Dating and assessing the recent sediments of three deep basins of the Baltic Sea: Indication of natural and anthropogenic changes

During a 3-years EU-MAST-3 project (Baltic Sea System Study, BASYS), short and long (> 5 m) sediment cores were collected from 3 deep basins of the Baltic Sea: Bornholm Basin, Gotland Basin and North Central Basin. As part of a paleoenvironmental study, lead-210 dating and geochemical data were generated at the Gamma Dating Center (GDC) of the Risø National Laboratory.
Dating of cores from the Bornholm Basin was generally hampered by the fact that lead-210 and Cs-137 depth profiles suggest disturbed sediment surfaces probably caused by human activity (e.g., trawling). Sedimentation rates are therefore in excess of 5 mm/a. The sediments from the North Central Basin exhibit the lowest rates, between 1 and 2 mm/a, while the rates for the Gotland Basin were at 2-3 mm/a. All the short core and long box core chemical analyses were used in the paleoenvironmental interpretations. From the large geochemical data base, however, only a few elements were chosen for more detailed discussions. Ca and Mn covary perfectly throughout the whole marine section of a long box core from the Gotland Basin. Ca-Mn accumulations are ascribed to the mineral rhodochrosite. Rhodochrosite formation is thought to be
coupled to saltwater inflows in that oxygen and HCO₃^- rich saltwater converts bacterially re-dissolved Mn²⁺ into the carbonate mineral. For the upper 1.5 m of the Gotland Basin, there is a clear indication for cyclic rhodochrosite deposition in that about 300 year long periods with relatively high Ca-Mn are followed by about 300 years lasting sections with low Ca-Mn. Presently, a coupling to trangressional/regressional phases (sealevel change) in the Baltic is favoured to explain the cyclicity. Mo accumulations with peak values exceeding 300 mg/kg are found in all cores from the three deep basins. Mo has previously been used as an anoxicity indicator in Baltic Sea sediments, but there is strong evidence that Mo is coupled with the biogenic phase in the sediments because there is a strong correlation with, e.g. organic carbon and total nitrogen. The Mo transport to the seafloor is thought to be coupled with the nitrogen fixation processes by cyanobacteria being known for their need of Mo as central element in the nitrogen fixing enzyme. Mo is finally settling with biogenic remains or in remnants of their grazers. It is worthwhile to mention that it is usually the uppermost flocculated sediment surface layer that shows high Mo in all three basins, but Mo accumulation does also occur at 1 m depth in the Gotland Basin box core and longer down in the core, suggesting that cyanobacterial blooms extracting Mo from seawater have occurred earlier in time. The question then is whether the present-day accumulations are a natural phenomenon or caused by eutrophication. Taking the occurrences of laminated and thickly bedded sediments, based on the C_org and Mo data being coupled with the Mo behaviour, a different model of the formation of laminated and homogeneous sediments in
the Baltic Sea is proposed A well-stratified water column and sufficient supply of nutrients leads to algal blooming in the central Baltic. Cyanobacterial blooms require however a distinct and relatively constant salinity range because such blooms are usually not observed in the more saline North Sea. Blooming periods generate finely laminated sediments by settling of larger flocs of biogenic remains on the seafloor. In periods of increased wind forcing (normal conditions in the central Baltic), a relatively thick and well-ventilated surface water layer is formed with normal primary production. Particle transport to the seafloor is then restricted and more homogeneous sediments are deposited.