The Stable Isotope Geochemistry of Molybdenum

> “The Answer to the Great Question... Of Life, the Universe and Everything... > > Is... Forty-two,” said Deep Thought, with infinite majesty and calm… > > “I checked it very thoroughly,” said the computer, “and that quite definitely is the answer.” > > — Douglas Adams, The Hitchhiker’s Guide to the Galaxy Molybdenum (Mo)—the element with atomic number 42—possesses unique properties that make it the answer to many questions in the geosciences, life sciences, and industry. In the geosciences, the redox sensitivity of Mo makes it particularly useful for answering questions about environmental redox conditions. In particular, it was first suggested as an ocean paleoredox proxy over 30 years ago (Holland 1984; Emerson and Huested 1991)—an application that finally came to fruition in the late 1990s and 2000s when understanding of Mo geochemical behavior in modern environments improved significantly (e.g., Crusius et al. 1996; Helz et al. 1996, 2011; Morford and Emerson 1999; Erickson and Helz 2000; Barling et al. 2001; Siebert et al. 2003, 2005; Arnold et al. 2004; Vorlicek et al. 2004; Morford et al. 2005; Nagler et al. 2005; Algeo and Lyons 2006; McManus et al. 2006; Poulson et al. 2006; Anbar et al. 2007; Wille et al. 2007; Pearce et al. 2008; Archer and Vance 2008; Neubert et al. 2008; Scott et al. 2008; Gordon et al. 2009; Poulson Brucker et al. 2009). In the life sciences, nature settled on Mo as the answer to the challenge of biological-N2 fixation at least ~ 2 billion years ago (Boyd et al. 2011), with the evolution of the Mo-dependent nitrogenase enzyme. Molybdenum is also at the heart of nitrate reductase enzymes, which are essential for assimilatory and dissimilatory nitrate reduction (Glass et al. 2009). Therefore, Mo is central …