Monsoon-related changes in surface hydrography and productivity in the Bay of Bengal over the last 45 kyr BP

Temporal variations of mixed-layer planktic foraminiferal abundances in a sediment core from the western Bay of Bengal (off Krishna - Godavari Basin) are used to reconstruct surface hydrographic structure and productivity in response to the changes in seasonal monsoon precipitation and wind intensity over the last 45 kyr BP. The faunal proxy records (abundances of eutrophic and oligotrophic species/group and G. bulloides / G. ruber ratio) exhibit millennial-scale variations during the late Glacial, deglacial and Holocene periods. Our results show that the surface hydrographic conditions and productivity varied in concert with the Northern Hemisphere climatic events (Dansgaard-Oeschger (D-O) oscillations and Heinrich (H) events). The productivity in the late Glacial period between 44 and 19 kyr BP in general was higher compared to the deglacial and Holocene. Within the late Glacial period, we record high productivity during the cold phases of the D-O cycles (43-41, 35-34, 28.5-28, 27-25 kyr BP), North Atlantic Heinrich events (39.5-38.5 kyr BP, H4; 31.5-30 kyr BP, H3; 24-23 kyr BP, H2) and during the Last Glacial Maximum (22-19 kyr BP). These productivity maxima are related to the pronounced advection of nutrient-rich subsurface waters to the euphotic zone induced by the intensified Northeast (NE) Monsoon winds in non-stratified/weakly stratified surface ocean conditions due to reduced Southwest (SW) Monsoon precipitation and freshwater influx. Our faunal record suggests a major reorganization of seasonal monsoon wind circulations after 16 kyr BP, when the SW monsoon intensified and concomitantly the NE monsoon weakened. Enhanced SW monsoon precipitation and freshwater flux to the Bay of Bengal between 15 and 13 kyr BP, a period broadly corresponding to the warm Bolling-Allerod (B/A) event, resulted in strong stratification of mixed-layer water and oligotrophic conditions. An increase in productivity between 13 and 11 kyr BP (equivalent to the cold Younger Dryas (YD) event) is attributed to intensified mixing of weakly stratified mixed layer waters by NE monsoon winds. During the early Holocene between 9 and 6 kyr BP, intensified SW monsoon precipitation and high fluvial discharge resulted in strongly stratified, nutrient-poor waters in the mixed-layer and a conspicuous reduction in productivity. However, a moderate increase in productivity is recorded between 5 and 3 kyr BP, when the SW monsoon intensity and associated fluvial discharge gradually declined and ocean surface stratification weakened resulting in conditions favourable for NE monsoon winds induced mixing. Overall, our results show that the alternating periods of intensified SW monsoon precipitation and NE monsoon wind circulation modulated the surface hydrographic structure and productivity patterns in the Bay of Bengal.

Automated summary

This study reconstructs the surface hydrographic structure and productivity in the western Bay of Bengal over the past 45 kyr BP using the temporal variations of mixed-layer planktic foraminiferal abundances in a sediment core. The results show that the surface hydrographic conditions and productivity were influenced by Northern Hemisphere climatic events and changes in monsoon precipitation and wind intensity.