Comparing Varve Counting And ¹⁴C-Ams Chronologies In The Sediments Of Lake Żabińskie, Northeastern Poland: Implications For Accurate ¹⁴C Dating Of Lake Sediments

Abstract

Varved lake sediments from Lake Żabińskie (northeastern Poland) provide a highresolution calendar-year chronology which allows validation of 14 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 +12 / -24 varves. The varve chronology was validated with the 137 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 14 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 14 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.

Determining the responses of vegetation to natural processes and human impacts in north-eastern Poland during the last millennium: combined pollen, geochemical and historical data

“…Down to 900 varve years, the varve chronology is supported by 32 AMS 14 C dates distributed along the sediment profile (Bonk et al 2015b; Fig. 2).…”

“…The composite sediment profile was obtained by stratigraphic correlation based on macro-and microscopic comparison of wellpreserved laminations and diagnostic horizons (Bonk et al 2015b). Half A of the composite sediment profile was subsampled continuously for thin sections while half B of the composite sediment profile was subsampled at 3-year resolution according to varve boundaries for discrete multiproxy analysis.…”

“…137 Cs activity peaks and Fig. 2 Chronology of the sediment core of Lake _ Zabińskie (Bonk et al 2015b). The 14 C-based age model (grey-shaded band passing through the probability distributions of the calibrated 14 C dates) is compared with the varve chronology represented by the dashed line.…”

Review and test of reproducibility of subdecadal resolution palaeoenvironmental reconstructions from microfossil assemblages

“…Varved sediments can capture the seasonal succession of diatom communities (Bonk et al, 2015b;Simola, 1977), although seasonal mixing can transport more diatoms to the centre of the lake (Raubitschek et al, 1999). This suggests that the largely planktic diatom record from the varved Baldeggersee (Lotter, 1998), marked by abrupt changes in the assemblages, might be an example of the more promising case of direct deposition of much of the assemblage.…”

Tracking climate signals in varved lake sediments: research strategy and key sites for comprehensive process studies in the Masurian Lakeland

“…A second major creek flows from the south and forms a small delta situated close to the deepest part of the lake. According to Bonk et al (2015b) it transports significant amounts of sediment particles and organic remains, especially after spring snowmelt and heavy rains in the summer. In contrast, inflow that enters the lake in the southeastern part is episodic and occurs mostly after heavy rainfalls.…”