Glacial-Interglacial Vegetation and Hydroclimate Changes in the Ganges-Brahmaputra Basin Recorded in the Deep Sea Sediments from the Bengal Fan

The Bengal Fan, the largest submarine active fan in the world, is fed by the Ganges and Brahmaputra (G-B) rivers, and corresponds to the final sink for the huge erosional system draining most of the Himalayan range. Although tectonic processes mainly control long-term changes in riverine sediment flux, and associated erosional carbon burial, the intensity of Himalayan weathering over Glacial-Interglacial (G-I) timescales may be modulated by changes in climate and vegetation cover in the G-B basin. Such influence has been suggested previously by geochemical and biomarker records of proximal sites (~16°N) covering the last 18 kyrs. Large changes in the Indian Summer Monsoon in response to orbital forcing operated during previous G-I periods, likely resulting in large rainfall and vegetation shifts in the G-B basin, which remains to be documented. Recently, the IODP expedition 354 (8°N) recovered sites from several levees of the southern part of the Bengal Fan. This study presents a pollen record that reveals more detailed information on vegetation changes in the G-B basins over the last 18 kyrs from the proximal cores together with preliminary results on pollen analyses of sediments from the IODP Exp. 354. We will use the pollen record from proximal sites as reference to explore the palynological content of older turbiditic sequences of the fan. Pollen assemblages from the proximal sites indicate the dominance of savannah-like vegetation prior to 11 ka, as evidenced by high percentages of grasses and moderate percentages of forest, dominated by temperate taxa. The Holocene vegetation is characterized by the expansion of forest, mainly tropical trees until 6ka, then by temperate taxa until the end of the Holocene. So far, our preliminary results on the analyses of the IODP sites suggest 1) pollen content large enough to work on few cc of sediment and 2) little bias on the pollen signal induced by the sedimentary transport between 16°N and 8°N. Therefore, the IODP Exp. 354 material will give access to detailed information on vegetation changes from climate cycles covering the entire Quaternary.