A whiff from blue-green algae gave oxygen to Earth

Toronto/Washington: Oxygen on Earth emerged in whiffs from a kind of blue-green algae in shallow oceans around 2.5 billion years ago, helping the origin of life with enough oxygen around to inhale, shows significant research from Canadian and US scientists.

These whiffs of oxygen likely happened in the following 100 million years, changing the levels of oxygen in Earth's atmosphere until enough accumulated to create a permanently oxygenated atmosphere around 2.4 billion years ago - a transition widely 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 Brian Kendall, professor of Earth and Environmental Sciences at the University of Waterloo in Ontario. 

"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 added.

Earth scientists from University of Alberta, University of Waterloo, Arizona State University, University of California Riverside and Georgia Institute of Technology found evidence that Earth's transition to a permanently oxygenated atmosphere was anything but smooth.

The team presented new isotopic data showing that a burst of oxygen production by photosynthetic cyanobacteria temporarily increased oxygen concentrations in Earth's atmosphere.

"One of the questions we ask is: 'did the evolution of photosynthesis lead directly to an oxygen-rich atmosphere? Or did the transition to today's world happen in fits-and-starts?" said Professor Ariel Anbar of Arizona State University. 

"How and why Earth developed an oxygenated atmosphere is one of the most profound puzzles in understanding the history of our planet."

In Western Australia, Anbar and team found preliminary evidence of these oxygen whiffs in black shales deposited on the seafloor of an ancient ocean.

The black shales contained high concentrations of the elements molybdenum and rhenium, long before the “Great Oxidation Event”.

These elements are found in land-based sulphide minerals, which are particularly sensitive to the presence of atmospheric oxygen. 

Once these minerals react with oxygen, the molybdenum and rhenium are released into rivers and eventually end up deposited on the sea floor.

In the new paper, researchers analysed the same black shales for the relative abundance of an additional element: osmium. 

Like molybdenum and rhenium, osmium is also present in continental sulfide minerals. 

The osmium isotope evidence found in black shales correlates with higher continental weathering as a result of oxygen in the atmosphere. 

The findings were presented in a paper published in the journal Science Advances.