UC Berkeley

Abstract Permafrost influences 25% of land in the Northern Hemisphere, where it stabilizes the ground beneath communities and infrastructure and sequesters carbon. However, the coevolution of permafrost, river dynamics, and vegetation in Arctic environments remains poorly understood. As rivers meander, they erode the floodplain at cutbanks and build new land through bar deposition, creating sequences of landforms with distinct formation ages. Here we mapped these sequences along the Koyukuk River floodplain, Alaska, analyzing permafrost occurrence, and landform and vegetation types. We used radiocarbon and optically stimulated luminescence (OSL) dating to develop a floodplain age map. Deposit ages ranged from modern to 10 ka, with more younger deposits near the modern channel. Permafrost rapidly reached 50% areal extent in all deposits older than 200 years then gradually increased up to ∼85% extent for deposits greater than 4 Kyr old. Permafrost extent correlated with increases in black spruce and wetland abundance, as well as increases in permafrost extent within wetland, and shrub and scrub vegetation classes. We developed an inverse model to constrain permafrost formation rate as a function of air temperature. Permafrost extent initially increased by ∼25% per century, in pace with vegetation succession, before decelerating to <10% per millennia as insulating overbank mud and moss slowly accumulated. Modern permafrost extent on the Koyukuk floodplain therefore reflects a dynamic balance between widespread, time‐varying permafrost formation and rapid, localized degradation due to cutbank erosion that might trigger a rapid loss of permafrost with climatic warming. Plain Language Summary Arctic rivers rework their floodplains, eroding permafrost from one riverbank while depositing unfrozen sediment on the opposite bank, where permafrost may eventually form. While permafrost is often considered a stable reservoir for carbon or platform for construction, permafrost extent in river floodplains is continually changing. To better understand the fate of permafrost in a warming world, we mapped and dated floodplain deposits, determined permafrost extent, and characterized vegetation types along the Koyukuk River in Alaska. Permafrost was present in floodplain deposits of all ages but was most abundant in older areas of the floodplain, where thick mosses and muddy flood deposits insulated the ground from warm summer air temperatures. Younger areas of the floodplain initially had patchy permafrost but rapidly formed permafrost in 50% of their area within 200 years. Permafrost formation then slowed and reached a maximum extent about 85% of deposit area after 4,000 years. We propose that early, rapid permafrost generation is driven by vegetation growth and succession while the later, slow rate of permafrost generation is determined by the rates at which muddy flood deposits and moss accumulate, providing insulation to the ground. Under a warming climate, our modeling shows that permafrost generation in new river deposits will be even slower or cease entirely, making river floodplains susceptible to rapid loss of permafrost via river erosion. Key Points River meandering resets permafrost development and vegetation succession by eroding old floodplain areas and depositing new land On the Koyukuk River, permafrost rapidly forms in new deposits, but it takes over 4 Kyr to develop to its full areal extent of 85% The rate of permafrost formation is likely linked to vegetation succession on younger and overbank deposition on older floodplain areas