Pushing back an oxygen-rich atmosphere

Tiny crystals of iron oxide in ancient Australian rocks offer evidence that the Earth’s atmosphere held significant amounts of oxygen far earlier than previously thought, a new study suggests.

Large quantities of oxide minerals in rocks around the world indicate that the atmosphere had at least small amounts of oxygen by 2.2 billion years ago (SN: 1/24/04, p. 61). And the presence of certain biomarkers in Australian rocks has been hailed as evidence that oxygen-making organisms had evolved by 2.7 billion years ago, but recent studies have cast some doubt on that earlier date (SN: 11/22/08, p. 5).

Now, analyses of rocks laid down 3.46 billion years ago in what is now Australia push back the oxygen era even further, Hiroshi Ohmoto, a geochemist at Pennsylvania State University in University Park, and his colleagues contend online March 15 in Nature Geoscience.

Hematite, one type of iron oxide, can form in a variety of ways, only some of which require an oxygen-rich atmosphere, says Ohmoto. If ultraviolet light strikes iron hydroxide minerals, it triggers a reaction that drives away water and leaves hematite behind. In an environment that lacks UV light, however, hematite only forms via a reaction between iron and oxygen.

The team’s analyses of hematite-containing samples from Australia’s Marble Bar Chert — a rock formation in the northwestern part of the country — suggest the hematite formed deep underwater, in the absence of UV light.

The chert formation, which is between 50 and 200 meters thick and about 30 kilometers long where it’s exposed at the Earth’s surface, is sandwiched between two thick layers of volcanic rock. The cooled lavas in those adjacent formations aren’t frothy with bubbles, a sign that the two strata formed under high pressure — probably on a seafloor at least 200 meters deep, Ohmoto says. A lack of erosion in any of these strata and of signs that waves or currents disrupted the sediments that made up the chert suggest that the material accumulated in deep water, far below where ultraviolet light penetrates.

The hematite in the uppermost layers of the chert — those laid down as sediments around 3.46 billion years ago — is the key evidence for plentiful oxygen, Ohmoto says. Those particles, which in many cases clump together to form thin layers, are single crystals of the mineral, indicating that they weren’t produced by the UV-driven degradation of an iron hydroxide mineral, he notes. The team’s geochemical analyses also suggest that the crystals formed as hot, iron-rich hydrothermal fluids spewed from an ocean floor into cool oxygenated waters. The concentration of dissolved oxygen in those waters almost rivaled those found in today’s ocean deeps, the team estimates.

Rock samples from the Marble Bar Chert “are rosy red” from oxidation, says Paul Knauth, a geochemist at Arizona State University in Tempe, but the presence of other easily oxidized minerals in the same rocks — pyrites, in particular — suggests that that the hematite oxidation occurred when the rocks first formed, not millions of years later. “I’m convinced the environment there was oxygenated,” he adds.

The implications of these findings are profound: If oxygen was present in near-modern-day concentrations in such a broad and deep body of water, the atmosphere above must have been heartily oxygenated as well. Presumably that oxygen was produced by photosynthetic organisms, possibly pushing back their first appearance beyond eras when they were known to exist.

Ohmoto says that the researchers can’t yet tell whether oxygen was available worldwide or only locally at the time, or whether oxygen concentrations declined in later eras only to bounce back to modern levels millions of years later. However, Ohmoto suggests, it is “possible to have limited amounts of anoxic water in an oxygenated ocean, but it’s not really likely to have an oxygen oasis in a large, anoxic ocean.”