The Holocene Black Sea reconnection to the Mediterranean Sea: New insights from the northeastern Caucasian shelf

Recent findings about the evolution of palaeogeographic conditions of the Black Sea during the Holocene have significantly improved our understanding of the profound environmental changes that took place around 9 ka ago, when the Neoeuxinian Lake reconnected to the global ocean. In contrast to the western and southeastern regions where numerous studies have been recently performed, the northeast region remains relatively under investigated. We carried out the first multi-proxy continuous study of a sediment core (Ak-2575) from the northeastern Black Sea shelf that includes benthic calcareous fossils (ostracods, molluscs and foraminifers), dinoflagellate cysts (dinocysts) and sedimentology, thus providing reconstructions of surface and bottom-water conditions. The age model of the core is based on 10 AMS-14C dates. Calibrated ages are used throughout the manuscript. The first appearance of Mediterranean elements is documented at 9.6 cal. ka BP. Our data provide evidence of sustained cohabitation of benthic species of Caspian and Mediterranean origins, represented by different ontogenetic stages, from at least ~ 7.8 (or even 8.8) to 6.7 cal. ka BP with the gradual disappearance of brackish species suggesting a gradual increase in salinity and most likely a change in the salt composition. Dinocyst assemblages show species succession that is coherent across the Black Sea basin, with brackish taxa dominating until ~ 8.5 cal. ka BP and being slowly replaced by euryhaline species. The occurrences of authigenic gypsum crystals, especially abundant at ~ 7.4 and 6.5 cal. ka BP, suggest the temporal appearance of hydrogen sulphide at the shelf edge which during certain periods appears to reduce the abundance of benthic fauna