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

Abstract

Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments1. The efficiency of organic carbon deposition in sedimentary 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 basin2,3,4. 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 atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales.

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Figure 1: Map of the Himalayan system.
Figure 2: TOC of riverine sediments as a function of their Al/Si ratio.
Figure 3: Radiocarbon composition of acid-insoluble Corg extracted from riverine depth profile sediments as a function of 1/TOC.
Figure 4: TOC of riverine and fan sediments as a function of their Al/Si ratio.

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Acknowledgements

This study was funded by CNRS-INSU programmes Eclipse and Relief de la Terre. We thank A. Gajurel and M. Rahman for help during the sampling of river sediments; C. Guilmette for technical assistance in the stable isotopes laboratory; and P. Burnard and L. Reisberg for improving the quality of the text.

Author Contributions V.G. and C.F.L. are the main authors. O.B. conducted the fossil Corg characterization. P.F. and F.P. were involved in the organic geochemistry part of the study. H.K. provided the Bengal fan sediments and the framework of sedimentological interpretation.

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Correspondence to Valier Galy.

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The authors declare no competing financial interests.

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Supplementary Information

The file contains Supplementary Discussion with additional references, Supplementary Figures 1-7 with Legends and Supplementary Tables 1-3. This section comprises a supplementary discussion on: (1) fossil Corg detection and, (2) sources of Corg in Bengal Fan sediments. The Supplementary Figures and Supplementary Tables support both the main text and the supplementary discussion. (PDF 2875 kb)

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Galy, V., France-Lanord, C., Beyssac, O. et al. Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature 450, 407–410 (2007). https://doi.org/10.1038/nature06273

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