1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
A 3-years EU-MAST-3 project (Baltic Sea System Study, BASYS) recovered short and long sediment cores from 3 deep basins of the Baltic Sea (Bornholm Basin, Gotland Basin and North Central Basin). During a paleoenvironmental study, lead-210 dating andgeochemical data were generated.Dating of cores from the Bornholm Basin suggests disturbed sediment surfaces probably caused by human activity. Sedimentation rates are in excess of 5 mm/a. Sediments from the North Central Basin have rates, between 3 and 5mm/a, while the rates for the Gotland Basin are at 2-3 mm/a. Ca-Mn accumulations are ascribed to rhodochrosite formation which is thought to be coupled to saltwater inflows in that oxygen and HCO_3- rich saltwater converts bacterially re-dissolved Mninto the carbonate mineral. 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. Mo accumulations with peak valuesexceeding 300 mg/kg are found in all cores. 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 finallysettling with biogenic remains or in remnants of their grazers. >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 algalblooming in the central Baltic which however reqire a distinct and relatively constant salinity range. Blooming periods generate finely laminated sediments by settling of larger flocs of biogenic remains on the seafloor. Under normal conditions (increasedwindforcing), 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.
Planteproduktion og stofomsætning; Risø-R-1077; Risø-R-1077(EN)