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Orogenesis - CO2 source or sink? Insight from quantification of fossil organic carbon weathering rates in Taiwan

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Physical erosion of the continents can result in the mobilisation of organic carbon (OC) from the terrestrial biosphere and its transfer by rivers to the ocean. If this OC is recently photosynthesised organic matter (OCnon-fossil), then its transfer and burial in sedimentary deposits contributes to geological sequestration of atmospheric CO2 . Mountain belts with steep topographic gradients and high precipitation totals experience the highest rates of OCnon-fossil erosion (1), and its transfer along with large volumes of clastic sediment can increase its likelihood of burial in sedimentary deposits (2). In this way, orogenesis can act to drawdown CO2 . However in many mountain belts, rapid fluvial incision and hillslope mass wasting processes expose fossil organic carbon (OCfossil), contained within sedimentary bedrock, to the modern atmosphere and hydrosphere. Oxidation of OCfossil during weathering is a source of CO2 that counters drawdown by silicate weathering and recent organic carbon burial (3). Despite this recognition, we have very poor constraint on the rates at which OCfossil weathering occurs in natural environments. While it is thought that physical erosion is the primary control on this re-flux of carbon from the lithosphere, the precise nature of the link remains unknown. Here, these questions are addressed using the trace element rhenium, whose affinity to OCfossil and redox-sensitive geochemical behaviour make it an ideal tracer of this process (4). The new data from Taiwanese rivers confirm high rates of physical erosion can enhance OCfossil weathering, but show that they only counter half of the CO2 drawdown by terrestrial OC burial in marine sediments offshore. In this setting, erosion and weathering induced cycling of OC results in a net sink of CO2 .

1. Hilton, R. G., A. Galy, N. Hovius, S. J. Kao, M. J. Horng, and H. Chen (2012), Climatic and geomorphic controls on the erosion of terrestrial biomass from subtropical mountain forest, Global Biogeochemical Cycles, 26, GB3014 , doi:10.1029/2012GB004314

2. Galy, V., C. France-Lanord, O. Beyssac, P. Faure, H. Kudrass, and F. Palhol (2007), Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system, Nature, 405, 407–410, doi:10.1038/nature06273

3. Hilton, R.G., A. Galy, N. Hovius, M. J. Horng, and H. Chen (2011), Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: An orogenic carbon sequestration mechanism, Geology, 2011, 39, 71-74, doi:10.1130/G31352.1

4. Dalai, T. K., S. K. Singh, J. R. Trivedi, and S. Krishnaswami (2002), Dissolved rhenium in the Yamuna River System and the Ganga in the Himalaya: Role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere, Geochimica et Cosmochimica Acta, 66, 29-43.

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