LINKING PERMAFROST AND ACTIVE LAYER SURFACE PROPERTIES TO THERMOKARST RISK

How do the structure, composition, and physical properties of permafrost and active layer soils affect their susceptibility to rapid thaw and thermokarst subsidence? Spatial variability in the hydraulic and physical properties of active layer soils influences shallow groundwater flow through cold-desert hydrological systems, which in turn determines how heat, water, solutes, and carbon are moved into and out of polar landscapes. We measured the saturated hydraulic conductivity (k), grain-size distribution, and thermal properties(e.g., diffusivity, D) of >50 soil samples from the McMurdo Dry Valleys (MDV), Antarctica, in order to determine what processes drive soil hydraulic conductivity in a simple, mineral soil-dominated natural hillslope laboratory, and how soil hydrological properties compare to soil thermal properties and landscape-scale processes such as thermokarst erosion (measured via ground-based and airborne imaging and LiDAR).

We find that both the hydraulic conductivity and the grain size distribution of soils are organized longitudinally along MDV valley axes. Soils down-valley near the coast have a higher percentage of fine-sized sediments (fine sand, silt, clay) and lower hydraulic conductivities than soils collected up-valley. Soils collected mid-valley have intermediate amounts of fines and conductivity values consistent with a hydrogeologic gradient spanning the valley from high inland to low near the coast. These results suggest that the organization of modern soil properties in Taylor Valley is a relict signature from past glaciations that have deposited tills of decreasing age towards the mouth of the valley, modified by fluvial activity acting over microclimate and geological gradients. Importantly, we show that water-holding soils show a strong, positive feedback between water content and thermal diffusivity, meaning that where low-k, high-D soil properties overlap in terrestrial Antarctic settings, rapid thawing and disruption of ground ice can occur as meltwater advects heat into permafrost-affected soils and accelerates conductive thaw.