Environmental changes on the inner northeastern Black Sea shelf, off the town of Gelendzhik, over the last 140 years

Abstract

A comprehensive multi-proxy study of mini-core Ash-2009-08 from the inner shelf of the northeastern Black Sea, allowed us to define recent environmental changes in the area, on a sub-decadal resolution. The age model of the core based on radionuclide (210Pb and 137Cs) measurements suggests an average sedimentation rate of 2 mm/y and, consequently, a sedimentary record spanning the last ∼140 years. Overall, four fossil groups studied from the core indicate subtle environmental changes. The dinocyst assemblages show stable surface water conditions with a possible cooling in the 1930s-1950s and a slight decline in salinity from the 1960s, which corresponds to an increase in precipitation and the fresh water balance of the Black Sea, known from the published instrumental data. A gradually increasing abundance of the heterotroph species suggests a nutrient increase from the 1920s. Ostracods and molluscs demonstrate rather stable bottom-water salinity and biotopes over the last ∼75 years. Changes in abundance of ostracods and benthic foraminifers, as well as changes in the mollusc diversity, most likely reflect variations in trophic conditions with the higher food supply to the sea floor in the 1940s–1960s and from the late 1990s. Enhanced bottom-water energy is inferred from the mollusc enrichment and a relatively high content of the fine sand grain-size fraction in the 1990s. In the pollen assemblages, trees mainly represented by Pinus strongly prevail over herbs throughout the record. However, the trees to herbs ratio demonstrates a slight decline from the 1920s, most likely resulting from human impact on the vegetation of the nearby land.