Ancient rocks provide clues to the beginnings of the Earth

So when and where did O2 start to accumulate on Earth?

By studying ancient rocks, researchers determined that some time ago between 2.5 and 2.3 billion years ago, the Earth was undergoing what scientists call the "Great Event". oxidation "or" GOE ". The O2 first accumulated in the Earth's atmosphere at that time and has been present ever since.

However, many studies in this area of ​​research have highlighted the presence of minimal amounts of O2 in small areas of the former shallow oceans of the Earth before the GOE. And in a study recently published in the journal Nature Geoscience, a research team led by scientists from the University of Arizona (ASU) provided compelling evidence of significant ocean oxygenation before the GOE, on a larger scale and at greater depths than previously recognized .

For this study, the team targeted a set of marine sedimentary rocks 2.5 billion years old from Western Australia, known as Mt. McRae Shale. "These rocks were perfect for our study because it was shown that they had been deposited during an episode of abnormal oxygenation before the big event of oxidation," said Dr. Main author Chadlin Ostrander from the School of Earth Exploration and Space ASU.

The shales are sedimentary rocks that were, at one time in the past of the Earth, deposited on the bottom of the ancient oceans. In some cases, these schists contain the chemical footprints of the ancient oceans in which they were deposited.

For this research, Ostrander dissolved shale samples and separated elements of interest into a clean laboratory and then measured isotopic compositions on a mass spectrometer. This process was completed with the help of co-authors Sune Nielsen of the Woods Hole Oceanographic Institution (Massachusetts); Jeremy Owens of Florida State University; Brian Kendall from the University of Waterloo (Ontario, Canada); scientists Gwyneth Gordon and Stephen Romaniello from the School of Earth Exploration and Space USS; and Ariel Anbar from the School of Earth and Space Exploration and School of Molecular Sciences of the ASU. The data collection lasted more than a year and used the facilities of the Woods Hole Oceanographic Institution, Florida State University and the ASU.

Using mass spectrometers, the team measured isotopic compositions of thallium and molybdenum from Mt. McRae Shale. This was the first time that both isotope systems were measured in the same set of shale samples. As assumed, a predictable pattern of isotopes of thallium and molybdenum emerged, indicating that manganese oxide minerals were buried in the ocean floor over vast areas of the former ocean. For this burial to take place, the O2 was to be present up to the bottom of the sea, 2.5 billion years ago.

These discoveries improve scientists' understanding of the history of oxygenation of the ocean on Earth. The accumulation of O2 was probably not limited to small parts of the ocean surface before the GOE. More likely, the accumulation of O2 has spread over vast areas of the ocean and has spread far into the depths of the ocean. In some of these areas, the accumulation of oxygen seems to have spread to the bottom of the sea.

"Our discovery forces us to rethink the initial oxygenation of the Earth," says Ostrander. "There is ample evidence to suggest that O2 began to accumulate in the Earth 's atmosphere about 2.5 billion years ago during the GOEs. is now evident that the oxygenation of the Earth is a story rooted in the ocean. The O2 's probably accumulated in significant levels, according to our data, well before doing so in the atmosphere. "

"Now that we know when and where O2 started to accumulate, the next question is why," says the professor and co-author of ASU's president, Anbar. "We believe that O2-producing bacteria were developing in the oceans long before they began to accumulate in the atmosphere." What has changed? to cause this build-up – it's what we're working on next. "

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More information:
Fully oxygenated water columns on the continental shelves before the big event of oxidation, Nature Geoscience (2019). DOI: 10.1038 / s41561-019-0309-7, https://www.nature.com/articles/s41561-019-0309-7