Palaeoecological Conditions in the South-Eastern and Western Baltic Sea during the Last Millennium

Abstract

We present the reconstruction of palaeoenvironmental conditions in the Gdansk, Bornholm, and Arkona Basins of the Baltic Sea over the last millennium. A multiproxy study (including geochemical, XRF, grain size, AMS, and micropalaeontological analyses) of five short sediment cores was performed. The relative age of the sediments was determined based on the Pb distribution along the sediment sequences, as radiocarbon dating has resulted in an excessively old age. The retrieved cores cover two comparable warm periods, the Medieval Climate Anomaly and the Modern Warm Period, for which the increase in surface water productivity was reconstructed. Notably, the production of diatoms was higher during the colder periods (the Dark Ages and Little Ice Age), but this was also the case within the Modern Warm Period. In the Gdansk Basin, the initial salinity increase during the Littorina transgression started after 7.7 cal. a BP. The increased inflow activity was reconstructed during the Medieval Climate Anomaly, even in the Gdansk Basin, despite, in general, very low foraminiferal amounts and diversity. The strongly positive North Atlantic Oscillation Index during this period led to the prevalence of westerly winds over the Baltic region and stronger saltwater intrusions. In the recent sediments, the reconstructed inflow frequency demonstrates a variability against the reduction trend, and a general decline compared to the Medieval Climate Anomaly is seen.

1. Introduction

The Baltic Sea is one of the world’s largest brackish bodies of water, and it is characterised by strong and permanent stratification, restricting vertical mixing. Sporadic inflows of dense, saline, oxygen-enriched waters of the North Sea have a strong influence on the hydrochemical conditions of the isolated bottom layer [1] and on the development and state of its benthic ecosystems [2].

Increasing anthropogenic load, under conditions of recent warming of the climate, has a pronounced negative influence on the oxygen content in these bottom waters [3,4,5,6]. Moreover, many studies on the Baltic Sea environment [4,5,7,8,9] report a decrease in the frequency of inflows, observed since the 1980s, and favouring the expansion of oxygen-deficiency zones in the bottom water layer. However, following the analysis of Mohrholz (2018) [10], it can be determined that the observed reduction in inflows is rather a variation within a 25–30-year period.

The sediment archives provide long sets of information on the environmental conditions, which are required for evaluation of the present state and the prediction of future changes, as well as for assessment of the role of anthropogenic contributions in these processes. The paleoenvironmental records of comparable climate regimes can serve as a context for the study of the present environmental evolution. Thereby, investigations of the Medieval Climatic Anomaly (MCA), the interval of warm conditions comparable to the recent warming, can contribute to a profound understanding of the latter [3,11].

Data on the distribution of benthic foraminifera in sediment cores can provide information on variations in the saline water intrusions from the North Sea in the past. This information is crucial to understanding the role of atmospheric circulation patterns in inflow formation, as well as for drawing conclusions on the current and future dynamics of inflows under climatic changes. Due to the very high sedimentation rate, the Baltic Sea cores represent promising material for the detailed reconstruction of recent environmental conditions, considering that 1 cm spans from 2.6 to 100 years. However, well-known issues regarding the radiocarbon dating of Baltic sediments and the scarcity of calcareous microfossils due to the brackish conditions, in addition to the high level of carbonate dissolution, require a multiproxy approach, one which includes the other indicators of the near-bottom dynamic and alternative dating methods. In the works of [12,13,14], the potential of studying foraminiferal assemblages by counting not only shells but also organic remains was shown.

The Gdansk Basin represents a relatively poorly studied area regarding long-term environmental changes in the context of the saline water inflows. Moreover, a review of sediment archives data showed an almost-complete lack of foraminiferal information for the Late Holocene, except for rare studies [15]. Most of the palaeoreconstructions were carried out in the Central Baltic and aimed at the reconstruction of the surface water conditions based on the diatom data, e.g., [16,17,18,19]. In the exclusive economic zone of Russia in the southeastern part of the Baltic Sea, reconstructions based on foraminifera studies are represented by a selection of very few works, all written more than 20 years ago [20,21]. In one study [22], an alternative method of reconstruction of the changes in the salinity of bottom waters based on the bromine content in sediments was applied. However, in all of these studies, the reconstructions are characterised by relatively low resolution.

In this paper, we focus on the Gdansk Basin environment during the last millennium in relation to the Western Baltic, with attention given to the comparable climatic events of modern warming and the MCA. The variations in the saline waters’ intrusions are reconstructed in the frame of the modelled changes in the North Atlantic Oscillation (NAO), as one of the factors influencing the water exchange between the North and Baltic Seas.

2. Study Area

Only Major Baltic Inflows (MBIs) transporting large amounts of highly saline (17–25 PSU) water into the Western Baltic can impact the deep-water conditions in the Central Baltic [10,23,24,25]. As a precondition for this, the MBIs require a sea-level difference between the Kattegat and the Western Baltic, resulting from the air pressure difference and the forcing by easterly winds. Afterwards, strong, persistent westerly winds push saline water into the Baltic Sea [9,10,26,27,28,29]. The natural variability in water exchange between the Baltic and North Seas, as well as the climate over the Baltic region, is strongly affected by the alteration in the wintertime NAO [11,28,30,31,32], representing the difference in atmospheric pressure between the Icelandic Low and the Azores High [33,34]. The increased pressure difference (“positive” NAO) results in enhanced zonal flow with stronger westerly winds across the North Atlantic and the Baltic Sea and warmer winters in Europe [33,34,35,36]. During the “negative” NAO mode, a blocking ridge forms in the East Atlantic, resulting in increased meridional flow, a decrease in westerly winds, and extremely cold winters [30,33,34,37].

Late Quaternary sediments of the Baltic Sea Basin are represented by three main lithostratigraphic units: brown and grey glacial clays and silts (sediments of the Baltic Ice Lake), bluish and grey transitional clays (sediments of the Yoldia Sea and Ancylus Lake), and post-glacial olive and black muddy sediments of the Littorina Sea [38,39,40]. The zones of accumulation of silts and muds are generally found in the hydro-dynamically inactive areas below the permanent halocline depth (50–80 m) [38,39,41]. The complex topography of the Baltic Sea bottom limits the propagation of inflow waters and reduces their influence due to mixing [25]. The main branch flows through the Bornholm Gat from the Arkona to the Bornholm Basin and further along the Stolpe Furrow to the Central Baltic where a small portion goes to the Gdansk Basin [1,25].

2.1. Arkona Basin

The Arkona Basin is characterised by a maximum depth of about 50 m. On the west and northwest, the basin is connected to the Kattegat Strait through narrow and shallow straits [42]. The sediment cover was formed by glacial sedimentation, post-glacial morphological development, and hydrodynamic activity [42,43]. The main sources of sediment material are coastal erosion, riverine material, bioproductivity, and suspended matter, inflowing from the North Sea. The finer muddy material accumulates in the depocenter at a depth of about 50 m [44], forming an organic-rich muddy layer with an average thickness of 4.7 m [45]. The halocline occupies the depth between 20 and 35 m [46], separating the well-mixed brackish (8 PSU) surface layer and the dense saline (15–20 PSU) bottom layer that fills the central part of the Arkona Basin [46,47].

2.2. Bornholm Basin

The Bornholm Basin has a relatively simple bathymetry with a maximum depth of approximately 100 m in its central part [48]. The bottom sediments of the deeper areas are represented by muddy soft material, while the shallower region, close to Bornholm Island, is covered with sand and hard clay [49]. The basin is characterised by high spatial variations in the accumulation rates of silty-clay material (0.5 to 2 mm year⁻¹) [50] and, consequently, in the thickness of the Holocene sediments (0–12 m) [51,52]. The sedimentary material is delivered to the seafloor of the Bornholm Basin predominantly via coastal erosion, biogenic production, riverine inflow, and suspended material inflow from the Arkona Basin [43,51,52]. The permanent halocline, within an average depth range of 50–70 m [18], separates surface water with a salinity of 7.5–8.5 PSU from the deep layer, in which salinity reaches 14–18 PSU [49].

2.3. Gdansk Basin

The Gdansk Basin, located in the southeastern part of the Baltic Sea, is characterised by an average depth of 40 m and a maximum depth of 114 m (Gdansk Deep) [53]. The basin is separated from the Gotland Basin by the Gdansk–Gotland Sill, characterised by a maximum depth of 86 m. The organic carbon content of deep-water muds reaches up to 5–11%. Outside the deep areas, silty and sandy sediments cover moraine deposits [53,54]. Due to the patchiness of the environment, the linear sedimentation rate (LSR), reported in various studies, changes greatly: 0.7–3.9 mm year⁻¹ (Gdansk Deep) and 0.9–2.1 mm year⁻¹ [55]; 0.1–2.0 mm year⁻¹ [53]; 0.6 mm year⁻¹ [56]; 1.8–2.1 mm year⁻¹ [57]. In the Gdansk Deep, the boundary between the surface brackish (6–9 PSU) and deep saline (10.4–14.4 PSU) layers is located at a depth of approximately 70–75 m [41,53]. The limited exchange of bottom water and the influx of large amounts of organic matter to the sediments lead to the frequent formation of anoxic conditions in the near-bottom environment [53,54,58].

3. Materials and Methods

For the present study, five sediment cores were sampled during the 43rd and 44th cruises of the R/V Akademik Boris Petrov (summer and autumn of 2018) in the three sub-basins of the Baltic Sea (Figure 1; Supplementary, Table S1;

Table 1. Results of radiocarbon dating. Radiocarbon ages were calibrated using Calib8.2 and the “IntCal20” calibration curve.

Lab. Code	Core Depth (cm)	Dated Material	14C Age (a BP)	Error ±	Calibrated Age Median (cal a BP)
	ABP-43026
Poz-121066	7–8	Bulk sediment	710	30	664
Poz-121841	41–42	Bulk sediment	6890	40	7721
	ABP-43035
Poz-121363	9–10	Bulk sediment	2320	30	2342
Poz-121067	39–40	Bulk sediment	3695	30	4036
	ABP-43105
Poz-121068	7–8	Bulk sediment	3225	30	3429
Poz-121070	49–50	Bulk sediment	2130	30	2098

). The cores were retrieved by a short gravity corer of the Niemistö type, equipped with a transparent plastic tube (6 cm in diameter). The corer allows sampling of sediments with an undisturbed surface layer together with the near-bottom water [59]. The latter requirement is crucial for studying recent changes in ecosystems [60]. Cores ABP-43035 and ABP-43105 were sampled in the Gdansk Deep at water depths of 104 m and 105 m, respectively. Core ABP-43026 was retrieved from the Gdansk–Gotland Sill at a water depth of 78 m. The sea depths at the sampling stations in the Arkona and Bornholm Basins were 45 m (ABP-44059) and 89 m (ABP-44063), respectively. Such an arrangement of the sampling stations allows for the study of the spatial heterogeneity of the influence of inflows and other local paleoenvironmental changes on the Baltic Sea ecosystem. The thicknesses of the sampled sediments were 56 cm (ABP-43026), 46 cm (ABP-43035), 54 cm (ABP-43105), and 48 cm (ABP-44059 and ABP-44063).

Onboard, the cores were lithologically described inside the plastic tubes and then sampled into 1 cm slices. The sediment colour was determined following the Munsell Soil Color Chart. The upper 5 cm of the sediments was stained with an 80% ethanol solution of Rose Bengal to identify live foraminifera [60]. In the following study, these samples were used only for the micropaleontological analysis. The rest of the material was frozen to prevent the dissolution of carbonate shells. Due to the small diameter of the corer tube, the amount of material in a 1 cm-thick sample was not enough to perform all analyses. Therefore, the samples taken below 5 cm of the core depth were split in the following way: the odd-numbered samples were used for micropalaeontological analysis (due to the extremely low foraminiferal concentration the whole sample was needed for the analysis); the even-numbered samples were used for the rest of the analyses.

3.1. Grain Size Analysis

The grain size composition of the sediments was determined with a laser diffraction particle size analyser SALD-2300 (Shimadzu, Kyoto, Japan) with a measurement range of 17 nm to 2500 μm and a particle concentration range of 0.1 ppm to 20%. The measurements were performed with a 2 cm resolution, excluding the upper layer of 5 cm, which was stained with Rose Bengal. Organic carbon was preliminarily removed from the samples by treatment with 20 mL of hydrogen peroxide. The HCl treatment was unnecessary as Baltic Sea waters are unsaturated with CaCO₃ [61], and the sediments are barren of biogenic carbonate, which was also confirmed by microscopic study. The dispersion of the sediments before the measurement was carried out in two steps: first, the sodium tripolyphosphate was added to the samples before subsequently being left for 24 h; second, the samples were placed in an ultrasonic bath for 5 min. Statistical processing of the results was carried out in the program Gradistat [62]. The type of sediment was determined following Folk classification [63]. The content of the sortable silt (SS, 10–63 µm) and the distribution of its mean size throughout the core were interpreted to obtain information about past changes in the intensity of the near-bottom current, following [64,65].

3.2. Geochemical Analyses

3.2.1. Loss on Ignition

The loss on ignition (LOI) estimation was performed at a 2 cm resolution, except for the upper 5 cm of the cores, which were stained with Rose Bengal and became unsuitable for analysis. Prior to the analyses, wet sediments were dried at 90 °C and ground to a powder. The ground samples were again dried at 90–100 °C and then placed in a desiccator to protect the material from humidity. The LOI was determined by ashing the 1 g of dried sediments at 550 °C for more than 3 h (until a constant weight was reached) and calculating the resulting mass difference. For the Baltic Sea sediments, the LOI values provide an estimate of the total organic carbon content [66,67].

3.2.2. XRF Analysis

For the determination of Mn, Ti, Fe, Si, Al, Zr, Rb, and Pb concentrations in the sediment cores, an X-ray fluorescence spectrometer (Vanta C Series, OLYMPUS) was applied. The measurements were performed on wet bulk sediments in GeoChem mode with a resolution of 1 cm. The single samples were placed in plastic crucibles and covered with a 5 μm thick plastic film to prevent the contamination of the analyser. The measurement time for each sample was 180 seconds. To recalculate the concentration of elements to the dry weight of sediments, the content of water in the samples was determined by calculating the ratio of the Rayleigh and Compton peaks following the modified method of Boyle et al. (2015) [68], according to Laskina et al. (2024) [69]. The concentrations of elements and their ratios were used as indicators of the environmental conditions.

Mn is a highly redox-sensitive element, and it can serve as an indicator of the diagenetic processes. The distributions of Mn concentrations in sediment cores, as well as Mn/Ti and Fe/Mn ratios, were applied as proxies for the redox transitions [70,71,72]. The normalisation of Mn to Ti (Mn/Ti ratio) allows compensating variations in Mn concentration caused by terrigenous input [18]. As was shown by [72], a high Mn/Ti ratio documents Mn enrichment as a result of diagenetic relocation. According to various studies [70,73,74], under anoxic conditions, Mn mobilises, separates from Fe, and diffuses along concentration gradients, until it precipitates at a new oxic/post-oxic boundary. Therefore, a stable Mn/Fe ratio indicates no fractionation of elements under oxic conditions [70]. On the contrary, peaks in Mn/Fe distribution reflect Mn mobilisation under suboxic diagenesis. The Si/Ti ratio was used as a proxy for biogenic silica content, reflecting changes in surface productivity in the past, mainly related to diatom abundance [70,75,76]. To eliminate the non-biogenic components, the bulk Si can be normalised to Al or Ti, which both are associated with the detrital (terrigenous or clastic) material [77]. Due to the very close patterns in the distribution of the Si/Al and Si/Ti ratios (Supplementary, Figure S1), only Ti was chosen as a normalisation element [78]. The Zr/Rb ratio can be used as a grain-size proxy, as Rb adsorbs mainly to clay minerals, whereas Zr resides in silts and coarser grains [79,80]. Here, we applied the Zr/Rb ratio to reconstruct changes in the intensity of the near-bottom currents. As stronger currents carry greater, coarse-grained, sediments, higher Zr/Rb ratios correspond to the periods of intensification of near-bottom currents.

3.3. Microfossil Analysis

For the benthic foraminiferal analysis, the wet counting method was applied, as described in [12,13,14,15]. Wet counting allows for identification of the tests in every state of dissolution, including inner organic linings (IOL). IOL counting is particularly important for the organic-rich sediments of the Baltic Sea, which are characterised by a high degree of dissolution of carbonate material and low concentrations of shells. The micropalaeontological analysis was performed at resolutions of 1 cm (0–5 cm interval) and 2 cm (rest of the cores’ lengths). Around 30 g of wet sediment was sieved through a 63 μm mesh, using fresh tap water. The usage of distilled water was proven to cause the dissolution of fragile calcareous shells [14,15]. Depending on the preservation state and size of the shell, all individuals were identified under a stereomicroscope Olympus SZX16 at the species or genus levels following [15,81,82]. The IOLs and individuals of Elphidium spp. were counted separately, and then the amounts were combined. Benthic foraminiferal concentrations are represented as the number of individuals of the same genera per gram of wet sediments (n/g), as the counting of IOLs allows us to determine only the genus of the dissolved shells. Variations in the number of Elphidium individuals were applied as an indicator of a salinity of bottom water of more than 12 PSU [83], which was used to reconstruct the variations in saline inflows from the North Sea.

3.4. Dating and Age Modelling

In the Poznan Radiocarbon Laboratory (Poland), the radiometric accelerator mass spectrometry (AMS) ¹⁴C dating of six samples was performed (two samples were taken from each core, retrieved in the Gdansk Basin). Owing to the lack of carbonate material, the bulk sediments showing sufficient organic carbon content (over 2%) were chosen for the dating. The AMS ¹⁴C dates were converted to calendar years (cal. a BP) using Calib software (Version 8.2) and a terrestrial (IntCal20) calibration curve [84]. For the Baltic Sea sediments, it is widely accepted to calibrate the dates with the terrestrial curve, e.g., [17,40,85,86,87,88], due to the shallowness of the basin and the high input of organic matter from the surrounding land to the sea and, in particular, to the Gdansk Basin. Moreover, the marine (planktonic) organic matter of the bulk sediments is equilibrated with relatively young atmospheric ¹⁴C in the surface water layer; therefore, the marine calibration curve, which takes into account the relatively slow circulation of the oceans, is not necessary [89]. The calendar age is presented as a median value; the year 1950 was taken as the zero point.

It is widely known that the dating of bulk sediments from the Baltic Sea is associated with multiple-source errors, induced by the down-slope material redeposition, contamination with older resuspended organic matter, the unknown ratio of terrestrial and marine material, and highly variable local reservoir effects [87,89,90,91,92,93]. As shown by [92], the dating error (overestimation) increases in the upcore direction (i.e., the younger the sediment, the larger the error). Therefore, when dating bulk sediments of the Baltic Sea, especially the upper layers of sedimentary sequence (up to 1 m depth), alternative methods are increasingly used. One of the most common methods for the determination of the relative age of the sediments is based on the distribution of Pb concentrations along the core sections, e.g., [3,40,85,87,91,94]. There are well-known historical Pb concentration peaks, resulting from anthropogenic pollution, which are assigned exact calendar ages: 1 AD (1949 cal. a BP, Roman peak); 1200 AD (750 cal. a BP, medieval increase); and the 1970s (−20 cal. a BP, modern pollution) [81,82,83]. These Pb-peaks form exact isochrones that can be used in the dating of Baltic sediments [91]. However, because of the great patchiness of the Baltic Sea environment, the distribution of Pb is not always uniform from core to core, there are no specific Pb concentrations in the sediments that mark the above-described peaks, and these peaks are not well recognised in all sediment sequences [94,95,96,97,98,99]. The combined graph of Pb distribution in the sediments of Baltic Region lakes [96,98,100] (Figure 2) demonstrates the steady increase in Pb concentrations, marking the inception of the Medieval pollution ~1000 cal. a BP, after which the concentrations never returned to the previous levels. Consequently, in this study, we used the continuous increase in Pb concentration as a marking point for ~1000 cal. a BP, similar to the findings in [94,95,96,101]. Additionally, the distribution of LOI values, reflecting variations in surface productivity under different climates, was used as another criterion for the subdivision of the cores. As the ~1000 cal. a BP date marks the transition to the warmer climate during the beginning of the MCA, the overlying sediments should show an increase in organic matter content.

4. Results

The retrieved cores were represented by olive and grey homogeneous muddy sediments, compacting downcore and covering the denser homogeneous clayey muds (gyttia) of lighter olive–grey colours. In the Gdansk Basin cores, the uppermost interval was represented by loose thin lamination of olive and black muds, saturated with water and covered with a black fluky layer of a few millimetres thick (Figure 2).

In all studied cores, the carbonate shells of benthic foraminifera were represented by two species of the genus Elphidium—E. excavatum and Cribroelphidium (Elphidium) incertum. Since both of these species indicate an increase in the salinity of bottom waters (to 12 PSU and more) [83], they were combined for the discussion of the results. Furthermore, it was possible to determine only the genus of the inner organic linings. The only exception was core ABP-44059, in which single shells of Gyroidinoides spp., Eponides sp., and Ammonia sp. were found in the sediment samples. In cores retrieved in the Gdansk and Bornholm Basins, in the surface sediments stained with Rose Bengal, no living benthic foraminifera were found. In the upper centimetres of the sections, the IOLs predominated, accounting for more than 90%. Only in core ABP-44059, obtained in the Arkona Basin, the living foraminiferal content was less than 4% within the 0–1 cm layer.

4.1. Gdansk–Gotland Sill (ABP-43026)

In the ABP-43026 core, the bottom interval (56–49 cm) was represented by grey–blue clays, characterised by low values of LOI (7–10%), high values of the Si/Ti ratio, and the highest values of the Mn/Ti and Mn/Fe ratios (Figure 3). The SS content changed sharply within the range of 7–27%, and both the SS mean size and Zr/Rb record exhibited high peaks at 53–52 cm. The bottom interval was barren of benthic foraminifera. In the overlying muddy sediments, two intervals in the data distribution were distinguished. In the lower layer (49–33 cm), the LOI values showed a steady increase from 9 to 22%, the generally high Si/Ti ratio changed in the sawtooth manner, and the SS content significantly increased. The SS mean size and Zr/Rb ratio demonstrated high peaks at 42–43 cm, where the first appearance of benthic foraminifera was recognised. Throughout the core, the Mn/Ti and Mn/Fe ratios varied in a narrow range of low values. In the upper interval (33–7 cm), the LOI values remained consistently high (19–24%). Relatively lower LOI values were recognised at 32–22 cm, simultaneously with a reduced Si/Ti ratio and higher SS content and mean size (SS parameters, from here onwards). The Zr/Rb repeated the distribution of the curves of SS parameters. The foraminiferal concentration rose stepwise until reaching 8 n/g at 22 cm, and then the values decreased again.

4.2. Gdansk Deep (ABP-43035)

The bottom interval (46–37 cm) was characterised by stable, relatively low, LOI values (19%) and moderate Mn/Ti and Mn/Fe ratios (Figure 3). The SS content and mean size were relatively high (21–24% and 17–19 µm, respectively). The Zr/Rb ratio fluctuated in a narrow range throughout the core. The maximum foraminiferal concentration (20 n/g) among the cores retrieved in the Gdansk Basin was recorded at a 40 cm core depth. The Si/Ti ratio fluctuated highly against the generally decreasing trend, which persisted in the upper interval. Further upcore (37–26 cm), the LOI demonstrated high values, increasing from 19% to a maximum of 22%. In the same interval, the foraminiferal concentration dropped to 1 n/g. All other parameters remained without significant changes. In the interval 26–16 cm, the LOI decreased to 18%, while the Mn/Ti, Mn/Fe, and Si/Ti ratios increased. The SS parameters showed the minimum values for the core. At the same time, the Zr/Rb ratio and foraminiferal concentration remained low. Up to the core top, the LOI and the Si/Ti ratio increased, while the other parameters repeated the distribution pattern, demonstrated in the 37–26 cm interval.

4.3. Gdansk Deep (ABP-43105)

The 54–42 cm interval was characterised by a high LOI (19–21%), elevated Mn/Fe and Mn/Ti ratios, and a decreased Si/Ti ratio (Figure 3). The Zr/Rb ratio, SS parameters (30% and 20–22 µm), and foraminiferal concentrations showed increased values. Further upcore (42–26 cm), the distribution of all indicators was the opposite: a lower LOI (17–19%), lower Mn/Fe and Mn/Ti ratios, and an elevated Si/Ti ratio, accompanied by a moderate decrease in the Zr/Rb ratio and SS parameters (24–29% and 17–19 µm). High foraminiferal concentrations (6–8%) and Si/Ti values coincided with the minimum LOI values at 34–28 cm. In the upper interval (26–0 cm), the distribution of parameters was close to the lowest one (54–42 cm): a high LOI (18–20%) and high Mn/Fe and Mn/Ti ratios corresponded to lower Si/Ti values. The Zr/Rb ratio was slightly elevated; the SS parameters sharply increased (to more than 40% and 22 µm) in the upper 16 cm; and the concentration of foraminiferal shells varied within a narrow range of 0–8 n/g and decreased to the core top.

4.4. Bornholm Basin (ABP-44063)

In the 48–33 cm interval, the LOI values changed in the low range of 14–16%, and the Si/Ti values were high (Figure 3). With the background of low Mn/Ti and Mn/Fe ratios, a single spike was seen at 47–46 cm, which coincided with the extremely low foraminiferal abundance (0–2 n/g). The latter increased (to 18 n/g) to the top of the interval. Throughout the core, the Zr/Rb ratio and SS parameters (16–26% and 16–21 µm) varied in a narrow range of low values. Further upcore (33–21 cm), the Si/Ti ratio decreased, the LOI increased (16–21%), and the Mn/Ti and Mn/Fe ratios showed high spikes at the depth of maximum LOI values (25–24 cm). The foraminiferal content demonstrated sharp variations in a wide range, rising up to 45 n/g. In the overlying sediments (21–15 cm), the LOI values moderately decreased, and the Mn/Ti and Mn/Fe ratios were low, while the Si/Ti ratio increased. The foraminiferal content fell to less than 20 n/g. In the lower part of the top interval (15–0 cm), a sharp increase in LOI (to 37%) was noted, as well as the growth of the Mn/Ti and Mn/Fe ratios. The Si/Ti ratio was higher throughout the interval. The foraminiferal content changed in a sawtooth manner (33–2 n/g), generally, declining to the core top.

4.5. Arkona Basin (ABP-44059)

The interval 48–41 cm was characterised by an increasing LOI (14–18%) and increasing Mn/Ti and Mn/Fe ratios (Figure 3). The Si/Ti ratio changed sharply in a sawtooth pattern over a wide range of high values, demonstrating a generally decreasing trend until almost the core top. Throughout the core, the SS content and mean size, as well as the Zr/Rb ratio, were considerably higher compared to the other cores. Notably, the SS parameters changed in the moderate range (42–60%, 24–27 µm) without a significant trend. Generally, the concentrations of benthic foraminifera were also considerably higher in relation to the other cores (6–54 n/g), and their distribution was in agreement with the SS parameters. In the bottom interval, shells’ concentration changed from 9 to 21 n/g. Further upcore (41–28 cm), the LOI increased to 17–19%, while the Mn/Ti and Mn/Fe ratios were relatively stable. The foraminiferal abundance was comparable to the lower interval. In the interval of 28–20 cm, the LOI values gradually decreased from 19 to 15%, and the Si/Ti ratio demonstrated lower values. The Mn/Ti and Mn/Fe ratios showed peaks in the lower part of the interval. The high foraminiferal concentrations (28–45 n/g) were accompanied by the increase in SS parameters. At the 20–0 cm core depth, the LOI values were relatively elevated (15–18%), and the Mn/Ti and Mn/Fe ratios were higher at the bottom of this interval. The Si/Ti ratio followed the distribution of benthic foraminifera, the abundance of which increased to the maximum (54 n/g) at a depth of 4–5 cm and then decreased again to 16–6 n/g in the upper part of the core.

4.6. Chronology

In the studied cores, the lithology of the cores’ tops, especially the ones collected in the Gdansk Basin, confirms the presence of intact surface sediments. Therefore, the tops of the cores were assumed to correspond to the year of coring—2018 CE (-68 BP). According to the AMS dating results (Table 1), the sediment core ABP-43026 dated back to 7721 cal. a BP at a depth of 42 cm. At the same time, the underlying sediments (56–49 cm) were represented by bluish-grey clays with very low LOI values and the absence of benthic foraminifera. Together with the overlying “old” date, these sediments’ characteristics allowed us to assume (following [39]) that the lower interval was accumulated during the fresh Ancylus Lake phase. In the overlying sediments, the AMS date of 664 cal. a BP was obtained above the sharp increase in Pb distribution at 11 cm, which was, therefore, recognised as the Medieval Pb pollution peak (750 cal. a BP) (Figure 2). Based on the combined dating results, we assumed the extremely low sedimentation rate for this core. The highly dynamic hydrology on the shallow Gdansk–Gotland Sill caused the non-deposition or even erosion of sediments. Given the aforementioned features, this core was only considered as describing the general conditions on the sill.

Based on the AMS dating (Table 1), the sediment core ABP-43035 represented the last 4036 cal. a BP (date at 40 cm core depth). The LSR decreased from 0.17 mm year⁻¹ (40–10 cm) to 0.04 mm year⁻¹ (10–0 cm). However, the core was obtained from the Gdansk Deep, which is characterised by a calm sedimentation regime and the absence of conditions for such a low LSR, as well as for a sharp drop of the LSR values, especially, in the upcore direction. Even when we applied the zero validation (as described in [103,104]) and then subtracted the result of 1861 years as the local reservoir effect, the calculated average LSR was still non-typically low (0.2 mm year⁻¹). The AMS dating of the ABP-43105 core gave the age of 2098 cal. a BP at 50 cm. At the same time, the age of the sediment at the 8 cm core depth was 3429 cal. a BP, which indicated contamination by the redeposited older material. The lateral transport of the material was also evident from the high values of SS parameters, reflecting the active palaeohydrodynamic. Therefore, the upper AMS date was excluded from the calculations. The resulting overall LSR, calculated considering only the lower AMS date, would be 0.2 mm year⁻¹, which is too low for the core location.

Taking into account the very low LSRs, high uncertainties of the results, and age reversal, which are all associated with the AMS dating of the Gdansk Basin sediments, the AMS results were excluded from the chronology. The cores of the Western Baltic, which were collected and analysed later, were subjected only to Pb dating. Due to the absence of Pb distribution data in the upper parts of the cores, the modern pollution peak (1970s, −20 cal. a BP) was not evident. Nevertheless, we still consider the upper intervals of the cores as corresponding to the MoWP according to the following criteria: the core-top sediments represent −68 cal. BP (year of coring); the LOI values are increased; the lamination reflects the undisturbed sediments and high surface productivity; and the still-high Pb values mark the modern pollution period. The MoWP is prolonged downcore due to the highly watered cores’ tops. As was shown in the new study performed for the Gdansk Basin [69], the Modern Pb increase could occupy an interval of up to 20 cm-thick at the top of the core.

In the core ABP-43035, the increase in Pb concentrations at 41–35 cm (Figure 2) was interpreted as the inception of the Medieval pollution, and the date of ~1000 cal. a BP was placed at 41 cm, as the point of the lowest Pb concentration within the interval. In the overlying sediments, the LOI values increased (Figure 3), reflecting intensified production likely corresponding to the MCA warming. Low LOI values at the inception of the pollution could be connected to the organic matter oxidation by the inflowing water, as evidenced by very high concentrations of benthic foraminifera. In the core ABP-43105, we could not recognise the inception of the Pb increase in the retrieved sediments. As we are aware that it could be placed deeper in the sediments, the date of ~1000 cal. a BP at 51 cm was given with a question mark. Nevertheless, the high LOI values in this bottom interval indicate higher surface productivity, which should reflect the warmer conditions of the MCA. In the ABP-44059 core, the ~1000 cal. a BP date was connected to the increase in Pb concentrations at 34 cm, which corresponded to the higher LOI values. In the ABP-44063 core, the ~1000 cal. a BP date was referred to 28 cm; this rise in Pb also coincided with the LOI increase (Figure 2 and Figure 3).

5. Discussion

The palaeoenvironmental conditions of the Late Holocene in the three basins of the Baltic Sea are discussed in the framework of the following climate regimes: the Dark Ages (DA) c. 1550–1150 cal. a BP; the Medieval Climatic Anomaly (MCA) c. ~1000–600 cal. a BP; the Little Ice Age (LIA) c. 600–100 cal. a BP; and the Modern Warm Period (MoWP) c. 100 cal. a BP to present [3,4,14,102,105]. The variations in the NAO over the studied period [11,33,102,106,107] are considered as one of the forces governing the water exchange between the North and Baltic Seas.

The foraminiferal diversity of the studied sediments from the Gdansk and Bornholm Basins was extremely low, as the assemblages were represented by two species of the carbonate genus Elphidium: E. excavatum and C. (E.) incertum. No agglutinated specimens were found. In the core retrieved in the Arkona Basin, single shells of Gyroidinoides spp., Eponides sp., and Ammonia sp. were detected. This distribution pattern is consistent with the general west–east diversity decrease in the Baltic Sea [108,109]. Notably, the overall diversity is still low, compared to six benthic foraminiferal species found in the Bornholm Basin [14] or thirteen species reported by [110] for the Mecklenburg Bay (the south-western part of the Baltic Sea) and the same number in the Kattegat [111]. Despite the impracticability of precise reconstruction of conditions due to the low foraminiferal concentrations in the sediments of the Gdansk Basin, some interpretations for bottom-water salinity could be provided by the records.

In the core ABP-43026 retrieved on the Gdansk–Gotland Sill, the bottom layer of bluish clay sediments was accumulated during the cold Ancilus Lake phase. The very low organic content and high Si/Ti ratio (Figure 3), corresponding to the diatom abundance, reflect the cold-water conditions. The absence of foraminifera corresponds to the freshwater conditions. The coarse material inclusions, which are evident from the abrupt peaks in the Zr/Rb and SS parameters in this interval and the overlying sediments, indicate the highly dynamic hydrological conditions probably associated with the Littorina transgressions. Most likely, the latter is reflected in the simultaneous peak of Mn parameters, corresponding to the change in the redox state during the transition to a brackish environment. The first appearance of foraminiferal shells, together with the upper peaks in Zr/Rb and SS mean size, supports the initiation of saline water intrusions after 7.7 cal. a BP. The overlying sediments cover the Littorina and Post-Littorina stages due to the extremely low sedimentation rates associated with the core location on the topographic height of the sill. According to the scheme of inflowing water propagation by [53], the Gdansk–Gotland Sill is located on the pass of the North Sea water coming to the Gdansk Basin from the Stolpe Furrow. On the sill, the proximity of the halocline to the bottom causes the erosion of sediments by the inner waves occurring at the halocline [112]. This core is only considered in the discussion of the Gdansk Basin environment as an example representing the contrasting condition on the sill.

5.1. The Dark Ages

The lower sections of the cores retrieved in the Western Baltic (ABP-44059 and ABP-44063) and in the Gdansk Deep (ABP-43035) were accumulated during the DA period, characterised by general cooling in Europe [113]. According to the air temperature reconstruction by [114], during the DA, the temperatures in Sweden were considerably low. In the studied cores, this period corresponds to the lower amount of organic matter in the sediments (Figure 3). The accumulation of less organic matter in the sediments could result from a decrease in the productivity of surface waters caused by the climate cooling. During the DA, the decrease in organic matter content was also noted in the other cores retrieved in the Baltic Sea [14,92]. The high Si/Ti ratios in the studied cores reflect the high role of the diatoms in primary production. As was shown by many studies [30,115,116], the diatoms have a competitional advantage after cold winters; therefore, their high abundance in the sediments is consistent with the general cooling during the DA. It is worth mentioning that the Si/Ti ratio in the sediments of the Arkona Basin (ABP-44059) is twice as high as in the Bornholm Basin (ABP-44063) and Gdansk Deep (ABP-44035). The earlier time of bloom onset due to the earlier establishment of thermal stratification in the much shallower coring site in the Arkona Basin could be the possible reason for the higher overall annual production as compared with other basins. During the early DA, in the Bornholm Basin sediments, the moderate peaks in Mn ratios together with the almost complete absence of foraminiferal shells could reflect a change in the redox state caused by stagnation. Generally, in the sediments of other basins, relatively stable Mn ratios correspond to the ventilated water column and the absence of element fractionation. Ventilation of the near-bottom layer by inflow waters could be an additional factor that lowers the concentration of accumulated organic matter due to its oxidation.

In the studied sediments accumulated in the Western Baltic during the DA (Figure 3), the abundance of Elphidium spp. is generally lower compared to the rest of the cores. In the Bornholm Basin core (ABP-44063), the low Zr/Rb ratio and SS parameters correspond to calm hydrodynamic conditions under moderate inflow frequency. Within the Arkona Basin (ABP-44059), the higher foraminiferal abundance, accompanied by the elevated Zr/Rb ratio and SS mean size and content, indicates an intensive near-bottom dynamic connected to the local inflow’s activity. In the Gdansk Deep (ABP-43035), the sediments were characterised by low concentrations of calcareous shells (Figure 3) reflecting a minor marine influence. However, in both these regions, an increasing trend in foraminiferal abundance could be recognised towards the MCA. Reconstruction of the NAO Index showed that, during the DA, the index changed from negative to strongly positive [33], thereby first blocking and then inducing extremely strong westerly winds over the Baltic region. According to [29,117], abnormally strong westerly winds over the Baltic Region are balanced by a pressure gradient that hampers the spread of the North Sea waters. Consequently, such a meteorological situation resulted in moderate near-bottom water exchange between the Baltic and North Seas, which affected mainly the Arkona Basin and to a lesser extent the Bornholm Basin.

5.2. The Medieval Climatic Anomaly

The following interval of the MCA corresponds to the relatively stable climate conditions of warm and dry summers. The air temperatures were about 0.5 °C higher than during the MoWP, and the warmest period occurred at 730–700 cal a BP [6]. The positive NAO phase during the MCA [11,102] induced the transport of the warm air masses to Europe, which resulted in warmer winter temperatures. In the studied cores of the Western Baltic (Figure 3), the organic matter content is high, as a consequence of the increased surface productivity associated with the warmer climate. According to the reconstructions of the environmental conditions in the Baltic Sea [3,88,92,105], the accumulation of organic-rich and often laminated sediments is noted during the MCA, as a consequence of the higher temperatures of surface water. At the same time, the Si/Ti ratio in the studied sediments of the Arkona and Bornholm Basins demonstrates decreasing and lower values, correspondingly, as compared to the DA, which indicates a decrease in the diatoms’ abundance. Most likely, higher surface water temperatures led to the dominance of nitrogen-fixing cyanobacteria in primary production. The same pattern in the distribution of the primary producents during the MCA was noted in the studies of the Gotland Basin sediments [3,88]. Moreover, in the Arkona Basin section, peaks of high Si/Ti values correspond to relative depressions in organic matter content, indicating the preference of diatoms for colder conditions. In both sediment cores, the peaks in the distribution of Mn content and ratios coincide with the higher organic content, reflecting Mn enrichment, as a result of possible diagenetic relocation under hypoxic conditions. A stronger halocline caused by an increase in bottom salinity could be a further condition favouring better organic matter preservation due to the poor ventilation.

The pronounced increase in the abundance of Elphidium spp. in the studied sediments of the Arkona and Bornholm Basins (Figure 3) corresponds to high near-bottom salinity due to the frequent inflows during the MCA. Furthermore, the simultaneous increase in the SS content and mean size in the Arkona Basin indicates the intensive near-bottom hydrodynamics, which results from the proposed intensification of the inflows. According to published micropaleontological data [14], in the Bornholm Basin, the MCA period was characterised by the most prominent intensification in bottom water-mass exchange compared with the entire Late Littorina Sea stage. The strongly positive NAO phase during the MCA [11,102] was responsible for the increase in westerly wind speed, which, in turn, contributed to the intensification of saltwater intrusions from the North Sea [5,118].

Similarly to the cores of the Western Baltic, in the Gdansk Deep sediments during the warm MCA interval, the increase in organic matter content is distinguishable, which reflects the intensification in the surface water productivity (Figure 3). Relatively low Si/Ti ratios imply a reduced role of diatoms in production. In the distribution of Mn parameters, no signs of suboxic diagenesis could be seen.

The sediments of the ABP-43035 core (Gdansk Deep) corresponding to the MCA are characterised by the maximal concentration of calcareous shells compared to the whole record, as well as compared to the other cores retrieved in the Gdansk Basin (Figure 3). The high concentrations of Elphidium spp. indicate the salinity increase, as a result of the intensification of near-bottom water exchange, which was prominently reflected in the studied cores of the Western Baltic. The lower foraminiferal abundance in the Gdansk Deep sediments, compared to the Western Baltic, reflects the salinity decrease, as a consequence of the further position of the coring sites from the source of the inflow. Moreover, the increase in foraminiferal abundance was also noticeable in the other cores from the Gdansk Basin, even in the one situated on the sill, which is generally characterised by the minor influence of salt waters [119,120]. In the ABP-43105 core, only a slight increase in Elphidium spp. is related to the influence of complex topography of the basin. The coring sites with similar depths are separated by elevation, which most likely hampers the inflowing water propagation or induces mixing with a subsequent decrease in salinity (Figure 1). In both Gdansk Deep cores, against the generally low Zr/Rb ratios (Figure 3), reflecting the calm conditions associated with the cores’ position well below the halocline, slightly elevated Zr/Rb values correspond to the MCA interval. The latter is also true for the distribution of SS parameters, in which the increase is more pronounced. On the sill, the higher values of SS parameters reflect more dynamic conditions, which are associated with the inner waves occurring at the halocline or with the propagation of the inflowing waters. Based on the above-mentioned, we can conclude a moderate-to-pronounced increase in salinity and near-bottom hydrodynamics under the influence of transformed inflowing water reaching the Gdansk Basin during the MCA.

5.3. The Little Ice Age

Unfortunately, the limitations of the obtained dates do not allow precise subdivision of the sediment sections. Presumably, sediments, lying above the MCA and demonstrating the relative decrease in the organic matter content (Figure 3) due to the reduced surface bioproductivity, could be assigned to the LIA, the period when the temperatures dropped again [6]. Generally, the lower LOI values accompanied by the elevated Si/Ti ratio indicate an increase in diatom production associated with the cooling. In the sediments of the Arkona and Bornholm Basins (Figure 3), as well as in the Gdansk Deep (Figure 3), the decrease in the concentration of Elphidium spp. indicates a reduction in the frequency of inflows, which was also found in studies performed for the other areas of the Baltic Sea [14,92,121]. During the LIA, the dominance of a negative NAO Index [33,102] led to a decline in westerly winds and the subsequent reduction in saline water intrusion into the Baltic Sea. Generally, in the Gdansk Basin sediments, lower Zr/Rb ratios and SS parameters support the assumption of the minor near-bottom dynamic.

5.4. The Modern Warm Period

The top intervals of the studied sediments were assigned to the period of modern climate warming, when sea surface temperatures were comparable to the MCA and were 2 °C higher relative to the preceding LIA [3]. Throughout the MoWP, in all studied sediment sections, the organic matter content is elevated, as a reflection of the high surface productivity under warmer sea conditions, similar to the MCA regime. Generally, the elevated Si/Ti ratio reveals a high production of diatoms contributing to the increase in organic matter content. The latter leads to an increase in oxygen consumption in the bottom layer of the water and, consequently, to the development of hypoxia [3,88,105]. The accumulation of laminated sediments together with the expansion of oxygen-deficiency zones in the near-bottom waters during the MoWP were also reported by several other studies of the deep basins of the Baltic Sea [3,5,92,105]. In the studied cores of the Western Baltic, no laminated sediments were found; therefore, a mainly oxic depositional environment is proposed.

Moreover, in the Arkona Basin, living foraminifera in the surface sediments imply oxygenated bottom water at the coring time. According to the comprehensive review of coring data [105], sediments of the Bornholm Basin were not affected by hypoxic conditions. Nevertheless, it is worth noticing the very high peak in the organic matter content and simultaneous high Mn parameters in the Bornholm sediments (ABP-44063 core, Figure 3), which could reflect the Mn mobilisation under the presence of at least short-lived hypoxia in bottom waters and then subsequent precipitation during oxygenation. Within the same horizon, the foraminiferal abundance is low, indicating the deficiency in bottom-waters’ renewal, which could be an additional factor contributing to the better preservation of organic matter. As shown by many studies, e.g., [5,8], throughout recent history, the Bornholm Basin was repeatedly affected by hypoxia and even anoxia. In the cores retrieved in the Gdansk Basin, only the uppermost centimetres, corresponding to the last ca. 10 years, were represented by laminated sediments affected by hypoxia.

In contrast to the MCA, sediments corresponding to the MoWP demonstrate a lower abundance of Elphidium spp., reflecting the ongoing period of moderate inflow activity initiated during the LIA (Figure 3). The only exception is the sediments of the Arkona Basin (ABP-44059, Figure 3), where foraminiferal concentrations were even higher than during the MCA. Considering the proximity of the basin to the entrance to the Baltic Sea, the pronouncedly high values of SS parameters throughout the core section reflect the active hydrodynamic environment due to the constant influence of saltwater intrusions. Most likely, the majority of these inflows do not reach the further deep basins due to their medium strength and baroclinic nature. Within the MoWP, a predominantly negative NAO Index [33,102] was responsible for the less frequent westerly winds, which resulted in a reduction in bottom-water renewal in the Baltic Sea [25]. Unfortunately, the age model does not allow a precise correlation of the signal in the cores to the existing data on the inflows’ statistic; nonetheless, some comparisons can be made. According to the revision of the data on inflow measurement [10], there exists a variability with a main period of 25–30 years, instead of the widely reported reduction in the inflows, started in the 1980s. During the MoWP, in the foraminiferal data of the studied cores obtained from the Western Baltic, the fluctuations are also recognisable; however, the declining trend is pronounced.

The combined effect of the restricted bottom-water renewal together with the ongoing climate warming during the MoWP, enhanced by an excess supply of nutrients (viz. phosphates) from anthropogenic sources, led to the widespread propagation of oxygen-deficiency zones in the modern Baltic Sea [3,5,8]. However, to draw a more certain conclusion, concerning the inflows dynamics and their role in the development of hypoxic and anoxic conditions, further detailed complex studies of well-dated recent sediment archives retrieved from different sub-basins are necessary.

6. Conclusions

The environmental conditions, concerning saline water inflows in the western and south-eastern Baltic Sea during the last millennium, were reconstructed based on multiproxy sediment analysis. The foraminiferal assemblage demonstrated particularly low diversity. Agglutinated specimens were absent, and the calcareous group was dominated by specimens of the Elphidium genus. Living individuals were found only within the maximal proximity to the inflow entrance—in the Arkona Basin. Despite the extremely high dissolution of carbonate material, counting of the foraminiferal IOLs allowed the application of the micropalaeontological method for the reconstruction of the inflows’ dynamic.

In the Gdansk Basin, the sedimentation was strongly affected by the local topography. Thereby, at the Gdansk–Gotland Sill characterised by high hydrodynamic activity, dense Ancylus clays were recovered in a short sediment sequence, and the upper layers were represented by Littorina and Post-Littorina materials of very low sedimentation rates. In turn, the sediments of Gdansk Deep were affected by lateral redeposition, which led to the radiocarbon age reversal in the upper part of the core ABP-43105. Due to the patchiness and complexity of the Baltic Sea environment, the sediments’ dating is highly challenging and requires a multiproxy approach, one which includes not only absolute (AMS) and relative (Pb) methods but also a control with the lithological and geochemical characteristics of the sediments. The extremely low sedimentation rates on the Gdansk–Gotland Sill allowed for reconstruction of the initial inflow of the salt waters to the Gdansk Basin during the Littorina transgression after 7.7 cal. a BP.

The studied sediments cover two comparable warm periods, the MCA and MoWP, during which the content of organic matter demonstrated a relative increase, as a result of enhanced surface water productivity due to higher sea surface temperatures. The Gdansk Deep sediments were characterised by an overall high organic matter content, resulting from the combination of the higher material input due to the coast vicinity and better preservation due to the calm hydrodynamic conditions. The production of diatoms was generally higher during periods of colder conditions (the DA and LIA), but also within the MoWP. However, biomarker studies are necessary to draw certain conclusions about the role of different producers in organic matter accumulation in the past. It is worth noting, in the sediments of the western Baltic Sea corresponding to these warm periods, the lamination was absent, but high peaks in Mn parameters could imply short-leaving hypoxic conditions, resulting from the combination of higher sea-surface bioproductivity and stronger water-column stratification. In the Gdansk Basin, the lamination in the top sediment layers reflects repeating hypoxia during the MoWP, probably, enhanced by increasing anthropogenic load.

During the MCA, the prominent marine influence was reflected by an increase in benthic foraminifera not only in the sediments of the Western Baltic, but also in the sediments of the significantly easterly located Gdansk Deep, indicating strong and frequent inflows. The predominantly positive NAO Index during this period was responsible for the strong westerly winds governing the saline water in the Baltic Sea. During the LIA and MoWP, the minor increase in bottom-water salinity was reconstructed, which corresponds to the negative NAO phase, leading to the weakening of the westerly winds and the reduction in inflows. Within the DA, the low inflow influence resulted from either negative or strongly positive NAO, both of which are unfavourable for the spread of the North Sea waters. However, in the Arkona Basin, moderate (during the DA) and pronounced (during the MoWP) increases in inflows reflected the regional marine influence connected to the vicinity of the Danish Straits. We can conclude there is a connection between the intensity of inflows of North Sea water and variations in the NAO Index over larger time periods. Further detailed study of well-dated sediment cores in conjunction with the data of direct measurements will improve the understanding of past and future environmental dynamics.