New research suggests Earth's oxygen-rich atmosphere was created by "whiffs" of blue-green algae present in shallow oceans around 2.5 billion years ago.
These whiffs of oxygen are believed to have accumulated over the next 100 million years until a permanently oxygenated atmosphere was created in what is known as the "Great Oxidation Event," the University of Waterloo reported.
"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, a professor of Earth and Environmental Sciences at the University of Waterloo. "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."
New isotopic data revealed that a burst of oxygen production by photosynthetic cyanobacteria once temporarily increased the oxygen concentration in the 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."
Past findings have found black shale deposits from Western Australia that were formed before the Great Oxidation Event contained high concentrations of the elements molybdenum and rhenium. These elements are found in land-based sulphide minerals, which react with oxygen and release molybdenum and rhenium into rivers where they are then deposited on the seafloor. In this new study, the researchers looked at the element osmium in these black shale deposits, and found isotopic evidence that higher continental weathering as a result of oxygen in the atmosphere occurred. The ratio of two osmium isotopes can tell us if the source of osmium was continental sulfide minerals or underwater volcanoes in the deep ocean. Comparatively slightly younger deposits with lower molybdenum and rhenium concentrations had osmium isotope evidence suggesting less continental output, which would indicate a disappearance of the oxygen.
The findings were published in a recent edition of the journal Science Advances.
