Earth’s acid test: When did ocean acidity allow life to commence?

New Haven, Conn. — Scientists at Yale and in Singapore have devised what may be the ultimate acid test — a comprehensive model for estimating the origins of Earth’s habitability, based in part on ocean acidity.

The new theoretical model applies previously published, Yale-led research to a wide range of interconnected geological and atmospheric processes. It may provide the clearest picture yet of how Earth evolved to a point where life was able to flourish.

“This is a tour-de-force theoretical endeavor, bridging a longstanding gap between surface processes and processes deep in the Earth,” said Jun Korenaga, a professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences, and co-author of a new study in the journal Nature Geoscience. “This work presents by far the most comprehensive whole-Earth system model to estimate how ocean pH likely evolved during Earth’s history.”

The term pH (“potential of hydrogen”) is a measure of the concentration of hydrogen ions in an aqueous — watery — solution. A lower pH level equals higher acidity. A solution with a pH lower than 7 is considered acidic; modern-day seawater has a pH of about 8.

But it is widely believed that Earth’s ancient ocean was much more acidic, making it harder to sustain life. Many scientists have found that the synthesis of organic molecules is extremely difficult in environments with a pH level lower than 7.

“To understand the origin of life, it becomes important to understand when and how Earth began hosting an ocean with a more neutral pH,” said Meng Guo, a former Yale graduate student in Korenaga’s lab who is now a presidential postdoctoral fellow at Nanyang Technological University in Singapore and first author of the new study.

“But modeling the long-term evolution of ocean pH is a notoriously difficult problem, as it involves almost all of the components of the Earth system: the atmosphere, the ocean, the crust, and the mantle,” Guo said.

For example, ocean pH depends to a large extent on atmospheric carbon dioxide (CO2), which, in turn, is influenced by a variety of other factors. The concentration of CO2 decreases, for instance, as a result of its chemical reaction with continents, deep-sea oceanic crust — and its eventual plunge into Earth’s interior via subduction. But levels of atmospheric CO2 increase when there is volcanic activity.

For their study, Korenaga and Guo carefully calibrated and set parameters for how each of these components functioned — and then had them interact. The researchers were guided by a series of early Earth studies previously published from Korenaga’s group.

“I think the main reason why we are able to do this modeling now is that our understanding of early Earth tectonics has been drastically improved in the last few years,” Korenaga said. “That work concentrated on the evolution of continental crust and the physics of magma oceans.”

Using their new model, Korenaga and Guo estimated that it would have taken Earth 500 million years to neutralize ocean acidity enough to support life. Pockets of water with more neutral pH levels may have existed earlier, but not on a large enough scale for life to take hold.

The researchers said their findings can shed light not on early Earth processes, but also on the role those processes play in modern day climate.

The research was supported, in part, by a NASA astrobiology grant.

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