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Investigations of interstadials during early stages of glacial periods are of special interest, because they featured large-scale transformations of the climate system and the build-up of land-based ice sheets. Lacustrine sediment sequences are considered to be important natural archives that register past climate and environmental signals. Here, we present new multi-proxy data obtained from a palaeolake succession preserved in the inner-alpine terrace of Unterangerberg, Eastern Alps. These sediments formed during the second Early Würmian Interstadial, equivalent to Marine Isotope Stage (MIS) 5a, and were used to reconstruct changes in lake conditions and to infer past air temperatures. The sediment geochemical data and subfossil aquatic biota provide evidence of a cyclic lake-fen-lake development during this interstadial. The proxy records reveal stable lacustrine conditions with dense charophyte meadows and abundant aquatic fauna during the early part of the interstadial, a progressive shallowing of the lake resulting in the spreading out of fen vegetation in the middle part, and a transition from wetland to a renewed shallow lake stage towards the end of the interstadial. Chironomids were used to reconstruct mean July air temperatures, employing a combined Norwegian-Swiss chironomid temperature inference model. The reconstruction indicates a temperature close to present-day values of ca. 18 °C in the middle part of the record, while temperatures of ca. 13-14 °C are recorded for the lower and upper parts. The proxy data from this palaeolake provide evidence of heat and drought in the middle part of MIS 5a, supported by the chironomid-based temperature reconstruction. Our reconstruction shows a climate pattern broadly similar to that found in pollen-based estimates of mean July air temperatures from sites in the northern Alpine foreland and compares well to other European palaeoclimatic reconstructions of MIS 5a climate.
