Mount McRae Shale Formation (IMAGE) University of Washington Caption Roger Buick in 2004 at the Mount McRae Shale in Western Australia. Rocks drilled near here show “whiffs” of oxygen occurred before the Great Oxidation Event, 2.4 billion years ago. New analyses show a slightly earlier spike in the element mercury emitted by volcanoes, which could have boosted populations of single-celled organisms to produce a temporary “whiff” of oxygen. Credit Roger Buick/University of Washington Usage Restrictions With credit License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.