Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

Abstract

The Baltic Sea (~393 000 km2) is the largest brackish sea in the world and its hydrographic and environmental conditions are strongly dependent on the frequency of saline water inflows from the North Sea. To improve our understanding of the natural variability of the Baltic Sea ecosystem detailed reconstructions of past saline water inflow changes based on palaeoecological archives are needed. Here we present a high‐resolution study of benthic foraminiferal assemblages accompanied by sediment geochemistry (loss on ignition, total organic carbon) and other microfossil data (ostracods and cladocerans) from a well‐dated 8‐m‐long gravity core taken in the Bornholm Basin. The foraminiferal diversity in the core is low and dominated by species of Elphidium. The benthic foraminiferal faunas in the central Baltic require oxic bottom water conditions and salinities >11–12 PSU. Consequently, shell abundance peaks in the record reflect frequent saline water inflow phases. The first appearance of foraminiferal tests and ostracods in the investigated sediment core is dated to c. 6.9 cal. ka BP and attributed to the first inflows of saline and oxygenated bottom waters into the Bornholm Basin during the Littorina Sea transgression. The transgression terminated the Ancylus Lake phase, reflected in the studied record by abundant cladocerans. High absolute foraminiferal abundances are found within two time intervals: (i) c. 5.5–4.0 cal. ka BP (Holocene Thermal Maximum) and (ii) c. 1.3–0.75 cal. ka BP (Medieval Climate Anomaly). Our data also show three intervals of absent or low saline water inflows: (i) c. 6.5–6.0 cal. ka BP, (ii) c. 3.0–2.3 cal. ka BP and (iii) c. 0.5–0.1 cal. ka BP (Little Ice Age). Our study demonstrates a strong effect of saline and well‐oxygenated water inflows from the Atlantic Ocean on the Baltic Sea ecosystem over millennial time scales, which is linked to the major climate transitions over the last 7 ka.

Benthic foraminifera distribution in the modern sediments of the Southeastern Baltic Sea with respect to North Sea water inflows

“…In the sediments of the Bornholm Basin, the average amount of shells per gram of dry sediment was higher than in the Gdansk Basin in both seasons and reached 36 shell/g. The species diversity of the benthic foraminifera in the study region was very low, which is also noted in the works, describing the western regions of the sea [Brodniewicz, 1965;Binczewska et al, 2017;Hermelin, 1987;Murray and Alve, 2011;Polovodova et al, 2009] (Figure 3). As was also observed in the studies of [Alve and Murray, 1999;Brodniewicz, 1965;Hermelin, 1987], agglutinated species are the most abundant in the assemblages.…”

“…The species diversity of the benthic foraminifera in the study region was very low, which is also noted in the works, describing the western regions of the sea [Brodniewicz, 1965;Binczewska et al, 2017;Hermelin, 1987;Murray and Alve, 2011;Polovodova et al, 2009] (Figure 3). As was also observed in the studies of [Alve and Murray, 1999;Brodniewicz, 1965;Hermelin, 1987], agglutinated species are the most abundant in the assemblages. They were dominated by small sized individuals (less than 100 m) of a simple single-chamber ES6001 ponomarenko and krechik: benthic foraminifera distribution ES6001 structure, which belong to Psammosphaera, Pseudothurammina and Saccammina genera, and also Reophax species.…”

“…They were dominated by small sized individuals (less than 100 m) of a simple single-chamber ES6001 ponomarenko and krechik: benthic foraminifera distribution ES6001 structure, which belong to Psammosphaera, Pseudothurammina and Saccammina genera, and also Reophax species. The decrease in the size of benthic foraminifera in the Baltic Sea was also mentioned in the other papers [Brodniewicz, 1965;Binczewska et al, 2017;Hermelin, 1987], as the response of the organisms to reduced salinity. In the Gdansk Basin, carbonate shells were found only at the stations, situated in the depressions and characterized by anomalously high salinity (> 13 PSU), while in the Bornholm Basin they were found at all stations.…”

“…In the Gdansk Basin, carbonate shells were found only at the stations, situated in the depressions and characterized by anomalously high salinity (> 13 PSU), while in the Bornholm Basin they were found at all stations. As was already shown in [Binczewska et al, 2017;Brodniewicz, 1965;Hausler, 2017] that calcareous species were dominated by Cribroelphidium (Elphidium) genus. Species of this genus are characterized as opportunistic and euryhaline occurring under the conditions of increased productivity and reduced oxygen and salinity (up to 11-12 PSU) [Binczewska et al, 2017;Brodniewicz, 1965;Hausler, 2017;Kaiho, 1994;Lutze, 1965;Murray, 2006].…”

“…Wet counting was additionally applied to the samples taken in the 33rd cruise of the R/V Akademik Nikolai Strakhov. As described in [Binczewska et al, 2017;Brodniewicz, 1965], this method of sample preparation is used to avoid agglutinated tests breakage, which represent the majority of the population in the Baltic Sea.…”