Warm and wet, cool and dry? : molecular paleohydrology based on biomarker δD and δ18O across Central Europe since the Late Glacial

A warmer climate is often described as a wetter climate, as warmer air can hold more moisture, leading to increased precipitation. However, the simple notion of “warm and wet” or “cool and dry” can be misleading, as seasonal changes in precipitation and evaporation are important for ecosystems. In Central Europe, summer droughts are becoming more severe, while heavy precipitation events are increasing simultaneously. This seasonal interplay of hydrological processes strongly affects ecosystems worldwide, and understanding them is essential for assessing future climate change. This thesis applies molecular paleohydrology to investigate the relationship between temperature and hydroclimate using four lake sediment records from Central Europe. Two records cover the Late Glacial–Early Holocene transition (16 until 11 cal. ka BP; Bergsee, Bichlersee), and two span the Mid to Late Holocene (last 7000 years; Moossee, Schliersee). Molecular paleohydrology is applied using compound-specific stable isotope analyses (δ18O and δD) of aquatic and terrestrial hemicellulose sugars and n-alkanes, enabling the application of the dual biomarker and coupled isotope approach to reconstruct past evapo(transpi)ration. The synthesis of all four biomarker records shows that wetter or drier conditions primarily depend on seasonality and hydrological settings. As the biomarker proxies are summer-sensitive, the results indicate that warmer periods were characterized by drier summers and enhanced evapo(transpi)ration in Central Europe. This contrasts with the common assumption that warmer climates are generally more humid. For future climate change, these proxy-based findings suggest more severe summer droughts, alongside an increased risk of extreme precipitation events such as localized convective rainfall and flooding. Molecular paleohydrology using biomarkers and compound-specific δ18O and δD analyses thus provides powerful tools to explore such relationships in detail.