Molybdenum reduction in a sulfidic lake: Evidence from X-ray absorption fine-structure spectroscopy and implications for the Mo paleoproxy

Marine euxinic sediments, particularly organic-rich black shales, are important sinks for oceanic molybdenum (Mo), and the determination of Mo concentration and isotopic composition are used to constrain oxygenation state and specifically expansion of marine anoxic and sulfidic (euxinic) waters in ancient oceans. The use of Mo as a paleo-redox tracer is based on its distinct geochemical behavior in oxic and sulfidic environments. Mo removal from sulfidic waters starts with MoO₄^2- reacting with aqueous H₂S to form particle reactive thiomolybdates, MoO_4-xS_x^2-, but the post-thiomolybdate steps and the ultimate Mo host in euxinic sediments are not well understood. We used X-ray absorption fine structure (XAFS) spectroscopy to determine the oxidation state and the molecular coordination environment of pristine, solid phase Mo in sediments from permanently euxinic Lake Cadagno, Switzerland. Samples were taken from a 9-meter piston core representing the last ten thousand years of deposition.The euxinic lake sediments provide unequivocal evidence that the post-thiomolybdate steps along the burial pathway involve Mo(VI) reduction to Mo(IV) before it is ultimately hosted in the sediments. Anoxic samples contain Mo(IV)-S compounds that, when oxidized upon air-exposure, are transformed into Mo(VI)-O, thus confirming our results. This observation is important to better understand the Mo burial pathway from oxic waters to euxinic sediments. Our results support a model where post-thiomolybdate Mo^VI reduction proceeds via reactions with zero-valent sulfur, S(0) (mainly S₈, Vorlicek et al., 2004). In this model, Mo is scavenged from sulfidic waters as reactive Mo-polysulfide species, Mo(IV)O(S₄)S^2- or Mo(IV)S(S₄)S^2-, and not as thiomolybdate (MoOS₃^2- or MoS₄^2-) as has been previously assumed. This result can have important implications for how paleoenvironmental redox conditions are inferred from sedimentary Mo enrichments, Mo/TOC ratios, and δ⁹⁸Mo, if the Mo accumulation rate is accelerated via the S₈-assisted pathway in settings where partial oxidation of S is possible. For example, euxinic Mo/TOC ratios are predicted not only as a function of Mo concentration in overlying seawater. Instead, the ratio will be higher in sediments deposited under highly sulfidic waters where S₈ is also abundant, such as in settings where the chemocline depth fluctuates considerably and/or oxygen intrusions occur to the overlying bottom waters. Partial oxidation of dissolved sulfide is an important step in polysulfide production. Finally, we conclude that XAFS measurements are a powerful way to study the Mo burial pathway in sediments and enhance our ability to infer past environmental conditions from the sedimentary record.