Previous models of the early Earth posited that the level of oxygen in the atmosphere was extremely low for the first two billion years of the planet’s history, over 100,000 times lower than the present level. Despite ephemeral increases in oxygen levels 2.7 to 2.6 billion years ago, oxygen remained sparse until the Great Oxidation Event about 2.3 billion years ago, when levels of free oxygen in the atmosphere swelled due to the rise of multicellular cyanobacteria not long before, in geological terms, which boasted more efficient metabolisms than their unicellular precursors.
However, a new study indicates that this picture of Earth’s greatest climatic revolution might be more complicated. The new research, led by Sean Crowe of the University of British Columbia, has revealed that oxygen began to build up in Earth’s atmosphere approximately 3 billion years ago, 700 million years prior to the Great Oxidation Event.
Crowe and colleagues from the University of Copenhagen investigated two geological layers in South Africa, both approximately 3 billion years old. The Nsuze paleosol is the most ancient preserved soil on Earth, and is sandwiched between volcanic rocks and sedimentary rocks that were deposited in the sea and river channels. The other geological unit examined was the Ijzermyn iron formation.
Crowe and team analyzed samples of the Nsuze paleosol and Ijzermyn iron formation to determine the ratios of chromium isotopes present in the rocks; isotopes are variants of an atomic element that differ in the number of neutrons in the nucleus. Chromium is an especially good indicator of atmospheric oxygen content in deep time because, as it is oxidized, the distribution of chromium isotopes in the environment changes. In the case of the Nsuze and Ijzermyn rocks, the researchers found higher proportions of a chromium isotope that results from oxidation. The team also found higher proportions of an isotope of uranium that also results from oxidation. The results indicate an interval of higher concentrations of atmospheric oxygen 3 billion years ago.
The new results from South Africa fill part of a gap in the geological record of oxygen concentration. Iron formations in Greenland indicate almost no atmospheric oxygen at 3.8 billion years ago. Prior to the aforementioned transient elevated oxygen levels 2.7 to 2.6 billion years ago, there was a gap in the record between 3.51 and 2.85 billion years ago. The new study points to another, older period of elevated oxygen approximately 3 billion years ago, in the middle of that gap.
“This study now suggests that the process began very early in Earth’s history, supporting a much greater antiquity for oxygen producing photosynthesis and aerobic life,” Crowe explained in a University of British Columbia press release.
The new study was published in the September 26 issue of the journal Nature.