How did Earth transform from a hostile, acidic ocean world to one that nurtured life?
Scientists at Yale and in Singapore have developed a groundbreaking model to estimate how ocean acidity evolved over time.
Unraveling Earth’s Habitability Through Ocean Acidity
Scientists at Yale and Singapore’s Nanyang Technological University have developed a groundbreaking model to estimate when and how Earth became habitable, with a key focus on ocean acidity.
This new theoretical model builds on previous Yale-led research, incorporating a wide range of geological and atmospheric processes. It offers one of the most detailed views yet of how Earth evolved to support life.
Connecting Surface and Deep Earth Processes
“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.”
pH, or “potential of hydrogen,” measures the concentration of hydrogen ions in water. A lower pH means higher acidity. A solution with a pH lower than 7 is considered acidic. While modern seawater has a pH of about 8, scientists believe Earth’s ancient oceans were much more acidic, which would have made it difficult for life to emerge.
The Complexity of Modeling Ocean pH
“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.
How Atmospheric CO2 Shapes Ocean Chemistry
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.”
How Long Did It Take for Earth to Support Life?
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 only on early Earth processes, but also on the role those processes play in modern-day climate.
Reference: “Rapid rise of early ocean pH under elevated weathering rates” by Meng Guo, and Jun Korenaga, 10 February 2025, Nature Geoscience.
DOI: 10.1038/s41561-025-01649-9
The research was supported, in part, by a NASA astrobiology grant.
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4 Comments
“While modern seawater has a pH of about 8, scientists believe Earth’s ancient oceans were much MORE ACIDIC, …”
The implication of the quote is that the oceans today are acidic. That is not the case. They are alkaline.
“To understand the origin of life, it becomes important to understand when and how Earth began hosting an ocean with a more neutral pH,”
I’m disappointed by the sloppy use of the term “more neutral.” Something is either neutral or not. It is akin to saying that a female is slightly pregnant. As used in the article, it should be “initially less acidic and subsequently alkaline.” A neutral aqueous solution is one with equal concentrations of hydrogen (actually hydronium) and hydroxyl ions. To a first-order approximation, that occurs at a pH of about 7. Therefore, solutions with a pH of less than 7 are, by definition, called acidic. Solutions with a pH greater than 7 are called basic or alkaline. Regardless, the boundary for acidic solutions is where the ratio of hydronium ions (H3O+) to hydroxyl ions (OH-) is 1:1, which is called neutrality, and neither ion is in excess. It is a point on the pH line; something is either at the point or not. On the other hand, acids can become “more acidic” or “less acidic” within the pH range of <0 and <7, with a pH less than 0 being a very concentrated, strong acid. (It might be surprising to some that an acid can have a negative pH.)
It is inappropriate to say that an acidic solution is becoming 'more neutral' if the pH increases. If the pH is changing, then it is enough to say that the pH is moving in the direction of neutrality to indicate it is becoming less acidic.
It is more informative to say whether the pH change is in the direction of acid or base, but an acid solution during the Precambrian, as discussed above, would have been experiencing a lowering in concentration of the hydronium ions, and an increase in the hydroxyl ions. Grammatically, one cannot have more of 'something' if that something doesn't exist (e.g. neutrality). For an acidic solution to become alkaline, it must first become neutral before it can become alkaline or, subsequently, more alkaline. The pH (0-14) scale isn't really a continuum because a solution interacts differently with metals, alkalies, acidic solutions, and basic solutions differently, depending on whether the the solution has a pH of 7. A pH of 7 is essentially a discontinuity where the nature of the behavior changes. (That is also why the careless practice of saying the oceans today are becoming “more acidic” is misleading. They are alkaline and probably always will be because carbonic acid is a weak acid that partakes in the (bi)carbonate buffering process.) The rate of the reaction (commonly called the strength) is determined by how far away from 7 the solution is.
Surprisingly, the article doesn’t mention the (bi)carbonate and borate buffering processes that keep the oceans in an alkaline state today.
I should also point out that they don’t specify their assumption of the minimum pH necessary for life to evolve. Today, extremophile organisms are known to exist in very acidic geothermal pools and hot pools that the acid has changed to ‘mud.’
https://scitechdaily.com/scientists-unlock-secrets-of-an-alga-that-shouldnt-survive-but-does/
“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.”
It is interesting that the number of significant figures in their estimate suggests that they think that their estimate has a precision of +/- 50 million years. The actual published article says that the oceans had a neutral pH by 4 billion years ago.
However, they say that plate tectonics was an important part of their model: “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.” However, the prevailing view is that plate tectonics didn’t start until 3 to 4 billion years ago. That would mean that neutrality wasn’t achieved until 2.5 to 3.5 billion years ago, NOT 4 billion years ago, assuming that the rest of their model assumptions and calculations are correct!
“Pockets of water with more [sic] neutral pH levels may have existed earlier, but not on a large enough scale for life to take hold.”
On what do they base that opinion?
It is surprising that work like this passes peer review.