| The rise of O2 in the atmosphere and oceans remains an intriguing question. Answering this question is important if we are to understand environmental and biological evolution through geological time. Geochemical investigations of ancient rocks may provide insight into the redox conditions in Earth's history. I used an isotopic approach by studying iron (Fe) and molybdenum (Mo) isotope systematics in a ∼85 m continuous drill core from the ∼2.5 Ga Mt. McRae Shale, Hamersley Basin, Western Australia, to examine environmental oxygenation before the Great Oxidation Event (GOE). The high-resolution profile of 556Fe of bulk samples reveals a significant Fe isotope excursion to negative values (as low as -2.06‰) in the upper black shale unit (SI), which correlates with reactive Fe enrichment factors. This correlation is similar to observations in a younger Phanerozoic (Devonian) basin in North America as well as the previously studied modern Black Sea, fingerprinting a benthic Fe source on the shelf and shelf-to-basin Fe transfer. Therefore, it suggests redox-stratification of the water column at that time, resulting in delivery of Fe oxides to the shelf and efficient transport of DIR-derived reduced Fe to the euxinic deep basin. The Mo isotope profile in the same sedimentary rock sequence exhibits delta98/95Mo values elevated above those of the average upper continental crust. Sedimentary Fe proxies suggest that S1 sediments were deposited under euxinic conditions which were conducive to quantitative Mo removal from bottom waters. Thus, delta98/95Mo in this interval likely records coeval seawater values. After evaluating a range of hypotheses, I conclude that the isotopically heavy delta 98/95Mo values seen throughout the Mt. McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces. This integrated study of Mo isotopes and Fe isotopes, in conjunction with previous geochemical data of Mo and Re abundances, sulfur and nitrogen isotopes, and Fe speciation, provides an emerging picture of mild oxidizing conditions at the Earth's surface during the Late Archean. Also, this study confirms the strength of Fe and Mo isotopes as paleoredox proxies in fingerprinting ocean cycles of these metals and delineating oxygenation events. |
