Most of our knowledge about the history of Indian and Asian monsoons has relied on delta18O records extracted from cave deposits in India and China. While these records are highly resolved and have the most robust age models, the reliance on a single proxy, delta18O, as an indicator of changes in precipitation amount harbors significant uncertainty. This is due to the difficulty in separating the various components influencing calcareous delta18O records. Here we present an independent and spatially integrated measure of past Indian summer monsoon precipitation extracted from a sediment sequence recovered from the northern Bay of Bengal, an area within a runoff-induced low salinity plume. The focus is on the time window spanning Termination II and Marine Isotope Stage 5 (145-70 kyr BP). We use delta18O, Ba/Ca, and Mg/Ca analyzed in tests of the surface-dwelling planktonic foraminifer Globigerinoides ruber (white).

Comparison of the temperature and ice volume corrected delta 18Osw record with the Ba/Ca record, a proxy for runoff, shows important differences. Consistent with the Kutzbach's hypothesis (Kutzbach, 1981), the Ba/Ca record indicates that strong monsoon intensities correspond with strong northern hemisphere summer insolation with a decreasing trend from strongest monsoon during MIS 5e (~130 kyr BP) and weakest during MIS 5a (80 kyr BP). In contrast, delta18Osw shows monsoon runoff to be comparable or stronger during MIS 5c and 5a relative to 5e. The delta18O-based observation shares some commonality with the results of East Asian and West African monsoon reconstructions. We removed the delta18Osw component related to runoff amount from the delta18Osw record and obtained delta 18Oresidual, which due to the setting we primarily interpret to reflect changes in the delta18O of precipitation (delta 18Oprecipitation).

delta18Oprecipitation is significantly depleted during MIS 5a and 5c relative to 5e and enriched during Termination II. Variations in the delta18O of precipitation in the Indian and East Asian domains, likely result from changes in moisture source, pathway, and the seasonality of precipitation. The major result of this study lies in revealing a large-scale imprint in calcareous delta 18O archives unrelated to the amount of precipitation and establishing an independent record of ISM precipitation during the last interglacial.