Summer temperatures and environmental dynamics during the Middle Würmian (MIS 3) in the Eastern Alps: Multi-proxy records from the Unterangerberg palaeolake, Austria

Several millennial-scale warm phases perturbed the glacial climate during Marine Isotope Stage 3 (MIS 3, ca. 57-29 ka BP). Little is known about the impact of these climatic changes on Alpine ecosystems due to the sparsity of undisturbed sediment records in the Alps and their foreland. In this study, multiple sediment-archived proxies (sediment geochemistry, stable oxygen and carbon isotopes of autochthonous carbonate, and subfossil remains of macrophytes and aquatic invertebrates) were examined in five drill cores from an ancient inner-Alpine lake at Unterangerberg (Eastern Alps) to reconstruct the palaeolake environment and to estimate summer temperature changes for the first half of MIS 3. The lacustrine sedimentation in the basin began ca. 54.6 ka, tentatively correlated with the start of Greenland Interstadial (GI) 14. We identified three distinct phases in the development of the lake. (1) A cold, oligotrophic water body influenced by snow/glacier meltwater ca. 54.6-52.2 cal ka BP. (2) A clear-water, macrophyte-dominated, productive lake ca. 52.2-44.9 cal ka BP. Submerged macrophytes were dominated by the charophyte alga Chara hispida and chironomid assemblages - by Corynocera ambigua , which is absent from the present-day fauna of the Alpine region. (3) A turbid-water, less productive lake ca. 44.9-41.5 cal ka BP. This shift towards a turbid-water state, as evidenced by the drastic reduction in the abundance of submerged macrophytes and associated invertebrates, likely occurred due to increased input of meltwater. The regime shift is tentatively correlated with the start of GI 11, for which the highest temperatures of the studied MIS 3 interval are inferred. Chironomid-based reconstructions of mean July air temperatures provide interstadial temperature estimates between ca. 11 and 12.5°C (i.e. ca. 5-6°C below present-day values), which concurs with reconstructions available from the northern Alpine foreland. Cooler July temperatures (ca. 9-10°C) are reconstructed for MIS 3 stadials. The Unterangerberg lacustrine records provide valuable new insights into MIS 3 climate dynamics inside the Eastern Alps and contribute to a better understanding of the effects of climate change on the Alpine environment.