Organic Carbon Cycling During Himalayan Erosion: Processes, Fluxes and Consequences for the Global Carbon Cycle

Abstract

The net effect of organic carbon cycling during continental erosion depends on the balance between rock-derived organic carbon oxidation and ­biospheric organic carbon burial in sediments. Himalayan erosion is dominated by physical transport and each year up to two billion tons of sediments eroded from the Himalaya are delivered to the Bengal Fan through the Ganga–Brahmaputra (G–B) fluvial system. We developed a sampling protocol that allows the heterogeneity of the sediment load to be accounted for. In the channel of large rivers, the total organic carbon content (TOC) is variable and decreases towards depth. TOC is positively correlated to Al/Si ratio, which characterizes the mineral and grain size sorting. In the delta of Bangladesh, sediments from Ganga, Brahmaputra and Lower Meghna have similar organic carbon loading. Coupling Raman Micro-spectroscopy and High Resolution Transmitted Electron Microscopy allows the unambiguous detection and characterization of petrogenic (rock-derived) carbon. Comparison of Himalayan rivers and G–B in Bangladesh indicates that the most graphitised forms are selectively preserved and delivered to the Bay of Bengal. Radiocarbon characterization of sediments along depth profiles yields values for the absolute concentration of petrogenic carbon in rivers sediments. Comparison of Himalayan rocks and G–B sediments in Bangladesh shows that 40% (±10) of the organic carbon contained in the Himalayan rocks is preserved and delivered to the ocean. The evolution of stable isotopic composition (δ13C) from the outflow of the Himalayan range to the delta of Bangladesh shows that during the Gangetic floodplain transit, more than 50% of organic carbon derived from the Himalaya is oxidized and replaced by organic carbon derived from the floodplain. The organic carbon loading of recent Bengal Fan sediments is comparable to that of G–B river sediments. Biomarker abundance and δ13C values show that organic carbon is dominated by terrestrial inputs. The terrestrial organic carbon burial efficiency is thus close to 100%. This strongly contrasts with other large deltaic system on earth, where ∼70% of terrestrial organic carbon is oxidized prior to burial. This extreme burial efficiency is sustained by high erosion rate in Himalaya that generates high sedimentation rate and low oxygen availability in the Bay of Bengal. The balance between biospheric organic carbon burial and petrogenic carbon oxidation indicates a net CO2consumption of 3.2 ± 0.8 × 1011mol/year. Atmospheric CO2consumption through organic carbon cycling during Himalayan erosion is thus an order of magnitude higher than the CO2consumption through silicate weathering in the Himalayan basin (6.4 × 1010mol/year). Efficient burial of organic carbon is a characteristic of high physical erosion typical of active orogenic systems. Enhanced physical erosion and consequent organic carbon burial buffer atmospheric CO2thereby exerting a negative feedback on the long-term climate.