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...

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The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan-Himalayan uplift. However, the existence of older Asian monsoons and their response to enhanced greenhouse conditions such as those in the Eocene period (55-34 Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan-Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34 Myr ago.