Lake-level changes in the Late Weichselian Lake Tøvelde, Møn, Denmark: induced by changes in climate and base level

Abstract

The lacustrine Tøvelde basin is a key locality for the study of Late Weichselian sedimentological, geochemical and climatic development in Denmark. It is situated on the south coast of the island of Møn 2.5–7.5 m above present day sea-level. The sediment record covers a time span from about 14,700 to 9000 cal yr BP and starts with resedimented till. Onset of climate amelioration and environmental stability is reflected by onset of lacustrine deposition of increasingly more organic-rich deposits at 14,300 cal yr BP following the deglaciation of the Young Baltic readvance. The lacustrine succession was deposited during overall lake-level rise interrupted by four short-lived falls. During Younger Dryas time, Greenland stadial-1 (event zone GS-1) lake-level rise resulted in the merging of several smaller lakes into one large lake. The early part of the Younger Dryas was characterized by a cold and dry climate and low precipitation, that did not favour the melting of stagnant or active ice. The marked Younger Dryas lake-level rise was probably driven by a rise in base- and ground-water level in the Baltic Basin caused by damming of the Baltic Ice Lake. This was followed by a significant water level rise in the southern Baltic basin during which the Faxe Bugt transgression occurred at around 10,600 14C yr BP. The expanded Younger Dryas Lake Tøvelde came to an end around 10,000 14C yr BP at the transition into the early Preboreal when the Baltic Ice Lake underwent a catastrophic tapping event. A renewed lake level rise took place after 9600 14C yr BP in Preboreal time and continued into the Boreal when a drastic lowering of Lake Tøvelde level took place. The resulting forced regression transformed the large lake into smaller lakes and a marker horizon with macrophyte debris, gastropods, bivalves and small vertebrates was formed by redeposition of lake-margin sediments. The lacustrine deposits of Lake Tøvelde contains abundant Late Glacial freshwater and terrestrial microvertebrates and molluscs. The low water levels in the Lake Tøvelde and in the Baltic Basin were contemporaneous resulting in the formation of land bridges between Sweden, Germany and Denmark, including the island of Bornholm. Such land bridges were established on three occasions allowing immigration of terrestrial microvertebrates to Denmark from the east and partly from the south as reflected in species composition, density and diversity in the Tøvelde succession. Sedimentological, geochemical and palaeoecological data show that Lake Tøvelde underwent significant fluctuations. Some of which can be directly linked to climate changes. Some rises correlate, however, with damming of the Baltic Basin caused by isostatic uplift of barriers. High-resolution stratigraphy may thus allow distinction between lake-level changes caused by climate fluctuations and base-level changes which are ultimately related to isostasy.