Indian summer monsoon (ISM) hydrology fuels biogeochemical cycling across South Asia and the Indian Ocean, exerting a first-order control on food security in Earth's most densely-populated areas. Despite projected ISM intensification under greenhouse forcing, large uncertainty surrounds future Indian Ocean stratification and primary production -- processes key to the health of already-vulnerable fisheries in the region. Here we present centennially-resolved records of ISM runoff and marine biogeochemical fluctuations in the Bay of Bengal (BoB) since the Last Glacial Maximum (LGM; ~21 ka). We find that ISM runoff was at its weakest during Heinrich Stadial 1 (HS1; 17.5-15.5 ka) and that peak freshwater discharge occurred during the early Holocene (EH; 10-9 ka). Counterintuitively, our records indicate that BoB productivity collapsed during both extreme states of peak monsoon intensity (EH) and failure (HS1). Using individual foraminiferal analyses (IFA) we demonstrate that both extremes were associated with upper-ocean stratification; whereas thermally-mediated stratification suppressed mixing and nutrient-delivery during HS1, outflow-induced stratification driven by strengthened ISM runoff curtailed productivity during the EH. In contrast to the latest Earth-system model projections, our paleoceanographic results raise the possibility of future deterioration in BoB productivity under strengthening monsoon seasonality.
Dr. Kaustubh Thirumalai is an Assistant Professor with the Department of Gesociences at the University of Arizona. Dr. Thirumalai received his Ph.D. in geological sciences from the University of Texas at Austin. He studies climate variability across Earth's history. Thirumalai is interested in the causes and mechanisms of past variations in the oceans and atmosphere and how such information can help anticipate future impacts under global change. To implement this research objective, he uses a combination of modern observations, geochemical measurements, and climate simulations. He is motivated to use the geological record as a laboratory to fully understand the machinery of Earth's climate system and explore its response to perturbations. Dr. Thirumalai received the Geosciences Advisory Board Outstanding Faculty Award and the AGU Nanne Weber Early Career Award in 2022.