Efficient carbon burial in the Bengal fan sustained by the Himalyan erosional system

Nancy Université, CRPG CNRS/INSU, BP 20, 54501 Vandoeuvre-lès-Nancy, France.
Nature (Impact Factor: 41.46). 12/2007; 450(7168):407-10. DOI: 10.1038/nature06273
Source: PubMed


Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. 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 basin. 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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    • "Pg organic carbon (OC) to the oceans per annum (Hedges et al., 1997; Lal, 2003). Rivers form a key connection between the terrestrial and marine carbon cycles, constituting the primary mode of carbon transport from continents to the ocean (Smil, 2007) and are thus key focal points in ''source to sink'' studies (Blair et al., 2004; Galy et al., 2007; He et al., 2013). Suspended particulate matter (SPM) is the most important carrier of particulate OC from continent to ocean. "
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    ABSTRACT: Insight into the content and composition of organic carbon (OC) in river systems contributes to our understanding of the global carbon cycle. The Changjiang (Yangtze River) plays a significant role in global carbon and hydrological cycles, as it is an important supplier of sediment, nutrients and OC to the East China Sea. To provide a preliminary insight on the source of OC transported by the Changjiang, we analyzed bulk (grain size, organic carbon content, δ13C), and molecular (lignin phenols, branched and isoprenoid GDGTs) characteristics of organic matter in bed sediments at eight locations along the river. The δ13C values and lignin phenol composition indicate that the OC originates from a mixture of primarily soils and plants. Comparison between branched GDGT (br GDGT) distributions in riverbed sediments and those in the different soil types in the drainage basin indicate that the br GDGT signal in the upper reaches is largely derived from soils from the Qinghai-Tibet Plateau. Downstream changes in br GDGT distributions can be linked to subsequent input of local soil material. The observed variation in br GDGT composition along the river implies that a portion of the OC may be oxidized or replaced by the local input during transit. Although the relation between δ13C and lignin phenol composition indicate that the contribution of phytoplankton is limited, comparison of GDGT-ratios (GDGT-0:crenarchaeol, iso GDGTs:br GDGTs, BIT index) in the sediments and surrounding soils indicates that at least part of the iso GDGTs has an aquatic origin. Overall, the downstream evolution of br GDGTs in the Changjiang implies that their use as paleoclimate proxy in down-core applications will likely yield a local, rather than a basin-integrated continental climate record.
    Full-text · Article · May 2015 · Organic Geochemistry
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    • "This needs to be coupled with insightful postexpedition studies but further supports the idea that Himalayan erosion has consumed atmospheric CO 2 through the burial of organic carbon, more than by silicate weathering (France-Lanord and Derry, 1997). • Preliminary estimates of organic carbon loading and behavior— such as preferential association of organic matter with clays— resemble observations made in the modern Ganga-Brahmaputra river system, suggesting efficient terrestrial organic carbon burial in the Bengal Fan (Galy et al., 2007). The relationship reveals , however, larger dispersion than documented in modern rivers, which suggests either variable source conditions or preservation during transport/deposition processes. "
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    ABSTRACT: International Ocean Discovery Expedition 354 to 8°N in the Bay of Bengal drilled a seven site, 320 km long transect across the Bengal Fan. Three deep-penetration and an additional four shallow holes give a spatial overview of the primarily turbiditic depositional system that comprises the Bengal deep-sea fan. Sediments originate from Himalayan rivers, documenting terrestrial changes of Himalayan erosion and weathering, and are transported through a delta and shelf canyon, supplying turbidity currents loaded with a full spectrum of grain sizes. Mostly following transport channels, sediments deposit on and between levees while depocenters laterally shift over hundreds of kilometers on millennial timescales. During Expedition 354, these deposits were documented in space and time, and the recovered sediments have Himalayan mineralogical and geochemical signatures relevant for reconstructing time series of erosion, weathering, and changes in source regions, as well as impacts on the global carbon cycle. Miocene shifts in terrestrial vegetation, sediment budget, and style of sediment transport were tracked. Expedition 354 has extended the record of early fan deposition by 10 My into the late Oligocene.
    Full-text · Article · May 2015 · Integrated Ocean Drilling Program: Preliminary Reports
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    • "Overall, TOC content is low, with an average value of 0.3 wt%. Within turbidites, TOC broadly co-varies with Al/Si ratios—a proxy for sediment grain size and mineral composition—reflecting preferential association of organic matter with clays previously documented in both the modern Ganges-Brahmaputra river system and in active channel-levee sediments in the Bay of Bengal deposited over the past 18 ky (e.g., Galy et al., 2007). Clay-rich turbiditic sediments are often characterized by significant organic carbon depletion compared with sediments from the modern Ganges- Brahmaputra river system and the active channel-levee system at 17°N. "

    Full-text · Technical Report · May 2015
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