Approximately 2.4 billion years ago, free oxygen began to fill Earth’s atmosphere for the first time in a period called the Great Oxidation Event (GOE). Following this period, it is commonly believed that oxygen levels collapsed for more than a billion years. However, a new study of ancient rocks that formed during this predicted decline actually shows evidence for elevated levels of oxygen – a rather puzzling discovery for the researchers involved.
“What we found contradicts the prevailing view,” Kaarel Mänd, a PhD candidate at the University of Alberta, Canada, and lead author of the study published in Nature Geoscience, said in a statement. “This will force the Earth science community to rethink what drove the carbon and oxygen cycles on the early Earth.”
What led this international team of researchers to the surprising conclusion was a 2-billion-year-old sample of shungite – a unique carbon-rich sedimentary rock predominantly found in Russia. Hidden in the rock are clues to Earth’s conditions when it was deposited. The researchers found strikingly high levels of molybdenum, uranium, and rhenium, in addition to elevated uranium isotope ratios. As Mänd notes, these are signs of a high concentration of oxygen on Earth’s surface.
“These trace metals are only thought to be common in Earth's oceans and sediments when oxygen is abundant,” Mänd explained. “These trace metal concentrations are unrivaled in early Earth's history, suggesting elevated levels of oxygen at the time when the shungite was deposited.”
However, this contradicts widely accepted models of Earth's carbon and oxygen cycles that predict a rapid decrease in oxygen levels after the Great Oxidation Event at the time when the shungite was deposited. These conflicting conclusions suggest to the authors that the Earth remained well oxygenated for millions of years after the termination of the GOE.
This finding could have implications for our understanding of when complex life first evolved. Eukaryotic cells, the precursor to all complex life, require high oxygen levels to thrive. Therefore, as this study proposes, the existence of a longer well-oxygenated early Earth suggests complex life may have evolved sooner than we expected. Further research at the University of Alberta and the University of Tartu, Estonia, will study the delay between the initial rise in oxygen and the spread of eukaryotes.