Holocene paleoecology and paleoceanography of the southwestern Black Sea shelf revealed by ostracod assemblages

The Holocene replacement of Ponto-Caspian ostracod assemblages by Mediterranean species is studied in two long composite cores, M02-45 (a composite of cores M02-45P, M02-45 T and M05-03P) and M05-50 (a composite of cores M05-50P and M05-51G), acquired at sites −69 m and −91 m deep on the southwestern Black Sea shelf. Composite core M02-45 was collected from the middle shelf and composite core M05-50 was acquired on the distal fringe of the eastern levée of a saline underflow channel emanating from the Strait of Bosphorus. Sixteen radiocarbon dates in M02-45 and nine in M05-50 are used to construct age models, which show recovery of sediments as old as 12,915 cal yr BP (M02-45 site) and 12,010 cal yr BP (M05-50 site). A total of 45 ostracod species are identified in the two cores. From ~12,000 to ~7425 cal yr BP, the ostracod assemblage is dominated by Ponto-Caspian species, mainly Loxoconcha sublepida, L. lepida and Tyrrhenocythere amnicola donetziensis. From ~7425 to ~6315 cal yr BP the assemblage consists of nearly equal abundances of Mediterranean species (Cytheroma variabilis in M02-45; Sagmatocythere littoralis in M05-50) and the Ponto-Caspian species. After ~6315 cal yr BP to the tops of the cores, the assemblage is dominated by Mediterranean species, including Palmoconcha agilis, Carinocythereis carinata, Hiltermannicythere rubra and Pterygocythereis jonesii. Cluster analysis further subdivides the stratigraphic succession into six bioecozones with different ostracod assemblages. The changes in the ostracod assemblages from one bioecozone to the next indicate that progressive environmental changes took place on the southwestern Black Sea shelf from at least 7500 cal yr BP to the present. The first hint of changing conditions at ~7500 cal yr BP lags the initial reconnection to the Mediterranean Sea through the Strait of Bosphorus by ~2000 yr, demonstrating that Black Sea salinity increased slowly and took that long to reach values tolerable to marine ostracod immigrants. Widespread colonization by Mediterranean species took even longer, ~3000 years from the time of the initial reconnection