The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales

Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.