Abstract
Molybdenum 98Mo/95Mo isotope ratios are a sediment paleo proxy for the redox state of the ancient ocean. Under sulfidic conditions, no fractionation between seawater and sediment should be observed if molybdate (MoO42-) is quantitatively transformed to tetrathiomolybdate (MoS42-) and precipitated. However, quantum mechanical calculations previously suggested that incomplete sulfidation could be associated with substantial fractionation. To experimentally confirm isotope fractionation in thiomolybdates, a new approach for determination of isotope ratios of individual thiomolybdate species was developed that uses chromatography (HPLC-UV) to separate individual thiomolybdates, collecting each peak and analyzing isotope ratios with multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially available MoO42- and MoS42- standards, the method was evaluated and excellent reproducibility and accuracy were obtained. For species with longer retention times, complete chromatographic peaks had to be collected to avoid isotope fractionation within peaks. Isotope fractionation during formation of thiomolybdates could be experimentally proven for the first time in the reaction of MoO42- with 20-fold or 50-fold excess of sulfide. The previously calculated isotope fractionation for MoS42- was confirmed, and the result for MoO2S22- was in the predicted range. Isotopic fractionation during MoS42- transformation with pressurized air was dominated by kinetic fractionation. Further optimization and online-coupling of the HPLC-MC-ICPMS approach for determination of low concentrations in natural samples will greatly help to obtain more accurate species-selective isotope information.
Highlights
Sedimentary Mo and its isotopes are increasingly used as a paleo proxy for the redox state of the ancient ocean. 1-4 Key to this application is the different behavior of Mo in oxic and anoxic environments
Theoretical calculations have suggested that each sulfidation step is associated with substantial fractionation, each step at around +1.5‰ for δ98Mo. 37 if transformation to MoS42- is incomplete, and if there is different removal of the individual thiomolybdate species, there is potential to imprint substantial isotopic fractionations in sediments deposited beneath water columns that are anoxic or only moderately euxinic
No isotope fractionation occurred within the chromatographic peak, when split into three parts (Fig. 1A), and the calculated mass balance (-0.29±0.10‰) was in agreement with the original MoO42- stock solution (Tab. 1A part I-III)
Summary
Sedimentary Mo and its isotopes are increasingly used as a paleo proxy for the redox state of the ancient ocean. 1-4 Key to this application is the different behavior of Mo in oxic and anoxic environments. Theoretical calculations have suggested that each sulfidation step is associated with substantial fractionation, each step at around +1.5‰ for δ98Mo. 37 if transformation to MoS42- is incomplete, and if there is different removal of the individual thiomolybdate species, there is potential to imprint substantial isotopic fractionations in sediments deposited beneath water columns that are anoxic or only moderately euxinic. This observation would go some way to explaining the observations of Mo in modern anoxic settings that are isotopically intermediate between fully euxinic and fully oxic. It could explain the extremely light Mo isotopes in ancient sediments, that are ubiquitous but very difficult to interpret in a quantitative way. 31, 32
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