Roughly 2.4 billion years ago, oxygen gas began suffusing Earth’s atmosphere in what scientists call the Great Oxidation Event. Much remains uncertain about this critical moment in history, but now researchers find that copper in ancient sediments could help track how oxygen levels have fluctuated in the past. The scientists detailed their findings online April 18 in PNAS.

Although oxygen gas currently makes up about a fifth of the air we breathe, there was little if any oxygen in the primordial atmosphere. Significant levels of the gas started permanently building up in our atmosphere with the Great Oxidation Event, most likely due to the biochemical activities of cyanobacteria—microbes that, like plants, photosynthesize and exhale oxygen. The rise of oxygen then helped support the evolution of oxygen-breathing life.

“The Great Oxidation Event set the stage for the initiation of the evolution of complex life that eventually led to the appearance of higher animals that use oxygen as a fuel to burn food to generate energy,” says lead author and geomicrobiologist Ernest Chi Fru at Stockholm University.

Most of what scientists know of past oxygen levels depends on analyzing concentrations of oxygen-sensitive elements, such as iron, in ancient rocks. Many of these proxies of ancient oxygen levels remain unexplored.

In hopes of learning more about the Great Oxidation Event, Chi Fru and his colleagues explored whether copper might shed light on past oxygen levels. Roughly 69 percent of all copper is found as the stable isotope copper-63, while about 30 percent comes in the form of the stable isotope copper-65. For reasons that remain unclear, iron oxides prefer binding to copper-65 more than they do copper-63.

Before the Great Oxidation Event, massive continental deposits of iron oxide would have clung onto copper-65, suggesting that seawater back then would have been enriched in copper-63. However, as oxygen levels rose, the gas would have reacted with continental deposits of sulfides, breaking them down so they could more easily seep into water. This sulfide-rich water would in turn have washed more copper-65 runoff into the oceans.

Chi Fru and his colleagues investigated copper isotope ratios in marine black shales dating 2.66 billion to 2.08 billion years old that spanned the Transvaal Supergroup in South Africa and the

Francevillian Series in Gabon. They found evidence of the predicted shift toward increasing marine levels of copper-65 starting about 2.4 billion years ago, corresponding roughly to the beginning of the Great Oxidation Event.

“Copper does not appear to tell us much about when exactly the Great Oxidation Event started, but its properties allow us to show that the Great Oxidation Event was a protracted process that went on from about 2.45 billion to 2.3 billion years ago,” Chi Fru says. He notes that isotope ratios of other elements such as sulfur might provide greater quantities of data regarding ancient oxygen levels, but that copper could offer specific details about how those levels changed over time—for instance, when it comes to ocean chemistry.

“I find it exciting that the shift in copper isotopes is so unambiguous and irreversible,” says geomicrobiologist Elizabeth Swanner at Iowa State University, who did not take part in the study. “I think this bodes well for the utility of copper isotopes.”

Future research could look at copper isotope ratios to “help explain mass extinction events that are related to changing oxygen content in various ecosystems,” Chu Fri says.