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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. [ABSTRACT FROM AUTHOR]

To detect climatic linkages between the Baltic Sea, the Skagerrak and the Nordic Seas, we present multi-proxy reconstructions covering the last 4500 years from three sediment cores taken in the Skagerrak and along the SW Norwegian margin. Foraminiferal assemblages at all three sites show a distinct change at c. 1700 years BP, associated with a transition from absence and rare occurrence of Brizalina skagerrakensis during c. 4500-2300 years BP to its subsequent abundance increase, suggesting a stronger influence of nutrient-rich water-masses during the last c. 1700 years. Increased nutrient availability, which probably stimulated higher primary productivity, is further supported by an increase in diatoms, total organic carbon and benthic foraminiferal species indicative of high productivity and carbon fluxes during the last c. 1700 years as compared to c. 4500-2300 years BP. The amplitude of the B. skagerrakensis signal is largest in the central Skagerrak and gradually becomes smaller towards the Norwegian Sea suggesting that the dominant source of the nutrient-rich water was the brackish outflow from the Baltic Sea. The generally lower abundances of planktonic foraminifera since c. 1700 years BP support the hypothesis of less saline surface water conditions in the Skagerrak. These results agree with other studies, which suggest a stronger Baltic outflow over the last 1700 years coinciding with a general cooling, increased wintertime westerlies bringing more winter precipitation to northern Europe, increased river runoff and higher frequency of floods. The increase in outflow also occurs during deposition of laminated sediments in the deep Baltic Sea. Leakage of dissolved inorganic phosphorus from anoxic sediments, as well as enhanced erosion due to deforestation in combination with higher runoff from Norway, coastal upwelling and more vigorous frontal dynamics may all have contributed to higher nutrient availability within the adjacent Skagerrak during the last 1700 years BP as compared to c. 4500-2300 years BP, when low productivity prevailed in the study area. [ABSTRACT FROM AUTHOR]