Molybdenum speciation and burial pathway in weakly sulfidic environments: Insights from XAFS

Journal Article · Thu Jun 01 04:00:00 EDT 2017 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2017.02.018· OSTI ID:1355033

Wagner, Meghan; Chappaz, Anthony; Lyons, Timothy W.

Sedimentary molybdenum (Mo) accumulation is a robust proxy for sulfidic conditions in both modern and ancient aquatic systems and has been used to infer changing marine redox chemistry throughout Earth’s history. Accurate interpretation of any proxy requires a comprehensive understanding of its biogeochemical cycling, but knowledge gaps remain concerning the geochemical mechanism(s) leading to Mo burial in anoxic sediments. Better characterization of Mo speciation should provide mechanistic insight into sedimentary Mo accumulation, and therefore in this study we investigate Mo speciation from both modern (Castle Lake, USA) and ancient (Doushantuo Formation, China) environments using X-ray Absorption Near Edge Structure (XANES) spectroscopy. By utilizing a series of laboratory-synthesized oxythiomolybdate complexes—many containing organic ligands—we expand the number of available standards to encompass a greater range of known Mo chemistry and test the linkage between Mo and total organic carbon (TOC). In weakly euxinic systems ([H₂S_(aq)] < 11 µM), or where sulfide is restricted to pore waters, natural samples are best represented by a linear combination of MoO₃, MoO_xS_4-x^2- (intermediate thiomolybdates), and [MoO_x(cat)_4-x]^2- (cat = catechol, x = 2 or 3). These results suggest a revised model for how Mo accumulates in weakly sulfidic sediments, including a previously unrecognized role for organic matter in early sequestration of Mo and a de-emphasized importance for MoS₄^2- (tetrathiomolybdate).

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE; National Science Foundation (NSF)
• OSTI ID: 1355033
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: C Vol. 206; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

58 GEOSCIENCES
Euxinia
Molybdenum
Speciation
XAFS