Dynamics of primary productivity in the northeastern Bay of Bengal over the last 26 000 years

Abstract. At present, variations of primary productivity (PP) in the Bay of Bengal (BoB) are responding to salinity-related-stratification which is controlled by the Indian Summer Monsoon (ISM). The relationships between PP, ISM, and to a broader scale, North Atlantic climate rapid variability in the past, are not clearly understood. Here, we present a new record of PP based on the examination of coccolithophore assemblages in a 26 000 years sedimentary record, retrieved in the northeastern BoB (core MD77-176). Comparisons with published climate and monsoon records, as well as outputs from the transient climate simulation TraCE-21 and experiments run with the Earth System Model IPSL-CM5A-LR, including marine biogeochemical components, helped us interpret our PP records in the context of ISM and Atlantic Overturning Meridional Circulation (AMOC) changes. We demonstrate that PP is influenced by vertical stratification in the upper water column over the last 26 000 years (26 kyr BP). It is controlled by wind-driven mixing from 26 to 19 kyr BP, i.e., when dry climate conditions and reduced freshwater inputs occurred, and by salinity-related-stratification over the last 19 kyr BP (since the Last Glacial Maximum), i.e., when humid conditions prevailed. During the deglaciation, salinity and stratification are related to monsoon precipitation dynamics, which are chiefly forced by both, insolation and the strength of the AMOC. The collapse (recovery) of the AMOC during Heinrich Stadial 1 (Bolling Allerod) weakened (strengthened) ISM and diminished (increased) stratification, thus enhancing (subduing) productivity.