The determination of lake bathymetry and thickness of bottom sediments is extremely important in palaeolimnological research to help locate appropriate coring sites. This can be achieved through the use of GPR sounding (Lin et al., 2009; Sambuelli et al., 2009; Sambuelli and Silvia, 2012). In Poland, winter is a particularly convenient season as the lake is covered with ice, making GPR profiling much easier and improving access and the speed of data collection (Hunter et al., 2003). Measurements along and across the lake were carried out in 2017, directly on the lake ice (Fig. 2). We used the radar system ProEx of the Malå Geoscience (http://www.malagpr.com.au/mala-professional-explorer.html, last access: 23 May​​​​​​​ 2021). A radar pulse was generated at a regular distance interval of 0.02 m (900 samples were recorded from a single pulse). The time window of recording was between 250 and 300 ns. Prospection was done with use of a shielded monostatic antenna with 250 MHz nominal frequency of the electromagnetic wave.

Based on the results of the GPR sounding, four drillings were undertaken a ca. 2 m water depth (Fig. 3) following the Givelet et al. (2004) collecting protocol. A piston sampler was used during drilling, which is very suitable for sampling in moderately cohesive sediments to a depth of 5 m. The sampler set consists of a 200 cm long sonde, which is constructed from a thin-walled, 40 mm diameter, stainless steel tube (https://en.eijkelkamp.com/products/sediment-samplers/Piston-Sampler-set.html, last access: 10 June 2021). Sediment cores were film-wrapped in 1 m plastic tubes and transported to the laboratory. The cores (M1 to M4) were then subjected to magnetic susceptibility measurements which enabled the selection of the core M1, the longest and most continuous, to carry out detailed analysis. The 3.5 m long core M1 (geographic coordinates: 53.82486∘ N, 19.72419∘ E) was sub-sampled at 5 cm intervals and used for multi-proxy laboratory analyses.

The cores from Lake Młynek were subjected to MS measurements using a SM-30 magnetic susceptibility meter (ZH Instruments). Due to very high sensitivity (1 × 10-7 SI units) this device was provided with an 8 kHz linear integrated circuit oscillator, and its pick-up coil sensor was large enough to measure sufficiently high volume of sediments with very low MS. The measurements were made every 5 cm along each core (M1–M4).

Radiocarbon dating was performed on four bulk samples from the core M1, collected either from organic-rich gyttja or gyttja with dispersed organic matter (Table 1). The organic matter seems to have been derived from both aquatic and terrestrial sources. AMS dating was carried out by the Poznań Radiocarbon Laboratory in Poland (for methodology see Goslar et al., 2004). The construction of age–depth models required an assessment of several factors that could disturb constant accumulation of bottom deposits in Lake Młynek, such as those from sedimentary and post-sedimentary processes (including a varied rate of deposition and compaction, and the impact of bioturbation). The varied influx of material delivered to the lake from the adjacent area is a very important factor of disturbance. Therefore, a Bayesian age–depth model was chosen as it takes into account the sedimentation rate and its variability (Blaauw et al., 2007) (Fig. 4). The model was based on default settings, except for section thickness, which was set at 0.05 cm given the length of this core. The Bacon software uses the IntCal3 curve (Reimer et al., 2013) to calibrate the radiocarbon data.

ICP-OES (inductively coupled plasma–optical emission spectrometry) was used for determination of basic (Al, Ca, Mg, Na, K, Fe, P) and trace elements (As, Cd, Mn, Th, Ti, U, V, Zn). Powdered samples were mineralized in a closed microwave Anton Paar Multiwave PRO reaction system. The mineralization procedure was based on the procedure of Lacort and Camarero (2004). Characteristics of lake sediments were determined by the extraction method of elements that are soluble in aqua regia (according to European Standard CEN/TC 308/WG 1/TG 1, slightly modified). Dry samples of about 0.2 g weight were transferred to the PTFE vessel and HNO3, and HCL Merck Tracepur^® was added. The vessels were placed in a rotor and loaded to a microwave. Finally, the samples were analysed in the Spectro Blue ICP-OES spectrometer at the Regional Research Centre for Environment, Agricultural and Innovative Technologies, Pope John II State School of Higher Education, in Biała Podlaska. A Bernd Kraft Spectro Genesis ICAL solution and VHG SM68-1-500 Element Multi Standard 1 in 5 % HNO3 were used.

Total organic carbon (TOC) analysis was done after sample acidification to remove carbonates in the SHIMADZU SSM 5000A analyser with a solid sample combustion unit. The method was the catalytically aided combustion oxidation at 900 ∘C with pre-acidification and oven temperature 200 ∘C. A measuring range TC was 0.1 to 30 mg carbon. The sample amount was 1 g and aqueous content < 0.5 g, with repeatability at SD ±1 % of the full-scale range (http://www.ssi.shimadzu.com/products/toc-analyzers/ssm-5000a, last access: 10 June 2021).

All selected samples were analysed using a scanning electron microscope (SEM) HITACHI TM3000 with an energy dispersive spectrometer (EDS) SWIFT ED 3000 from Oxford Instruments. The samples were not covered with any conductive material. Magnification range was used 20× to 30 000×. This method was used to perform basic microscopic observations of samples of the core M1 with point determination of their chemical composition of major elements.

A georadar transect across the lake reflects both its bathymetry and composition of its bottom (Figs. 2–3). The superficial layer of the transect is represented by lake ice that is ca. 25 cm thick, and although it is almost not visible on radar images due to its thickness being smaller than a vertical resolution of measurements, beneath there are multiple abundant horizontal reflections of energy from the bottom of the ice. Two narrow and vertical zones with small diffraction hyperboles at 23 and 29 m of the transect indicate upward deformation of bottom sediments at the location sites of the sounding core and the core M1 (Fig. 2a). The top of the underlying mineral deposits (so-called hard bottom) is indicated as a distinct downward-deflected reflection surface (Fig. 2b). In the central part of the lake, it occurs at 2.6 m depth (two-way travel time 290 ns) and indicates the top of the Holocene organic sediments. Unfortunately, beneath there is a signal-absorption zone (Fig. 2d), resulting from the fact that most sediments are composed of fine-grained organic material (gyttja). However, the thickness of this layer was determined by drillings to about 5 m. A relief of the lake bottom in the GPR image reflects a cross section of a buried glacial tunnel valley that was eroded mainly in sandy and sandy-gravel deposits. Close to the lake shore (0 to 20 m in the north-west and 110 to 140 m in the south-east), there are numerous oblique and chaotically parallel reflection surfaces dipping towards the channel axis. They reflect bedding of the Pleistocene sandy-gravel series that partly filled a subglacial channel (Fig. 2c).

MS is highly dependent on lithology and grain size of deposits (Dearing, 1994; Sandgren and Snowball, 2001). It reflects the presence and size of ferromagnetic particles in a sample (Verosub and Roberts, 1995). Increased content of ferromagnetic minerals such as magnetite, Fe-Ti oxides or pyrrhotite generates higher MS whereas biotite, pyrite, carbonates and organics result in its lower values. The total volume of magnetic minerals in lake sediments mostly reflects climatic changes in a catchment (Bloemdal and deMenocal, 1989; Snowball, 1993; Peck et al., 1994). MS in the core M1 is varied but due to the organic character of the sediments, its values are relatively low, from 0.002 to 0.034 × 10-7 units SI. In grey-brown gyttja with organic matter at 3.50–2.58 m depth, MS rises and drops in turn from 0.01 to 0.02 × 10-7 SI. MS drops from 2.60 m depth, reaching a minimum at 1.63 m. Higher up, MS rises again, with the highest value at 1.35 m; then there is a minimum at 1.05 m, and the next maximum is at 0.69 m depth (Fig. 6).

The age–depth model of the core M1 from Lake Młynek indicates (Fig. 4) that the M1 core chronologically covers the last 2400 years. Bottom deposits of Młynek Lake are organic-rich. The core M1 is composed of grey-brown gyttja at 1.8–3.6 m depth (Fig. 5). At 1.45–1.80 m depth there is grey-brown gyttja detritus, and at 1.10–1.45 m depth algal gyttja is recorded. The uppermost part of the core is composed of grey-brown (depth 0.4–1.1 m) and detritus gyttja (0.0–0.4 m). The sedimentation rate was calculated based on the age–depth model. Results reflect quite a stable sedimentary environment with a general rate of 1.5 mm yr-1. The rate is stable at 3.40–1.77 m depth and equal to ca. 1.5 mm yr-1. It drops to 1 mm and then rises to 1.3–1.8 mm yr-1 at 1.77–0.30 m. At 0.0–0.3 m the sedimentary rate is the highest and equal to ca. 3 mm yr-1 (Fig. 5).

Based on percentage contents of main trees and terrestrial herbs five local pollen assemblage zones (LPAZ M1-M5) were established in the pollen sequence of the Lake Młynek. The pollen contents were determined based on changes in the percentage of individual taxa, confirmed by a cluster analysis (Table 2, Fig. 9).

Studies of the Lake Młynek bottom sediments revealed the presence of more than 200 diatom taxa belonging to 54 genera (Zalat et al., 2018) (Fig. 8). Diatoms were generally abundant and well to moderately preserved in most samples, although with a mixture of mechanically broken valves, especially in the topmost part of the core. The results of the diatom analysis and relative abundance of the most dominant taxa enabled subdivision of the M1 core section into 11 diatom assemblage zones (Fig. 8) that reflected six phases of lake development (Zalat et al., 2018). Moreover, changes in chrysophyte cysts distributions along with variation in diatom composition could be related to changes in pH, climate and trophic status. Stomatocysts can be used as the index of lake-level changes, habitat availability, metal concentrations and salinity.

Various factors influence distribution and accumulation of geochemical elements in lake sediments. Most important are texture, mineral composition, oxidation/reduction state, absorption/desorption and physical transportation processes (Ma et al., 2016). Curves of representative elements are generally used to characterize sedimentary environments. Most analysed elements do not indicate any clear trend with depth in Lake Młynek. The curves of S and TOC show significant increases at 2.0–1.4 m depth that are slightly correlated with decreased Al, Fe, K, Ca, Mg and MS (Fig. 6). Sulfur content is correlated with the existence of iron sulfides. In the studied core, Fe is positively correlated with Al and Ti (Fig. 7). Fe–Ti oxides are noted in SEM (scanning electron microscopy) EDS (energy dispersive spectroscopy) analysis. They are resistant to surface weathering and carry trace elements (Bauer and Velde, 2014). At ca. 3 m, high-frequency peaks of Al, K, Ca, Na, Mg, Fe and S occur (Fig. 6).

This phase is recorded in LPAZ M1, which represents closed forest communities dominated by hornbeam and alder, which colonized marshlands near lake shores. Plants of open stands are only rarely noted as well as indicators of anthropogenic activity (e.g. Plantago lanceolata). Vegetation at that time was relatively natural and not disturbed. The diatom assemblage at the start of this record (3.45–3.40 m depth) was distinguished by diatom subzone DZ1a (Fig. 8) dominated by the periphytic taxa such as Staurosira construens, Staurosira venter, Staurosirella pinnata, Gyrosigma acuminatum associated with the planktonic Aulacoseira granulata, A. ambigua and Puncticulata radiosa, which indicates a shallow and slightly alkaline lake. This interval was followed by a great abundance of the planktonic Alkaliphilus diatoms of subzone DZ 1b (3.35–3.15 m, Fig. 8) dominated by Aulacoseira granulata, Cyclotella sensu lato species, Cyclostephanos dubius and Stephanodiscus species. The diatom assemblage suggests a rising lake level with increasing nutrients (Douglas and Smol, 1999; Zalat, 2015). The predominance of A. granulata suggests a high trophic status and slightly alkaline freshwater environment with high silica concentration (Kilham et al., 1986; Zalat et al., 2018). Magnetic susceptibility is high and corresponds to high contents of Fe, Ti and Al, indicating an increasing influx of terrigenous material, presumably activated by intensive rainfall.

During this phase changes in the environment around the lake were caused by significant human impact. This phase corresponds with the LPAZ M2, characterized by the reduction and fragmentation of the hornbeam-dominated forest. Birch, pine and hazel expanded under better lighting conditions in a partly open forest. Mid-forest pastures occupied rather small-scale open areas, as can be inferred from higher percentages of Plantago lanceolata and other herbaceous plants, e.g. Gramineae, Artemisia and Rumex acetosa/acetosella. Cultivated plants such as Cannabis and Secale are rare; however, their occurrence is entirely consistent with other human indicators present during this phase. This phase is commonly noted and similarly expressed in numerous palynological sequences in neighbouring areas (see for example Noryśkiewicz, 1982, 1987; Bińka et al., 1991; Ralska-Jasiewiczowa et al., 1998). Pollen data indicate that societies of that time cultivated rye and probably hemp. It is the oldest settlement phase at Janiki Wielkie stronghold and corresponds to the termination of the La Tène and the time of the early Roman period. Human communities in the vicinity of the lake can be connected with settlements of the eastern Baltic Kurgan culture (Rabiega et al., 2017). During this phase, planktonic diatoms were replaced by benthic taxa, (DZ2) such as Staurosira construens, S. venter and Staurosirella pinnata accompanied by a significant abundance of Gyrosigma acuminatum indicating a lower lake level and dominance of a mesotrophic alkaline freshwater environment. The lower stands were interrupted by a short rise in water level at 2.90–2.85 m (ca. 1930–1896 cal BP) where the abundance of planktonic eutrophic indicator Aulacoseira spp. increased suddenly at the expense of the benthic taxa. During this phase climatic conditions were still similar to ones in the previous phase, but it was drier than is reflected by shallowing of the lake. This phase can be correlated with the so-called Roman Climatic Optimum (see McCormick et al., 2012).

This phase corresponds to LPAZ M3 when a forest restoration occurred. Absence of human indicator plants suggest that the settlement in the catchment was abandoned. There are also no traces of human activity nearby (Rabiega et al., 2017). Reduction of human impact and human-generated semi-open habitats allowed for a short-term expansion of birch into empty, open areas, which were later replaced by hornbeam that rebuilt its position to the level it was at in the LPAZ M1. Elm also expanded again in a riparian forest. This restoration of the natural forest was followed by abrupt expansion of beech in the second half of the LPAZ M3. The area of open herbaceous plants communities, previously widespread, was limited.

Abundant planktonic diatoms including Aulacoseira spp. and Puncticulata radiosa and the common occurrence of small Cyclotella spp. occurred in the lake (Fig. 8), which indicates its deepening, enhanced thermal stratification, reduced mixing and increased thermal stability (Zalat et al., 2018). Intensified development of vegetation cover and higher lake levels are indicated by geochemical indices. A gradual drop of MS corresponds with decreased content of detrital elements such us Fe, Ti, Al and K, accompanied by a gradual increase in TOC and the Fe / Ca ratio. Lower MS and Al content (acting as a major constituent of soils) accompanied by the higher TOC suggest limited erosion, in spite of gradually higher precipitation in the lake catchment and therefore a rise in its water level (Fig. 6). The climate in this phase has become more humid. Increased rainfall and decreased evaporation are reflected in lake sedimentation as the lake got deeper, resulting in reduced deposition and greater stability. This phase could be associated with a global cooling of the Bond 1 event (1.5 ka BP) (Bond et al., 1997; Welc, 2019).

This phase is correlated with the LPAZ M4 and is divided into two sub-phases 4a and 4b (Fig. 10). The sub-phase 4a marks the onset of another settlement phase, resulting in forest clearing. Disturbances took place firstly in a beech forest and less in a hornbeam-dominated one. The anthropogenic activity is reflected by presence of Gramineae, Artemisia, Cannabis/Humulus, Plantago lanceolata, Rumex acetosella, Secale and Cerealia undiff. Diatom assemblages suggest a deepening of the lake (Zalat et al., 2018) as indicated by the abundance of Aulacoseira associated with Puncticulata radiosa in the upper part of the diatom zone 5 at 1.85–1.70 m depth (ca. 1070–941 cal BP). The diatom assemblage suggests a rising lake level, higher trophy and stronger turbulent mixing conditions (Rühland et al., 2008; Zalat et al., 2018). Moreover, the greatest reduction of abundant Fragilaria sensu lato accompanied by abundant A. granulata, could be a result of forest clearing around the lake. Higher TOC corresponds with lower content of detrital material (Fe, Ti, Al and K) and lower MS, and it can reflect a progressing humidity (Fig. 6). This phase can be correlated with the Migration Period and the early Middle Ages. A wooden-loamy defence rampart was raised at the end of the phase in a settlement close to the lake (archaeological phase III), after removal of a natural soil developed during abandonment of the site in the early Roman Period. After a short period, this stronghold was destroyed. Charcoal from a fired wall that represents destruction at the end of the archaeological phase IIIA was dated at 1245 ± 25 cal BP (i.e. 682–870 CE; 95.4 % probability) and 1090 ± 30 cal BP (i.e. 892–1014 CE; 95.4 % probability) (Rabiega et al., 2017).

Human impact declines during the subphase 4b (1.70–1.45 m depth, ca. 940–782 cal BP). At this time birch and less intensively poplar occupied temporarily abandoned open areas, especially toward the end of the zone, when human activity was less intensive. Alder became more abundant, probably expanding into exposed marginal areas of the lake. The sub-phase 4b corresponds to the diatom zone 6 (Fig. 8), which is characterized by abundant benthic Fragilaria sensu lato with sporadic occurrence of planktonic taxa. A high proportion of benthic to plankton assemblages was reported as indicative of long-lasting ice cover (Karst-Riddoch et al., 2005). As well as this, a great abundance of the benthic Staurosira venter and Staurosira construens with marked decline in the planktonic diatoms such as Aulacoseira spp. and Puncticulata radiosa reflects lowering water levels and slightly alkaline freshwater, lower nutrient concentrations and low silica content (Kilham et al., 1986; Stevens et al., 2006; Zalat et al., 2018).

In the stronghold at the lake shore, the next phase of human activity took place at the end of the 11th century CE when a new rampart was raised. Wooden constructions were also built, traces of which were excavated in the gate passage. The settlement was finally abandoned, presumably in the first half of the 13th century, and then its ramparts were strongly eroded, with their material moving towards a yard and the moat (Rabiega et al., 2017). The sub-phase 4b is characterized by a gradual warming, which correlates with a gradual shallowing of the lake and increased rate of sedimentation. Human impact on the environment in this sub-phase is already so great that reconstruction of a climate change is not clear. There is no doubt; however, that this is a warm period, which should be correlated with the Medieval Warm Period (MWP) (Mann et al., 2009).

This phase starts about 1200 CE and is connected with the early Modern Period. Cultivation and treatment of hemp has been terminated but cultivation of cereals and presence of synanthropic plants indicate human activity near the lake. The water level changes only slightly and is not high, which is evidenced by a great abundance of benthic diatom taxa over the planktonic forms (DZ 8–10, Fig. 8). There is a drop in TOC and rise in MS caused by increasing input of terrestrial material at 1.4 m depth (ca. 751 cal BP), resulting presumably from human deforestation. The small watercourse which enters the lake from the north-east appeared most probably during this phase and had a strong impact on its water environment (see, Bińka et al., 2020). As previously mentioned, in the 15th century CE a mill was built near the lake, and damming of the water in the mill reservoir probably contributed to periodical blooms of dinoflagellate populations in Lake Młynek. Major blooms of Tetraedron, which usually precede blooms of the dinoflagellate, were most probably the main factor that contributed to the decline of settlement at the stronghold (Bińka et al., 2020). This zone is also characterized by increased precipitation which is reflected by more intensive terrestrial inflow to the lake and is confirmed by quasi-linear correlation of MS with contents of Fe and Ti in sediments (Fig. 6). The modern lake is shallow (2–3 m) and gradually overgrowing. Summing up, the phase 5 is marked by intensive human activity around the lake, and therefore most “natural” environmental and climate changes are obliterated.