Contemporary formation of ice-wedge pseudomorphs during the expansion of a thermokarst lake in Old Crow Flats, Yukon, Canada

Abstract

Ice-wedge pseudomorphs (IWPs) are thermokarst structures that form by the secondary infilling of cavities left by the slow melting of ice wedges. Contemporary IWP formation in periglacial environments informs our understanding of past processes and dynamics implied by their presence in the stratigraphic record. However, contemporary IWPs are seldom studied as they are difficult to locate and observe due to the general lack of surface expression and because they can deform with time or be confused with sediment wedges. This study presents detailed information about the morphology and soil composition of IWPs that formed beneath a thermokarst lake in Old Crow Flats, northern Yukon (Canada), and provides the first conceptual model of lacustrine IWP development. Seven IWPs were unearthed along a transect perpendicular to a former eroding lakeshore in a recently (2019), partially drained lake basin. The IWPs were between 76 cm and 122 cm high and had varying morphologies, generally forming triangles pointing down or T shapes. They had a much higher organic matter content and were composed of slightly coarser sediment than their host sediment, characteristics that were shared by basin floor surface sediment found in the near-shore zone (<15 m) of the former lake. Within five meters of the former shoreline, some troughs showed pronounced subsidence due to the melting of remnant wedge ice following lake drainage. Based on ice-wedge dimensions, bank height, shore erosion rates, and field measurements of sublake active-layer thickness prior to lake drainage, it is likely that ice wedges were entirely melted by the development of the sublake active layer, prior to talik development. Together, the results indicate that the IWPs formed within 5 to 10 m of the eroding shore bank, 9 to 18 years after lake submergence, and were filled with the organic-rich lacustrine sediment that accumulated in the near-shore zone due to sediment sorting by wave action following the erosion of peaty polygonal tundra. This study highlights that IWPs can form during the natural evolution of thermokarst lakes in regions with stable permafrost and are not necessarily indicators of conditions leading to generalized permafrost degradation.