Approximately 2.4 billion years ago, the Great Oxidation Event dramatically increased the oxygen content in Earth’s atmosphere and paved the way for the rise of all lifeforms that use oxygen to break down nutrients for energy.
Scientists agree about when the event happened, but they are less certain about exactly how.
Marek Locmelis, assistant professor of geosciences and geological and petroleum engineering, believes he’s discovered that “how.” He studies if and how oxygen bound in minerals and magmas in the interiors of planets can affect the composition of oceans and atmospheres and contribute to whether a planet is habitable.
“Without the Great Oxidation Event, there would be no plant and animal life on Earth or at least no life on Earth as we know it — including us,” says Locmelis. “We provided really solid evidence that Archean mantle oxidation contributed to it, which was something that was ruled out for the past 20 or 30 years.”
In the August 2019 issue of American Mineralogist, Locmelis describes how he used a technique known as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis to study minerals that contain clues to how Earth’s interior changed over time. This technique enables much more sensitive isotopic and elemental analysis of solid samples than earlier methods.
Locmelis studied samples of the mineral olivine in igneous rocks called komatiites that were derived from Earth’s mantle during the Archean Eon — a geologic time period 4 billion to 2.5 billion years ago.
Locmelis gathered samples of komatiites from cratons in southern Africa, Australia and Canada. A craton is an old, stable part of the Earth’s topmost layers — the crust and upper mantle — that range from 2.7 billion to 3.5 billion years old.
The researchers compared the trace element chemistry of olivine from komatiites that are 3.5 billion to 3.3 billion years old to ones that formed more recently — about 2.7 billion years ago. They found that Earth’s mantle appears to have gradually become more oxidized between 3.5 billion and 2.7 billion years ago, Locmelis says. This oxidation, in turn, possibly triggered the Great Oxidation Event.
Locmelis says previous studies dismissed the Archean Eon mantle oxidation because they used different analytical techniques. Researchers used to crush and analyze whole rocks instead of isolating minerals, such as olivine, and analyzing them via modern techniques, such as the one used in this study.
“Take eating a sandwich. You take a bite and just want to taste a pickle, but sometimes the pickle is so small that you’re overwhelmed by all the other flavors,” says Locmelis. “That’s the difference between whole rock studies and mineral studies. When you analyze minerals directly, you circumvent all the problems associated with bulk analysis, which really are just a mix of different flavors or geological processes. If you analyze minerals that crystallized early from the magma, you really have much more robust information, especially with all the modern techniques we have available today.”
Locmelis says his study suggests that Archean mantle oxidation may have contributed to, or even triggered, the oxidation of the atmosphere, which led to the lifeforms on Earth today. He notes that a recent study submitted at the same time as his yielded similar results, but used a different analytical approach.
“If two studies come to the same conclusion, independently and with different techniques, it suggests that we are really on to something, and that we have to rethink our understanding of the redox evolution of our planet,” says Locmelis.