Earth’s oxygen-rich atmosphere emerged in whiffs from a kind of cyanobacteria in shallow oceans around 2.5 billion years ago, according to new research.
“The onset of Earth’s surface oxygenation was likely a complex process characterized by multiple whiffs of oxygen until a tipping point was crossed,” said Brian Kendall, a professor of Earth and Environmental Sciences at the University of Waterloo. “Until now, we haven’t been able to tell whether oxygen concentrations 2.5 billion years ago were stable or not. These new data provide a much more conclusive answer to that question.”
The new data supports a hypothesis proposed by Anbar and his team in 2007. In Western Australia, they found preliminary evidence of these oxygen whiffs in black shales deposited on the seafloor of an ancient ocean.
The black shales contained high concentrations of the elements molybdenum and rhenium, long before the Great Oxidation Event.
The osmium isotope evidence found in black shales correlates with higher continental weathering as a result of oxygen in the atmosphere. By comparison, slightly younger deposits with lower molybdenum and rhenium concentrations had osmium isotope evidence for less continental input, indicating the oxygen in the atmosphere had disappeared. More.
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Here’s the 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 187Os/188Os of 0.34 ± 0.19 were obtained for rhenium- and molybdenum-rich black shales. The initial 187Os/188Os 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 187Os/188Os 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. (Public access) – B. Kendall, R. A. Creaser, C. T. Reinhard, T. W. Lyons, A. D. Anbar. Transient episodes of mild environmental oxygenation and oxidative continental weathering during the late Archean. Science Advances, 2015; 1 (10): e1500777 DOI: 10.1126/sciadv.1500777