Comparing Varve Counting and C-ams Chronologies in the Sediments of Lake Żabińskie, Northeastern Poland: Implications for Accurate C Dating of Lake Sediments

Varved lake sediments from Lake Żabińskie (northeastern Poland) provide a highresolution calendar-year chronology which allows validation of C dating results. Microscopic analysis of the varve microfacies revealed that laminations found in Lake Żabińskie were biogenic (calcite) varves. Three independent counts indicated a good preservation quality of laminae in the 348 cm long sediment profile which contained 1000/-24 varves. The varve chronology was validated with the Cs activity peaks, the tephra horizon from the Askja eruption at AD 1875 and with the timing of major land-use changes of known age inferred from pollen analysis. 32 AMS C dates of terrestrial macrofossils distributed along the profile were compared with the varve chronology. After identification of outliers, the free-shape model performed with 21 C dates provided the best possible fit with the varve chronology. We observed almost ideal consistency between both chronologies from the present until AD 1250 while in the lower part (AD 1000–1250) the difference increases to ca. 25 years. We demonstrate that this offset can be explained by too old radiocarbon ages of plant remains transported to the lake by the inflowing creek. Results of this study highlight that careful interpretation of radiocarbon age-depth models is necessary, especially in lakes where no annual laminations are observed and no independent method are used for cross-validation.