The Study of Uranium Accumulation in Marine Bottom Sediments: Effect of Redox Conditions at the Time of Sedimentationстатья
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Аннотация:To evaluate the effect of redox conditions at the sedimentation stage on uranium contentand 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 thecontinental run‐off. The upper layer of sediments (0–50 cm) were characterized by oxidizing condi‐tions (Eh ~ 400 mV); with the increase in depth, redox conditions changed from oxidizing to reduc‐ing (െ0 ÷ െ200 mV). The uranium concentration in the upper layer was 1–1.5 ppm, U/TOC ratiovaried in the range of 0.8–1.1 ppmU/%TOC. The uranium content and U/TOC ratio increased up tothe values of 2.6 ppm and 1.4 ppmU/%TOC at a depth of 0.5−2.5 m, respectively, but the generalcontent of uranium in the studied environment was close to the values characterizing continentalrun‐off. The results obtained for the White Sea sediments were compared with the sediment of theBlack 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.