A 25ka monsoon variability record from the Bay of Bengal: a key concept.

We are using high resolution, marine sediment records to study the effects of the migration of Intertropical Convergence Zone (ITCZ) on surface ocean conditions over the past 25 ka BP in northeastern Indian Ocean. We have reconstructed sea-surface temperature (SST) by making paired ^18O and Mg/Ca measurements in Globigerinoides ruber (white variety) in cores from the Andaman Sea (RC12-344; water depth 2140 m) and western shelf of the Bay of Bengal (VM29-19; water depth 3183 m). Our initial results suggest that SST and seawater ^18O (^18Osw) in the Bay of Bengal were ~2oC cooler and ~2‰ higher, respectively, during the Last Glacial Maximum (LGM) relative to the Holocene. The higher ^18Osw values during the LGM are interpreted to reflect less freshwater dilution of surface-ocean waters caused by less direct rainfall over the Bay of Bengal during the monsoon period and/or reduced outflow from the Ganges-Brahmaputra-Irrawaddy Rivers. A southern displacement of mean ITCZ position during the LGM could result in a reduction of cross-equatorial moisture transport from the tropical Indian Ocean, which would decrease rainfall on the Indian subcontinent and riverine influence on surface-ocean conditions. The transition from glacial to interglacial conditions in our records includes a distinct and expanded Bolling-Allerod warming event but the following Younger Dryas cooling is truncated.