Millennial-Scale Variability in the Indian Monsoon and Links to Ocean Circulation

Abstract:

Millennial-scale variability in the Indian monsoon was temporally linked to changes in global ocean circulation during the last glacial period, as evidenced by planktic-benthic foraminiferal stable isotope and trace element results from an intermediate depth sediment core from the northwestern Bay of Bengal. Paired planktic foraminiferal Mg/Ca and δ¹⁸O_c constrain sea surface temperatures and isolate millennial-scale variations in the δ¹⁸O of surface waters (δ¹⁸O_sw), which resulted from changes in river runoff in the northwestern Bay. Concurrently with low δ¹⁸O_sw events, benthic foraminiferal δ¹³C decreased, suggesting an increased influence of an aged water mass at this intermediate depth site during the low salinity events. Benthic foraminiferal Cd/Ca results support the identification of this water mass as aged Glacial Antarctic Intermediate Water (GAAIW). Lagged correlation analysis (r= 0.41) indicates that changes in subsurface properties led changes in surface properties by an average of 380 years. The implication is that Southern Hemisphere climate exerted a controlling influence on the Indian monsoon during the last glacial period.