The onset of a widespread absence of oxygen in the world's deep oceans 94.5 million years ago may have been triggered by sulphur emitted from volcanoes, according to a report published online this week in Nature Geoscience.
Using marine sediments deposited during this event ― known as Ocean Anoxic Event 2 ― Matthew Hurtgen and colleagues found that unusually low background sulphate levels increased dramatically at the start of the event. Sulphate is a key ingredient for the recycling of organic matter, allowing the bodies of sinking plankton to decompose. The team suggests that once more sulphate was available, it triggered a chain reaction: the nutrients released by the decomposing organisms allowed more and more organisms to grow at the surface. As their bodies sank, they too were decomposed ― a process that rapidly consumes oxygen. The chain only stopped once widespread oxygen depletion allowed the sulphur mineral pyrite to develop, consuming the excess sulphate.
The sulphate was most likely supplied by the heightened volcanic activity that is reported to have occurred at this time.