Significant Floodplain Soil Organic Carbon Storage Along a Large High‐Latitude River and its Tributaries

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. Plain Language Summary Large amounts of organic carbon (OC) are stored in the northern permafrost (perennially frozen) soils. Accurately estimating the amount of carbon is important for understanding the movement of carbon between the land, the ocean, and the atmosphere. Soil OC in northern regions may be released to the atmosphere with future warming and permafrost thaw, further indicating the need to accurately understand how much OC is stored in soils. Floodplains, as sites of sediment and nutrient deposition, store significant amounts of soil OC and to date have not been adequately characterized. We estimate the amount of OC in floodplain soils in the Yukon Flats region in interior Alaska, finding that there may be significantly more soil OC in northern floodplains than previously thought. Key Points We use a large field data set to quantitatively estimate floodplain soil organic carbon stock along the Yukon River The stock is ~68% higher than that estimated by the Northern Circumpolar Soil Carbon Database Dating of organics in floodplain sediment indicates that floodplains can store sediment and OC for thousands of years