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

The results of a clay mineralogy study combined with major element geochemi stry, strontium, neodymium and oxygen isotopes, and C-14 AMS stratigraphy a re reported for deep-sea gravity cores located in the Bay of Bengal (MD77-1 80) and the Andaman Sea (MD77-169). epsilon(Nd)(0) and Sr-87/Sr-86 from Hol ocene 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 detr ital material issuing from these sources has been constrained by the geogra phic distribution of Nd and Sr isotopic ratios. The LGM sediments are chara cterized by a significant increase of Sr-87/Sr-86, small changes in epsilon (Nd)(0), and a general decrease of smectite/(illite + chlorite), which toge ther imply a decrease in weathering intensity. The increased 87Sr/86Sr, att ributed to a decrease in chemical weathering, which should release preferen tially Sr-87 during weathering processes. In the Andaman Sea sediments, sme ctite/(illite + chlorite) and kaolinite/quartz ratios combined with a chemi cal index of alteration (CIA*) indicate that the weathering intensify of th e 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). (C) 1999 Elsevier Science B.V. All rights reserved.