Millennial-scale variability of Indian summer monsoon during the last 42 kyr: Evidence based on foraminiferal Mg/Ca and oxygen isotope records from the central Bay of Bengal

Abstract

Studying marine sedimentary archives from the Indian Ocean have provided great opportunities to assess the forcing/response relationship between the Indian monsoons, millennial-scale climate variability as revealed in Greenland ice cores, and possible roles of the Atlantic meridional overturning circulation (AMOC) changes that are most likely responsible for driving both. Here we present new evidence of sea surface temperature (SST) estimates and δ18O of seawater (δ18Osw, presumably salinity-driven) from the paired data of Mg/Ca and δ18O measured from planktonic foraminifer Globigerinoides ruber (white) in core BoB-24 collected from the central Bay of Bengal (BoB), tropical Indian Ocean. Eleven accelerator mass spectrometry (AMS) radiocarbon dates on planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate a gradual surface warming with several short-lived fluctuations since ~42 kyr ago, and a ~3 °C cooling during the Last Glacial Maximum (LGM) and a ~3 °C warming from the last glacial to the Holocene. Our δ18Osw revealed an increased salinity during the Marine Isotope Stage (MIS) 2, which is likely attributable to a weakening of Indian Summer Monsoon (ISM) precipitation and associated river discharge in the BoB. Our record also indicates clearly a phase of strengthening ISM during the mid-MIS 3 (37.5–32 ka) as evidenced by low foraminiferal δ18Osw values of ~0.2‰. Lower than present δ18Osw values during the Bølling/Allerød (B/A) event (14.5–12.6 ka) and the early Holocene (10–5 ka) in core BoB-24 indicate lower salinity that in turn suggest enhanced ISM precipitation and increased freshwater output from the Ganges-Meghna-Brahmaputra-Irrawaddy (GMBI) rivers together with peninsular rivers and Irrawaddy-Salween rivers from Myanmar. This study brings data evidence that implies weakened ISM was linked with Northern Hemisphere cooling events, especially during the MIS 2. Furthermore, the millennial-scale hydroclimate changes in the BoB presented in this study support the dynamic role of AMOC in driving a teleconnection between the North Atlantic and Indian Ocean monsoon regions.