The Os and Sr isotopic record of Himalayan paleorivers: Himalayan tectonics and influence on ocean chemistry

Weathering products in Himalayan paleosols provide a unique archive of changes in Os-187/Os-188 and Sr-87/Sr-86 records of Himalayan rivers in the Neogene and can provide important constraints on the potential role of Himalayan weathering in controlling seawater isotopic compositions. Covariation in the Sr and Os isotopic records of the paleosols from the Indus and Ganges river systems indicates that these elements are derived from the same source terranes. The Os isotopic ratios from the Indus paleosols are less radiogenic than that of the paleo-oceans until similar to 5 Ma, whereas Os in the Ganges system paleosols is significantly more radiogenic than seawater ratios throughout the last similar to 20 Myr. Os isotopic ratios in the paleosols have increased by over 100% over the last 11 Myr in both the Indus and Ganges river systems. Variations in both the Os and Sr records of paleosols from the Ganges river system can be explained utilizing the tectonic history of the Central Himalayas. The paleosol record provides important new constraints on the potential role of Himalayan weathering in controlling seawater Sr and Os. If the Himalayan rivers systems are responsible for the observed changes in the marine Sr and Os records, then large changes in the fluxes of both these elements must be invoked to accommodate the paleosol record. Future models of marine isotopic variation need to account for potentially large and rapid changes in the riverine isotopic ratios over time. (C) 2000 Published by Elsevier Science B.V. All rights reserved.