Some 2.5 billion years ago, Earth’s oxygen-rich atmosphere emerged on the scene, all thanks to the photosynthesis of blue-green algae, according to a new study from a team of Canadian and American researchers.
For the next 100 million years, the oxygen continued to build, eventually leading to a permanent change in the oxygen levels around 2.4 billion years ago—which is 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 co-author Brian Kendall, a professor of Earth and Environmental Sciences at the University of Waterloo, in a statement.
“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.”
Blue-green oxygen fiends
The data, as published in Science Advances, lends credence to the notion that the sudden burst of oxygen arose thanks to photosynthetic cyanobacteria—or blue-green algae.
“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?’” asked co-author 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.”
Anbar himself previously studied the matter in 2007, when he and his team found evidence of these oxygen whiffs in black shales that had once been sediment deposited on the ancient ocean floor before the Great Oxidation Event.
These shales contained high concentrations of two chemical elements that are sensitive to the presence of atmospheric oxygen: molybdenum and rhenium. When these elements react with oxygen, they are released into rivers, leading into to the ocean—where they deposit on the ocean floor and eventually become shale.
Now, the same shales were studied again—this time for the presence of osmium isotopes. The ratio of two specific isotopes, osmium 187 and osmium 188, indicates where the osmium came from—either land or an underwater volcano. Osmium sourced from land would indicate that it was pushed into the water by the presence of oxygen.
The ratio of the isotopes indicates that the osmium was deposited in the ocean thanks to oxygen-driven continental weathering—helping to confirm that algae released whiffs of oxygen that eventually led to the Great Oxidation event around 2.4 billion years ago.
—–
Image credit: Thinkstock
Comments