Summer temperatures from the Middle Pleistocene site Schöningen 13 II, northern Germany, determined from subfossil chironomid assemblages

We present the first climatic and environmental reconstruction based on subfossil chironomid head capsules from the Middle Pleistocene Reinsdorf sequence, Schöningen, northern Germany, corresponding to Marine Isotope Stage 9e-a. The sequence is characterized by interglacial forest successions followed by alternating woodland and steppe phases. Higher levels of runoff formed lacustrine habitats during post-interglacial, cool steppe (woodland) phases. These were characterized by diverse chironomid assemblages with up to 27 chironomid morphotypes occurring simultaneously. Warmer forest phases were mostly void of chironomids when the site Schöningen 13 II fell dry owing to higher vegetational coverage and therefore lower runoff. Transitional periods between woodland and steppe phases show higher abundances of profundal, bottom-dwelling chironomid taxa, suggesting oligo-mesotrophic aquatic conditions, while steppe phases are dominated by shallow-lake taxa with higher tolerance to increasing productivity. We applied temperature inference models to the chironomid assemblages based on a Swiss–Norwegian and a Swiss–Norwegian–Polish chironomid–temperature calibration data set to reconstruct mean July air temperatures for the Reinsdorf sequence. The Swiss–Norwegian–Polish training set (TS) seems better suited owing to a longer temperature gradient and the presence of the dominant taxon, Propsilocerus lacustris-type, which is missing from the Swiss–Norwegian TS. In sections of the record with low taxon richness (Shannon index <2) and a dominance of P. lacustris-type, indicating increased nutrient impact, summer temperatures may have been overestimated by the Swiss–Norwegian–Polish TS. In the other sections, the chironomid-based reconstructions based on the Swiss–Norwegian–Polish TS were in line with ostracod and plant remains-based temperature reconstructions, suggesting summer temperatures of the post-interglacial Reinsdorf oscillations between 16.5 and 22 °C. Our results show that summer air temperatures were lower during warmer, wetter transitional zones (−0.5–0.2 °C colder/warmer than today) and increasing during cooler, dry steppe phases (1 °C warmer than today), most likely caused by higher continentality.