For about the first 2.1 billion years of Earth’s history, its atmosphere didn’t have enough oxygen to support complex life.
Scientists believe that Earth’s first atmosphere,formed after its creation, was probably made up of hydrogen and helium. Then as a result of volcanic eruptions that spewed gases from Earth’s interior, in a process known as “outgassing”, a secondary atmosphere became filled with those such as carbon dioxide, sulfur dioxide, ammonia and water vapor.
Then about 2.3 to 2.5 billion years ago, during the Proterozoic Eon, blue/green algae called cyanobacteria, living in Earth’s shallow oceans, began emitting enough oxygen through photosynthesis to create the permanently oxygenated atmosphere that keeps us alive today.
Researchers from five North American universities have collaborated on a study that found the process in fully oxidizing the atmosphere didn’t happen suddenly, but instead took place in scattered bursts over an approximately 100-million-year period in what has become known as the Great Oxidation Event.
“The onset of Earth’s surface oxygenation was likely a complex process characterized by multiple whiffs of oxygen until a tipping point was crossed,” said one of the study’s authors, Brian Kendall from Canada’s University of Waterloo in a university press release. “Until now, we haven’t been able to tell whether oxygen concentrations 2.5 billion years ago were stable or not. These new data provide a much more conclusive answer to that question,” he said.
Prior to these large bursts of oxygen from the cyanobacteria, scientists say most of the oxygen that had been produced by early microbial life was simply chemically captured by materials such as dissolved iron or organic matter before having the chance to escape into the atmosphere.
But O2 began to collect in the atmosphere after these oxygen sinks filled up and couldn’t absorb any more.
The researchers were able to make their findings after discovering certain chemicals in black shale that had been deposited in the seafloor of an ancient ocean in West Australia.
The elements that provided the link to the researcher’s findings are osmium, molybdenum and rhenium. They are produced by a reaction of oxygen with land-based sulfide minerals. The scientists say that after the chemicals are produced they then make their way into rivers, which flow into oceans and are eventually deposited on the sea floor.
The research collaborators from Canada’s Universities of Alberta and Waterloo along with Arizona State University, University of California Riverside, and Georgia Institute of Technology, all in the U.S., published their findings in the journal Science Advances.