Initially, 15 Russian sampler cores were extracted along four transects in the north, middle and south of the research area. The exact location of the cores was determined by GPS, and their height was read from lidar data. Soil sampler cores were described in the field. Master core M12 was taken using a Cobra TT mechanical drill. The compaction rate within core M12 was calculated based on the penetration depth compiled with the length of the core. Compaction varies with depth, depending on the nature of the deposit, and within the peat itself, depending on the degree of decomposition of the peat and the admixture of mineral matter. Compaction ranged from 4 to 13 cm for each 1 m long core section. Master core M12 was described in detail and sampled for further analyses. Laboratory analyses were also performed on 6 end member samples taken from the beach (3 samples) and dune (3 samples).
The chronology of master core M12 and its twin core was based on combined accelerated mass spectrometry (AMS) 14C and gamma spectrometry 210Pb and 137Cs dating. Fourteen bulk peat samples and a single wood sample were selected for radiocarbon dating to provide a general core chronology and to date particular event layers. In the latter case, the samples were collected from below and above selected sandy layers. The samples were measured in the Poznań Radiocarbon Laboratory (Poland), Beta Analytics Laboratory (USA) and GADAM Centre (Gliwice, Poland). Radiocarbon ages were calibrated with CALIB Rev. 8.1.0 (48) using the IntCal20 calibration dataset (49). The youngest sediments (upper 12 cm) were analyzed for short-lived radionuclides 210Pb and 137Cs using a Canberra BE3830 gamma spectrometer at the Institute of Geology, Adam Mickiewicz University in Poznań, Poland. The sediment accumulation rate was calculated by applying a constant flux sedimentation rate model using the serac package (50).
The combined age-depth model for core M12 was constructed using the available results and applying the Bacon software package (51). The sandy event layers were treated as instantaneous deposits, and the 4 ages were not considered, as they provided significantly older ages than samples taken from sediments below them.
Physical core description, loss-on-ignition and grain-size analyses
Master core M12 was photographed and described in the laboratory in terms of physical appearance, namely, sediment type, texture, color, and the type of boundary between identified sediment types. The loss-on-ignition technique, performed on 185 samples, was used to establish the percentage amount of organic versus mineral matter within the deposits (52).
Sediment grain size from 60 samples from core M12 and 8 end member samples were established using a Morphology G3 optical automated microscope at the Faculty of Geographical and Geological Sciences, Adam Mickiewicz University in Poznań, Poland. Prior to analysis, the samples were burned in a muffle furnace at 550°C for 3 hours to remove organic material. As the amount of siliciclastic material smaller than 63 µm was negligible prior to the analysis on Morphology G3, samples were wet sieved on a 63 µm sieve. The grain-size distribution was plotted with Gradistat 8.0, using the logarithmic method of moments (53).
Selected sections of master core M12 (8 thin sections), comprising sandy layers, were subjected to micromorphological analysis. The deposits extracted from the coring tube were dried and impregnated using epoxy resin. After impregnation, the samples were cut into 8 thin sections. Slides were scanned on a high-resolution scanner to produce images of all samples and analyzed under an optical/petrographic microscope under plane light conditions. The description protocol followed Leszczyńska et al. (54) and the terminology developed by pedologists (55). Particular attention was given to depositional and erosional structures typical for flood deposits (56).
A handheld X-ray Tracer III ED-XRF (Bruker AXS, USA) spectrometer was used to assess the concentrations of 9 selected major and 7 selected trace elements in 109 samples from master core M12 and 6 end member samples. The spectrometer was operated in the quantitative mode described by the manufacturer. Two calibration standards were used: Bruker Mudrock Major for analysis of Ca, Ti, S, Si, Na, K, Cr, Zn and Mg and Bruker Mudrock Trace for determination of Mn, Sr, Fe, Zr, Rb, Ni, and As as described by Kozak et al. (57).
Micropaleontological analysis for the presence/absence and the type of diatoms was conducted for selected sandy event layers on 9 samples; in one case, the organic deposits below and above the minerogenic layer were also analyzed. Preparation of collected material for diatom analysis was conducted according to standard procedures. Diatom valves were identified to the lowest possible taxonomic level with reference to (58).
Heavy minerals analysis
The heavy minerals were analyzed from 4 samples from the sand layers within master core M12 and 2 end member samples (from the beach and from the dune). Samples were dried and sieved, and minerals from fractions of 125–250 µm were separated using sodium polytungstate liquid of density <2.85 g/cm3. Microscopic slides were prepared using Canada Balsam and analyzed under a petrographic microscope with cross-polarized optics.
Factor analysis was undertaken on 29 samples from core M12 and 6 end member samples (3 beach and 3 dune samples). The calculations were made in R software (59)using 2 factors: Bartlett’s score and rotation varimax and autoplot function. The aim was to establish the similarities between the discrete samples.