Chironomidae as Indicators of (Paleo-) Temperature and Environmental Change at Sites of Pleistocene and Holocene Human Habitats

This thesis presents paleoclimatological and -ecological case studies spanning three periods of the Late Quaternary based on subfossil chironomidae analysis. The aquatic ecosystems presented are characterized by shallow-lake chironomid communities which react sensitive to climatic and environmental fluctuations ca. 300, 132 and 1.1 k years ago. The first part of this thesis deals with chironomid-based July mean air temperature reconstructions from the Mid- and Late Pleistocene. The study sites are located at the archaeological sites Schöningen (52°8'N; 11°0'E, 300 ka BP) and Lichtenberg (52°55′ N, 11°15′ E, ca. 132 k years BP), both situated in Lower Saxony, Northern Germany. Both sites represent outstanding evidence of human activities. Schöningen is famous for the discovery of the oldest wooden hunting spears associated with Homo heidelbergensis. Lichtenberg provides the first specific evidence of the presence of Neanderthals north of the Central Uplands of Germany during glacials and interglacials. The second part of this thesis presents two studies dealing with the development of paleolake Gayal el Bazal (Yemen), one of the few freshwater ecosystems in the arid region, which is characterised by strong anthropogenic interventions. This thesis highlights the value of chironomid records in understanding past interglacial-glacial climatic and environmental variability, as well as hydrological fluctuations and anthropogenic influence, which is critical for predicting future trends of the ongoing climate change. The Middle Pleistocene Reinsdorf sequence from Schöningen 13 II corresponds to Marine Isotope Stage 9e-a and includes phases of interglacial forest successions followed by alternating woodland and steppe-phases. During cool steppe (woodland) phases, higher runoff created lake habitats with diverse chironomid assemblages characterized by up to 27 morphotypes. In contrast, warmer forest phases saw reduced chironomid presence due to dry conditions caused by a more extensive vegetation cover resulting in lower runoff. Transitional woodland-steppe phases are characterized by higher abundances of bottom-dwelling chironomid taxa, indicating oligo-mesotrophic aquatic conditions. Steppe phases featured littoral chironomid taxa, tolerant of increased productivity. Using two modern chironomid-temperature calibration training sets (TS; Swiss-Norwegian and Swiss-Norwegian-Polish), summer air temperatures during the Reinsdorf sequence were reconstructed: the reconstructed July mean temperatures for the entire sequence ranged between 16.5–22 °C. Warmer and wetter transitional zones show temperatures similar to or slightly below modern levels (-0.5 to +0.2 °C), while drier steppe phases were about 1 °C warmer, likely due to increased continentality as a consequence of low sea levels and an increase in land masses. The Swiss-Norwegian-Polish TS provided more reliable results due to its broader temperature range and the presence of relevant taxa (e.g., Propsilocerus lacustris-type). In sections with low taxon richness and nutrient-driven dominance of P. lacustris-type, summer temperatures may have been overestimated. The new chironomid record from the archaeological site Lichtenberg covers the Saalian Late glacial to the Early Weichselian. The application of the Swiss-Norwegian-Polish TS to reconstruct summer temperatures for MIS 5e–5b. The Eemian was dominated by thermophilic chironomid taxa (e.g., Cladopelma and Glyptotendipes pallens/severini-type), indicating a productive, littoral environment. Reconstructed July air temperatures during this period were up to 1.8 °C warmer than today. The early Weichselian was characterized by a high variability in July air temperatures (12–20 °C). Chironomid assemblages alternated between cold-stenothermal taxa (e.g., Tanytarsus lugens-type, Sergentia coracina-type, and Micropsectra radialis-type), associated with low-nutrient conditions, and thermophilic taxa typically found during the Eemian interglacial. Eemian summers were on average 19 °C warm, which is in the upper range of published Eemian temperature records (16–20 °C). In comparison to other chironomid-based temperature reconstructions from the same time period, summer temperatures from the Lichtenberg site are around 5 °C warmer, due to the use of a different modern training set which has a wider temperature range and includes more temperate, lowland lakes. This highlights the need of independent, regional data sets, which have a higher similarity to the fossil assemblages. The studies from paleolake Gayal el Bazal provide insights into hydrological fluctuations on the Arabian Peninsula, which are mainly controlled by the Indian Summer Monsoon. The first paper examines how the ecosystem of the paleolake responded to climatic and hydrological changes over the last ~1,200 years, with focus on the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). N-Alkanes correlated well with moisture changes and indicated pluvial times during the MCA with higher nutritional input resulting in increased in-lake productivity. Ostracods were suitable to indicate lake level changes. Chironomids were used to track changes in salinity and in-lake productivity. This work highlights the need for both qualitatively and quantitative paleoclimatic studies in arid regions. The second study presents the first subfossil chironomid record on morphotype level for Yemen. Shifts in chironomid assemblages were associated with hydrological fluctuations based on Redundancy Analysis of both chironomid assemblages and environmental variables (Ti/Al) associated to higher precipitation and runoff. Dry intervals (ca. 1575–1670 CE and 1770–1930 CE) were dominated by Dicrotendipes nervosus-type and Polypedilum nubeculosum-type indicating a shallow, productive environment with high macrophyte cover. A rise in Tanytarsus cf. formosanus-type Bazal indicates rising lake levels and higher runoff between 1670–1770 CE and since 1930 CE. While precipitation data from the region show an annual decrease in precipitation, early summer (May–June) and autumn (September–November) precipitation increased from time periods 1961–1990 and 1991–2020 CE. This suggests a seasonal wetness increase rather than an overall trend, likely linked to a strengthening of the Indian Summer Monsoon (ISM), driven by anthropogenic warming. The lake dried up due to anthropogenic groundwater abstraction, which led to a rise in Ceratopogonidae Dasyhelea-type during the last three decades, indicating an expansion of semi-terrestrial habitats. This study suggests a rather dry Little Ice Age and a seasonal wettening during the last decades, which is masked by anthropogenic activities.