Holocene history of the Baltic Sea as recorded in a sediment core from the Gotland Deep

• Published in: Marine geology 1996, Vol.134 (3), p.183-201
• Main Authors: Sohlenius, G., Sternbeck, J., Andrén, E., Westman, P.
• Format: Article
• Language: English
• Subjects: Archaeology subjects; Arkeologiämnen; Authigenic minerals; Baltic Sea; Earth sciences; Geovetenskap; Historisk-filosofiska ämnen; History and philosophy subjects; Holocene; HUMANIORA och RELIGIONSVETENSKAP; HUMANITIES and RELIGION; Laminated sediments; NATURAL SCIENCES; NATURVETENSKAP; Siliceous microfossils
• ISSN: 0025-3227; 1872-6151; 1872-6151
• DOI: 10.1016/0025-3227(96)00047-3

A 4 m long sediment core from the Gotland Deep, Baltic Sea, was investigated by means of biostratigraphical and chemical parameters and 14C datings. This multidisciplinary approach allows us to evaluate the regional changes in salinity and redox conditions during the Holocene. According to the diatom assemblages and 14C datings, the sediments were deposited in the brackish and freshwater phases of the Yoldia Sea (10,300–9600 14C yr B.P.), the freshwater Ancylus Lake (9600-8000 14C yr B.P.) and in the brackish water Litorina Sea (8000-3000 14C yr B.P.). Of several palaeosalinity indicators evaluated, the diatoms are the least ambiguous. Nevertheless the B content and, under some premises, the C S ratio may be used for palaeosalinity information. Iron sulphides in the lowermost freshwater sediments formed below the prevailing, oxic sediment-water interface. Pyrite in the uppermost freshwater sediments was to a large extent formed in Litorina time, by diffusion of ΣH 2S from overlying brackish water sediments. The Ancylus Lake/Litorina Sea transition can be traced by the marked changes in both the siliceous microfossil assemblages and in the chemical properties of the sediments. An early Litorina phase is characterised by increased primary production, progressively increasing salinity and the development of euxinic conditions. In the Litorina Sea laminated clay gyttjas (def. as a clay containing 6–30% organic matter) containing pyrite and laminae of Ca-rhodochrosite, (Mn,Ca)CO 3, accumulated. Both these minerals formed in close connection with sediment deposition. The rhodochrosite laminae formed during occasional inflow of oxygen rich, marine water through the Danish Straits. Rhodochrosite is by far the most common mineral in the laminae and formed on average with a periodicity of two to three years.