GEOPHYSICAL INVESTIGATION OF TALIK STRUCTURE BENEATH AN ACTIVELY EXPANDING THERMOKARST LAKE IN CONTINUOUS PERMAFROST

摘要

Thermokarst lakes occupy depressions caused by the subsidence of thawed, ice-rich permafrost. Such lakes dominate the Arctic Coastal Plain of northern Alaskas landscape. Areas of thawed sediment, called taliks, can form beneath lakes which have a mean annual bottom temperature greater than 0 °C. Measuring degraded permafrost geometries below lakes contributes to understanding interactions between groundwater, surface water, and carbon cycling. Non-invasive geophysical methods are conducive to investigations of talik sediments as the contrast between unfrozen and frozen sediments is an ideal geophysical target and borehole studies are difficult to undertake and result in few data points. We conducted a geophysical transect across an actively expanding thermokarst lake to investigate the associated talik configuration. Over the past sixty years, the expanding lake surface area has been detected; and a talik of previously unknown thickness likely exists below the lake. The transect consisted of fifteen transient electromagnetic (EM) soundings across the lake and extending into the terrestrial environment. Since permafrost has relatively high resistivity compared to the thawed sediments of the talik, the interpreted electrical conductivity structure of the subsurface likely reflects the talik geometry. We also conducted nuclear magnetic resonance soundings at representative locations along the EM transect. These measurements provide information on sub-lake sediment properties including water content. Together, these measurements image the talik structure and show evidence of varying talik thickness from lake edge to center. There is no evidence of talik development at the terrestrial sites. These results can help constrain talik development models and thus provide insight into arctic and permafrost processes in the face of climate change.