Spatial and seasonal variations in the particulate sinking flux in the Bay of Bengal

Sediment traps have been widely used to study particles sinking through the oceanic water column. As part of the SIBER-INDIA program, sediment traps were deployed at three depths − 430 m (shallow), 1020 m (middle), and 1555 m (deep) on a mooring line in the northern BoB (Bay of Bengal Sediment Trap, BoBST). We present here data on fluxes, concentrations, and isotopic composition of carbon and nitrogen, for the period 2011–2012 for all three traps, and the shallow (505 m) and middle (1035 m) traps for 2019–2020. The mooring site was largely influenced by riverine water in 2011–2012 and the increase in flux during the southwest monsoon was attributed to both autochthonous production and a high influx of river-supplied material. On the other hand, the contribution of the lithogenic matter was less in 2019–2020. The weak sea level anomaly (SLA) signal showed that the influence of cyclonic eddies at the mooring location was minimal during the deployment period in 2011, whereas a cyclonic eddy seems to have contributed to the higher flux seen in May-June 2019. To delineate the long-term changes in the component fluxes, data collected during 2011–2012 and 2019–2020 were compared with earlier available data from the Northern Bay of Bengal Traps (NBBT-N). The annual carbonate fluxes in the present study (9.3 and 7.1 g m−2 y-1 in 2011 and 2019, respectively in the ∼ 1000 m trap, and 7.7 g m−2 y-1 in the deep trap in 2011) were low compared to earlier (1987–1997) data of NBBT-N (10.9–14.7 g m−2 y-1; avg. 13.1 ± 1.4 g m−2 y-1 for ∼ 1000 m traps and 12.0–16.1 g m−2 y-1; avg. 13.3 ± 1.3 g m−2 y-1 for deep traps). On the other hand, the opal fluxes were higher in 2011 as compared to earlier (1987–1997) and 2019 data. This is also reflected in the abundance of silicifiers, which was much higher in 2011 mainly supported by significant dissolved silica in the river runoff and from the lithogenic material dissolved in the seawater. The present study highlights the seasonal and inter-annual variations in riverine suspended particulate material in the BoB and warrants the continuation of time-series studies to infer long-term changes in the component fluxes.