7 . 10 . 6 . Mo and U Geochemistry and Isotopes

The molybdenum and uranium isotopic systems can potentially be used to reconstruct the oxidation state of an ancient the global ocean as recorded in the FAR-DEEP samples. As previous studies have shown (e.g. Anbar et al. 2007; Kendall et al. 2010), the use of multiple proxies can help lift uncertainties and avoid the pitfalls associated with the use of a single redox proxy. Molybdenum is the most abundant transition metal in modern seawater, occurring dominantly as the molybdate anion (MoO4 2 ; Morford and Emerson 1999). Along with many other transition metals, Mo becomes authigenically enriched in sulphidic environments, where molybdate is converted to particle-reactive oxythiomolybdates (MoOxS4-x 2 ) and sequestered in sediments (Erickson and Helz 2000; Helz et al. 1996; Tribovillard et al. 2004). Although well-oxygenated marine sediments are characterised by approximately crustal Mo concentrations (~1–2 ppm), and reducing sediments overlain by oxygenated water do not typically show Mo concentrations in excess of ~10–20 ppm, most modern and Phanerozoic euxinic sediments show high Mo concentrations (often in excess of 100s of ppm) that correlate strongly with the total organic carbon (TOC) content of the sediments. However, scavenging can be efficient enough in some environments to remove Mo from solution quantitatively (Erickson and Helz 2000), such as in the modern Black Sea, so that muted euxinic enrichments can also reflect drawdown of the Mo inventory on an oceanic scale during times of widespread oxygen deficiency (Algeo and Lyons 2006; Emerson and Huested 1991; Scott et al. 2008), as suggested for the Proterozoic (Canfield 1998; Lyons et al. 2009). Molybdenum depletion can have consequences for biological pathways that are dependent on Mo, such as nitrogen fixation (Anbar and Knoll 2002; Zerkle et al. 2006) and inorganic nitrogen assimilation (Milligan and Harrison 2000). Recent work (Scott et al. 2008) showed a dramatic increase in Mo concentration in euxinic shales at ~2.2–2.1 Ga, around 200 million years after the Great Oxidation Event (GOE). While almost certainly tied to an increased weathering flux of Mo to the ocean, the existing deep marine record is sparse in this interval, and ocean redox conditions for the early Palaeoproterozoic in general are not well known. In sum, Mo concentration relationships provide an additional constraint on local palaeoenvironments. When viewed at the same time from the perspective of global mass balance, magnitudes of Mo enrichment in black shales can speak to ocean-scale palaeoredox. The ocean has a homogeneous present-day dMo value (expressed in conventional ‘delta’ notation, where dMo 1⁄4 1,000 [(Mo/Mo)sample/(Mo/Mo)standard – 1]) of about +2.4 ‰ relative to the Specpure®Mo plasma standard (Barling et al. 2001; Siebert et al. 2003). Although uncertainties still remain on the modern oceanic budget of Mo isotopes (e.g., the Mo isotopic composition of rivers and estuarine systems, the role of suboxic sediments with respect to Mo removal from the ocean), Mo-isotopes are promising tracers of ocean-scale palaeoredox conditions (e.g. Anbar 2004; Anbar and Rouxel 2007; Arnold et al. 2004; Siebert et al. 2003). Also, new constraints are emerging, for example, for the riverine contribution and Mo cycling and associated isotope effects in suboxic settings (Archer and Vance 2008; Poulson et al. 2006). Mo-isotope fractionation is associated with the transition between the conservative molybdate ion (MoO4 ) and the more reactive thiomolybdate (MoS4 ) species through intermediate oxythiomolybdate species (MoOxS(4-x) ). The total reaction can be written as follows: