By Karin Valentine
Newswise — TALLAHASSEE, Fla. — Earth’s vast oceans started becoming oxygenated on large scales and great depths long before scientists previously recognized, according to new research from Arizona State University, Florida State University, Woods Hole Oceanographic Institution and The University of Waterloo.
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 where did O2 begin to build up on Earth?
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. But this most recent study, published in the journal Nature Geoscience, provides compelling evidence for extensive ocean oxygenation that potentially predates the GOE by millions of years.
“This research uses thallium and molybdenum isotopes to investigate the earliest ocean oxygenation,” said study co-author Jeremy Owens, an assistant professor in FSU’s Department of Earth, Ocean and Atmospheric Science. “Our work provides strong evidence for a rise of ocean oxygenation, which precedes the Great Oxidation Event by 50 million years.”
For their 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.
Shales are sedimentary rocks that were, at some time in Earth’s past, deposited on the sea floor of ancient oceans. In some cases, these shales contain the chemical fingerprints of the ancient oceans they were deposited in.
For this research, Ostrander dissolved shale samples and separated elements of interest in a clean lab, then measured isotopic compositions on a mass spectrometer located at the National High Magnetic Field Laboratory.
The team then measured the thallium and molybdenum isotope compositions of the Mt. McRae Shale. This was the first time both isotope systems had been measured in the same set of shale samples. As hypothesized, a predictable thallium and molybdenum isotope pattern emerged, indicating that manganese oxide minerals were being buried in the sea floor over large regions of the ancient ocean. For this burial to occur, O2 needed to have been present all the way down to the sea floor 2.5 billion-years-ago.
These findings improve scientists’ understanding of Earth’s ocean oxygenation history. Accumulation of O2 was probably not restricted to small portions of the surface ocean prior to the GOE. More likely, O2 accumulation extended over large regions of the ocean and extended far into the ocean’s depths. In some of these areas, O2 accumulation seems to have even extended all the way down to the sea floor.
“Our discovery forces us to re-think the initial oxygenation of Earth,” said Ostrander. “Many lines of evidence suggest that O2 started to accumulate in Earth’s atmosphere after about 2.5 billion years ago during the GOE. However, it is now apparent that Earth’s initial oxygenation is a story rooted in the ocean. O2 probably accumulated in Earth’s oceans — to significant levels, according to our data — well before doing so in the atmosphere.”
“Now that we know when and where O2 began to build up, the next question is why,” said ASU President’s Professor and co-author Ariel Anbar. “We think that bacteria that produce O2 were thriving in the oceans long before O2 began to build up in the atmosphere. What changed to cause that build-up? That’s what we’re working on next.”
Sune Nielsen at Woods Hole Oceanographic Institution also contributed to this study, along with Brian Kendall at the University of Waterloo and scientists Gwyneth Gordon and Stephen Romaniello of ASU’s School of Earth and Space Exploration.