Seasonal subsurface thaw dynamics of an aufeis feature inferred from geophysical methods

Aufeis are sheets of ice unique to cold regions that originate from repeated flooding and freezing events during the winter. They have hydrological importance associated with summer flows and possibly winter insulation, but little is known about the seasonal dynamics of the unfrozen sediment layer beneath them. This layer may support perennial groundwater flow in regions with otherwise continuous permafrost. For this study, ground penetrating radar (GPR) were collected in September 2016 (maximum thaw) and April 2017 (maximum frozen) at the Kuparuk aufeis field on the North Slope of Alaska. Supporting surface nuclear magnetic resonance (NMR) data were collected during the maximum frozen campaign. These point-in-time geophysical data sets were augmented by continuous subsurface temperature data and periodic Structure-from-Motion digital elevation models (DEM) collected seasonally. GPR and difference DEM data showed maximum ice thicknesses of up to 6 m over the sediment surface. Below the ice, GPR and NMR identified regions of permafrost and regions of seasonally frozen sediment (i.e., the active layer) underlain by a substantial perennially unfrozen zone or “lateral talik” that ranged from 0 m to over 13 m thick. The seasonally frozen cobble layer above the talik was typically 3 to 5 m thick, with freezing apparently enabled by relatively high thermal diffusivity of the overlying ice and rock cobbles. The large talik beneath the aufeis and active layer suggests that year-round groundwater flow and coupled heat transport occurs beneath much of the feature. Highly permeable alluvial material and discrete zones of apparent groundwater upwelling indicated by geophysical and ground temperature data allows direct connection between the aufeis and the talik below.