Lost in dating - Problems with the absolute chronologies and sedimentation rates of Late Glacial and Early Holocene oxbow lake deposits in Central Europe

This article presents potential errors related to the construction of absolute chronologies for Late Glacial and Early Holocene sediments accumulated in oxbow lakes in Central Europe and their influence on palaeoecological reconstruction. Our study is based on two sediment cores: (i) from the San River (Stubno-Nakło site in south-eastern Poland; S-N) and (ii) from the Dniester River (Luka site in western Ukraine) valleys. Accelerator mass spectrometry (AMS) 14C dating was performed on terrestrial plant macrofossils from each site to produce Bayesian age-depth models and to calculate sedimentation rates (SRs). The SR in both sites was characterised by high variability reflecting the developmental stage of oxbow lake. The highest SR values (1.18-1.23 cm yr-1) were identified at the S-N site at the beginning of the Holocene (ca. 11,550-11,470 cal. BP) and related to enhanced overbank deposition. The lowest SR values (0.01-0.02 cm yr-1) were identified in the transitional layer between the Younger Dryas and the Preboreal chronozone (ca. 11,810-11,160 cal. BP) in the Luka profile; this may have been related to the sedimentary gap(s) caused by falls in the water table and/or temporary desiccation of the water body. Moreover, our study revealed the presence of three distinctly aged AMS 14C dates in the Younger Dryas section of the Luka profile. These radiocarbon inversions were completely undetected by pollen analysis, which indicated the domination of Pinus and Betula without any significant fluctuations throughout the entire subsection. The presence of such 'cryptic' insertions of older material could lead to erroneous palaeoenvironmental reconstruction. Similarly the S-N site revealed anomalously 'aged' dates which were also difficult to detect by pollen analysis.