The concentration of oxygen in Earth’s atmosphere dropped for an extended time about 1.9 billion years ago, following the Great Oxidation Event, researchers now report.

Evidence for the drop in oxygen levels comes from the analyses of minerals taken from banded iron formations, large repositories of iron oxide that accumulated  billions of years ago (SN: 6/20/09, p. 24). Those minerals contain large amounts of trace elements, which can provide details about 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, the ratio of stable isotopes of chromium reveal the level of oxygenation in the ancient atmosphere.

Analyses of chromium isotopes in samples from banded iron formations that accumulated during various intervals between 3.7 billion and 570 million years ago show oxygen trends generally consistent with those seen in previous studies. But levels unexpectedly differ during one interval, the time around 1.9 billion years ago. Chromium ratios seen in samples of minerals deposited around that time — and particularly in those from a banded iron formation in Ontario, Canada — are similar to those seen in deposits that formed well before Earth’s atmosphere became well oxygenated about 2.5 billion years ago, during what’s called the Great Oxidation Event.

The new findings are a sign that oxygen concentrations in the atmosphere 1.9 billion years ago dropped substantially for an extended period, 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 once the Great Oxidation Event occurred, atmospheric oxygen levels never dropped back, says Timothy Lyons, a biogeochemist at the University of California, Riverside. But, he notes, the team’s new findings “are compelling evidence that oxygen made a dip again about 1.9 billion years ago. … That’s an observation that’ll have legs.”

The new technique for inferring atmospheric oxygen levels works like this: When rocks bearing manganese and chromium are exposed to an oxygenated atmosphere, a series of chemical reactions releases the chromium, which makes its way to the sea via rivers. The higher the concentration of oxygen in the air, 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 —which has a strong affinity for iron — gets locked away in the mineral formations.

For now, Lyons isn’t concerned by the fact that the team’s chromium-isotope data disagree with other proxies, such as the ratios of various sulfur isotopes used in other studies, that don’t show the dip. Future work may show that other techniques for inferring oxygen concentrations work only within certain ranges of oxygen levels and therefore results obtained using those techniques may not be valid in all cases.

But other researchers suggest caution. “It’s a bit premature to make a big deal of this [chromium-isotope] proxy because we don’t understand a lot about it,” says Kurt Konhauser, a geobiologist at the University of Alberta in Edmonton, Canada. “If the new findings are true, we’d have to reinterpret everything we know about environmental conditions” 1.9 billion years ago, he adds.

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

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