Changes in siliceous paleoproductivity during the early Middle Pleistocene in the Northwest Bay of Bengal

The Northwest Bay of Bengal (BoB) is located in the core precipitation zone of the Indian Summer Monsoon (ISM), and an ideal area to study Indian monsoon activity and its impact on the environment. We reconstructed the history of siliceous productivity evolution in the study area during late Early Pleistocene to early Middle Pleistocene (920～370 ka) by analyzing biogenic silica content and sediment fluxes of cores from Site U1446 of IODP Expedition 353 in the Mahanadi Basin, Northwest BoB, and explored the mechanism of ISM precipitation and runoff activities on the paleoproductivity changes in the study area. Results indicate that siliceous productivity reflects a trend from low in the Mid-Pleistocene Transition (MPT) to high after MPT in the northwest BoB during 920～370 ka in an obvious glacial-interglacial cycle. It decreased during the glacial period and increased during the interglacial period, and the fluctuation increased after the MPT. A comparison of existing summer monsoon precipitation and runoff indicators such as δ¹⁸O_sw and Rb/Ca ratio at this site reveals that siliceous productivity increased (decreased) when the ISM strengthened (weakened). It is hypothesized that the enhanced ISM led to a dramatic increase in runoff from the Indian peninsula and precipitation in the BoB, and the nutrients from rivers led to a proliferation of surface siliceous organisms and a rapid upsurge in siliceous productivity in the northwest BoB. However, the weakened ISM and the reduction of nutrients inhibited the flourishing of planktonic siliceous organisms, which then decreased the surface productivity. Spectral and wavelet analyses of biogenic silica fluxes at Site U1446 also indicate that siliceous productivity was modulated by orbital cycle variations in this study area.