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

Abstract

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...

Runoff-driven export of particulate organic carbon from soil in temperate forested uplands

“…The rate of export of non-fossil POC from the Erlenbach (14.0 ± 4.4 tonnes km -2 yr -1 ) 478 is broadly comparable to yields of non-fossil POC reported from Taiwan (21 ± 10 479 tonnes km -2 yr -1 ) (Hilton et al, 2012a) and New Zealand (~39 tonnes km -2 yr -1 ) 480 (Hilton et al, 2008a), and an order of magnitude greater than from the Ganges-481 Brahmaputra basin (~3 tonnes km -2 yr -1 ) (Galy et al, 2007b). However, the real 482 significance lies in the contrasting processes responsible for these fluxes and their 483 geographical scope.…”

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“…Although the fate of this physically eroded C is difficult to trace, it is likely to be refractory at the centennial timescale 49 and is most likely channelled through inland waters and estuaries to the open ocean without significant exchange with the atmosphere. It is thus treated separately in Fig.…”

Key Future Directions For Research On Turbidity Currents and Their Deposits

“…The timing of large flows that reach basin plains may be temporally random, suggesting limited sea-level control for such events (Clare et al 2014). The frequency of sediment transport also has important implications for the efficiency of burial of organic carbon in submarine settings, and hence for the global carbon cycle (Galy et al 2007). It is important to analyze the controls on larger scale system evolution on longer time scales, in order to constrain the influence of climate and tectono-morphologic controls on deep-sea sedimentation.…”

“…This topic is important for understanding the climatic record in submarine fan sequences as well as how these systems are perturbed by humans. It has important implications for predicting the frequency, location, and geometry of subsurface oil and gas reservoirs, as well as the efficiency of burial of organic carbon in submarine settings (Galy et al 2007). …”