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The Great Oxidation Event (GOE), an event marking the rise of oxygen in the early Earth’s atmosphere, is estimated to have happened between 2.5 and 2.3 billion years ago. In a study led by researchers at Arizona State University, and supported in part by the NASA Astrobiology Institute, scientists analyzing ancient shale samples found in Western Australia have discovered evidence for significant ocean oxygenation occurring before the GOE, and as far down as the sea floor. This opens up new questions about the GOE and how and why oceanic build-up of O₂ happened.

The paper, “Fully oxygenated water columns over continental shelves before the Great Oxidation Event” is published in Nature Geoscience.

The press release is available through Arizona State University.

Excerpted from ASU:

Oxygen in the form of the oxygen molecule (O2), produced by plants and vital for animals, is thankfully abundant in Earth’s atmosphere and oceans. Researchers studying the history of O2 on Earth, however, know that it was relatively scarce for much of our planet’s 4.6 billion-year existence.

So when, and in what environments, did O2 begin to build up on Earth? Stromatolite in Shark Bay, Western Australia. These stromatolites are thought to be some of the most ancient forms of life on Earth and are comprised of organisms that probably contributed to the O2 scientists are inferring existed on ancient Earth (i.e., cyanobacteria). Credit: Ariel Anbar, ASU Download Full Image

By studying ancient rocks, researchers have determined that sometime between 2.5 and 2.3 billion years ago, Earth underwent what scientists call the “Great Oxidation Event” or “GOE” for short. O2 first accumulated in Earth’s atmosphere at this time and has been present ever since.

Through numerous studies in this field of research, however, evidence has emerged that there were minor amounts of O2 in small areas of Earth’s ancient shallow oceans before the GOE. And in a study published recently in the journal Nature Geoscience, a research team led by scientists at Arizona State University has provided compelling evidence for significant ocean oxygenation before the GOE, on a larger scale and to greater depths than previously recognized.

For this study, the team targeted a set of 2.5 billion-year-old marine sedimentary rocks from Western Australia known as the Mt. McRae Shale.

“These rocks were perfect for our study because they were shown previously to have been deposited during an anomalous oxygenation episode before the Great Oxidation Event,” said lead author Chadlin Ostrander, of ASU’s School of Earth and Space Exploration.

Read the full press release.