It has long been debated how exactly our world came to be breathable to so many forms of life. Was there was a smooth transition to an oxygen-rich atmosphere following on from the rapid increase in global photosynthesis? Or was there a series of “fits and starts” on the road to planet-wide oxygenation? A new study in Science Advances agrees with the latter, revealing that the Great Oxygenation Event (GOE) became stable not immediately after the emergence of the photosynthesizing bacteria, but after a series of smaller spikes that gradually built up the amount of free oxygen in our atmosphere.
The GOE was undoubtedly one of the most important events in the history of the world. Scientists agree that the GOE was caused by photosynthesizing bacteria called cyanobacteria, which emerged about 200 million years before, but the way in which it occurred has been heavily debated.
Roughly 2.5 billion years ago the concentration of oxygen (O2) in the atmosphere spiked, permitting the emergence of oxygen-using (aerobic) life. The event caused massive, widespread extinctions too; in fact, it has sometimes been referred to as the Oxygen Catastrophe. Free oxygen is actually toxic to a wide range of lifeforms (anaerobic organisms) that do not require it for growth, so its dramatic increase caused most of them to die out. Without the GOE, life on Earth would have evolved to be radically different.
The widely acknowledged evidence for the GOE comes in many forms. For example, certain compounds of uranium can only exist in the world if there is a sufficient concentration of free oxygen in the atmosphere. Uranium has an impressive half-life, meaning that we can use it to date substances containing it back billions of years. Using this technique, scientists have agreed that the GOE occurred 2.5 billion years ago.
However, it has also been argued that significant levels of oxygen were being produced by photosynthesizing life long before this, but because the concentrations at the time were relatively low, they are far more difficult to detect in the geological record. The authors describe these small oxygen generation events as “whiffs,” and they set out to find them in the ancient rocks of Western Australia.
Image credt: These stromatolites in Western Australia were created by ancient cyanobacteria responsible for producing the GOE. Rob Beyer/Shutterstock
In doing so, they found high concentrations of three elements – molybdenum, rhenium and osmium – within once-marine rocks called black shales. These elements are normally found in land-based minerals, and they are highly reactive to free oxygen. The only way they could have made it into these deep ocean black shales was if they reacted with atmospheric oxygen; this would release them into nearby rivers, which would transport them into the sea.
Of course, this weathering process requires there to have been oxygen production in the first place, indicating that there was photosynthesizing life around at the time. The age of these rocks could be ascertained using the same type of radiometric dating used on the aforementioned uranium deposits – and it turns out that they are more than 50 million years older than the onset of the GOE.
Tracking these elements’ concentrations over time, it appeared to show fluctuations in the amount of atmospheric oxygen available for the weathering process, rather than a gradual, smooth increase. This indicated that the GOE was the final result of “fits and starts” of oxygen generation before the global concentration finally stabilized. The cause of these fluctuations, however, is still under debate.