Oxygen Abundance over Time (image) Science in Public Caption The red line represents the estimated oxygenation curve based on the abundance of the trace element selenium (Se) in pyrite samples. Oxygen levels are presented as %PAL (Present Atmospheric Levels), where 100% PAL represents the Earth's atmospheric oxygen at the present day. Credit University of Tasmania Usage Restrictions None 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.