Molybdenum isotope fractionations observed under anoxic experimental conditions

Copyright © 2012 by The Geochemical Society of Japan. al., 1996, 2004; Zheng et al., 2000; Algeo and Lyons, 2006; Tribovillard et al., 2004, 2006), and formation or subsequent removal of these intermediates may be one way of driving variations in Mo isotope fractionation under reducing conditions (Neubert et al., 2008). In the open ocean, which is generally devoid of dissolved sulfide, Mo is primarily present as molybdate (MoO4 2–; e.g., Emerson and Huested, 1991). Here seawater Mo concentrations are relatively high (~105 nM) and quasi-uniform (Collier, 1985; Fig. 1A), and the modern seawater Mo isotope composition also appears to be reasonably uniform as well (Barling et al., 2001; Siebert et al., 2003). In well-oxygenated sediments of the open ocean, Mo is primarily associated with solid-phase Mn and Fe–oxides (e.g., Bertine and Turekian, 1973; Calvert and Pedersen, 1993; Chappaz et al., 2008). Experimental work and natural samples have shown that Mo adsorption to Mn–oxides results in sediment Mo isotope values that are fractionated relative to parent seawater Mo (Barling et al., 2001; Siebert et al., 2003, 2006; Barling and Anbar, 2004; Wasylenki et al., 2008; Poulson Brucker et al., 2009; Figs. 1B and C). The process responsible for this particular fractionation appears to be related to a change in coordination from tetrahedral to octahedral (e.g., Wasylenki et al., 2011; Kashiwabara et al., 2011). Molybdenum isotope fractionations observed under anoxic experimental conditions