posted on 2017-02-02, 00:00authored byCarolin
F. Kerl, Regina Lohmayer, Elvira Bura-Nakić, Derek Vance, Britta Planer-Friedrich
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.