Cooling ages of rock in the Himalayas imply that rapid exhumation between the Main Central thrust system and the South Tibetan detachment system occurred between 21 and 17 Ma. The generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium, carbon, phosphorus, and other chemical weathering products to the ocean (Richter et al., 1992). The increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the Monterey formation and other deposits from this interval. A downdraw of atmospheric CO2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the Antarctic ice sheet by 15 Ma. The above scenario differs from the ''Monterey hypothesis'' of Vincent and Berger in that CO2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling. A carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 Ma predicts that absolute organic carbon burial increased over this interval while, at die same time, the fraction of carbon buried as organic matter versus carbonate decreased. This implies that the organic carbon cycle has acted as a net source of CO2 to the atmosphere over the late Cenozoic.
THE HIMALAYAS, ORGANIC-CARBON BURIAL, AND CLIMATE IN THE MIOCENE
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