NEOGENE HIMALAYAN WEATHERING HISTORY AND RIVER SR-87 SR-86 - IMPACT ON THE MARINE SR RECORD/

Clastic sediments in the Bengal Fan contain a Neogene history of erosi on and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Mi ocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data s how that the Himalayan provenance for the eroded material has varied l ittle since > 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical e rosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The Sr-87/Sr -86 ratios of pedogenic clays (vermiculite, smectite) record the isoto pic composition of Sr in the weathering environment, and can be used a s a proxy for Sr-87/Sr-86 in the paleo-GB basin. The Sr data from pedo genic clays shows that river Sr-87/Sr-86 values were near 0.72 prior t o 7 Ma, rose rapidly to greater than or equal to 0.74 in the Pliocene, and returned to less than or equal to 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal vari ability of Sr-87/Sr-86 in a major river system. The high Sr-87/Sr-86 v alues resulted from intensified chemical weathering of radiogenic sili cates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river Sr-87/Sr -86 values require a ca. 50% decrease in the Sr flux from the GB syste m in the Pliocene. The relationship between weathering intensity, Sr-8 7/Sr-86 and Sr flux is similar to that observed in modem rivers, and i mplies that fluxes of other elements such as Ca, Na and Si were also r educed. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, f ollowed by a return to physically dominated and rapid erosion in the P leistocene. In contrast to the view that increasing seawater Sr-87/Sr- 86 results from increased erosion, Mio-Pliocene to mid-Pleistocene cha nges in the seawater Sr budget were the result of reduced erosion rate s and Sr fluxes from the Himalaya.