(2020) Fluvial Sediment Transit Time Regulates the Fate of Organic Carbon between Source and Sink
Repasch M, Scheingross J, Hovius N, Wittmann H, Lupker M, Eglinton T, Haghipour N & Sachse D
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10f: Plenary Hall, Thursday 25th June 23:48 - 23:51
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Do you consider DOC transport and oxidation when calculating the Carbon In/ oxidized/ and out (Slide 9) ? Is most carbon in the river transported in the particulate fraction (POC)?
Hi Regina, that's a great question. I do not consider DOC in my calculations. I focus mainly on POC, which is known to have impacts over geological timescales because it can be preserved and then buried in marine sediments, effectively removing carbon form the atmosphere. We did measure DOC concentrations in the Rio Bermejo during the high flow season. Our data show that the DOC makes up just 30% of the total annual OC export. We also know that water will take just ~14 days to travel the length of the river, and given that DOC decomposition rates are on the order of ~2 years, DOC is not likely to be turned over during fluvial transit, but maybe consumed downstream in the higher productivity estuary.
What hydroclimatic and geomorphic conditions would be needed for a river to be a source rather than sink? is the sediment transit time the dominant factor in determining the rivers role as a net source or sink?
Hi Anne, these are great questions. Determining the carbon budget of a large river system is quite complicated because there are many internal feedbacks, and every river might experience these feedbacks in different ways. In a simple sense, increasing the sediment transit time would result in more POC oxidation, but it would also result in more organo-mineral complexation, allowing more POC to be stabilized. A warmer and wetter climate may increase decomposition rates, causing more OC oxidation, but such a climate change may also increase floodplain primary production, allowing the oxidized OC to be replaced. Changing runoff and water discharge would likely increase the POC flux, but might also increase channel migration rates and therefore increase floodplain deposition of POC. These feedbacks need to be explored more rigorously to more confidently model fluvial POC transport and turnover.
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