“A little oxygen made a huge difference,” geologist Hugo Moreira said, Eos reported on Friday describing how Earth operated as a geological sponge for oxygen more than a billion years before the Great Oxidation Event.
That milestone event transformed the primordial oxygen-free atmosphere roughly 2.3 billion years ago into a life-sustaining envelope. Researchers established how long ago such oxygen started being absorbed by geological processes in Earth’s mantle through an analysis of a new study in Nature Geoscience.
Early Earth’s mantle contained specific amounts of oxygen. The first whiffs of gaseous oxygen from photosynthesizing cyanobacteria added to that total, leaving trackable evidence inside zircon crystals.
Researchers examined zircon crystals taken from coarse-grained igneous rocks known as sanukitoids. These rocks appear in cratons, which are ancient geological cores located near the centers of modern continents.
Sanukitoids function as geochemical windows into craton formation. They routinely contain zircon crystals trapping inclusions of apatite, a phosphorus-based mineral carrying traces of sulfur.
The oxidation state of that sulfur indicates exact oxygen amounts in the mantle during apatite formation. Zircons also contain uranium, which decays radioactively at a known rate and allows scientists to date crystal formation to within a million years.
“These minerals are really exciting to scientists, as they are essentially time capsules,” Moreira said.
The study focused on subarc regions of the upper mantle situated beneath active volcanoes. Researchers theorized these zones experienced the heaviest impact from surface oxidation.
The wider implications extend down to Earth’s core. Rocks created in subarcs circulate over geological time into much deeper regions.
Billions of years ago, anaerobic microbes produced energy by consuming iron and other minerals in shallow waters and near hydrothermal vents. The evolution of cyanobacteria photosynthesis liberated life from this reliance on anaerobic processes.
Colonies grew wherever water and sunlight were present. The resulting oxygen byproduct eventually accumulated in the atmosphere, triggering the Great Oxidation Event and setting off a mass extinction of anaerobic microbes.
The study determined that Earth’s mantle reacted to atmospheric oxygen whiffs as long as 3.4 billion years ago. Researchers observed mantle oxygen activity increase between 3.4 billion and 3 billion years ago.
Geologist Richard Palin at the University of Oxford, who wasn’t involved in the study, called the work a “clever approach.” Palin studies rocks from Earth’s Archaean era that are between 4 billion and 2.5 billion years old and formed before the Great Oxidation Event, during the period covered by the study.
“The dataset is quite sparse,” Palin said, noting the study used 14 rock samples worldwide yielding 72 zircon crystals and 39 apatite inclusions.
“Sanukitoids are rare.…If they were more abundant, you would be able to do a more comprehensive study,” Palin told Eos.
