Origin and evolution of the Bezedna lake-mire complex in the Lublin area (East Poland): a case study for permafrost lakes in karstic regions.

The Bezedna site represents a unique lake-mire complex in the Lublin chalkland, eastern Poland. This karst region contains Upper Cretaceous carbonate rocks under the influence of continental climate. Using sedimentologic, palaeobotanical, and fossil ostracode analyses as well as radiocarbon dating on cores, we were able: (1) to reconstruct the morphogenetic mechanisms related to lake formation, (2) to describe the main stages of lake evolution, and (3) to identify the environmental conditions during this evolution. The first stage was the formation of an accumulation basin in a periglacial setting related to permafrost thawing in the last phase of the Weichselian and at the beginning of the Holocene. During the second stage, in the successive phases of the Holocene, lake evolution was determined by climate changes and local conditions, and in the youngest Subatlantic period-also by human activity in several settlement phases. Natural water bodies in a karstic region create unique conditions for water circulation, especially for groundwater. During the change from the periglacial to temperate climate of the Holocene in the Lublin chalkland, the response of substratum (deeply frozen during the Weichselian) was influential in the development of surface water conditions in the study area. The strong fracturing of the Upper Cretaceous bedrock and general change of groundwater circulation associated with the activation of an ascending groundwater supply favored the intensive dissolution of carbonate and the progressive ground subsidence. Based on the reconstruction and correlation with paleoenvironmental investigations of the adjacent Volhynia chalkland, we were able to estimate and compare the influences of local (morphologic and geologic) conditions versus global climate changes on the paleoenvironmental record. Climate-driven lake-level fluctuations coincided over great areas of the Northern Hemisphere and are well reflected in changes of the Ostracoda fauna. [ABSTRACT FROM AUTHOR]