The young Earth supported little multicellular life until its atmosphere acquired a healthy portion of oxygen. That change has been credited to the rise of cyanobacteria, known as blue-green algae, that produce oxygen by photosynthesis. Now, scientists argue that oxygen couldn’t have built up in the atmosphere until a crucial geological mechanism kicked in and set the scene for the rise of more-complex forms of life.
By relating atmospheric composition to the chemistry of various ancient rock types, geologists have inferred that Earth went from largely oxygenfree to oxygen-rich 2.4 billion to 2.5 billion years ago (SN: 1/24/04, p. 61). But the fossil record shows that cyanobacteria existed about 2.7 billion years ago, leaving scientists to wonder why 200 million to 300 million years of oxygen production by these bacteria resulted in no accumulation of the gas.
The answer, says Lee Kump of Pennsylvania State University in University Park, is that during that time, Earth’s oceans and land acted as a “chemical sink” that mopped up oxygen as fast as cyanobacteria produced it. What eventually permitted oxygen to accumulate in air was a broad change in the location and nature of volcanoes, Kump and Mark Barley of the University of Western Australia in Perth propose in the Aug. 30 Nature. After examining geological data on the composition, age, and by-products of volcanoes from different eras, the researchers concluded that around 2.5 billion years ago, there was a general shift from underwater volcanoes to volcanoes on land.
That this shift was followed by a rise in an oxygen-rich atmosphere is more than coincidence, Kump says. His and Barley’s analysis shows that rocks of the Archean eon, which are more than 2.5 billion years old, formed when gases such as hydrogen sulfide and methane were abundant. Underwater volcanoes, whose eruptions don’t attain such high temperatures as their above-water counterparts, spew those gases profusely. In both the oceans and the atmosphere, these reactive gases create “oxygen-hungry” conditions, says Kump, preventing that gas from accumulating.
The start of the next eon, the Proterozoic, was marked by changes in rock formation that favored thicker, lighter crust and in particular, “the development of large, stable continents,” says Kump. With higher eruption temperatures, volcanoes on these burgeoning continents would have ejected gases such as carbon dioxide and sulfur dioxide directly into the atmosphere. These gases don’t readily react with oxygen, Kump says, which would have allowed it to build up.
The move to an oxygen-rich atmosphere is “an Earth-system event,” says Kump, that “reflects the interplay between biological and nonbiological factors.”
Geochemist Tim Lyons of the University of California, Riverside, who wrote an editorial accompanying the new study, says that “Kump and Barley have put together a really elegant tectonic argument.” Up-and-down swings in the amount of oxygen in the atmosphere might have preceded the volcano-induced transformation to a stable, oxygen-rich world, Lyons adds.