MADISON, Wis., Oct. 7 (UPI) -- Scientists have estimated that oxygen became abundant some three billion years ago. But when did it first appear? That question has been harder to answer.
But rock samples recovered from South Africa have offered scientists the earliest evidence of oxygen and photosynthesis. By measuring the presence of iron isotopes and uranium, researchers at the University of Wisconsin confirmed the presence of oxygen in Earth's ancient oceans, 3.23 billion years ago.
The scientists published their findings in the journal Earth and Planetary Science Letters.
The core sample was sourced from a 3.23-billion-year-old rock found in a geologically stable region in eastern South Africa. The sample's layered bands show the various types of sediment that fell to the bottom of the ocean over time. A mass spectrometer revealed the first presence of oxygen-rich iron oxides, likely from sediments stirred up by the waves in the ocean's shallows.
"The grainier rock that formed from shallow, wave-stirred sediment looks rusty, and contains iron oxide that required much more oxygen to form," lead study author Aaron Satkoski, an assistant scientist in Wisconsin's geoscience department, said in a press release.
Scientists say a boost in oxygen levels could have only come from photosynthesis performed by a living organism. But for now, the evidence only proves oxygen and photosynthesis were present in one specific spot, long ago.
Once organisms developed photosynthesis, however, researchers say it would have quickly spread.
"There was evolutionary pressure to develop oxygenic photosynthesis," said co-author Clark Johnson, a professor of geoscience at Wisconsin. "Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live."
Scientists used uranium to confirm their analysis of iron oxide isotopes. Because uranium is only soluble in the oxidized form, its presence also connotes ancient oxygen levels. And because uranium decays at a predictable rate, scientists were able to confirm the timing of oxygen's appearance.
Researchers say the cyanobacteria was likely to the first living organism to develop photosynthesis, but their current work can't reveal the exact source of the first oxygen spike.
"We are trying to define the age when oxygenic photosynthesis by bacteria started happening," said co-author Brian Beard, a senior scientist. "Cyanobacteria could live in shallow water, doing photosynthesis, generating oxygen, but oxygen was not necessarily in the atmosphere or the deep ocean."