Holocene environmental history of Thomas Island, Bunger Oasis, East Antarctica, inferred from a lake sediment record

Over the last decades, climate change led to only moderate changes of the East Antarctic Ice Sheet (EAIS). In recent years, however, modelling approaches and field experiments have shown that also the EAIS is increasingly affected by ice mass losses. In order to obtain information on temporal/spatial ice sheet changes at the margins of the major glaciers in East Antarctica, northern Bunger Hills were visited within the frame of the EASI3 "RV Polarstern" expedition in the period between 19. and 29. February 2024. From a ca 1-km-long lake, here informally named Western Ear Lake (S 66.10621°, E 100.95924°), on Thomas Island, several gravity cores and hammering cores of up to 112-cm-length were recovered in the central part of the ice-covered lake at 14.4 m water depth. Very stiff, greyish to olive sediments sticking to the outer core barrel at sediment depths >110 cm imply that the recovered sediment successions contain the entire environmental history of the lake since the deglaciation of the basin. Greyish and clastic, coarse to fine grained sediments at the base of the sediment succession represent the retreat of the ice sheet after deglaciation of the lake basin. Sediments with fine lamination ranging from submillimeter to centimeter scale characterize the uppermost ~80 cm of the recovered sediment succession. Individual layers show distinct changes in granulometric and geochemical characteristics, particularly with respect to organic matter and calcite contents. The detailed study of these layers will allow a better understanding of lake internal sedimentation processes and related environmental changes. Bulk organic matter from nine horizons throughout the core is used for radiocarbon dating and will set the chronological framework for the reconstructed environmental changes. The radiocarbon ages may support an ice retreat during the early Holocene, as it is reported from geomorphological evidence and glacial deposits from the closer surrounding of the lake. Despite distinct changes in lamination with respect to lamination thickness or internal structures and geochemical composition in the uppermost ~80 cm of the sediment succession, large scale environmental changes that might be related to a marine transgression after ice retreat cannot be observed. The lack of evidence for marine conditions in the basin supports a marine limit several meters below the lake level of 14.8 m asl during the time of visit and/or the sill height of the outflow of the outflow at 16.3 m asl. Moreover, despite a long-term trend of sediments more enriched in organic matter towards the sediment surface, there is no indication for distinct long-term changes in environmental conditions. This may indicate that sedimentation conditions in the lake remained relatively constant after the ice retreat until today and were mainly controlled by small scale changes, such as lake ice coverage, meltwater supply, light and/or nutrient conditions. It also indicates that a major glacial advance of the ice sheet or of outlet glaciers into the lake catchment after the presumed early Holocene ice retreat can be discarded.