From Science Daily: "What’s alarming to us as scientists is that there were only very slight natural changes that resulted in the onset of hypoxia in the deep ocean," said Professor Kennedy. "This occurred relatively rapidly — in periods of hundreds of years, or possibly even less — not gradually over longer, geological time scales, suggesting that Earth’s oceans are in a much more delicate balance during greenhouse conditions than originally thought, and may respond in a more abrupt fashion to even subtle changes in temperature and CO2 levels."
M. J. Kennedy, T. Wagner. Clay mineral continental amplifier for marine carbon sequestration in a greenhouse ocean. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1018670108