A potential relationship between molybdenum speciation and its isotopic signature in sedimentary records: New insights from old shales

Abstract:

Molybdenum has emerged as a powerful paleo-indicator of sulfidic conditions in studies of the evolution of Earth’s early oxygenation, either by examining patterns of Mo enrichment and/or the δ⁹⁸Mo isotopic signature in sedimentary records. However, the processes leading to Mo incorporation in sulfidic sediments are still unknown, limiting its use as a proxy.

The Mount McRae Shale, deposited ~2.5 billion years ago (Ga) in the Hamersley Basin, Western Australia, provides a unique opportunity to examine Earth surface conditions during the Archean just prior to the Great Oxidation Event, and an important example of the deep time application of Mo geochemistry. Trace element [1] and iron speciation [2] data measured in the upper shale interval indicate deposition under euxinic conditions and significant aqueous transport of redox-sensitive trace elements.

δ⁹⁸Mo measured in the upper euxinic interval of the Mt. McRae Shale ranges from 0.99 to 1.86 ‰ [3]. These heavy δ⁹⁸Mo values have been attributed to the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces. To further explore the implications of these data and to identify possible mechanisms controlling Mo burial, we analyzed samples from the upper euxinic shale using XANES and EXAFS. First, our data suggest an association between Mo and organic matter implying that metamorphic processes have not altered this interval. Perhaps more surprisingly, we find a strong relationship between Mo speciation and δ⁹⁸Mo isotopic signature (r² = 0.90). We suggest an alternative mechanism for explaining the Mo isotope systematics of the upper Mt. McRae Shale involving Mo reduction. If correct, our results add new interpretive texture to sedimentary Mo isotope records and imply a primary role for speciation in the Mo isotope composition of sulfidic marine environments.

[1] Anbar et al., 2007. Science 317, 1903-1906

[2] Reinhard et al. 2009. Science 326, 713-716

[3] Duan et al. 2010. GCA 74, 6655-6668