MOLYBDENUM CONCENTRATION AND ISOTOPIC COMPOSITION IN THE ~2.5 GA MCRAE SHALE, HAMERSLEY BASIN, WESTERN AUSTRALIA: AN EARLY WHIFF OF OXYGEN?

The detailed timing and pattern of oxygenation of late Archean and early Proterozoic ocean-atmosphere remain enigmatic and controversial. The abundances and isotopic compositions of redox sensitive metals in sedimentary rocks reflect contemporaneous redox conditions and hence provide important paleoredox records. Molybdenum (Mo) is emerging as a particularly powerful global paleoredox proxy. We are investigating the concentrations of Mo and other metals and the isotopic composition of Mo at high stratigraphic resolution in the ~ 2.5 Ga McRae Shale sampled from the Astrobiology Drilling Program Hamersley-Fortescue core (ABDP-9) in Western Australia. The primary motivation is a better understanding of environmental conditions immediately preceding the major rise in oxygen in the Paleoproterozoic.

The samples investigated were all nominal black shales with TOC > 1%. The concentration of Mo in the upper ~ 400 m of this core is typically < 5 ppm. However, a spike of extremely enriched Mo is well contained within the upper ~ 200 m based on 82 separate analyses in this interval. In particular, there is a significant Mo excursion to values as high as ~ 40 ppm spanning a ~ 20 m interval. Other indicators, including TOC, δ¹³C, S content, δ³⁴S and Δ³³S also shift at approximately this horizon. Preliminary Mo isotope measurements on 10 representative samples from the McRae Shale yield δ^97/95Mo fractionated from the presumed average crustal Mo by +0.88 to +1.05 ‰ (relative to JMC standard).

Mo concentrations in Archean black shales are low because oxidative weathering and subsequent transport of continental Mo was limited under a dominantly anoxic atmosphere. In the marine environment, Mo isotope fractionation of this magnitude occurs when isotopically light Mo is removed from solution to oxic or suboxic sediments. Mo remaining in solution is isotopically heavy and can be preserved in sediments that accumulate in strongly reducing settings, such as that of the McRae Shale. Therefore the observation of transient Mo enrichment correlated with heavy δ^97/95Mo and with other shifts in other proxies, including indications of elevated sulfate concentrations, may suggest a short-lived oxygenation episode prior to the Paleoproterozoic rise in oxygen.