Deposition of Mo, Re and U under contrasting redox conditions; assessment of the [Re/Mo]sw redox proxy

Abstract

We have analyzed concentrations of Mo, Re and U in the suspended particles, water column, underlying carbonate-dominated sediments and pore waters of two contrasting marine lakes situated on the Adriatic Coast of Croatia. Our data are the first for Re in these lakes. Rogoznica Lake is mainly euxinic, but subject to occasional oxygenation events, whereas Mljet Island’s Malo Jezero is currently oxic year around but historically was seasonally euxinic. These characteristics make the lakes useful testing grounds for redox proxies. We devote particular attention to Re/Mo ratios, which have great promise as redox proxies. In marine sediments, the range of log[Reauth/Moauth] exceeds 2, and typical analytical uncertainty is ±0.05, providing resolving power similar to δ98Mo. Authigenic [Res/Mos] ratios in sediments depend on varying supplies of three components: FeMo,Re)S4 coprecipitates, Fe(III) oxyhydroxides (plus Mn(III,IV) oxyhydroxides in some cases) and a still-uncharacterized Re-rich, Mo-poor component possibly of biological origin. Removal of the latter component explains a 50% depletion of Reaq without Moaq from Rogoznica’s oxic waters. Water column concentrations of Moaq and Reaq, as well as [Res/Mos] ratios in Rogoznica’s suspended particles are predicted successfully by a model that attributes these elements’ removal from sulfidic seawater solely to coprecipitation in Fe(Mo,Re)S4, assuming a partition coefficient of 0.6. However, to predict sediment compositions, additional authigenic Mo supplied by mainly Fe(III) oxyhydroxides during sporadic oxygenation events must be considered. In Malo Jezero, sulfide concentrations are too low even in pore waters for precipitation of Fe(Mo,Re)S4. Consistent with earlier work, Uaq profiles are best explained by eddy diffusive transport to the sediment rather than capture by a precipitate in the water column. In Rogoznica pore waters, Moaq extracted through 0.45 μm pore size filters exceeds by 50-fold concentrations predicted by the model and measured previously with 0.15 μm filters. This observation implies presence of ferrothiomolybdate colloids, which could be important, overlooked participants in the sedimentary geochemistry of Mo.