A Holocene dinocyst record of a two-step transformation of the Neoeuxinian brackish water lake into the Black Sea

An exceptionally high-resolution and species-rich dinoflagellate cyst record from core M02-45 collected from the southwestern Black Sea shelf provides strong evidence of a gradual reconnection between the Black (BS) and Mediterranean (MS) seas at the beginning of the Holocene. Two main assemblages, one dominated by brackish species, Spiniferites cruciformis and Pyxidinopsis psilata, and freshwater algae, and a subsequent one, characterised by euryhaline species (Lingulodinium machaerophorum, Brigantedinium spp., Protoperidinium ponticum), document a progressive change in sea-surface conditions from low saline (∼7–12 psu) to present-day conditions. A first major pulse of marine waters is recorded at around 8.46 ka BP, with a maximum of L. machaerophorum. The occurrence of this species from the bottom of the core, dated at 9.3 ka BP, supports the hypothesis that water levels were already high on the southwestern shelf by that time. Fully present-day conditions are recorded at around 5.6 ka BP, when brackish species and morphotypes of S. belerius, S. bentorii and L. machaerophorum disappeared. Arrivals of Mediterranean species (Operculodinium centrocarpum and S. mirabilis) are observed simultaneously in the southwest and southeast region of the BS at around 7 ka BP. Despite a different protocol for palynomorph preparation and presentation of data, previous studies from the northern shelf also document the arrival of euryhaline species at 7 ka BP, and marine influence prior to that time. The history of harmful algal blooms (HABs) shows a correlation with warmer mid-Holocene temperatures, followed by a succession of introductions possibly associated with early Greek exploration, then merchant shipping.