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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1 Continental chemical weathering is central in Earth's surface biogeochemical 2 cycles as it redistributes elements across reservoirs such as the crust and the oceans. 3 However the evolution of weathering through time and its response to external forcing 4 such as changes in climate remain poorly constrained. In this work, a composite 5 sediment record from the Bay of Bengal is used to document the evolution of chemical 6 weathering in the Himalayan system (Himalayan range and Indo-Gangetic floodplain), 7 the world largest sediment conveyor to the oceans, since the Last Glacial Maximum 8 (LGM). The degree of weathering of the sediments is documented using mobile to 9 immobile ratios such as K/Si and H2O + /Si as well as detrital calcite abundance. Robust 10 weathering proxies are derived by correcting the chemical composition of sediment for 11 sorting effects that occur during transport and deposition. The Bay of Bengal record is 12 also further compared to the chemical composition of modern river sediments from the 13 Ganga & Brahmaputra basin. Weathering proxies all indicate that the sediments 14 exported by the Ganga & Brahmaputra Rivers became increasingly weathered over the 15 past ~21 kyr, whereas, Sr, Nd and major elements suggest a constant sediment 16 provenance in the system over the last 21 kyr. These changes in the degree of weathering 17 of the sediments show that the weathering flux exported by the system to the Indian 18 Ocean during the LGM was significantly lower than at present and demonstrate that 19 chemical weathering in continental scale basins such as the Ganga & Brahmaputra 20 responds to Late Quaternary climate changes. 21 137 Cs, 210 Pb and 228 Ra gamma spectrometry (Michels et al., 1998). Scarce hemipelagic layers 65 were excluded from the study in order to focus on turbiditic sediments representing the Ganga 66 & Brahmaputra detrital signal (Pierson-Wickmann et al., 2001; Weber et al., 1997). We 67 interpret our data by constructing a composite Log from the shelf and levees core following 68 the approach detailed in Galy et al. (2008). 69 River suspended river sediments were filtered through 0.22 µm PES membrane filters 70 within 24 h of sampling and dried in the lab. Major element concentration was determined by 71 Lupker, M., France-Lanord, C., LavÈ, J., Bouchez, J., Galy, V., MÈtivier, F., 577 Gaillardet, J., Lartiges, B., Mugnier, J.-L., 2011. A Rouse-based method to integrate the 578 chemical composition of river sediments: Application to the Ganga basin. A., Wiedicke, M., 1998. The 586 submarine delta of the Ganges-Brahmaputra: cyclone-dominated sedimentation 587 patterns. Marine Geology 149, 133-154. 588 Pierson-Wickmann, A.-C., Reisberg, L., France-Lanord, C., Kudrass, H., 2001. Os-Sr-589 Nd results from sediments in the Bay of Bengal: Implications for sediment transport and 590 the marine Os record. Paleoceanography 16, 435-444. 591

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