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Weathering rocks have traces of the Earth’s great oxidation event

Weathering rocks have traces of the Earth's great oxidation event
Lead author James Andrew Leong near a very alkaline source in Oman. The modern source lies along massive deposits of calcium carbonate formed from thousands of years of spring emissions. In the background, mountains are composed of ultramafic rocks. Credit: Everett Shock / ASU

About 2.4 billion years ago, the Earth's atmosphere underwent what is known as the Great Oxidation Event (GOE). Before GOE, the early Earth had far less molecular oxygen than we have today. After GOE, molecular oxygen began to rise in abundance, eventually making life as ours possible.

For decades, researchers have been trying to understand why and how GOE originated.

A team of scientists, led by James Andrew Leong with Tucker Ely, who both received their doctorates from Arizona State University (ASU)'s School of Earth and Space Exploration in 2020, and ASU Professor Everett Shock, have determined that weathering rocks could have contributed to GOE. Their results were recently published in Nature communication.

Molecular oxygen is produced by plants and photosynthetic microbes, but molecular oxygen is also consumed by organisms and by oxidation of iron, sulfur, carbon and other elements in rocks. Molecular oxygen can also be consumed through reaction with reduced gases such as hydrogen, which can be formed during weathering of rocks.

Scientists studying the Early Earth assume that the consumption of oxygen may have been faster than the production of oxygen by photosynthesis, so oxygen was not able to accumulate in the atmosphere.

"It's like when your bills exceed your income, money can not accumulate in a savings account. This seems to have been the situation on the early Earth," said co-author Shock, from ASU's School of Earth and Space Exploration and the School of Molecular Science.

For GOE to occur under this hypothesis, the consumption of oxygen had to decrease over time so that oxygen could build up in the atmosphere.

With that in mind, Leong and his team set out to determine what processes could slow the consumption of oxygen on the early Earth to produce an increase in oxygen.

Weathering rocks have traces of the Earth's great oxidation event
Chemical reactions between ultramafic rocks and groundwater led to the formation of strongly alkaline water, which can be discharged back to the surface. At the surface, these liquids react with carbon dioxide from the atmosphere to form calcium carbonate, which are the white minerals seen here from a field study the authors conducted in Oman. Credit: Leong / Shock / ASU

"We know it's probably not biological consumption that does a decent job of keeping pace with oxygen production through photosynthesis," Shock said. "So we might have thought that the rate at which oxygen was consumed by the weathering of rocks created this change."

To test their hypothesis, Leong and his team focused on the weathering of a type of rock known as "ultramafic", a igneous rock rich in magnesium and low-silica iron.

Ultramafic rocks comprise most of the Earth's upper mantle, where they were formed at high temperatures. When these rocks are brought to the surface and come in contact with water, the waterless minerals that make up these rocks are converted into minerals that contain water. This process is called serpentinization, after the most important substitute mineral, serpentine. The process also converts the reacting groundwater into a strongly alkaline water with elevated gas content; especially hydrogen.

They were inspired to do this by research they had previously conducted on hyperalkaline and gaseous fluids found in the ultramafic mountains of present-day Oman, which was published in the AGU's Journal of Geophysical Research in 2021.

"Our previous field research in Oman made us wonder what the Earth's early surface and atmosphere would have looked like, as high pH and hydrogen-rich fluids were as common as today's almost neutral pH groundwater and rivers," Leong said. "Ultramafic rocks such as those found in Oman are rare on the earth's surface today, but were abundant beneath the warmer early earth."

For their analysis, they performed computer simulations, based on a computer code developed by co-author Ely, to predict the hydrogen generation potentials of thousands of rock formations common in early Earth. From there, they could then draw connections between rock compositions and their potentials to generate hydrogen and consume oxygen.

With these simulations, the team was then able to reconstruct global hydrogen production and oxygen consumption rates via early underground serpentinization and determine that the weathering of ultramafic rocks could have helped ease GOE.

"We were able to model the change of thousands of rock compositions that are likely to be present on the early Earth," Leong said. "Our calculations show that many of these rocks, especially those that are truly ultramafic in composition or rich in magnesium, such as those found in Oman today, have very high potentials for generating hydrogen gas and helping to prevent the accumulation of "The decline in the abundance of ultramafic rocks in the earth's surface towards the end of the archaic eon could have helped ease the great oxidation event."

How low did it go? Study seeks to settle debate on oxygen in Earth's early atmosphere

More information: James Andrew M. Leong et al., Declining extent of archaic serpentinization contributed to the rise of an oxidized atmosphere, Nature communication (2021). DOI: 10.1038 / s41467-021-27589-7
Provided by Arizona State University

Citation: Weathering rocks have traces of the Earth's Great Oxidation Event (2022, January 19) retrieved January 20, 2022 from

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