Weathering rocks hold clues to Earth's Great Oxidation Event

The lead author is near a spring. The calcium carbonate that formed from thousands of years of spring discharge lies along the modern spring. The mountains are made of ultramafic rocks. Credit:ASU

The Great Oxidation event took place about 2.4 billion years ago. The early Earth had less oxygen than we do today. Life became possible after the GOE because of the increase in oxygen in the air.

Researchers have tried to understand how the GOE happened.

A team of scientists, led by James Andrew Leong with Tucker Ely, both of whom earned their doctorate degrees from Arizona State University's School of Earth and Space Exploration in 2020, have determined that weathering rocks might have contributed to the GOE. Their results were published in Nature Communications.

Oxygen is produced by plants, but also consumed by organisms, and by the oxidation of iron, sulfur, carbon and other elements in rocks. Reduced gases like hydrogen can form during rock weathering and can be used to consume oxygen.

The consumption of oxygen was more rapid than the production of oxygen, so oxygen was not able to accumulate in the atmosphere.

Money can't accumulate in a savings account when bills exceed income. This appears to have been the situation on the early Earth, according to co-author Shock.

The consumption of oxygen had to be slowed so that oxygen could build up in the atmosphere.

Leong and his team set out to determine what processes could be slowing down the consumption of oxygen on the early Earth to produce an increase in oxygen.

Highly alkaline water can be discharged back into the surface from chemical reactions between ultramafic rocks. The white minerals seen here are calcium carbonate and carbon dioxide from the atmosphere, which are formed when fluids react with carbon dioxide. Credit: Leong/Shock/ASU

"We know it's probably not biological consumption, which does a decent job of keeping up with oxygen production by photosynthesis," Shock said. The rate at which oxygen was being consumed by the weathering of rocks was thought to be the cause of the change.

To test their hypothesis, the team focused on the weathering of a type of rock called "ultramafic," which is rich in magnesium and iron.

Most of the Earth's upper mantle was formed at high temperatures. The waterless minerals that make up the rocks transform into water-bearing minerals when they are brought to the surface. The process is called serpentinization. The process transforms the reacting groundwater into a highly alkaline water with elevated gas content.

They were inspired to do this by the research they had done on fluids found in the ultramafic mountains of present-day Oman that was published in the AGU's Journal of Geophysical Research.

When high pH and hydrogen-rich fluids were common, what the early Earth surface and atmosphere would have looked like. Ultramafic rocks like those found in Oman are rare in the Earth's surface at the present-day, but were abundant during the hotter early Earth.

They used computer simulations to predict the hydrogen generation potentials of thousands of rock compositions that were common during the early Earth. They could draw connections between rock compositions and their potentials to generate hydrogen and oxygen.

The team was able to reconstruct the global hydrogen production and oxygen consumption rates via serpentinization during early Earth and determine that the weathering of ultramafic rocks could have helped facilitate the GOE.

"We were able to model thousands of rock compositions that are likely to be present on the early Earth." Many of the rocks that are rich in magnesium and ultramafic have high potentials to generate hydrogen gas and help prevent the build up of oxygen, according to our calculations. The Great Oxidation Event could have been aided by the decline in the amount of ultramafic rocks in the Earth's surface.

The rise of an oxidation atmosphere was caused by decreasing extents of Archean serpentinization. There is a DOI of 10.1038/s41467-021-27589-7.

There are clues to Earth's Great Oxidation Event in the weathering rocks.

The document is copyrighted. Any fair dealing for the purpose of private study or research cannot be reproduced without written permission. The information provided is for educational purposes.