Molybdenum stable isotopes: Observations, interpretations and directions

This chapter summarizes the results of the first wave of examinations of Mo stable isotopes in nature. These efforts reveal large, systematic variations in Mo isotope compositions in the ocean system that are apparently associated with the redox-sensitive marine geochemistry of this element. Laboratory studies of fractionation mechanisms, while limited in number, converge with predictions from natural observations. Combined with the relative simplicity of the Mo ocean budget and the likely preservation of oceanographically-interpretable Mo isotope signatures in both black shales and ferromanganese oxides, applications of this isotope system for paleoredox research are already apparent. Further work is needed to validate such applications, and to develop a framework to quantitatively relate δ⁹⁷/⁹⁵Mo variations in sedimentary records to changes in δ ^9/95Mo_SW and hence in the relative importance of oxic and anoxic sedimentary reservoirs through time. Better constraints on the modern Mo element budget, which would permit more robust "calibration" of the Mo isotope budget, would be extremely helpful in this regard. However, even in advance of such refinements, the available data make clear that the Mo isotope system is poised to be among the more immediately useful of the emerging "non-traditional" stable isotope systems.