The first oxygen-producing life-forms appeared hundreds of millions of years earlier than previously known, new evidence suggests.
Analyzing iron and uranium embedded inside primeval rocks, researchers discovered that shallow seawater contained whiffs of dissolved oxygen around 3.2 billion years ago. The new date places the appearance of Earth’s oxygen around 200 million years earlier than previous studies, the researchers report online August 24 in Earth and Planetary Science Letters.
This early oxygen pins the evolution of oxygen-producing photosynthesis to nearly a billion years before cyanobacteria flooded Earth with oxygen during the Great Oxygenation Event, says Penn State geoscientist James Kasting, who was not involved with the study. “This is further evidence that cyanobacteria came early,” he says, noting that the finding raises more questions. “The earlier cyanobacteria evolved, the more we have to explain what delayed the rise of oxygen.”
The presence of even a tiny bit of oxygen allows for a slew of chemical reactions. Geochemist Aaron Satkoski of the University of Wisconsin–Madison and colleagues hunted for the products of these chemical reactions in ancient red- and pink-striped rocks collected in South Africa. The rocks formed on the seafloor around 3.23 billion years ago as sediments piled up. Iron-enriched red bands probably formed in the deep ocean while lower-iron pinkish layers came from shallow waters where sediments accumulate more quickly.
The iron in all of these bands probably originated from hydrothermal vents. As the iron floated through the ocean, any dissolved oxygen in the water would react with it and cause the iron to oxidize, essentially rusting. The oxidized iron then fell to the seafloor.
Some varieties of iron oxidize more readily than others. Typically about 92 percent of the iron in seawater is in the form of iron-56, which is more prone to oxidation than its lighter sibling, iron-54 (about 6 percent of the ocean’s iron). At very low oxygen concentrations, a disproportionate amount of iron-56 will oxidize and build up in seafloor sediments. At higher oxygen concentrations, all of the iron will oxidize.
By measuring the ratio of the two iron isotopes in the rock layers, researchers can calculate how much oxygen was dissolved in the water. Deep ocean water was oxygen-free 3.2 billion years ago, the analysis found. Shallow waters, however, contained as much as 0.1 percent of the oxygen concentrations found in modern seawater. Since photosynthetic organisms congregate near the sea surface to gather light, the oxygen serves as indirect evidence that “tells us that an oxygen-producing biosphere had already developed,” says coauthor Clark Johnson, a Wisconsin–Madison astrobiologist.
Uranium atoms found alongside the iron ruled out the possibility that the oxidation came from another source —such as bacteria that oxidize iron, rather than using photosynthesis, for energy. The uranium atoms require oxygen to break free from the mineral uraninite.
While the new work provides important clues about when oxygen first appeared in Earth’s oceans, determining when exactly the first photosynthetic organisms appeared “is a tricky thing,” says geochemist Dimitri Sverjensky of Johns Hopkins University. Early oxygen-producing organisms probably created localized patches of oxygenated water called oxygen oases, he says. Detecting the presence or absence of oxygen in one spot therefore can’t provide a global picture of oxygen conditions.
Editor’s Note: This story was updated on September 8, 2015, to clarify in the caption that cyanobacteria exhale oxygen.