A new composite core from the Ossówka palaeolake (eastern Poland): sedimentary facies and XRF-based stratigraphy across MIS 12–11c

A new composite sediment core (OSS 21) was recovered in summer 2021 from the Ossówka palaeolake sediment succession (eastern Poland), close to earlier key drill sites. Three parallel boreholes with overlapping segments were correlated to establish a 53.98 m long composite sequence. This study focuses on the interval between 34.30 and 53.98 m, which includes the transition from the Elsterian Glacial (MIS 12) into the Holsteinian Interglacial (MIS 11c). Core handling included splitting, detailed macroscopic logging, targeted thin-section microfacies analysis, and high-resolution XRF core scanning combined with hierarchical cluster analysis and principal component analysis to summarize compositional variability and support facies interpretation.Five sedimentological units (I–V) are distinguished and broadly corroborated by the XRF-based clustering. Basal unit I comprises structureless sandy–clayey material with scattered coarse sand and gravel, interpreted as Elsterian till. Unit II consists of laminated clayey silt with frequent fine sand layers and retains a siliciclastic geochemical signature (high Si–K–Ti; low Ca and organic-related signals). A major shift occurs at the onset of unit III, where carbonate mud with reduced siliciclastic input and increased endogenic components appears. Unit III is subdivided into IIIa (faint cm-scale lamination) and IIIb (more distinct sub-mm lamination) with pronounced sulfur variability and mixed “organic” cluster dominance.Unit IV contains distinctly varved Holsteinian carbonate mud with near-continuous sub-mm light–dark couplets (average couplet thickness ~0.75 mm). Thin sections show light laminae dominated by micritic calcite and darker laminae enriched in organic matter and clay. Geochemically, carbonate associated elements Ca and Sr gradually increase through unit IV, which is consistent with CaCO₃ concentrations that increase from ~50% to ~65–70% while TOC decreases about ~2–3%. Unit V continues as laminated carbonate mud with generally weaker varve expression and further carbonate enrichment (CaCO₃ up to ~81% in the analyzed part). Together, OSS 21 provides a refined sedimentological and geochemical framework for future high-resolution palaeoenvironmental reconstructions of the Holsteinian, including potential investigation of intra-interglacial variability.This research is funded as part of the NCN project “Novel multi-proxy approaches for synchronization of European palaeoclimate records from the Holstein Interglacial”, funded by the Polish National Science Centre through grant no. 2019/34/E/ST10/00275.