A paleoproductivity shift in the northwestern Bay of Bengal (IODP Site U1445) across the Mid-Pleistocene transition in response to weakening of the Indian summer monsoon

Abstract

The long-term variability of the Indian monsoon in the Bay of Bengal remains inconclusive due to the lack of proximal sedimentary records. To further elucidate the long-term variability of the Indian monsoon, we analyzed the paleoproductivity regime over the last 2.3 Myr at the International Ocean Discovery Program (IODP) Expedition 353 Site U1445 located near the Mahanadi Basin in the northwestern Bay of Bengal. We measured the downcore concentrations and mass accumulation rates (MARs) of biogenic opal, CaCO3, total organic carbon (TOC), and total nitrogen over the Mid-Pleistocene Transition (MPT) to identify the links between surface water marine biogenic production and the Indian summer monsoon. TOC MARs were found to reflect both surface water marine productivity and terrestrial organic matter through the measurements of sediment C/N ratios and δ13CSOM values. Nonetheless, we identified a shift in the paleoproductivity regime from a dominance of biogenic opal deposition prior to the MPT to the dominance of CaCO3 deposition following the MPT. The shift in biogenic marine productivity across the MPT was closely related to riverine discharge, which was primarily controlled by the intensity of the Indian monsoon. Our results, therefore, infer a decrease in riverine discharge to the Bay of Bengal across the MPT in response to a weakened Indian summer monsoon (and/or strengthened Indian winter monsoon). In addition, changes in the intensity of the Indian monsoon across the MPT were more closely linked to the global climate cooling rather than the gradual uplift of the Himalaya and Tibetan Plateau.