Dawen (image) Arizona State University Caption The research team analyzed carbonate rock samples collected from the Dawen Permian-Triassic boundary section located on the Great Bank of Guizhou, a carbonate platform in China's Guizhou Province. Uranium isotopes measured in these ancient rocks showed that the world's oceans experienced a rapid shift toward strongly oxygen-deficient conditions at the time of the end-Permian mass extinction. Credit Tom Algeo 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.