title: Transient episodes of mild environmental oxygenation and oxidative continental weathering during the late Archean.
abstract: It is not known whether environmental O2 levels increased in a linear fashion or fluctuated dynamically between the evolution of oxygenic photosynthesis and the later Great Oxidation Event. New rhenium-osmium isotope data from the late Archean Mount McRae Shale, Western Australia, reveal a transient episode of oxidative continental weathering more than 50 million years before the onset of the Great Oxidation Event. A depositional age of 2495 ± 14 million years and an initial (187)Os/(188)Os of 0.34 ± 0.19 were obtained for rhenium- and molybdenum-rich black shales. The initial (187)Os/(188)Os is higher than the mantle/extraterrestrial value of 0.11, pointing to mild environmental oxygenation and oxidative mobilization of rhenium, molybdenum, and radiogenic osmium from the upper continental crust and to contemporaneous transport of these metals to seawater. By contrast, stratigraphically overlying black shales are rhenium- and molybdenum-poor and have a mantle-like initial (187)Os/(188)Os of 0.06 ± 0.09, indicating a reduced continental flux of rhenium, molybdenum, and osmium to seawater because of a drop in environmental O2 levels. Transient oxygenation events, like the one captured by the Mount McRae Shale, probably separated intervals of less oxygenated conditions during the late Archean.
authors.affiliation: Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors.forename: Brian
authors.initials: B
authors.lastname: Kendall
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Creaser
authors.affiliation: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA.
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Reinhard
authors.affiliation: Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, USA.
authors.forename: Timothy W
authors.initials: TW
authors.lastname: Lyons
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA. ; Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Ariel D
authors.initials: AD
authors.lastname: Anbar
keywords: Archean; Earth sciences; Geochronology; Hamersley Basin; Mount McRae Shale; atmospheric oxygen; molybdenum; osmium; oxidative continental weathering; rhenium