Orogenic sources and sinks of CO2: The Himalayan example

The Himalayan orogeny has generated major changes in the Earth surface processes that affect the carbon cycle through various processes. Erosion triggered long term enhancement of silicate weathering and organic carbon burial that tend to consume atmospheric CO2. Erosion also tend to return CO2 by oxidation of fossil organic carbon. Finally, orogeny also generates metamorphism of carbonated formation that release massive fluxes of CO2. While these processes have been active for at least 20 million years, their modern fluxes are still difficult to assess. Geochemical signatures of modern river sediments allow quantifying fluxes of organic carbon and losses of mobile elements. Their compositions are governed primarily by mineral sorting processes occurring during transport. However these are well correlated to simple parameters such as Al/Si ratios and allow very accurate inter-comparison of sediments. Differences between river sediments and source rocks allow to quantify losses and gain of elements resulting of weathering, soil, and transport processes. Following this approach, silicate weathering is mostly linked to alkaline silicate weathering. This strongly limits the efficiency in term of long-term CO2 uptake as alkaline cations further undergo reverse weathering which returns all the linked alkalinity to the atmosphere. On the reverse, the overall budget of organic carbon oxidation, fixation and burial is highly efficient. This efficiency is not due to exceptional organic carbon concentration in sediment which does not exceeds 0.4 wt.% on the average. Rather, its importance is due to the intensity of physical erosion that generates intense particulate flux. This favours rapid burial in the Bengal fan preventing oxidation of organic carbon and induce major flux. The Himalayan orogenesis also acts as a source of CO2 to the atmosphere through metamorphic decarbonation. Direct evidences of these fluxes have been measured through dissolved and gas CO2 discharge associated to thermal spring along the MCT in Himalaya [1&2]. While inaccurately determined, these fluxes are significant and certainly counter balance those linked to silicate weathering.