Os-Sr-Nd results from sediments in the Bay of Bengal: Implications for sediment transport and the marine Os record

Sr, Nd, and Os isotopic data are presented for sediments from diverse locat ions in the Bay of Bengal. These data allow the samples to be divided into three groups, related to their sedimentary contexts. The first group, mainl y composed of sediments from the shelf off Bangladesh and the currently act ive fan, has Sr and Nd characteristics consistent with a dominantly Himalay an source. Their Os-187/Os-188 ratios (similar to1.2-1.5) show that the ave rage detrital material delivered by the Ganga-Brahmaputra (G-B) river syste m is not unusually radiogenic. A large difference in Os-187/Os-188 ratio ex ists between these Bengal Fan sediments and Ganga bedloads (Os-187/Os-188 s imilar to2.5, Pierson-Wickmann et al. [2000]). This difference mainly refle cts addition of a less radiogenic Brahmaputra component, though mineralogic al sorting and loss of radiogenic Os during transport may also play some ro le. The second sample group contains sediments from elsewhere in the Bay, p articularly those located on the continental slope. They display Os isotopi c compositions (0.99-1.11) similar to that of present seawater and higher O s and Re concentrations. These characteristics suggest the presence of a la rge hydrogenous contribution, consistent with the lower sedimentation rate of these samples. Sr and Nd ratios indicate that a significant fraction of these sediments is derived from erosion of non-Himalayan sources, such as t he Indo-Burman range. These observations could be explained by the deflecti on of sediments from the G-B river system by westward currents in the head of the Bay. The third group contains only one sample, but shows that in add ition to a Himalayan source, sediment discharge from Sri Lanka may influenc e the detrital component in the distal part of the fan. The similarity betw een the isotopic compositions of the group I RN Sonne samples and those of Ocean Drilling Program Leg 116 [France-Lanord et al., 1993, Reisberg et al. , 1997] suggests that the material eroding in the Himalayas has been roughl y constant since the Miocene. The high Os isotopic ratios of leachates of b oth Sonne group I and Miocene Leg 116 sediments imply that much of the leac hable highly radiogenic Os component was conserved during transport through the estuary or interaction with seawater. In constrast, samples with lower , but still relatively high, sedimentation rates (Sonne groups II and III a nd Pliocene Leg 116) seem to have significantly adsorbed or exchanged Os an d Re with seawater. This suggests that in some cases the Os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient ma rine Os record, or conversely, to date unfossiliferous sediments.