High-latitude permafrost regions store large stocks of soil organic carbon (OC), which are vulnerable to climate warming. Estimates of subsurface carbon stocks do not take into account floodplains as unique landscape units that mediate and influence the delivery of materials into river networks. We estimate floodplain soil OC stocks within the active layer (seasonally thawed layer) and to a maximum depth of 1 m from a large field data set in the Yukon Flats region of interior Alaska. We compare our estimated stocks to a previously published data set and find that the OC stock estimate using our field data could be as much as 68% higher than the published data set. Radiocarbon measurements indicate that sediment and associated OC can be stored for thousands of years before erosion and transport. Our results indicate the importance of floodplains as areas of underestimated carbon storage, particularly because climate change may modify geomorphic processes in permafrost regions.

Floodplains accumulate and store organic carbon (OC) and release OC to rivers, but studies of floodplain soil OC come from small rivers or small spatial extents on larger rivers in temperate latitudes. Warming climate is causing substantial change in geomorphic process and OC fluxes in high latitude rivers. We investigate geomorphic controls on floodplain soil OC concentrations in active-layer mineral sediment in the Yukon Flats, interior Alaska. We characterize OC along the Yukon River and four tributaries in relation to geomorphic controls at the river basin, segment, and reach scales. Average OC concentration within floodplain soil is 2.8% (median = 2.2%). Statistical analyses indicate that OC varies among river basins, among planform types along a river depending on the geomorphic unit, and among geomorphic units. OC decreases with sample depth, suggesting that most OC accumulates via autochthonous inputs from floodplain vegetation. Floodplain and river characteristics, such as grain size, soil moisture, planform, migration rate, and riverine DOC concentrations, likely influence differences among rivers. Grain size, soil moisture, and age of surface likely influence differences among geomorphic units. Mean OC concentrations vary more among geomorphic units (wetlands = 5.1% versus bars = 2.0%) than among study rivers (Dall River = 3.8% versus Teedrinjik River = 2.3%), suggesting that reach-scale geomorphic processes more strongly control the spatial distribution of OC than basin-scale processes. Investigating differences at the basin and reach scale is necessary to accurately assess the amount and distribution of floodplain soil OC, as well as the geomorphic controls on OC.