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- Andrén, Thomas, et al.
(författare)
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Late Quaternary development of the north-western Baltic Proper : Results from the clay-varve investigation
- 1995
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Ingår i: Quaternary International. - 1040-6182 .- 1873-4553. ; 27, s. 5-10
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Tidskriftsartikel (refereegranskat)abstract
- Within the project "Late Quaternary development of the north-western Baltic Proper" a total of 36 piston core samples have been collected. The results presented in this study are based on analyses of 7 cores with emphasis on the late glacial part of the sequence. The varved glacial clay sequences have been analyzed for calcareous micro-fossils and mineral magnetic properties as well as measurements of clay-varves. The results from these analyses indicate that there is a close correlation between the occurrence of brackish water ostracods and foraminifera and a distinct change in the mineral magnetic parameters. The lithological composition of the clay-varves also differs in this sequence. The duration of this marine phase of the Yoldia Sea stage is 60 to 120 clay-varve years. It is concluded that correlations between the cores can be made using fossil assemblages and mineral magnetic parameters. It has not yet been possible, however, to make any reliable clay-varve connections.
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| 4. |
- Sohlenius, Gustav, et al.
(författare)
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Holocene history of the Baltic Sea as recorded in a sediment core from the Gotland Deep
- 1996
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Ingår i: Marine Geology. - : Elsevier. - 0025-3227 .- 1872-6151. ; 134, s. 183-201
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Tidskriftsartikel (refereegranskat)abstract
- A 4 m long sediment core from the Gotland Deep, Baltic Sea, was investigated by means of biostratigraphical and chemical parameters and C-14 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 C-14 datings, the sediments were deposited in the brackish and freshwater phases of the Yoldia Sea (10,300-9600 C-14 yr B.P.), the freshwater Ancylus Lake (9600-8000 C-14 yr B.P.) and in the brackish water Litorina Sea (8000-3000 C-14 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 Sigma H2S 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)CO3, 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.
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