Nature
PUBLISHED
Earth’s first (water) oceans are thought to have been too acidic for life. A new study has estimated the timelines of the processes that changed this and puts a figure on when they reached the right pH to host the first organisms.
Modern oceans are slightly alkaline, with a pH of around 8.1. The addition of carbon dioxide to the atmosphere is shifting that – a process known as ocean acidification – but the value is expected to stay above the neutral point of 7, which is just as well. Even a slightly more acidic ocean is bad news for some shell-forming creatures, but a truly acidic ocean, even slightly so, is considered incompatible with life because it hinders the formation of organic molecules.
Yet many scientists have argued conditions on the early Earth would have made for acidic oceans, until processes driven by plate tectonics changed oceanic chemistry. “To understand the origin of life, it becomes important to understand when and how Earth began hosting an ocean with a more neutral pH,” the study's first author Dr Meng Guo, formerly of Yale University, said in a statement.
Guo and Professor Jun Korenaga have estimated the time those processes would have taken to reach a point compatible with life. If they’re right, for its first 500 million years, Earth’s oceans were uninhabitable by any lifeform. Pockets of lower pH may have existed in semi-isolated bays where there was little interchange with the global ocean, but not far beyond.
The finding creates a paradox, because the Earth is 4.54 billion years old, and molecular clocks put the first life forms 4.2 billion years ago, ie before Guo and Korenaga think the oceans could support it. There are several possible solutions to this. Not only could the new estimate be wrong – not uncommon for a first try – but the molecular clocks’ validity is questioned once you go back before fossil evidence to calibrate them. Alternatively, however, this could be the answer to the question of whether life began at hydrothermal vents on the sea floor, or in island ponds.
Guo said; “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.” Korenaga called the work; “A tour-de-force theoretical endeavor,” but those challenges leave plenty of room for error.
Indeed, the authors acknowledge that on some estimates Earth’s oceans were alkaline from the beginning. Most of the few rocks that survive from those times are not of the sort that preserve a record of the pH in which they formed.
By modeling the rates of weathering of sea floor silicates and the first protocontinents, and ocean capture of carbon dioxide from the atmosphere, Guo and Korenaga estimate a starting pH around 5, reaching neutrality 4 billion years ago. Although taking place over an immense period by our standards, the shift is rapid compared to the oceans’ subsequent stability. The authors think the oceans were initially very hot and rich in magnesium, leading to the accelerating weathering of rocks that produce alkaline products or lock up acidic carbon dioxide.
“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,” which preceded the watery version.
This work is good news for the quest for life (although not necessarily intelligence) on other worlds. One of the strongest arguments for extraterrestrial life is that life appeared on Earth almost as soon as it was possible to do so, suggesting the bar to get over is not that high. If Guo and Korenaga are right, that’s even more true than we thought, with excess acidity barring the way until just before.
The study is published in the journal Nature Geoscience.
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