Resolving abrupt Indian summer monsoon (ISM) changes in the past is critical to understanding major forces that modulate Indian summer monsoon variability. Mixed layer dwelling planktonic foraminifer Globigerinoides ruber (white) from core material retrieved from the northern Bay of Bengal, tropical Indian Ocean, were used to create a record of Ba/Ca, Mg/Ca, and delta 18O variations. Time-series of Ba/Ca-based sea surface salinity (SSS), delta 18O of sea water, and Mg/Ca-based sea surface temperature are used to investigate millennial-scale variability of the ISM and its relationship to Bay of Bengal sea surface temperature through the last glacial (95--25 kyr BP). The proxy records indicate that the ISM responded sensitively to rapid changes in oceanic and atmospheric perturbation. Intensification of ISM, as indicated by a decrease in runoff-induced SSS, corresponds to episodes of warm air temperature over Greenland, known as Dangaard-Oeschger (DO) interstadials.

Concomitant to Heinrich events our record indicates an abrupt weakening of ISM. A weak correlation between Ba/Ca-based SSS and Mg/Ca-based sea surface temperature indicates a decoupling between SST and Indian summer monsoon precipitation during millennial-scale events. Divergences in the timing, pace, and magnitude of Ba/Ca-based SSS and delta18Olocal seawater (temperature and ice volumes components removed) reveals local delta 18O changes related to ISM variability are likely caused by many factors. Furthermore, the record of delta18Oresidual that has been established by removing the ice volume, temperature, and salinity components from foraminiferal delta18O, suggests that decline in monsoon strength was accompanied by an increase in the delta18 O of precipitation during northern high-latitude stadials.

Dissimilarity between the Ba/Ca-based SSS and delta18Olocal seawater records as well as significant delta18Oresidual variability suggest the imprint of millennial-scale ISM changes on the delta18O record may be more complicated than previously thought. An enrichment of delta18Oresidual is present during Heinrich events. One possible explanation for this observation is that concomitant to the decline in monsoon precipitation the isotope composition of precipitation was significantly enriched, most likely suggesting changes in source and pathway of the moisture. Therefore, reconstruction of past ISM changes solely on basis of delta18O records may lead to inaccurate assessment of precipitation change. Overall, the record from the northern Bay of Bengal indicates that the ISM responds very sensitively to changes in millennial-scale northern high-latitude temperature and ice sheet instability.