Recent multi-tracer dating of the Black Sea sediments: Recovery of the late post-Chernobyl trends of radioactive contamination

The combined use of artificial (¹³⁷Cs, ^{238,239+240,241}Pu, ²⁴¹Am) and naturally occurring (²¹⁰Pb/²²⁶Ra, ⁴⁰K) radionuclides as tracers for dating of the Black Sea sediments has allowed to reveal the recent increase in radioactive contamination of this basin with ¹³⁷Cs after the Chernobyl accident. This is presumably caused by the prolonged input of the man-made radionuclides that showed a rise in the late 1990s - early 2000s, and by a possible post-deposition remobilization of ¹³⁷Cs from the seabed.

Distribution and migration of 239+240Pu in abiotic components of the Black Sea ecosystems during the post-Chernobyl period

Distribution of ^239,240Pu in abiotic components (water and bottom sediment) of the Black Sea ecosystems was studied during the post-Chernobyl period at different offshore and near-shore locations. The trends of these radionuclides accumulation by sediments were analyzed. The spatial-temporal changes in the ^239,240Pu distribution as well as effective half-life for these radionuclides in the Black Sea surface water in deep-sea area are presented. The estimations of the average annual removal fluxes of the ^239,240Pu into the bottom sediments were obtained. The Black Sea sediments were characterized by a higher ^239,240Pu concentration factor (C_f ≈ n·10⁴-n·10⁶) and radiocapacity factor (F(^239,240Pu) was about 99.9% on the shelf, 94.5-99.1% on deep-sea basin for silty and 94.6-98.9% on the shelf for sandy bottom sediments) as compared with C_f and F for ¹³⁷Cs and ⁹⁰Sr. Silty bottom sediments play the role of ^239,240Pu main depot in the Black Sea ecosystem. The studied radioecological characteristics of Pu allowed us to define the type of plutonium biogeochemical behavior in the Black Sea as a pedotropic one. The results of this complex radioecological monitoring of ^239+240Pu contamination in the Black Sea and their analysis makes it possible to understand the plutonium redistribution pathways which will enable to carry out the tracing of its migration within the ecosystems.

[Microbial Processes and Genesis of Methane Gas Jets in the Coastal Areas of the Crimea Peninsula]

Hydroasoustic techniques were used for detection and mapping of gas jet areas in the coastal regions of the Crimean peninsula. Gas seep areas in the bays Laspi, Khersones, and Kazach'ya were chosen for detailed microbiological investigation. The first type of gas jets, observed in the Laspi Bay, was probably associated with discarge of deep thermogenic methane along the faults. Methane isotopic composition was char- acterized by Δ13C of -35.3 degrees. While elevated rates of aerobic methane oxidation were revealed in the sandy sediments adjacent to the methane release site, no evidence of bacterial mats was found. The second type of gas emission, observed in the Khersones Bay, was accompanied by formation of bacterial biofilms of the "Thiodendron" microbial community type, predominated by filamentous, spirochete-like organisms, in the areas of gas seepage. The isotopic composition of methane was there considerably lower (-60.4 degrees), indicating a considerable contribution of modern microbial methane to the gas bubbles discharged in this bay. Activity of the third type of gas emission, the seeps of the Kazach'ya Bay, probably depended directly on modern microbial processes of organic matter degradation in the upper sediment layers. The rates of sulfate reduction and methanogenesis were 260 and 34 μmol dm(-3) day(-1), respectively. Our results indicate different mechanisms responsible for formation of methane jets in the Laspi Bay and in the coastal areas of the Heracles Peninsula, where the bays Kazach'ya and Khersones are located.

(40)K in the Black Sea: a proxy to estimate biogenic sedimentation

An approach to estimate the rate of biogenic sedimentation in the Black Sea using the naturally occurring radionuclide (40)K has been considered. It allows assessment of the contribution of suspended matter of biological origin to the overall sediment accumulation in the Black Sea coastal, shelf and deep-water areas. Based upon this method, a relationship between the biogenic fraction of the seabed sediments and the water depth has been established with a view to differentiating the contributions of allochthonous and autochthonous suspended matter to the sedimentation rate. Overall, (40)K can be considered as an easily applicable proxy to assess sedimentation rate of biogenic fraction of particulate matter in marine environments.

Contemporary radioecological state of the North-western Black Sea and the problems of environment conservation

Review is devoted to the analysis of a radioecological situation in the North-western Black Sea and concerns the levels of contamination of the components of an ecosystem by the main artificial radioactive isotopes ((90)Sr, (137)Cs, (239,240)Pu). The long-term accumulation trends of these radionuclides were analyzed in components of the Black Sea ecosystem after the Chernobyl nuclear power plant accident. Zones that have an increased ability to accumulate these radioisotopes were revealed. The assessment of irradiation dose rates formed by (90)Sr, (137)Cs and (239,240)Pu in Black Sea hydrobionts was obtained. The strategy for biodiversity conservation and sustainable management of natural resources should include monitoring of the radioecological state of the marine ecosystems, and the formation of a complex of biogeochemical criteria for assessment of an ecological situation in the sea. This approach is important for marine protected areas, since it allows the formation of a basis for scientific and practical function.

Secondary radioactive contamination of the Black Sea after Chernobyl accident: recent levels, pathways and trends

The recent radionuclide measurements have showed that concentrations of the Chernobyl-derived (137)Cs and (90)Sr in the surface Black Sea waters are still relatively high, reaching 56 and 32 Bq m(-3), respectively. This is comparable or even exceeds the pre-Chernobyl levels (∼16 Bq (137)Cs and 22 Bq (90)Sr per m(3) as the basin-wide average values). The measurements have revealed that the Black Sea continues to receive Chernobyl radionuclides, particularly (90)Sr, by the runoff from the Dnieper River. An additional source of (90)Sr and (137)Cs was found in the area adjacent to the Kerch Strait that connects the Black Sea and the Sea of Azov. This may be caused by the inflow of the contaminated Dnieper waters, which come to this area through the North-Crimean Canal. The long-term monitoring of (137)Cs and (90)Sr concentration in the Black Sea surface waters and in the benthic brown seaweed Cystoseira sp., in comparison with the earlier published sediment records of the radionuclides, have showed signs of a secondary radioactive contamination, which has started to increase since the late 1990's. This may be the result of the combined effect of a higher input of radionuclides from the rivers in 1995-1999 due to an increased runoff; and a slow transport of the particulate bound radionuclides from the watersheds followed by their desorption in seawater from the riverine suspended matter and remobilization from the sediments adjacent to the river mouths.

(137)Cs concentrations in Atlantic and western Antarctic surface waters: results of the 7th Ukrainian Antarctic Expedition, 2002

The latitudinal distribution of (137)Cs in the Atlantic--western Antarctic surface waters was studied during the 7th Ukrainian Antarctic Expedition in January--May 2002. The (137)Cs concentrations have also been measured in the upper ice of the coastal glacier Woozle Hill located near the Ukrainian Antarctic station "Akademik Vernadsky" (western Antarctica, 65 degrees 15' S-64 degrees 16' W). Comparison of these data with results of previous same-route expeditions SWEDARP (Swedish Antarctic Research Expedition, 1988/1989) and the French R/V "Jeanne d'Arc" (1992/1993), has shown practically parallel changes of (137)Cs surface concentrations between 40 degrees N and 20 degrees S, pointing to decrease of (137)Cs radioactivity at these latitudes with an apparent half-life of 10--15 years (12.5+/-2.1 years on average). This suggests that decrease of (137)Cs surface concentration within this latitude band is controlled, besides the radioactive decay of (137)Cs (half-life=30 years), by vertical mixing of the upper water masses. South of 20 degrees S, the (137)Cs concentrations in surface water have decreased more rapidly because of the influence of the less contaminated Antarctic waters. At 50--60 degrees S and near the Antarctic coast, the (137)Cs activity in 2002 was similar to those measured during the SWEDARP and "Jeanne d'Arc" expeditions, suggesting an additional input of (137)Cs to these waters from the melted ice from the adjacent glaciers.

Recent changes of biogenic carbonate deposition in anoxic sediments of the Black Sea: sedimentary record and climatic implication

Recent changes of carbonate deposition were traced in a Black Sea sediment core taken in the western abyssal basin. The sediments were dated from a vertical profile of excess 210Pb. The 210Pb geochronology corresponded well to the 137Cs fallout record. A 20-year cyclic variability of carbon deposition has been traced in the dated sediments and has been related inversely to the long-term changes in temperature of air over the basin, forcing the convection in the upper water column, which may bear influence upon the coccolithophorid blooms by bringing nutrients from deeper water to the surface.