QUANTIFYING THE EXPORT OF FLOODPLAIN SOIL CARBON TO ARCTIC RIVERS BY BANK EROSION (Invited Presentation)

Recent efforts to quantify terrestrial carbon budgets highlight the importance of inland waters in the transport, loss to the atmosphere, and storage of carbon. Many of the estimates of carbon loss and storage along river systems are largely based on differencing the estimated input to rivers in headwaters from the outflows at measured at river mouths. However, studies of sediment exchange between river and floodplains, on systems large and small, have reported that the total amount of sediment being moved between river and floodplains can exceed the amount being discharged at the river mouth on an annual basis. This back and forth of sediment also likely impacts the storage, release and fate of particulate carbon in systems with floodplains. Here we present the results of a study that seeks to quantify the flux of soil carbon from floodplains into rivers across the Arctic. Using satellite imagery and aerial photography collected over 4 decades, we quantified rates of lateral floodplain erosion on approximately 5,000 km of rivers in 12 Arctic watersheds. An empirical model for lateral erosion rates was developed using topographic, climatological and permafrost distribution datasets. This model was then used to extrapolate erosion rates from the measured other sections of rivers across the Arctic. This extrapolation was restricted to only rivers with detectable alluvial floodplains. We then coupled these pan-Arctic erosion estimates with recently published maps of soil carbon across permafrost regions to quantify the annualized flux of soil carbon from the upper few meters of floodplains into rivers. Preliminary results indicate that annually tens of Terragrams (Tg) of carbon enter arctic rivers from their bounding floodplains. The fate of this carbon is presently unknown. Much of the carbon may be redeposited downstream or respired to the atmosphere. However, based on limited data of particulate carbon fluxes measured at the mouths of arctic rivers, it appears that more carbon is cycled between rivers and their floodplains than is exported to the ocean.