Life on Earth was transformed following the permanent rise of atmospheric oxygen. Previous research timed this transition, a period dubbed the Great Oxidation Event, to about 2.32 billion years ago. According to a new study in Nature, this timing needs a revision.
The authors propose that atmospheric oxygen fluctuated for 200 million years during the Great Oxidation Event and didn’t become constant until about 2.22 billion years ago. The new findings push the permanent arrival of oxygen to 100 million years later than previously thought.
“One hundred million years is a long time period—even for a geologist,” said Simon Poulton, a geochemist at the University of Leeds in the United Kingdom and lead author of the study.
In addition to providing insights into our planet’s history, information about oxygen on Earth could guide the search for life beyond the solar system. “Say we detect oxygen in an exoplanetary atmosphere and Earth-like planet—we want to be able to know what exactly that means,” said Noah Planavsky, a geochemist at Yale University in New Haven, Conn., not involved in the study.
Drilling into Earth’s Past
Previous studies already suggested that the oxygenation of Earth’s atmosphere wasn’t a one-time transition. Scientists obtained this historical information from drill cores and mass-independent fractionation of sulfur isotopes, or MIF-S.
“The community is pretty well agreed that [MIF-S] is a solid, robust tracer of atmospheric oxygen levels,” said Michael Kipp, a geochemist at the California Institute of Technology in Pasadena not involved in the study. Specific sulfur isotopes in rock samples show stretches of Earth’s history when the atmosphere lacked oxygen (less than 100,000 times the present-day amount).
In samples from the Paleoproterozoic era, the MIF-S signal disappeared and reappeared multiple times. Rising oxygen levels, indicated by loss of the MIF-S signal, may have disrupted greenhouse gases and altered temperatures planetwide. In agreement with this model, oxygen fluctuations correspond to three of the glaciation events that occurred between 2.43 billion and 2.22 billion years ago. A fourth glaciation, however, occurred 60–100 million years after oxygen supposedly became permanent.
“That’s where you start to get a puzzle,” Poulton said. He and his colleagues analyzed two drill cores from the Transvaal Supergroup in South Africa. These samples provided evidence of the atmospheric conditions before and after the final Paleoproterozoic glaciation. The team detected a new MIF-S signal in rock layers deposited after about 2.32 billion years ago, indicating that Earth’s atmosphere returned to an anoxic state—one lacking oxygen—later than expected.
“Now we have an explanation for the final glaciation,” Poulton said.
The team confirmed that the MIF-S signals reflect a true atmospheric record and weren’t just artifacts of recycled sediments. In the study, the authors verified that the isotope signals match those expected under anoxic conditions. “That’s a really critical part of the paper,” said Joshua Krissansen-Totton, a planetary scientist at the University of California, Santa Cruz not involved with the study.
In the future, the authors plan to continue investigating what drove these changes in atmospheric oxygen and how feedback produced its permanent rise. “There’s plenty to do,” Poulton said.
—Jack Lee (@jackjlee), Science Writer
21 May 2021: This article has been updated to include a more recent photo of a researcher sampling a Precambrian drill core in South Africa.