Tracking the Younger Holsteinian Oscillation in laminated lake sediments of Holsteinian (Mazovian) interglacial (MIS11c). Cladocera evidence from Ossówka (E Poland)

The Holsteinian interglacial (terrestrial equivalent of MIS 11c in central Europe) is often viewed as a long (~15–16 ka), warm and humid interval formed under an orbital configuration comparable to the Holocene, with global temperatures ~1.5–2°C above pre-industrial levels (Koutsodendris et al., 2012; Kühl & Litt, 2007). High-resolution varved records show, however, that it was punctuated by two major oscillations: the Older (OHO) and Younger Holsteinian Oscillation (YHO) (Koutsodendris et al., 2012). Here, we assess lake ecosystem responses to the YHO using an exceptionally well-preserved Cladocera record.A 54 m sediment core from Ossówka (eastern Poland) preserves the Holsteinian interval in partially laminated, carbonate-rich lacustrine deposits. The interglacial and the position of the oscillations were constrained by high-resolution pollen analysis (1–2.5 cm), providing the stratigraphic framework for proxy interpretation. Although Cladocera analysis is widely used to infer trophic state, water depth, and habitat structure, it is rarely applied to sediments older than MIS 3 due to poor preservation of chitinous remains. At Ossówka, preservation is outstanding, including delicate planktonic Daphnia remains. We identified in total 29 cladocera taxa; species richness ranges from 8 to 16 and total abundance from 1,700 to 8,600 specimens/cm⁻³.The high-quality material enables a high-resolution reconstruction of ecosystem change across the YHO based on taxonomic composition, complemented by ephippia production as an indicator of ecological stress. Continuous Daphnia spp. occurrence also permits morphometric measurements of the postabdominal claw as a body-size metric potentially linked to temperature variability.This research has been supported by the Narodowe Centrum Nauki (National Science Center) (grant. no. 2019/34/E/ST10/00275). Koutsodendris, A., Lotter, A.F., Kirilova, E., Verhagen, F.T.M., Brauer, A. and Pross, J. (2013), Evolution of a 12-ka-long Holsteinian (MIS 11c) palaeolake. Boreas, 42: 714-728. https://doi.org/10.1111/bor.12