In this study, we present first results from an ongoing investigation on the stable barium (Ba) isotope fractionation in the natural barium cycle. Stable Ba isotope signatures of international IAEA reference materials (synthetic barium sulfate, IAEA-SO-5, -6, and barium carbonate, IAEA-CO-9), natural Ba minerals and experimental Ba precipitates have been analyzed as a first approach to evaluate potential discriminating processes in the global geochemical barium cycle. Ba = − 0.5‰ was found in a diagenetic barite sample from ODP Leg 207. The observed natural discriminations are clearly larger than the analytical uncertainty of the stable isotope measurements, indicating significant isotope discrimination in the natural barium cycle. Precipitation experiments from aqueous barium chloride solutions at temperatures of~21°C and 80°C indicate that the light Ba isotope is enriched in pure barium carbonate and barium sulfate compared to the aqueous solution. A maximum isotope fractionation of − 0.3‰ is observed for both barium carbonate and sulfate, that -in the case of BaCO 3 -seems to be influenced by precipitation rate and/or the aqueous speciation, but less by temperature.

We investigate the redox-sensitive isotope system of molybdenum (Mo) in marine carbonates to evaluate their potential as archive of the Mo isotopic composition of coeval seawater. We present Mo isotope data (δ 98/95 Mo) of modern skeletal and non-skeletal carbonates as well as a variety of precipitates from the mid and late Carboniferous. The external reproducibility is determined by repeated analyses of two commercially available carbonate standards. The resulting uncertainty of the low concentration samples is ±0.1‰ (2σ). Analysis of modern ooid sands from the Bahamas shows a consistently heavy Mo isotopic composition (δ 98/95 Mo between 2 and 2.2‰), approaching modern mean seawater values (δ 98/95 Mo = 2.3‰ ± 0.1‰ (2σ)). This suggests that isotope fractionation during Mo uptake into non-skeletal carbonate precipitates is small. In contrast, modern skeletal carbonates show variable isotopic compositions (0.1 to 2.2‰) which suggests a biologically controlled fractionation process. The varying Mo signatures found in Carboniferous cement phases point to a strong response to local changes in fluid composition from which they precipitated. Overall, we recognized three important factors to cause an offset relative to ocean water: Mo derived from skeletal components, input of detrital Mo and admixture of light, hydroxide derived Mo via diagenetic fluids. All of these factors cause a lighter Mo isotopic composition relative to seawater. Due to the apparent small isotope fractionation during Mo uptake into non-skeletal carbonates, their δ 98/95Mo closely reflects the ambient fluid composition. From these results we conclude that carbonates represent a promising new tool to characterize the water mass Mo isotopic composition of marine paleo-environments.

We present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface seawater and coral skeleton. Mediterranean surface seawater was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authigenic and terrestrial minerals. In aquarium experiments with a continuous supply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (d 137/134 Ba between +0Á16 AE 0Á12& and +0Á41 AE 0Á12&) were isotopically identical or lighter than that of the ambient Mediterranean surface seawater (d 137/134 Ba = +0Á42 AE 0Á07&, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of D 137/134Ba coral-seawater = À0Á26 AE 0Á14& at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonitestructured pure BaCO 3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, D (Ba/Ca) , thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk seawater. Complementary coral samples from natural settings (tropical shallowwater corals from the Bahamas and Florida and cold-water corals from the Norwegian continental shelf) show an even wider range in d 137/134 Ba values (+0Á14 AE 0Á08 to +0Á77 AE 0Á11&), most probably due to additional spatial and/ or temporal seawater heterogeneity, as indicated by recent publications.