VOLUME 53 (2)/August 2024

The main point of the study is the juxtaposition in the characteristics of the Black Sea, such as salinity and water level, for the last ~20,000 years. These characteristics have been depending on climate changes and geological events that occurred during this time interval. The Black Sea Basin has undergone significant ecological and geological changes since the end of the Pleistocene (following the Last Glacial Maximum). The study’s main motif is to synchronize events and data from previous and new studies. This study includes some new data on rice-like aragonite at the base of sapropel sediments and radiocarbon dating of reef mussels. A medium surface salinity curve for the Black Sea Basin is derived for the studied time period. The sudden change in salinity is associated with rapid increase in water level in the basin following the penetration of saltier waters through the Bosporus. The conclusion was drawn after comparing current hypotheses.

The falsiovalis, rhenana, triangularis, marginifera and praesulcata conodont zones have been recognized within the Upper Devonian (Frasnian–Famennian) carbonate succession in the Preslavtsi-2 well from the Moesian Terrane in NE Bulgaria. Five microfacies types (MFT 1–5) are also distinguished and described within the Frasnian carbonate deposits: fenestral peloidal microbial bindstone (MFT 1a); fenestral microbial bindstone with porostromate microstructures (MFT 1b); paleosiphonoclad wackestone and packstone (MFT 2); bioclastic-peloidal packstone and grainstone (MFT 3); bioturbated wackestone to grainstone (MFT 4); and lithoclastic rudstone (MFT 5). They are interpreted as formed in arid peritidal and lagoon environments or deposited in deep-water setting, located below the storm wave base. A sea-level transgression is documented during the late Frasnian (Late rhenana Zone) corresponding to the Lower Kellwasser Event, which was followed by rapid regression. At the beginning of the Famennian, a climate change occurred when the Moesian Terrane was removed from the Subtropical Arid to the Warm Temperate Climatic Zone, when various shallow- and open-marine carbonates were formed under warm temperate climate conditions.

Toxic Hg species pose a global ecological problem due to the significant anthropogenic emissions of Hg into the atmosphere, especially during coal combustion. A study about the content, association and potential modes of occurrence of mercury and their ecological significance in eight biomass varieties belonging to woody, herbaceous, agricultural and aquatic biomass groups was conducted based on a combination of different chemical and mineralogical analyses and leaching procedures. The Hg contents in these biomasses vary between 0.003 ppm and 0.043 ppm (mean 0.013 ppm), which are about an order of magnitude lower than the Clarke value of coal (0.10 ppm). It seems that Hg occurs in both inorganic and organic matter of biomass, as the preferable association and potential modes of occurrence of this element include inorganic matter, hemicellulose, and water-soluble Cl-, S-, N-, and Na-bearing chlorides, sulphates, and nitrates such as halite, sylvite, arcanite, Ca nitrate, and nitrocalcite in the biomass system studied. Alternative renewable and C-neutral solid fuels poor in Hg such as appropriate sustainable biomasses are suggested to substitute partially or completely the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants to minimize the ecological problems related to this element.