Influence of monsoon variability on hydrographic and productivity dynamics in the western Bay of Bengal during the late Holocene: A planktic foraminiferal perspective

Abstract

Late Holocene paleoceanographic changes and the paleoenvironmental evolution of the Western Bay of Bengal (WBoB) were reconstructed using planktic foraminifera obtained from the NGHP-15A core sediments. The planktic foraminiferal record shows significant variations in surface hydrography and productivity due to seasonal monsoon rainfall and wind strength during the late Holocene. The foraminiferal records combined with several paleoceanographic indices provide insights into the centennial scale hydrographic and trophic level evolution and ecosystem dynamics of WBoB. Our findings demonstrate four different regimes of monsoon-induced fluctuations in the upper water column structure and productivity changes during the Roman Warm Period (RWP), Dark Age Cold Period (DACP), Medieval Climate Anomaly (MCA), and Medieval Climate Anomaly-Little Ice Age (MCA-LIA) transition. We concluded that oligotrophic conditions along with stratified upper water column prevailed during RWP and MCA due to intensified southwest monsoon and related river discharge. Enhanced eutrophication during the DACP and MCA-LIA transition, associated with the significant upward movement of nutrient-rich subsurface water to the photic zone caused by the strengthened Northeast (NE) monsoon winds. Overall, our findings indicate that the ocean surface hydrographic structure and productivity dynamics in the WBoB were modulated by the concurrent phases of enhanced precipitation during the SW monsoon and dry reverse wind circulation during the NE monsoon respectively.