Atmosphere took roller-coaster ride around time of Earth's oxygenation: minerals hint that levels of the gas rose, dipped, rose again.

Oxygen levels in Earth's atmosphere dropped for an extended time about 1.9 billion years ago, after the atmosphere became oxygenated in what is known as the Great Oxidation Event, researchers report.

Evidence for the oxygen drop comes from minerals in banded iron formations, large iron oxide repositories that accumulated billions of years ago (SN: 6/20/09, p. 24). Those minerals contain trace elements that reveal environmental conditions at the time, says Don Canfield, a geobiologist at the University of Southern Denmark in Odense. In particular, he and his colleagues argue in the Sept. 10 Nature, stable isotopes of chromium reveal the level of oxygenation of the ancient atmosphere.

Analyses of chromium isotopes in banded iron formations that accumulated during various intervals between 3.7 billion and 550 million years ago show oxygen trends similar to those seen in previous studies. But levels unexpectedly differ during one interval, around 1.9 billion years ago. Chromium ratios in minerals deposited around that time-particularly a formation in Ontario, Canada--are similar to those in deposits that formed well before the Great Oxidation Event began about 2.5 billion years ago.

The new findings are a sign that oxygen concentrations in the atmosphere 1.9 billion years ago dropped substantially for several million years, the researchers say. The environmental circumstances behind this decline in atmospheric oxygen aren't clear, however.

Although researchers debate the exact cause and timing of the Great Oxidation Event, the evolution of photosynthetic microorganisms almost certainly was required to generate large amounts of oxygen. Scientists have long thought that after the Great Oxidation Event, atmospheric oxygen levels never dropped.

The new technique for inferring atmospheric oxygen levels works like this: When rocks bearing manganese and chromium are exposed to oxygen in the air, a series of chemical reactions releases the chromium, which makes its way to the seavia rivers. The higher the oxygen, the higher the ratio of chromium-53 to chromium-52 is in the river water. When those waters flow into an iron-rich sea where banded iron formations are accumulating, the chromium gets locked away in the formations.

"If the new findings are true," says Kurt Konhauser of the University of Albertain Edmonton, Canada, "we'd have to reinterpret everything we know about environmental conditions" 1.9 billion years ago. Furtherwork may determine which of the existing oxygen-inferring techniques is most accurate for this time period.

The team's data also hint that oxygen concentrations were low but on the rise for at least 300 million years before the Great Oxidation Event. That finding may reinvigorate debate about whether oxygen-making organisms evolved as early as 2.7 billion years ago, as suggested by biomarkers in Australian rocks (SN: 11/22/08, p. 5).