History of development and terrestrialization of Lake Starowlany in the Eemian Interglacial on the basis of Cladocera analysis (Sokółka Hills, NE Poland)

This paper presents development of the paleolake Starowlany existing in the Eemian Interglacial (MIS 5e) in north-eastern Poland. The lake history is based on Cladocera analysis. The paleolake Starowlany is located in the central part of the Sokółka Hills, approximately 9 km north of the town of Sokółka in the North Podlasie Lowland. The sediments studied filled a southern part of a tunnel valley formed during the Warta Glaciation. The coring was carried out with a geological drilling and constituted a part of the framework aimed at preparing the Sokółka sheet for the Detailed Geological Map of Poland in scale 1:50 000. The core was 7 m long. Peaty silt, peats and organic silt were present at the depth of 4.0–6.6 m, under deluvial sands, tills and silt. Organic series was overlain by a clayey silt with no glacial cover. Cladocera analysis was conducted with 20 samples from a depth interval 6.6–4.1 m. Remains of 17 species of Cladocera were found, representing the families: Chydoridae, Sididae and Bosminidae. A vast majority of them were the littoral species. In the paleolake Starowlany four development zones were distinguished that reflected changes as regards temperature, trophic levels and changes of water level in the lake. The paleolake existed from the early until the middle Eemian. The highest water level and the most favorable conditions for zooplankton development occurred in zones SCZ I and II. The highest trophic level took place in SCZ III (middle Eemian). In the late Eemian the water level dropped and the paleolake was transformed into a wetland. At certain points, the higher water level enabled existence of a few Cladocera species. The reasons for disappearance of the paleolake Starowlany included a local decrease in groundwater levels as well as morphometric and geological characteristics of the lake.