Impacts of climate and fire on thermoerosion in the Alaskan tundra over the late Holocene
Anthropogenic climate warming in the Arctic may increase fire activity and enhance permafrost thaw. Thermoerosion is an important pulse disturbance by which carbon stored in frozen soil is rapidly exposed to microbial degradation. These catastrophic events may be facilitated by increases in the permafrost active layer, which may occur after wildfires that warm tundra soils for decades. We examine modern and past thermoerosion in the Noatak River Watershed, a highly flammable tundra ecoregion in Alaska where fire frequency is expected to increase in the future. To help identify geochemical indicators of thermoerosion, we use X-Ray fluorescence to analyze soil profiles of modern thaw slumps of varying ages. Sediment cores were obtained from two lakes, one with and one without thermoerosion along the shoreline, to reconstruct local fire and thermoerosional events in each watershed over the past 4000 years. We use peak analysis to estimate the frequency of past thermoerosional events at each site and compare those records to charcoal-inferred fires in order to assess fire-thermoerosion linkages. Understanding the role of fires in facilitating thermoerosion can help predict feedbacks to climate warming in the Arctic where fire frequency is predicted to increase in the future.
Sediments from younger thaw slumps had higher Ca:K and Ca:Sr values than older features, and these values increased with depth in exposed slump headwalls, suggesting higher carbonate relative to silicate content in deeper soils. At Wolf Track Lake, the site with shoreline thermoerosion, sediments reveal distinct peaks in Ca:K and Ca:Sr through time. Bulk-sediment δ13C values are significantly correlated to Ca:K (R=0.79, p<0.0001, n=59), further suggesting peaks reflect weathering of carbonate-rich soils. Peak analysis reveals 22 thermoerosional and 24 fire events over the past ~4000 years. Three thermoerosional events occurred in the same decade as a fire and four events occurred within 40 years of a fire. The site without modern thermoerosion showed no distinct peaks in Ca:K. However, values were elevated between ~800 and 2000 cal BP, when several fires occurred. This site also burned less frequently in the past (mean return interval = 216 years) compared to Wolf Track Lake (mean return interval = 139 years), suggesting a more active thermoerosional regime at the site that experienced more frequent fires. These results are among the first for reconstructing past thermoerosion, and thus they represent a substantial first step in assessing controls of long-term permafrost dynamics in Arctic systems.