Climatic and geological controls on the hydrological response of two neighbouring Eemian palaeolakes (Central Poland): A multi-proxy study

• High-resolution pollen analysis identifies distinct vegetation shifts during the Eemian. • Stable isotope data reveal contrasting hydrological regimes in two neighbouring Eemian lakes. • Sediment texture shows variations in energy and depositional processes over time. • Data suggest a more stable Eemian climate than previously thought. • Palaeoclimate reconstructions reveal new insights into interglacial temperature and precipitation. This study aims to compare two neighbouring Eemian palaeolakes in central Poland—Kozłów (hydrologically open) and Struga (hydrologically closed)—to improve our understanding of palaeohydrological changes during the last interglacial period (MIS 5e). Using a multi-proxy approach, including pollen and plant macrofossil data, stable carbon and oxygen isotopes, and grain size distributions, we assess the impact of local hydrological conditions on sedimentation and climatic signals. Biogenic sequences record vegetation succession and climatic fluctuations throughout the Eemian, with Kozłów preserving a more continuous archive, whereas Struga exhibits hydrological instability affecting depositional patterns. Pollen data enabled the identification and detailed characterisation of regional pollen assemblage zones (RPAZ), facilitating correlations with regional frameworks across Western and Eastern Europe. Isotopic records at Kozłów indicate evaporative enrichment linked to river inflow and intensified evaporation. In contrast, Struga functioned as a closed basin, with prolonged water residence time and isotope values reflecting internal hydrological balance. Grain size results highlight contrasting sedimentation regimes, with Kozłów showing a transition towards coarser material, suggesting growing fluvial influence, while Struga reveals episodic hydrological fluctuations within a closed-lake system. Climate reconstructions using PPPbase indicate pronounced seasonality and shifts in precipitation patterns during the interglacial. Our findings underscore the contrasting hydrological regimes of Kozłów and Struga, illustrating how local geomorphology and water balance shaped lake evolution. The palaeobotanical record further enhances understanding of catchment dynamics. By integrating isotopic and biological proxies, this study provides a refined reconstruction of Eemian climate evolution and a valuable framework for future interglacial research in Central Europe and beyond.