An authigenic calcite layer in the sediments of the Sea of Marmara—A geochemical marker horizon with paleoceanographic significance

Three sediment cores taken from the Central Sea of Marmara were sedimentologically and geochemically investigated. All the cores show a continuous sedimentation record. According to isotopic age-dating measurements, two of the cores cover an age range of more than 20,000 years. Since the Sea of Marmara was an isolated lacustrine system during the last glacial maximum (about 20 ka BP), the two cores should include the transition from lacustrine to marine environmental conditions. The third core has a very high sedimentation rate (70 cm/ka), and therefore does not reach the transitional section. In two cores, however, a very calcite-rich layer (Unit C) was observed. Carbon-isotope investigations show that this calcite is not of diagenetic or biogenic origin but may represent a direct inorganic precipitate out of a deepwater layer. In order to prove this hypothesis, we used a geochemical model based on thermodynamic calculations. This model shows that the calcite precipitation can only be explained by mixing of anoxic deep lacustrine water with oxic marine Mediterranean water. The necessary aqueous conditions are also obtained. The model presented thus provides an important contribution to the evolution of the Sea of Marmara during the transition from lacustrine to marine conditions. The thickness of the Unit C layer indicates that this environmental change lasted approximately 1.5 ka.