Geochemical characteristics of frozen active layer and near-surface permafrost in the Anaktuvuk River Fire scar, Alaska

Abstract:

The Anaktuvuk River Fire (ASF) in 2007, combusted a vast area of tundra on the North Slope underlain by ice-rich permafrost. The tundra fire triggered and enhanced thermokarst resulting in prolonged ground deformation to a varying extent. Estimation of thermokarst development and carbon release upon surface disturbances such as wildfires in permafrost regions is one of the critical challenges to understand future Arctic terrestrial conditions and project the future climate. To understand the spatial distribution of carbon/ice content in the near-surface ground and development/degradation history of the ice-rich permafrost, we analyzed the geochemical characteristics of the near-surface ground (frozen active layer and near-surface permafrost) in the northern area of the ARF scar.

Significantly higher organic carbon content was found in the upper 1-1.5 m profiles, with a continuous or discontinuous distribution of peat in deeper profiles, regardless of conditions of surface disturbance and topography. Below the 1-1.5 m horizon, permafrost became less organic carbon-rich and more ice-rich. Cryostructure of the lower profiles was mostly ataxitic or thick lenticular-layered cryostructure, with silty clay of high carbonate contents. While the ice-rich sediment layer had volumetric ice contents ranging from 60-80 %, horizons with markedly low ice content (around 20-40 %) were often found in the middle to lower horizons of the active layer. The desiccated soil layer was developed by water migration due to cryosuction during two-directional freeze-up in early winter. The overall tendency of decreasing ice content in the upper active layer indicates predominant seasonal thaw settlement in this area happens at the beginning of the thawing season. In general, the lower ataxitic layer showed a larger variation in stable water isotopes than the upper horizons. Spatial variation in the geochemistry of permafrost water masked the difference between intact and disturbed tundra ground. On the other hand, geomorphological factors, such as slope angles and whether the ground is the primary surface or thermokarst basin, seem to play an essential role in determining stable water isotope profiles in the current near-surface frozen ground.