Terrestrial volcanoes appear to have contributed to the rise of oxygen in Earth's atmosphere more than 2 billion years ago. The atmospheric shift, which allowed life to flourish, had long been attributed to ancient bacteria.
The change in Earth's atmosphere occurred about 2.5 billion years ago. And the bacteria theory has long been problematic, because fossilized cyanobacteria first appeared 200 million years earlier. Scientists wondered why they took so long to fill the atmosphere with oxygen. The answer appears to be that underwater volcanoes were undoing their work.
To put the story together, geoscientists Lee Kump of Pennsylvania State University in State College and Mark Barley of the University of Western Australia in Crawley relied on some recent clues. Improvements in radioactive isotope dating techniques have timed the influx of oxygen more precisely, and it coincides with the formation of most of the continents, 2.5 billion years ago. Kump and Barley suspected there might be a link.
The pair analyzed published studies of volcanic deposits and found something intriguing: a significant shift from submarine volcanoes to volcanoes on land around the same time that oxygen showed up. Prior to 2.5 billion years ago, the oceanic crust was much thicker than today, and continental crust was thinner. This meant that more land mass was flooded and most volcanic eruptions were underwater. Because these eruptions are cooler than those on land, they release a lot of reduced gases that consume oxygen, such as hydrogen and hydrogen sulfide.
As the continents thickened and stabilized, the ocean crust thinned. Ocean basins could hold more water, and much more land bobbed above sea level. Consequently, more eruptions took place on solid ground. Unlike underwater volcanoes, the gases from land-based volcanoes soak up far less oxygen. Kump and Barley believe that this explains the time lag between the appearance of bacteria and oxygen's prominence. The shift in volcanoes allowed oxygen produced by bacteria to begin accumulating and, 2 billion years later, to give rise to animals. The research appears in the 30 August issue of Nature.
This volcanic buffering scenario is winning fans. "There's a real cause-and-effect relationship," says geochemist Timothy Lyons of the University of California, Riverside.