Quantifying Molybdenum Isotopic Speciation in Sulfidic Water: Implications for the Paleoredox Proxy

Molybdenum (Mo) isotopic signature and concentrations can be a geologic indicator of changes in molecular oxygen concentrations in Earth's oceans and atmosphere. However, in natural waters that are weakly to strongly sulfidic, few studies have been able to determine the exact mechanisms controlling partitioning and speciation related to isotopic fractionation of Mo. To understand the isotopic contribution of each Mo species to the overall bulk Mo isotope signature, we report a method that measures both the concentration and the isotopic composition of each Mo-bearing species during thiolation using synthesized sulfidic water. We accomplish this by using reversed-phase ion chromatography to isolate individual Mo species and then measure the isotopic ratio of each species with a multicollector inductively coupled plasma mass spectrometer. Our findings show that progressively thiolated species contribute to an increasingly negative isotopic value (i.e., -3.16 to +3.14) compared to the precursor stock solution. Additionally, these results corroborate Tossell's theoretical predictions, where lower Mo isotope values correlate to increasing sulfide presence. Furthermore, the formation of thiolated species and evolution of Mo isotopic signatures in our experiments do not occur via a stepwise formation or a quantitative conversion model for molybdate to tetrathiomolybdate. Overall, our study improves our understanding of Mo systematics, shows the potential of measuring specific Mo species, and thereby strengthens the application of Mo paleoproxy through consideration of all Mo species.