Holocene ostracod and foraminiferal assemblages of the Romanian Black Sea shelf

Abstract

Qualitative and quantitative study of the ostracod and foraminiferal assemblages were performed on two cores, around 0.30 m depth bsf in thickness, collected from the Romanian Black Sea shelf. The lithology of the studied cores is characterised by grey and black muds, topped by a centimetre thick fluffy mud layer. One of the cores, MN 103_09, is placed on the Romanian inner shelf, at water depth of 17.80 m. The sediments of this core contain very few and small bivalve fragments and no gastropods. In this core, the ostracod and foraminiferal assemblages are influenced by a quite high-energy environment, due to the Sulina arm of the Danube Delta. This feature is mirrored in the poor preservation and low abundance of the species, as well as the mixed Caspian and Mediterranean character of the ostracod assemblages. The other studied core, MN 103_04, is situated on the Romanian outer shelf, at a water depth of 78 m. Its deposits commonly contain gastropods and bivalves, as well as microfauna displaying a low diversity and a very high abundance; the character of the ostracod assemblages is a Mediterranean one. Overall, the identified microfaunas of the two investigated cores reflect the environmental conditions of modern and/or latest Holocene sea-floor assemblages and their relationships to salinity, sediment input and water depth. Even the depositional time of the 30 cm cored interval in the two studied cores is quite different, i.e., possibly several hundreds of years, up to 1000 years, on the inner shelf and several thousands (from 3000 BP to the Present), in the outer shelf, the results are indicative for modern Black Sea conditions. A stable marine environment with a salinity around 17–18‰, comparable with the present one, could be assumed for the whole cored interval of the Romanian shelf. Besides, the age structure, i.e., the ratio between adults and juvenile taxa of ostracod population in the outer shelf indicates a moderate to high energy autochthonous thanatocoenosis, with some post-mortem disturbance by currents. By contrast, the ratio between adults and juvenile ostracod taxa in the inner shelf mirrored a taphocoenosis with post-mortem disturbance, sorting and transport. This assemblage, dominated mainly by disarticulated valves of juveniles, does not represent the environment in which the ostracods lived.