Erosional history of the Himalayan and Burman ranges during the last two glacial-interglacial cycles

摘要

The results of a clay mineralogy study combined with major element geochemistry, strontium, neodymium and oxygen isotopes, and ~(14)C AMS stratigraphy are reported for deep-sea gravity cores located in the Bay of Bengal (MD77-180) and the Andaman Sea (MD77-169). #epsilon#_(Nd)(0) and ~(87)Sr/~(86)Sr from Holocene and last glacial maximum (LGM) sediments of fifteen other cores have also been investigated to identify sediment sources and to estimate oceanic sedimentary transport. The data show the contribution of three sources: (1) Ganges/Brahmaputra rivers; (2) Irrawaddy River; and (3) sediment derived from the western part of the Indo-Burman ranges. The dispersion of the detrital material issuing from these sources has been constrained by the geographic distribution of Nd and Sr isotopic ratios. The LGM sediments are characterized by a significant increase of ~(87)Sr/~(86)Sr, small changes in #epsilon#_(Nd)(0), and a general decrease of smectite/(illite + chlorite), which together imply a decrease in weathering intensity. The increased ~(87)Sr/~(86)Sr are attributed to a decrease in chemical weathering, which should release preferentially ~(87)Sr during weathering processes. In the Andaman Sea sediments, smectite/(illite + chlorite) and kaolinite/quartz ratios combined with a chemical index of alteration (CIA~*) indicate that the weathering intensity of the Irrawaddy River basin is mainly controlled by the summer monsoon rainfall intensity. The wet summer monsoons increase vegetation cover in the plains and favor soil development by the production of pedogenic clays (smectite and kaolinite).