Abstract
Abstract. Permafrost underlies one-quarter of the Northern Hemisphere but is at increasing risk of thaw from climate warming. Recent studies across the Arctic have identified areas of rapid permafrost degradation from both top-down and lateral thaw. Of particular concern is thawing syngenetic “yedoma” permafrost which is ice-rich and has a high carbon content. This type of permafrost is common in the region around Fairbanks, Alaska, and across central Alaska expanding westward to the Seward Peninsula. A major knowledge gap is relating belowground measurements of seasonal thaw, permafrost characteristics, and residual thaw layer development with aboveground ecotype properties and thermokarst expansion that can readily quantify vegetation cover and track surface elevation changes over time. This study was conducted from 2013 to 2020 along four 400 to 500 m long transects near Fairbanks, Alaska. Repeat active layer depths, near-surface permafrost temperature measurements, electrical resistivity tomography (ERT), deep (> 5 m) boreholes, and repeat airborne light detection and ranging (lidar) were used to measure top-down permafrost thaw and map thermokarst development at the sites. Our study confirms previous work using ERT to map surface thawed zones; however, our deep boreholes confirm the boundaries between frozen and thawed zones that are needed to model top-down, lateral, and bottom-up thaw. At disturbed sites seasonal thaw increased up to 25 % between mid-August and early October and suggests measurements to evaluate active layer depth must be made as late in the fall season as possible because the projected increase in the summer season of just a few weeks could lead to significant additional thaw. At our sites, tussock tundra and spruce forest are associated with the lowest mean annual near-surface permafrost temperatures while mixed-forest ecotypes are the warmest and exhibit the highest degree of recent temperature warming and thaw degradation. Thermokarst features, residual thaw layers, and taliks have been identified at all sites. Our measurements, when combined with longer-term records from yedoma across the 500 000 km2 area of central Alaska, show widespread near-surface permafrost thaw since 2010. Projecting our thaw depth increases, by ecotype, across the yedoma domain, we calculate a first-order estimate that 0.44 Pg of organic carbon in permafrost soil has thawed over the past 7 years, which, for perspective, is an amount of carbon nearly equal to the yearly CO2 emissions of Australia. Since the yedoma permafrost and the variety of ecotypes at our sites represent much of the Arctic and subarctic land cover, this study shows remote sensing measurements, top-down and bottom-up thermal modeling, and ground-based surveys can be used predictively to identify areas of the highest risk for permafrost thaw from projected future climate warming.
Highlights
Permafrost underlies ∼ 40 % of central Alaska, a 500 000 km2 region stretching east to west from the Canadian border to the Seward Peninsula and north to south from the Brooks Range to the Alaska Range
Patterned ground is readily evident in the airborne lidar in the mixed forest along the first ∼ 150 m of the transect (Fig. 2b). This area is characterized by high-centered polygons with up to 2 m of local relief that form when ice wedges melt
The majority of the warming and thaw degradation are occurring in mixed-forest ecotypes with low-ice-content sandy-silt soils; remote sensing evidence shows thermokarst features, including bog initiation, pits, troughs, sinkholes, irregular depressions and mounds, and moats, have been initiated in all of the ecotypes represented
Summary
Permafrost underlies ∼ 40 % of central Alaska, a 500 000 km region stretching east to west from the Canadian border to the Seward Peninsula and north to south from the Brooks Range to the Alaska Range. This is expected to mostly disappear from the near surface (upper 1 m) by 2100 (Pastick et al, 2015). Yedoma contains almost a third of the permafrost carbon pool despite underlying only 625 000 km of central Alaska and Russia, ∼ 7 % of the total global permafrost land area (Heslop et al, 2019). Yedoma permafrost contains large organic carbon stocks that are extremely biolabile (Vonk et al, 2013; Strauss et al, 2017; Heslop et al, 2019) and highly vulnerable to thaw due to high ice content and the prevalence of massive ice bodies, ice wedges (Strauss et al, 2013, 2017)
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