CHANGING CARBON DYNAMICS ACROSS A RETROGRESSIVE THAW SLUMP SOIL CHRONOSEQUENCE IN NORTHWESTERN ALASKA

Warming of Arctic permafrost threatens the release of vast stores of carbon (C). On Arctic hillslopes, thermokarst features can grow very rapidly, altering soil structure, temperature (T), moisture and soil organic matter (SOM) distribution, and impacting the rate and nature of C release. To assess changes in soil C dynamics during formation and stabilization of these thermokarst features, we studied a very large, active retrogressive thaw slump located along the Selawik River, NW Alaska. The active slump has rapidly grown since initiation (2004), mobilizing > 0.5M m³ of ice and sediments. We defined a three-stage slump chronosequence composed of: (1) undisturbed peat and tussock tundra overlying thick glacial diamict and loess deposits; (2) the active slump, and; (3) an older, adjacent stabilized slump, re-colonized by black spruce, alder and various grasses. Over two field seasons, the chronosequence was measured for: peak summer soil CO₂ efflux, depth profiles (to 50cm) of soil CO_2, CH₄, and SOM concentrations and δ¹³C, T, bulk density, calculated porosity, water filled pore space (WFPS), and effective diffusivity. In the active slump, soil T were 10-15°C warmer, and profile CO₂ and CH₄ were 3-100X higher than in the undisturbed tundra or stabilized slump. Yet, CO₂ efflux from the active slump was <0.5X efflux from other parts of the chronosequence. Low effective diffusivity, particularly in the upper 20 cm, is attributed to settling of fines from thawed material flowing from the headwall, plugging porosity and dampening CO₂ efflux. Across the chronosequence, values of δ¹³C-CO₂, -CH₄, and -SOM suggest CH₄ and CO₂ are forming under anaerobic conditions, except within the drier 2011 stabilized slump, where δ¹³C-CO₂ profiles are consistent with well-drained, aerobic conditions. Our findings suggest that rapidly-formed, hillslope thermokarst features can profoundly alter soil environmental conditions and biophysical controls on C cycling in warming permafrost soils, and ultimately lead to stable soil states that support different vegetation and higher CO₂ efflux. During active slumping, however, newly exposed mineral soils remain relatively saturated, with low effective diffusivity, dampening C efflux during thermokarst formation, despite permafrost warming.