The decline of permafrost after the Last Glaciation—Relicts of Thermokarst Lakes in Northern Poland

The presence of permafrost at the foreland of the Last Glacial Maximum (LGM) is evidenced by a range of landforms and structures characteristic of the periglacial zone. In areas once covered by the LGM ice sheet, lake and peatland sediments serve as excellent archives for paleoclimatic reconstruction and environmental change analysis. Current knowledge suggests that the melting of buried dead ice blocks and the formation of deep kettle lakes occurred in tandem with the degradation of permafrost. In the Polish Lowlands, the first phase of lake formation corresponds to the Bølling–Allerød complex (14.7–12.8 ka cal BP). However, our understanding remains limited regarding the geomorphological and hydrological dynamics immediately following ice sheet retreat—particularly in relation to air temperature changes. Until recently, no in situ biogenic sediments had been found to record environmental changes in this highly dynamic postglacial system. The period of more than 1000 years between the ice sheet's disappearance and the formation of deep kettle lakes was occupied by thermokarst lakes—depression landforms caused by permafrost degradation and ground collapse under periglacial conditions. Preliminary studies have mapped over 200 relict thermokarst lakes (RTL) within the LGM extent in Poland. These are characterized by distinctive morphologies, including aligned orientations, marginal ridges, and location on sandy outwash plains (sandurs). The lack of previous studies in the European Lowlands may be due to their subtle landscape expression. The analysis of LiDAR data, combined with field verification, has greatly facilitated their identification and geomorphological interpretation.