Layers of clues: The Contessa Quarry in central Italy where rock for part of the study was drilled from. The different coloured sediment layers represent deposition of organic carbonate under oxygenated versus anoxic ocean conditions.
Credit: S.Turgeon
Two competing theories
Two theories, which are not mutually exclusive, emerge to explain the chemistry of what happened next, commented Tim Bralower, a geologist at Pennsylvania State University, in the U.S., who reviewed the paper.
One possibility is that the volcanoes spewed out metal-rich fluids that seeded the upper level of the ocean with micronutrients, he said. Tiny plantlife on the sea surface, called phytoplankton, gorged on the food, and storing up carbon as they grew. They then sank to the sea floor and decayed, stripping the ocean of oxygen.
The other is that the volcanoes disgorged clouds of carbon dioxide to the atmosphere, said Bralower, stoking global warming to the extent that Earth's ocean circulation system ground to a near-halt. Beyond the surface layers, water was no longer turned over and anoxia resulted.
Bralower said that figuring out the post-volcanism scenario could help scientists wrestling with unknowns about global warming today.
The knowledge gaps include the impact of higher temperatures on marine circulation and whether controversial schemes to sow the ocean with iron filings, to spur phytoplankton growth and thus soak up carbon dioxide from the atmosphere, would ease warming or cause oxygen starvation in the sea depths.
