Abstract

Several sedimentary, geochemical, and isotopic records indicate that the hypothesized intensification of the Asian monsoon at about 8 Ma triggered a physical and chemical weathering event in the Himalayan-Tibetan Plateau. Records of sediment input and clay composition from the northern Indian Ocean reveal clear weathering changes in the plateau at this time, and Ge/Si ratios of opaline silica as well as biogenic sedimentation rates indicate that increased dissolved element fluxes from this weathering event had an oceanwide effect. It is likely that weathering intensity also increased in the Andes and Amazon Basin in this interval as well. Perhaps the most important impact of this weathering event was to increase the net flux of the biolimiting nutrient phosphorus to the ocean, as evidenced by a peak in phosphorus accumulation rates at this time. The temporary increase in nutrient inputs to the ocean triggered increased oceanic productivity and organic carbon burial (recorded in carbon isotopic records and paleooxygen concentrations). The net result of this weathering event may have been an increase in the rate of drawdown of atmospheric CO2 through the late Miocene–early Pliocene, via direct silicate weathering reactions and increased burial of organic carbon in the ocean. This increased rate of atmospheric CO2 drawdown may have destabilized the climate system by the late Pliocene, thus initiating a period of intense cooling and ice buildup leading to the present.

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