Organic carbon burial forcing of the carbon cycle from Himalayan erosion

Abstract

1-4 . Recent attention has focused on increased silicate weathering of tectoni- cally uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era 5-7 . The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 con- sumption from the net burial of organic carbon during Himala- yan sediment deposition was 2-3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes. Silicate weathering is typically incongruent, yielding both a solute and a secondary mineral phase, so direct evidence of chemical weathering can be found in the record of secondary minerals in sedimentary basins. The Bengal Fan and Ganges-Brahmaputra (GB) delta contain a huge volume of sediment derived from erosion of the India-Asia collision zone, with 6 3 10 6 km 3 deposited in the past 20 Myr (ref. 8). Isotopic data for Nd, Sr and O from Bengal Fan sediments show that the source for over 80% of the detritus since 20 Myr ago has been the high-grade metasedimentary rocks of the High Himalayan crystalline (HHC) sequence 9 . Clastic and carbo- nate sediments of the Precambrian Lesser Himalaya (LH) and Palaeozoic-Mesozoic Tethyan Himalaya (TH) are the other impor- tant sources of sediment to the Bengal Fan during the Neogene. Carbon dioxide consumption from silicate weathering can be represented schematically by: CaSiO3 ˛ 2CO2 ˛ 2H2O ! Ca 2˛ ˛ 2HCO 2 3 ˛ H4SiO4