Indian Summer Monsoon variability 140–70 thousand years ago based on multi-proxy records from the Bay of Bengal

Abstract

Understanding the Indian Summer Monsoon (ISM) behaviour during the late Pleistocene has been largely based on the wind-driven upwelling records from the Arabian Sea. However, it remains unclear the extent to which these records can also be used to infer a concomitant signal of monsoon rainfall, or how the two ISM components, rainfall and wind, are linked on millennial timescales. In order to isolate a primary signal of ISM rainfall, we exploit two deep sea sediment cores from the northern Bay of Bengal (Site U1446) and Andaman Sea (Site U1448), both situated proximal to the South Asian continent, and thus ideally situated for capturing ISM rainfall and fluvial runoff. By comparing our multi-proxy ISM rainfall and runoff records with published ISM wind-driven records from the Arabian Sea, we observe pronounced decoupling of the rainfall and wind components of the ISM across Marine Isotope Stage 5/6 (∼140–70 thousand years ago). We reveal that the relative dominance of barometric dynamics (wind) and the thermodynamic (rainfall) components of the monsoon shifts with changes in background climate state. This finding constitutes an important consideration for the interpretation of past monsoon reconstructions. By comparing our new ISM rainfall records with high latitude climate records, we show that moisture export from low-latitudes, via the monsoon, could have preconditioned the high latitudes for ice sheet growth during glacial inceptions.