SOURCES OF THE LATE QUATERNARY SEDIMENTS IN THE ANDAMAN SEA: IMPLICATIONS FOR MONSOON VARIABILITY AND OCEAN CIRCULATION

Geochemical and Sr-Nd isotopic studies were carried out on a sediment core collected from the Andaman Sea to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Chronology of the core was established using radiocarbon ages and δ¹⁸O stratigraphy. We find that the rate of sedimentation was strongly controlled by fluctuations of the monsoonal intensity. Three major sediment provenances are identified, viz. (1) the Irrawaddy catchment, (2) the western slopes of the Indo-Burman-Arakan (IBA) mountain ranges and the Andaman Islands and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30 to 60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ¹⁸O of planktic forams are observed at seven time periods (11-14, 20-23, 36, 45, 53, 57 and 62 ka) of which five are synchronous with the northern hemisphere cooling - suggesting a link between the changes in sediment provenances with the shifting of locus of the Indian summer monsoon, southwards from the Himalayas, without substantial reduction in the intensity. Our data and that from other cores in the region suggest that an east moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the last glacial maximum (LGM). The existence of this current during the LGM implies that, contrary to the general belief, the Andaman Sea and the north-eastern Bay of Bengal were well connected during the last glacial period.