A 600 years sediment record of heavy metal pollution history in the Danube Delta.

Heavy metal pollution in the Danube Delta (in sediments, water and living organisms) has recently received increasing attention due to its impact on ecosystems health and water quality. However, long term records of heavy metal contamination are not available to date. In this study radiometric dating and geochemical analyses for major elements (Al, Fe, Ca and S) and metals (Cu, Zn, Pb, Ni, Cr and Cd) were performed on the top 4 m of a 9-m sediment core retrieved from the alluvial plain of Sulina distributary channel aiming to reconstruct the heavy metal geological background and contamination history and discuss the possible origins (natural vs. anthropogenic) of metals and the main factors driving their temporal variation. Chronological analysis revealed that the top 4 m of the core span the last ~600 years. Three distinct sediment units (U1: 400-200, U2: 200-140, U3: 140-15) were identified based on the downcore element concentration variation. The lower unit (400-200 cm, ~1450-1700 CE) shows an upward increase of Cu, Zn, Pb, Ni, Cr and Cd metal contents, which are strongly correlated with Al. Enrichment factor (EF) analysis indicates that metals detected in this unit are derived primarily from natural sources. In contrast, metals show elevated EF values within the middle (200-140 cm, ~1700-1770 CE) and upper unit (140-15 cm, ~1770 CE to present). The highest degrees of enrichment of Cu, Zn and Cd occur in the peat layer of the middle unit (U2) which displays higher organic carbon and sulfur contents, indicating that diagenetic enrichment of detected metals occurs under reducing condition. Overall, sediment contamination is moderate while the level increases with time. This study provides new insights into the metal contamination history of deltaic environments and yields baseline values for heavy metal contents in pristine sediments deposited prior to the onset of anthropogenic impact.

Fallout isotope chronology of the near-surface sediment record of Lake Bolatau.

Fallout isotope (210Pbex,137Cs and 241Am) based dating has been carried out on the near-surface sediment core collected from Lake Bolatau-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 Bolatau, including multiple radioisotopes, can serve as a regional benchmark for similar studies.