Oxide minerals discovered in past geologic studies around the world had indicated that Earth's atmosphere had at least small amounts of oxygen about 2.22 billion years ago. New studies of the sediments in northern South Africa's Rooihoogte formation suggest that oxygen was present nearly 100 million years earlier than that.
The ratio of sulfur isotopes in sulfide minerals obtained at middle levels of the Rooihoogte formation–deposited in a shallow sea about 2.32 billion years ago–suggest that the oxygen concentration in the atmosphere had grown to at least 1/100,000th of that found in today's air. Large quantities of iron oxides present in strata even higher up verify the presence of oxygen at later dates, says geochemist Heinrich D. Holland of Harvard University.
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