Before the winter came: The Early Pleistocene pre-glacial history of the Baltics illuminated by facies analysis and cosmogenic nuclide dating of borehole cores from Lithuania (NE Europe)

Understanding the evolution of past ice sheets is essential for comprehending Earth’s climate and improving its prediction. However, information about the oldest Quaternary continental glaciations is scarce due to preservation limitations and methodological challenges in dating. This study focuses on the Lower Pleistocene succession underlying the oldest preserved tills in the Baltic region, NE Europe, providing a unique opportunity to reconstruct environmental conditions prior to the first advance of the Fennoscandian Ice Sheet (FIS) into the area. As the succession occurs mostly in subsurface, we analyzed borehole cores using polished specimens and thin sections to identify depositional processes, and applied a combination of burial 26 Al/ 10 Be and authigenic 10 Be/ 9 Be dating to establish a chronostratigraphic framework. Our results reveal a complex depositional history preceding the first FIS advance. The lowermost Daumantai Formation represents alluvial sedimentation, later disrupted by a mass-wasting event that produced the Kalviai diamicton. Previously misinterpreted as glacial till, the Kalviai diamicton is redefined here as a cohesive debris-flow deposit, likely reflecting increased hydrological instability associated with climatic perturbations during the Middle Pleistocene Transition. Overlying the pre-Elsterian interdiamicton beds indicate renewed alluvial sedimentation within the same basin. Burial 26 Al/ 10 Be dating evaluated using an inverse modeling approach yields a weighted mean age of 998 ± 62 ka for the pre-Elsterian interdiamicton alluvial succession, indicating deposition during the Early Pleistocene and predating the Elsterian glaciation in the Baltic region. In contrast, the authigenic 10 Be/ 9 Be ratios point to a discernible variability across the Pleistocene. Assuming the inverse model burial age, the temporal change of ratios could be estimated by back-calculation from 10.19–11.69 × 10 -9 (Daumantai Formation), decreasing to 5.56–8.35 × 10 -9 (pre-Elsterian interdiamicton beds), and further declining to the present-day range of 4.47–6.45 × 10 -9 . The contrasting performance of burial 26 Al/ 10 Be and authigenic 10 Be/ 9 Be dating highlights the sensitivity of meteoric 10 Be systems to sediment recycling and variable initial ratios in low-accommodation alluvial settings. The documented paleoenvironmental changes provide a significant refinement of the regional Quaternary stratigraphy.