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Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Valier Galy, Christian France-Lanord, Olivier Beyssac, Pierre Faure, Hermann Kudrass & Fabien Palhol Nature, Vol 450, 407-410|15 November 2007

Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sedi- ments1. The efficiency of organic carbon deposition in sediment- ary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmo- spheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales.

Figure : TOC of riverine and fan sediments as a function of their Al/Si ratio. Filled black diamonds, TOC of the Bengal fan sediments, with best fit (solid line) and 95% confidence interval (dashed lines). Marine sediments define a positive trend highly comparable to that defined by river sediments (filled grey squares), suggesting they have similar Corg loads.