes in marine productivity induced by oceanographic processes. This is most obvious for SBBT, where the advection of upwelled water from the west coast of India in some years results in highest SWM fluxes and a distinct shift towards biogenic opal.