Photosynthesis of organic matter is a major pathway for consumption of atmospheric CO 2 . Although most photosynthetic organic carbon (C org ) is re-oxidized and returns to the atmosphere, a small fraction is buried in sedimentary basins and stored over geological timescales 5 . This burial represents the second largest atmospheric CO 2 sink (after silicate weathering coupled to carbonate precipitation) and contributes to long-term climate 2 regulation 6 . Continental erosion exerts a primary control on C org burial through sediment transport and detrital deposition in sedimentary basins. However, assessing the role of continental erosion in this part of the C cycle is complex, as several processes control its efficiency. First, C org transported by rivers is composed of both recent organic matter and fossil refractory C org derived from erosion of carbonaceous rocks. Erosion-burial of the latter has no effect on the long term C cycle and it is therefore necessary to determine its proportion. Second, it is generally believed that ~70 % of C org exported by global rivers (for example, the Amazon) is oxidized in the continental margins before burial and thus returns to the atmosphere 2-4 . Last, assessing the riverine C org flux is not straightforward, because the C org content of sediment is highly variable owing to transport segregation processes.Understanding the impact of continental erosion on the C org cycle requires identification of the controls on the flux of riverine C org , the proportion of rock-derived fossil C org and the burial efficiency.Himalayan erosion generates the largest flux of sediments to the oceans. Today, this represents between 1 and 2 billion tons of sediments exported each year from the Himalayas through the Ganges-Brahmaputra (G-B) system and buried in the Bengal fan sedimentary unit 7-9 . The total C org concentration (TOC) in G-B fluvial sediments was hitherto estimated using only surface suspended sediments and without correction for the fossil C org contribution 10,11 . Over the past 15 Myr, the Bengal fan has buried an average of about 0.6 10 12 mol C org yr 1 , that is, 15% of the global burial flux 12 . The Himalayas are thus a key locality for isolating the role of major orogens on the C cycle.A major sample set covering the whole basin from the Himalayas to the distal part of the Bengal fan has been analysed for C org , 14 C and chemistry (Supplementary Tables 1 and 2).The two major rivers (Ganges and Brahmaputra) and their confluence (Lower Meghna) were sampled during three monsoon seasons when ~95% of the sediment flux is exported 13 (Fig. 1, Methods). Bed loads and depth profiles of suspended sediments were collected to integrate the total sediment variability. Simultaneous acoustic Doppler current profiler measurements were performed in order to characterize the hydrodynamic conditions. The Bengal fan sedimentary units are documented by subsurface sediments cored in the shelf, in the mid-fan active channel-levee system and in the distal part of the fan during RV Sonne c...

Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales. On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs. Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels. The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of 157(+74)(-50) and 43(+61)(-25) megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.

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