Integrated sediment multiproxy studies and modeling were used to reconstruct past changes in the Baltic Sea ecosystem. Results of natural changes over the past 6000 years in the Baltic Sea ecosystem suggest that forecasted climate warming might enhance environmental problems of the Baltic Sea. Integrated modeling and sediment proxy studies reveal increased sea surface temperatures and expanded seafloor anoxia (in deep basins) during earlier natural warm climate phases, such as the Medieval Climate Anomaly. Under future IPCC scenarios of global warming, there is likely no improvement of bottom water conditions in the Baltic Sea. Thus, the measures already designed to produce a healthier Baltic Sea are insufficient in the long term. The interactions between climate change and anthropogenic impacts on the Baltic Sea should be considered in management, implementation of policy strategies in the Baltic Sea environmental issues, and adaptation to future climate change.

The focus of this study was the 11.55 m long sediment core 303700-7, which was retrieved from the Gdansk Basin during a cruise of RV 'Poseidon' within the frame of the Russian-German Project GISEB. The core was analysed for grain size, elemental chemical composition, organic carbon and palynological spectra. The age control was based on palynostratigraphy and 7 radiocarbon datings of bulk sedimentary organic matter. These data provide a high-resolution record of climatic and marine palaeoenvironments in the Gdansk Basin for the last ca. 13 kyr, from the Bølling to late Holocene time. Sedimentation rates were estimated to vary between 0.37 and 1.62 mm yr -1 . Major variations in palaeosalinity were estimated from bromine concentrations in the sediment. This method allows the first quantitative reconstruction of palaeosalinity changes in the Baltic Sea, especially profound during the Littorina and Postlittorina periods (middle to late Holocene). In addition, grain size data indicated several Littorina transgressive-regressive stages and a few episodes of increased near-bottom current activity. Our results from the Gdansk Basin are consistent with palaeoceanographic data from other deep basins of the Baltic Sea and provide new insights into the regional Holocene history.

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