Floodplains, permafrost, cottonwood trees, and peat: What happened the last time climate warmed suddenly in arctic Alaska?

We use the stratigraphy of floodplains on Alaska’s North Slope to describe how tundra watershedsresponded to climate changes over the last 15,000 calibrated years BP (15 cal ka BP). Two episodes ofextremely rapid floodplain alluviation occurred during the PleistoceneeHolocene transition, onebetween 14 and 12.8 cal ka BP and the other between 11.5 and 9.5 cal ka BP. These aggradation episodescoincided with periods of warming in summer when cottonwood (Populus balsamifera L.) expanded itsrange, peatlands became established, and widespread thermokarst occurred. The two aggradationepisodes were separated by a period of floodplain incision during the Younger Dryas under cooler andpossibly drier conditions. At times of increasing summer warmth, melting permafrost and enhancedprecipitation probably triggered widespread mass wasting on hillslopes that overwhelmed the capacityof streams to transport sediment downstream, and rapid floodplain aggradation resulted. After peatlandsbecame widespread in the early Holocene, rivers slowly incised their valley fills. Because major pulses ofsediment input were limited to times of rapid thaw and increasing moisture, many floodplains on theNorth Slope have been effectively decoupled from upstream hillslopes for much of the past 15,000 years.Our findings: (a) confirm the sensitivity of arctic watersheds to rapid warming in summer, (b) emphasizethe importance of hillslope mass wasting in landscape-scale responses to climate change, and (c) suggestthat the presence of peatland on this arctic landscape today has raised its geomorphic response thresholdto climate warming compared to what it was 14,000 years ago.