Microfabric study of diatomaceous and lithogenic deposition in laminated sediments from the Gotland Deep, Baltic Sea

The deep basins of the Baltic Sea are commonly anoxic, and finely laminated diatomaceous sediments have been deposited at intervals throughout the last 8000 yr. The origin and composition of individual laminae in Gotland Deep sediments have often proved difficult to characterise using traditional micropalaeontological and sedimentological studies. Here, we present a scanning electron microscope study in which lamina down to 30 μm in thickness with distinct mineralogical or micropalaeontological composition have been identified and described. Depositional laminae sequences in the form of couplets, triplets and quadruplets of diatomaceous and lithogenic laminae are observed with an average thickness of approximately 700 μm. Diagenetic Ca-rhodochrosite laminae also occur within these depositional sequences. Examination of the diatom assemblages suggests that these bundles of laminae represent annual deposits, or varves. Varves are relatively uncommon, and typically occur in small intervals of two to five varves, which are interrupted by more diffusely laminated and homogenous sediments. The origin of these more massive sediments probably relates to periodic re-oxygenation of the basin on inter-annual time scales and destruction of varves by bioturbation.