Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides

A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

“…In previous investigations of Mo isotopic fractionation in adsorption experiments, Mo showed little or no fractionation during adsorption to ferrihydrite (Malinovsky et al, 2007), whereas preferential adsorption of the lighter isotope to δ-MnO 2 with heavier isotope remaining in solution was found, which is similar to the offset between natural ferromanganese oxides and seawater Wasylenki et al, 2008). On the other hand, quantum mechanical insights demonstrate that the heavier isotope preferentially partitions into stronger bonding environments (e.g., higher oxidation state, highly covalent bonds, low-spin configuration for transition elements, or lower-coordination number) in the equilibrium isotopic fractionation (Bigeleisen and Mayor, 1947;Schauble, 2004).…”

“…Many previous studies performed quantum mechanical calculations of fractionation factors between the dominant species in seawater (MoO 4 2-) and various candidates of adsorbed species to explain the Mo isotopic offset observed between seawater and ferromanganese oxides theoretically (Tossell, 2005;Weeks et al, 2007;Wasylenki et al, 2008). The calculated fractionation factors, however, were significantly smaller than the experimental values in most cases, which may be due to poor knowledge of the adsorbed Mo species.…”

“…Molybdenum is one of these elements, that is, around 70% of output is presumed to be incorporated into ferromanganese oxides in the budget of Mo in the marine system (Bertine and Turekian, 1973;Siebert et al, 2003). This element is a redox-sensitive trace metal with seven stable isotopes and is essential in many bio-many studies have focused on this promising tool (Anbar, 2004;Barling and Anbar, 2004;Weeks et al, 2007;Wasylenki et al, 2008;Arnold et al, 2004;Pearce et al, 2008;Poulson et al, 2006;Voegelin et al, 2009).…”

Molybdenum evidence for expansive sulfidic water masses in ~750Ma oceans

“…These values reflect 361 far less metal enrichments relative to crustal average than in previously analyzed Meso-362 to Neoproterozoic black shales (~24 ppm Mo [11,17] been suggested to record <5 nM Mo in the water column [17,50]. This is an order of 371 magnitude lower than the Proterozoic average of 6.4 ppm/wt% between 1700-550 Ma, 372 which may represent seawater Mo concentrations of ~20 nM [17]. 373 Mo isotope fractionation occurred during diagenetic alteration in this case.…”

“…Sulfate reduction causes 443 seawater pH to decrease relative to the surface ocean, with values of 7.1-7.7 in the 444 deepest parts of modern euxinic basins (see summary in [20] burial. In these cases, the euxinic removal pathway would have operate an order of 470 magnitude faster than today (5-15%, [17,26,56] there would be little influence on the isotope mass balance. We can conclude, however, 488 that since oxic removal pathways were likely limited during the time of Chuar deposition 489 [7,46], the removal of Mo into euxinic environments was substantial, and euxinic 490 environments were much more abundant than today.…”