- © The Mineralogical Society Of America
The unusual chemistry of molybdenum (Mo) makes this trace element interesting to both geochemists and biochemists. Geochemically, Mo is relatively unreactive in oxygenated, aqueous solutions, and hence is a nominally conservative element in the oceans. In fact, Mo is removed so slowly from seawater that it is the most abundant transition metal in the oceans despite being a ppm-level constituent of the crust. In contrast, Mo is readily removed from solution in anoxic-sulfidic (“euxinic”) settings, so that Mo enrichments in sediments are considered diagnostic of reducing depositional conditions. Few elements possess such bimodal redox behavior at the Earth’s surface.
Biochemically, Mo draws attention because it is an essential enzyme cofactor in nearly all organisms, with particular importance for nitrogen fixation, nitrate reduction and sulfite oxidation. Such biochemical ubiquity is surprising in view of the general scarcity of Mo at the Earth’s surface.
Isotopically, Mo initially catches the eye because it has seven stable isotopes of 10–25% abundance, covering a mass range of ~8% (Fig. 1⇓). Thus, from an analyst’s perspective, Mo offers both an unusually large mass spread and a number of options for isotope ratio determination. Combined with rich redox chemistry and covalent-type bonding, both of which tend to drive isotope fractionation, these factors make the Mo isotope system a particularly promising target for stable isotope investigation.
In the environment, Mo isotope research began in earnest with the application of multiple-collector inductively coupled plasma mass spectrometry. While much work remains to be done, this early research points to promising applications in paleoceanography, and beyond.
This review is intended to provide an overview of this emerging stable isotope system in the context of Mo environmental biogeochemistry. Special attention …