The Study of Uranium Accumulation in Marine Bottom Sediments: Effect of Redox Conditions at the Time of Sedimentation

To evaluate the effect of redox conditions at the sedimentation stage on uranium content and U/TOC ratio in marine source rocks, we analyzed the accumulation of uranium in modern marine bottom sediments formed in different redox conditions. The behavior of uranium from bottom sediments formed in oxidizing and sub-oxidizing settings has been studied on the sediments of the Upper Pleistocene–Holocene age accumulated in the coastal area of the White Sea (Kandalaksha Gulf). We studied the content of uranium, Eh, pH, TOC, C, H, N, and S element and isotope compositions and other parameters in two sampled columns of bottom sediments at a depth of 0–2.5 m. The composition of sediments was typical for the shelf zone where marine genesis mixes with the continental run-off. The upper layer of sediments (0–50 cm) were characterized by oxidizing conditions (Eh ~ 400 mV); with the increase in depth, redox conditions changed from oxidizing to reducing (−0 ÷ −200 mV). The uranium concentration in the upper layer was 1–1.5 ppm, U/TOC ratio varied in the range of 0.8–1.1 ppmU/%TOC. The uranium content and U/TOC ratio increased up to the values of 2.6 ppm and 1.4 ppmU/%TOC at a depth of 0.5–2.5 m, respectively, but the general content of uranium in the studied environment was close to the values characterizing continental run-off. The results obtained for the White Sea sediments were compared with the sediment of the Black Sea, formed in the anoxic conditions of hydrogen sulfide contamination. In these conditions, the uranium content varied from 10 to 20 ppm. The obtained data were interpreted using thermodynamic modeling of the uranium forms in the seawater at different pH and Eh. This study demonstrated that the change of redox conditions from oxidizing to reducing leads to increased uranium content due to a decrease in uranium’s solubility in water. These results show that oxidation–reduction potential could be one of the most important factors controlling uranium content in black shales formed in the marine environment

Organic matter geochemical characteristics of bottom sediments of the northern Barents Sea as an indicator of hydrocarbon migration from deep source

Group and molecular compositions of organic matter from bottom sediments of the northern sector of the Barents Sea were analysed. The sites where bottom sediments contain organic matter of molecular composition similar to that of oil are identified. Thermally mature hydrocarbon compounds detected in extracts from sediment samples indicates ongoing migration from deeply buried strata and fluid discharge processes on seafloor within the studied areas. Molecular compositions of extracts from bottom sediments were compared with and showed geochemical characteristics similar to some series described for the closest onshore to the study areas. That allows an assumption that the source for migrated hydrocarbons identified in studied bottom sediments can be Mid-Triassic organic-rich claystone intervals which are a part of sedimentary succession of the North Barents deep depression

Structure of Benthic Microbial Communities in the Northeastern Part of the Barents Sea

The Barents Sea shelf is one of the most economically promising regions in the Arctic in terms of its resources and geographic location. However, benthic microbial communities of the northeastern Barents Sea are still barely studied. Here, we present a detailed systematic description of the structures of microbial communities located in the sediments and bottom water of the northeastern Barents Sea based on 16S rRNA profiling and a qPCR assessment of the total prokaryotic abundance in 177 samples. Beta- and alpha-diversity analyses revealed a clear difference between the microbial communities of diverse sediment layers and bottom-water fractions. We identified 101 microbial taxa whose representatives had statistically reliable distribution patterns between these ecotopes. Analysis of the correlation between microbial community structure and geological data yielded a number of important results—correlations were found between the abundance of individual microbial taxa and bottom relief, thickness of marine sediments, presence of hydrotrolite interlayers, and the values of pH and Eh. We also demonstrated that a relatively high abundance of prokaryotes in sediments can be caused by the proliferation of Deltaproteobacteria representatives, in particular, sulfate and iron reducers