Floodplain architecture governs organic carbon storage for a meandering river in discontinuous permafrost

Patterns of Arctic river channel migration may change as thawing permafrost destabilizes riverbanks, allowing rivers to mobilize sediment and carbon from permafrost that was previously sequestered far below the ground surface. Therefore, in addition to threatening infrastructure built on permafrost river floodplains, changes to river migration rates may alter downstream fluxes of sediment and carbon to the Arctic Ocean, modulating the carbon cycle and consequently the climate system. In order to quantify floodplain-scale organic carbon (OC) fluxes due to river channel migration, we combined field, remote sensing and chemical measurements from the floodplain of the Koyukuk River, a major meandering tributary of the Yukon that flows through discontinuous permafrost in central Alaska. We mapped geomorphic landforms, floodplain relative depositional ages and riverbank erosion rates from remote sensing imagery to develop the context for soil OC measurements to quantify the OC eroded and deposited by river meandering. We found that near-surface permafrost is located predominantly on older floodplain deposits, while younger river deposits have well-preserved scroll bars and a higher frequency of meander bends translating downstream rather than expanding outwards. Sediment radiocarbon content does not correlate with deposit relative age, and spans the radiocarbon content of woody debris from the same location. By comparing soil OC stocks across different geomorphologic units on the floodplain, we identified oxbow lakes as important OC reservoirs. We also used channel erosion and deposition masks generated from Landsat imagery to calculate that total OC fluxes into and out of the Koyukuk due to bank erosion and bar deposition are balanced over the interval 1978-2018. Our results illustrate how the Koyukuk River governs patterns of permafrost succession as it migrates across its floodplain, and provide a framework for linking geochemical budgets to lateral transport processes across a landscape.