A new study of prehistoric ocean sediments from an era of high carbon dioxide concentrations shows that warm oceans with high CO2 levels and low-ocean conditions have experienced mass extinctions of marine organisms. Scientists from the UK and Australia examined ocean sediment samples off the coast of western Africa from the late Cretaceous period, 85 million years ago, an epoch of high atmospheric CO2 levels. The researchers found a significant amount of organic matter from marine organisms buried within the deoxygenated sediment layers, indicating that these organisms suffered mass die-offs as CO2 levels rose, ocean temperatures increased, and the oceans held less oxygen. Martin Kennedy of the University of Adelaide in Australia said the research showed these extinctions occurred over periods of only hundreds of years or possibly less, and took place with only modest changes in CO2 and oxygen levels in the oceans. Kennedy said the results, published in the Proceedings of the National Academy of Sciences, demonstrate that the rapidly rising atmospheric CO2 levels of our era pose a serious threat to marine life, as already evidenced by growing marine dead zones. “Earth’s oceans are in a much more delicate balance during greenhouse conditions than originally thought,” said Kennedy.