Paleo-productivity reconstruction in Bay of Bengal during the past 1.3 Ma: Implications for glacial-interglacial dynamics and southern hemispheric processes

Productivity variations in the Bay of Bengal is often linked to the Indian summer and winter monsoons, influenced by Northern Hemispheric (NH) climate variability. However, limited instrumental data, scarcity of long-term proxies, and sparse climate reconstructions in the Southern Hemisphere (SH) pose challenges to studying its influence on the Indian Summer Monsoon (ISM) and in turn on productivity. To further elucidate the driver of the productivity in the western Bay of Bengal we analysed various paleoproductivity proxies over 1.3 Myr at the International Ocean Discovery Program (IODP) Expedition 353 Site U1446 located near the Mahanadi Basin in the Northwest (NW) Bay of Bengal. We measured the downcore variation in Mass Accumulation Rates (MARs) of CaCO₃, Total Organic Carbon (TOC), Total Foraminifera Number (TFN), Biogenic Barium (Ba_bio), Organic Phosphorus (P_org) during and post Mid-Pleistocene Transition (MPT) to decipher the cause of productivity change in the region. We found that δ¹³C_org, δ¹⁵N, and other Rock-Eval parameters, reflect the contributions of both marine and terrestrial Organic Matter (OM). However, a shift in OM from marine to terrestrial was observed across MPT. The increase in productivity after the MPT was most pronounced during the glacial periods and was closely linked to climate variability in the SH. The productivity in the interglacial periods was low owing to intensified monsoon associated with NH climate variability. Our results, therefore, infer that NH climate variability (during interglacials,) in tandem with SH climate variability (during glacials) modulates productivity in NW Bay of Bengal. In addition, changes in the productivity post-MPT were more closely linked to the intensification of glacial periods and increase in polar ice volume, rather than the increased ISM intensity.