Magazine article Science News

Atmosphere Took Roller-Coaster Ride around Time of Earth's Oxygenation: Minerals Hint That Levels of the Gas Rose, Dipped, Rose Again

Magazine article Science News

Atmosphere Took Roller-Coaster Ride around Time of Earth's Oxygenation: Minerals Hint That Levels of the Gas Rose, Dipped, Rose Again

Article excerpt

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. …

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