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

Abstract

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.

Uranium in Source Rocks: Role of Redox Conditions and Correlation with Productivity in the Example of the Bazhenov Formation

“…The intervals formed in reducing conditions show significantly higher uranium concentration and TOC values. The observed pattern of the uranium behavior in BF rocks is similar to that observed for the modern marine sediments [43].…”

Environmental conditions in the preglacial late Ordovician subtropical Iapetus Ocean: geochemistry of the type section of the Lower Hartfell Shales (Katian) at Hartfell, Dumfries & Galloway, Scotland.

“…Thus; Th/U authigenic = Th measured / (U measured -Th measured /3) Only two Th/U authigenic values for Hartfell could be calculated; 2.1 (sample 5) and 0.83 (sample 8), and both are dysoxic to anoxic compared with Black Sea sediments. Pleistocene Black Sea anoxic grey clayey and black sapropel silts, with low redox potentials, have TOC of 0.22-8.95%, and U/TOC ratio from 0.96 to 2.83 ppm; while grey and black silts with high redox potentials close to oxidizing, have TOC of <1.0, and average U/TOC ratios of 2.31 (Khaustova et al, 2021). Th/U ratios as indicators of anoxia, however, are more complex than earlier studies indicated (Algeo et al, 2011), and the importance of organic matter in U geochemistry is only now being thoroughly investigated (Cumberland et al, 2016).…”