Late Holocene Vistula River floods recorded in grain size distributions and diatom assemblages of marine sediments of the Gulf of Gdańsk (Baltic Sea)

Abstract

During the large flood of the Vistula River in 2010, the riverine brackish water surface plume extended up to 70 km into the Gulf of Gdańsk (Baltic Sea), leaving a thin layer of medium-grained sand deposits. It inspired a search for palaeoflood records in marine sediments. Thus, we aimed to identify the most useful flood indicators and apply them to reveal palaeoflood records in sediment cores from the Gulf of Gdańsk. The study is based on analyses of surface samples, collected during and one year after the 2010 flood, and two long sediment cores, which were subjected to high-resolution grain size, diatom, and geochemical analyses, while chronology was based on the combined AMS 14C, 210Pb and 137Cs dating. It was found that, in a water depth of less than 30 m, modern large flood deposits were not preserved after a year. Sediment cores retrieved from greater water depths (over 60 m) were composed of sandy mud, and most of the 1 cm thick sediment samples were characterized by unimodal grain size distribution. However, some of the samples were bimodal, with the additional mode in fine-grained fractions, which is interpreted to be the result of direct deposition from riverine flood surface water plume. The diatom assemblages revealed a moderate downcore variability, except for the intervals characterized by bimodal grain size distributions. They contained elevated amounts of benthic oligohalobous (freshwater) and decreased euhalobous and mesohalobous taxa, supporting the likely interpretation of these layers as deposited during river flood events. During the last c. 4 ka, a dozen major flood events were identified. However, their application to flood climate reconstruction is challenging because of relatively frequent and partly unknown changes in major river mouth positions in the past. We suggest that thin deposits of major floods left on the seafloor and subjected to further mixing maybe still recognized using a combination of high-resolution grain size distribution and diatom analyses supplemented by a good understanding of the depositional system history.