Investigating the molybdenum and uranium redox proxies in a modern shallow anoxic carbonate rich marine sediment setting of the Malo Jezero (Mljet Lakes, Adriatic Sea)

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.