Abstract The molybdenum (Mo) and uranium (U) isotope compositions recorded in carbonate rich sediments are emerging as promising paleo-redox proxies. However, the effects of early diagenetic effects within the sediments on these isotope systems are not well constrained. We examined the Mo and U isotopic systematics in anoxic carbonate rich sediments in a semi enclosed karstic marine lake (Malo Jezero) of the Island of Mljet, Adriatic Sea. Measurements of water column redox behavior in the lake since the 1950s, have shown a transition from anoxic-sulfidic conditions in the deeper water column to more oxic conditions and anoxia refined to the sediment and pore-waters. A 50 cm long sediment core from the deepest part of the lake, show a transition from moderate to high authigenic Mo and U accumulation with depth, consistent with the changing lake redox environment in the past. In the deep euxinic part of the core, the authigenic Mo and U are isotopically lighter and heavier, respectively, than seawater, following similar systematics as observed in other modern euxinic basins, with high, but non-quantitative, Mo and U uptake into the sediments. Based on Bahamas bank carbonate sediments, it has been suggested that the 238U/235U ratio is ~+0.25‰ higher compared to seawater from the effects of early carbonate sediment diagenesis and this carbonate vs. seawater off-set is applicable to carbonate rich sediments across the geological past. The shallower part of lake sediment core was deposited under similar redox conditions as the Bahamas sediments, and these sediments show an average 238U/235U ratio +0.31 ± 0.01‰ (2SE) higher than seawater. Although the average 238U/235U ratios for these two carbonate rich settings are similar, caution is necessary when inferring seawater 238U/235U compositions from such sediments, as they contain U from different sources (e.g. diagenetic uptake and carbonate-bound). The Mo isotope compositions within the same Malo Jezero sediments are variable but approaches the seawater composition at low pore-water H2S concentrations. This show the potential of using the Mo isotope composition from carbonate rich sediments to infer the seawater composition, however, further work is required to establish the link between the Mo isotope composition and the chemistry of the pore water environment.

中文翻译：

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在 Malo Jezero（Mljet 湖，亚得里亚海）的现代浅层缺氧富含碳酸盐的海洋沉积物环境中调查钼和铀的氧化还原代理

摘要 在富含碳酸盐的沉积物中记录的钼 (Mo) 和铀 (U) 同位素组成正在成为有希望的古氧化还原代理。然而，沉积物中早期成岩作用对这些同位素系统的影响并没有得到很好的限制。我们研究了亚得里亚海姆列特岛半封闭岩溶海洋湖 (Malo Jezero) 中富含缺氧碳酸盐的沉积物中的 Mo 和 U 同位素系统。自 1950 年代以来对湖中水柱氧化还原行为的测量表明，从较深水柱中的缺氧-硫化物条件过渡到更多的含氧条件和对沉积物和孔隙水细化的缺氧条件。湖最深处的一个 50 厘米长的沉积岩心，随着深度的增加，显示出从中度到高度的自生 Mo 和 U 积累的转变，与过去不断变化的湖泊氧化还原环境一致。在核心的深层富生部分，自生 Mo 和 U 在同位素上分别比海水更轻和更重，遵循与在其他现代富生盆地中观察到的类似系统学，具有高但非定量的 Mo 和 U 吸收到海水中沉积物。根据巴哈马海岸碳酸盐沉积物，由于早期碳酸盐沉积物成岩作用的影响，238U/235U 比值比海水高~+0.25‰，并且这种碳酸盐与海水的偏移适用于整个地区富含碳酸盐的沉积物。地质过去。湖泊沉积物核的较浅部分在与巴哈马沉积物相似的氧化还原条件下沉积，这些沉积物的平均 238U/235U 比值比海水高 +0.31 ± 0.01‰ (2SE)。尽管这两种富含碳酸盐的环境的平均 238U/235U 比率相似，但在从此类沉积物中推断海水 238U/235U 成分时仍需谨慎，因为它们含有来自不同来源（例如成岩吸收和碳酸盐结合）的 U。同一 Malo Jezero 沉积物中的 Mo 同位素组成是可变的，但在低孔隙水 H2S 浓度下接近于海水组成。这显示了使用富含碳酸盐沉积物的 Mo 同位素组成来推断海水组成的潜力，但是，需要进一步的工作来建立 Mo 同位素组成与孔隙水环境化学之间的联系。因为它们含有来自不同来源的 U（例如成岩吸收和碳酸盐结合）。同一 Malo Jezero 沉积物中的 Mo 同位素组成是可变的，但在低孔隙水 H2S 浓度下接近于海水组成。这显示了使用富含碳酸盐沉积物的 Mo 同位素组成来推断海水组成的潜力，但是，需要进一步的工作来建立 Mo 同位素组成与孔隙水环境化学之间的联系。因为它们含有来自不同来源的 U（例如成岩吸收和碳酸盐结合）。同一 Malo Jezero 沉积物中的 Mo 同位素组成是可变的，但在低孔隙水 H2S 浓度下接近于海水组成。这显示了使用富含碳酸盐沉积物的 Mo 同位素组成来推断海水组成的潜力，但是，需要进一步的工作来建立 Mo 同位素组成与孔隙水环境化学之间的联系。