Erosion, Transport and Burial Of Petrogenic Organic Carbon In the Himalayan System: A Closed Loop In The Carbon Cycle?

The burial of terrestrial organic carbon (Corg) in marine sediments represents the second largest atmospheric CO2 sink. Corg exported by rivers is a mix of recent organic carbon (e.g. plant debris and soil Corg) and fossil Corg derived from erosion of carbonaceous material bearing rocks. Burial of petrogenic Corg is a simple recycling of reduced C and does not participate to atmospheric CO2 consumption. Moreover, its oxidation represents a net source of CO2. It is therefore crucial to determine the proportion of fossil Corg in river sediments as well as its fate during transport and burial. Recently, we realised a comprehensive Corg budget for the Himalayan system including source rocks, river sediments and marine sediments buried in the Bengal Fan. We showed that this system is characterised by an extreme burial efficiency of Corg, and represents 10 to 20% of total terrestrial Corg burial. In this study, we present a systematic characterisation of fossil Corg contained in source rocks, river sediments and marine sediments using Raman microspectrometry and High-Resolution Transmitted Electron Microscopy (HRTEM). Fossil Corg has been detected in suspended and bed sediments, as well as in sediments deposited in the Bengal Fan turbiditic system including its more distal part, 3000 km south of the river mouth. Fossil carbonaceous particles are present under 3 main forms: (1) independent, (2) associated with minerals as aggregates, (2) included in minerals. In Himalayan rivers, structural organization of carbonaceous particles is similar to that observed in the source rocks, from highly disorganised C to pure graphite. On the other hand, in the delta and Bengal Fan, sediments contain almost only highly graphitized carbonaceous particles. This suggests that disorganised carbonaceous particles are oxidised during transport in the floodplain, while highly graphitised C is selectively preserved and finally buried in Bengal Fan sediments. This is consistent with an increase of mechanical and chemical resistance during graphitization. Selective preservation of graphitic C forms a closed loop in the long term C cycle since graphitic C may escape to oxidation for several orogenic cycles. Over the geological timescale, the consequence must therefore be an increase of the reduced C reservoir.