Physical and Chemical Characteristics of the Near-Bottom and Pore Waters, and the Bottom Sediments in the Northeastern Part of the Black Sea Shelf

Purpose. The investigation is purposed at studying the features of the vertical distribution of geochemical parameters and chemical profile of pore waters in bottom sediments in the coastal zone of the northeastern part of the Black Sea.Methods and Results. 24 samples of the bottom sediments surface layer were obtained in three cruises on the R/V "Professor Vodyanitsky" in September 2018, July 2019 and August 2021. 5 samples of cores and 9 samples of water from the near-bottom layer were taken in August 2021. The profiles of vertical distribution of oxygen and hydrogen sulfide, the iron oxidized and reduced forms in pore waters, and also oxygen concentration and the degree of oxygen-saturation of the near-bottom waters were studied; the geochemical characteristics of bottom sediments in the surface layer (0-5 cm) and in the cores were also determined. It was noted that the sediments surface layer in the northeastern region was formed by a silty material, the maximum portion of which (83-97%) was recorded in the areas where large rivers of the Krasnodar region flowed into the sea. In the sediments surface layer, the Corg content varied within 0.2-1.9% dry mass. The vertical distribution of Corg in the bottom sediments cores is characterized by a tendency to decrease in its content with depth; its maximum concentrations were observed in the upper (0-4 cm) layer.Conclusions. It is established that in the area under study, the sediments upper layer is characterized by the aerobic conditions (oxygen penetrated up to 10 mm); in the sediments, the organic matter is oxidized mainly with the participation of iron (Fe(II, III)), that leads to developing the suboxic conditions. In some areas, the redox conditions in the sediments changed from suboxic (the region of Anapa and the area between the cities of Gelendzhik and Tuapse) to anoxic (the region of Novorossiysk and Tuapse), that is conditioned by occurrence of the sulfur reduced forms in the bottom sediments layer. It has been revealed that in the northeastern part of the Black Sea, the distribution of Fe(II, III) in the pore waters of bottom sediments is inversely proportional to the distribution of pH value (the correlation coefficients are -0.68 and -0.73, respectively).