SALINITY AND REDOX ALTERNATIONS IN THE NORTHWESTERN BALTIC PROPER DURING THE LATE HOLOCENE

Selected geochemical parameters and siliceous microfossil assemblages in Baltic Sea sediments are presented which reflect past variations in redox conditions, salinity and primary production. The sediments were deposited during the freshwater Ancylus Lake (9500-8000 C-14 BP) and brackish Litorina Sea (8000-3000 BP) stages of the Baltic. The diatom record shows that surface-water salinity increased further at c. 7000- 6500 BP, although smaller amounts of brackish water entered the basin from c. 8000 BP onwards. Attempts to use exchangeable Mg as a palaeosa linity indictor were not applicable. Gross primary productivity increa sed along with salinity, which has been interpreted as an effect of nu trient enrichment in the photic zone. This led between c. 6500 and 450 0 BP to a high accumulation of organic carbon, anoxic or nearly anoxic bottom conditions and formation of laminated deposits. Certain lamina e consist of alternating layers of organic and minerogenic material an d were probably formed annually, i.e. in the manner of varves. The lam inated successions are distinguished by enrichments of V, Cu and espec ially Mo. The highest Mo content occurs in the core from the greatest water depth, an effect of anoxic conditions during deposition. The Fe/ Mn ratio was shown to be ambiguous as an indicator of past redox condi tions. Since biogenic silica shows large variablity in contemporaneous ly deposited sediments, this parameter cannot be used as a proxy for t he past production of siliceous algae in the Baltic Sea.