The organic, inorganic and isotope geochemistry of the holocene sapropel units in the sea of Marmara and their paleoceanographic significance

The ocean redox structure and nitrogen utilization are investigated in the Holocene sapropel depositions in the Sea of Marmara, based on inorganic, organic, and isotopic analyses and grain-size distribution of the sediments of a piston core from Çınarcık Basin. Two sapropel units are identified in the core studied, the lower (main) sapropel of the early Holocene and the upper sapropel of the middle-late Holocene. Relatively high C/N ratios (10–11) and smaller δ13Corg values (~−26‰) in the lower Holocene sapropel unit indicate that the organic matter is mainly of terrestrial origin. Up to 5.5‰ δ15N values and Mo concentration (6.5 ppm) above crustal values in the bottom of the lower sapropel unit strongly suggest that bottom-water conditions were denitrifying and suboxic-dysoxic, suitable for the preservation and burial of the organic matter. Such bottom-water conditions were induced by water stratification that resulted from the transgression from the Aegean Sea coupled with a riverine influx from the Black Sea that provided the terrestrial organic matter. A progressive upward decrease of C/N ratios and increase of δ13Corg values in the upper sapropel unit might have been caused by the additional supply of organic matter of mainly marine origin and improved ventilation of the lower water, which resulted in the oxidation of the organic nitrogen pool and a decrease of δ15N values (below 3‰) under nitrate-rich conditions. Mn enrichment in the upper sapropelic sediments also supports a relatively oxidative environment. •Nitrogen isotopic compositions in the Holocene sapropels from the Sea of the Marmara are reported for the first time.•The redox condition changed from suboxic-dysoxic to relatively oxidative during the Holocene sapropel deposition.•The nutrition condition shifted from low nitrate to temporally nitrate-rich during the sapropel deposition.•Holocene sapropel deposition in the Sea of Marmara was closely related to the regional paleo-hydrological condition.