Molecular isotopic and dinoflagellate evidence for Late Holocene freshening of the Black Sea

The Black Sea is the world's largest anoxic basin with oxygen-free conditions below water depths of approximately 100 m resulting from strong density stratification. The salinity of its surface water likely varied substantially over time due to variations in freshwater input from large rivers and in the saline bottom water of Mediterranean origin coming across the shallow sill of the Bosporus. However, long-term reconstructions of surface water salinities are lacking. The invasion of the coccolithophorid Emiliania huxleyi in the Black Sea at approximately 2720 a, responsible for a marked change in sediment composition (lithology), has been commonly attributed to salinity levels rising above 11. We analyzed the δD values of long-chain alkenones produced by haptophyte algae, mainly E. huxleyi, in a core from the eastern basin of the Black Sea to reconstruct past variations in sea surface salinities, and combined this with relative salinity changes generated from organic-walled dinoflagellate cyst (dinocyst) distributions from the same core. Combined results indicate a substantial freshening of Black Sea surface waters in the last 3000 years, suggesting that sea surface salinity was substantially higher than the present-day salinity of approximately 18 at the time E. huxleyi invaded the Black Sea.