Geomorphological and geochemistry changes in permafrost after the 2002 tundra wildfire in Kougarok, Seward Peninsula, Alaska

Abstract

Geomorphological and thermohydrological changes to tundra, caused by a wildfire in 2002 on the central Seward Peninsula of Alaska, were investigated as a case study for understanding the response from ice‐rich permafrost terrain to surface disturbance. Frozen and unfrozen soil samples were collected at burned and unburned areas, and then water isotope geochemistry and cryostratigraphy of the active layer and near‐surface permafrost were analyzed to investigate past hydrological and freeze/thaw conditions and how this information could be recorded within the permafrost. The development of thermokarst subsidence due to ice wedge melting after the fire was clear from analyses of historical submeter‐resolution remote sensing imagery, long‐term monitoring of thermohydrological conditions within the active layer, in situ surveys of microrelief, and geochemical signals recorded in the near‐surface permafrost. The resulting polygonal relief coincided with depression lines along an underground ice wedge network, and cumulative subsidence to 2013 was estimated as at least 10.1 to 12.1 cm (0.9–1.1 cm/year 11 year average). Profiles of water geochemistry in the ground indicated mixing or replenishment of older permafrost water with newer meteoric water, as a consequence of the increase in active layer thickness due to wildfire or past thaw event. Our geocryological analysis of cores suggests that permafrost could be used to reconstruct the permafrost degradation history for the study site. Distinct hydrogen and oxygen isotopic compositions above the Global Meteoric Water Line were found for water from these sites where permafrost degradation with geomorphological change and prolonged surface inundation were suggested.

Tundra fire alters vegetation patterns more than the resultant thermokarst

“…1). The field investigations confirmed that the landscape was broadly homogenous in any slope direction (Iwahana et al 2016) if fires did not occur for long term (Narita et al 2015).…”

“…These reports suggest that fire affects vegetation structures at various spatio-temporal scales. Satellite imagery confirms that thermokarsts occur frequently in the burned areas of Kougarok where polygonal networks of high-centered polygon derived by melting ice wedges are well developed (Iwahana et al 2016). However, no subsidence was observed in adjacent unburned sites.…”

“…In an area close to our study site, the fire intensity was moderate to severe and consumed 50% of organic layer (Liljedahl et al 2007). Thermokarsts were observed firstly on the burned area in 2006 (Iwahana et al 2016). The soil profiles had peat sediments or mixed layers of peat and silty mineral soils.…”

“…Fire removes plants from the ground surface, and creates low albedo that increases the active layer (Beringer et al 2001, Tsuyuzaki et al 2009. Ground subsidence occurred in a polygonal network on thawing after the 2002 tundra fire at Kougarok in the Seward Peninsula (Iwahana et al 2016). Ice wedge degradation occurs over a decadal time scale in both continuous and discontinuous permafrost zones (Tsuyuzaki et al 1999, Jorgenson et al 2006.…”

Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

“…Fire frequency and severity in the Arctic are expected to increase in the future and can have large-scale and long-lasting effects on hydrological and biogeochemical cycling (Flannigan et al, 2005;Hu et al, 2015). For instance, fire can change the landscape locally by enhancing erosion and thermokarst development (Chipman & Hu, 2017;Iwahana et al, 2016;Jones et al, 2015), and can have global impacts by releasing soil carbon that contributes to global climate change (Abbott et al, 2016;Balshi et al, 2007;Schuur et al, 2015). In continuous permafrost settings, these changes are primarily driven by increases in the thickness of the active layer (the soil above the permafrost which thaws and refreezes annually) following fire.…”