Black Sea paleosalinity evolution since the last deglaciation reconstructed from alkenone-inferred Isochrysidales diversity

•Isochrysidales diversity provides new constraints on Black Sea salinity for the past 16 kyr.•Group I Isochrysidales is discovered in Unit III sediments for the first time.•RIK37 and C38Me/C38Et alkenone ratios provide quantitative salinity assessment.•Black Sea surface salinity changed gradually during marine transgression.•Alkenone hydrogen isotope data re-interpreted based on new results. The Black Sea underwent dramatic changes in salinity from the last glacial maximum to Holocene as it evolved from a large inland lake to become a part of the global ocean due to post-glacial sea level rise. However, the detailed history of the re-connection of the Black Sea to the Mediterranean and the resulting Black Sea salinity variations have been heavily debated. We take advantage of our recent study on alkenones from phylogenetically classified haptophyte groups and their association with variable salinity levels in modern environments to reconstruct detailed salinity changes in the Black Sea over the past 16,000 years. We report the first discovery and documentation of alkenones from Group I Isochrysidales in relation to past salinity changes in the Unit III sediment. Our data indicate that the Black Sea surface salinity gradually increased from near fresh at ∼16 ka (∼1-2 psu) to oligohaline (∼ 5-6 psu) at ∼ 9.4 ka, the time of initial marine inflow, suggesting a negative regional hydrological balance. Although the IMI (initial marine inflow) occurred at 9.4 ka, the relatively low surface water salinity levels persisted to the beginning of sapropel formation at 7.6 ka, suggesting a strongly positive regional hydrological balance. Alkenone profiles also indicate a period of significantly reduced salinity between 3.5 and 1.6 ka, differing from the previous proposed peak salinity at 2.7 ka based on alkenone hydrogen isotope values. We demonstrate that the mismatch originates from the predominance of previously unidentified Group II Isochrysidales over E. huxleyi during this time interval, and when species-specific hydrogen isotopic fractionation of alkenones is taken into consideration, the isotope-inferred salinity between 3.5 and 1.6 ka is consistent with our proposal. Our new assessment of the Black Sea salinity evolution since the last glacial shows broad agreement with other regional records and provides a new basis for evaluating the impact of regional hydrological changes on Neolithic human migrations to Europe.