Impact of Climate and Fires on Abrupt Permafrost Thaw in Alaskan Tundra

Abstract:

Thermo-erosion from abrupt permafrost thaw is a key pulse disturbance in the Arctic that may impact the global carbon cycle. Abrupt thaw can occur when the permafrost active layer expands in response to climate warming and/or increased wildfire activity. Understanding these drivers of thermo-erosion is necessary to anticipate feedbacks in the Arctic, where summer temperature and fire frequency are predicted to increase. We examine modern and late-Holocene thermo-erosion in high-fire (Noatak) and low-fire (North Slope) tundra ecoregions of Alaska using a combination of remote-sensing and paleo-records. Lakes with active thaw features were identified through Landsat-7 image classification and time-series analysis based on observed 0.52-0.60 μm reflectance peaks following slump formation. We identified 1067 and 1705 lakes with active features between CE 2000-2012 in the Noatak and North Slope ecoregions, respectively. The density of features was higher in the highly flammable Noatak (0.04 versus 0.01 features km^-2, respectively), suggesting that warmer climate and/or fires likely promote high thermo-erosional activity at present. To assess modern signals of thermo-erosion and identify past events, we analyzed soil profiles and lake-sediment cores from both ecoregions using X-ray fluorescence. The ratios of Ca:K and Ca:Sr increased with depth in permafrost soils, were higher in soils from younger versus older slump surfaces, and were significantly correlated with the ratio of carbonate to feldspar and clay minerals in lake sediments (r=0.96 and 0.93, P<0.0001, n=15). We interpret past increases in Ca:K, Ca:Sr, and δ¹³C as enhanced weathering of carbonate-rich permafrost soils associated with thermo-erosion. At the North Slope site, we identified ten episodes of thermoerosion over the past 6000 years and found strong correspondence to summer temperature trends. Events were more frequent at the Noatak site, where 15 thermo-erosional episodes and 26 fires occurred over the past 4000 years. Superposed epoch analysis shows a significant response (90% CI) of Ca:K to catchment fires within 45-50 years, suggesting that fires promote positive feedbacks that facilitate thermo-erosion. Our results suggest that tundra fire regimes may play a key role in Arctic permafrost dynamics as climate continues to warm.