Fallout isotope chronology of the near-surface sediment record of Lake Bolătău

Publication date: January 2018
Source:Journal of Environmental Radioactivity, Volume 181
Author(s): Árpád Bihari, Máté Karlik, Marcel Mîndrescu, Zoltán Szalai, Ionela Grădinaru, Zoltán Kern
Fallout isotope (²¹⁰Pbex,¹³⁷Cs and ²⁴¹Am) based dating has been carried out on the near-surface sediment core collected from Lake Bolătău-Feredeu (Bukovina, Romania). The motivation was to improve the chronology of this recent section in connection with significant fluctuations observed in sediment accumulation rates, particle size distribution and primordial radioisotope (i.e. ⁴⁰K and ²³²Th) composition. Previously only an extrapolation of a broad-range OxCal age-depth model, which was based on 8 AMS radiocarbon dates from the deeper part of a parallel sediment sequence and tentatively validated for the upper part using the double peaks of the ¹³⁷Cs activity concentration distribution, was available for the studied section (1–24 cm). Parallel to the previous ¹³⁷Cs measurement, ²¹⁰Pb and ²²⁶Ra (for a more detailed, ²¹⁰Pbex-based chronology), ²⁴¹Am (for an additional time-marker), as well as ⁴⁰K and ²³²Th concentrations have also been determined by gamma-spectrometry. In case of the ²¹⁰Pbex-based chronology, due to a large deviation from a pure exponential distribution, the Constant Flux (CF) model has been used for the calculation of sediment ages and accumulation rates. Although the broad-range OxCal and the CF model were broadly similar down to 22 cm, the ²¹⁰Pbex-based ages are clearly superior in terms of uncertainty in the uppermost 12 cm, while the broad-range model has smaller uncertainty below 20 cm (>150 years). The CF model gave an average mass accumulation rate of (0.08 ± 0.03) g cm⁻² yr⁻¹ for sections 0–11 cm, and (0.03 ± 0.01) g cm⁻² yr⁻¹ for sections 12–22 cm, respectively. Significant changes have been observed in the depth distribution of both the particle size distribution and the elemental/isotopic composition of the sediment record, most likely related to the variation observable in the intensity and volume of precipitation in the catchment. The obtained high-resolution records of Lake Bolătău, including multiple radioisotopes, can serve as a regional benchmark for similar studies.

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