MAGNETIC MINERALS AS INDICATORS OF MAJOR ENVIRONMENTAL CHANGE IN HOLOCENE BLACK SEA SEDIMENTS: PRELIMINARY RESULTS

Abstract

Four of the 38 sediment cores, collected from the northwestern part of the Black Sea (French–Romanian BLASON Programme, 1998), were analysed in order to study the major hydrological and environmental change of the sea during the Holocene. This change was caused by the inflow of the marine Mediterranean waters into the freshwater basin of the Black Sea. Two cores from the northwestern shelf and two from the deep part of the sea, representing distinct sedimentary environments along the same profile, were chosen. Lacustrine sediments from deep-water cores and uppermost marine sediments in all studied cores are characterised by the presence of pyrite (FeS2), which indicates anoxic, sulphate reducing conditions in sediment and in water column (as it is observed now in the Black Sea). Pyrite formation in this anoxic environment takes place below the oxic/anoxic limit, by reduction and sulphidation of iron. The transition from freshwater to marine conditions is marked in all studied cores by the presence of ferrimagnetic iron sulphide––greigite (Fe3S4). Greigite is an intermediate mineral in the pyritisation reactions during early diagenesis, which can be preserved in the sediment if pyritisation cannot be accomplished, due to a high Fe/S ratio. In shallow water cores from the shelf, where marine transgression is marked by a discontinuity, greigite is present below and above it. This suggests that the anoxic, sulphate reducing conditions, allowing preservation of greigite, started in the sediment after marine transgression. In cores from the deep part of the sea, the transgression is represented by a level of sediments with high organic matter content (sapropel). The greigite is present only in the lower part of this level and its formation is related to the arrival of marine water. The presence of pyrite in the upper part of the lacustrine sediments indicates that anoxic conditions probably prevailed here during marine transgression. These results show the sensitivity of magnetic parameters to major environmental change in the Black Sea, and allow a correlation between cores from different depths, by recognition of greigite in the sediments marking the marine transgression.