Earth may have contained fresh water 500 million years earlier than previously thought, according to a new study of tiny grains of zircon from Australia dated to about 4 billion years ago. The minerals contain evidence of rain falling on land, according to scientists, providing new evidence that early Earth had continental crust in addition to oceans.
It’s a new glimpse into the Hadean eon, which ended about 3.8 billion years ago—a time when our young planet held little more than rock, magma, and water. Data on this ancient period are sparse, making this new analysis a welcome addition to scientists’ understanding of how our planet formed.
“We pushed back the line 500 million years.”
The discovery, published in Nature Geoscience, could also help researchers better pinpoint when life emerged on Earth. Fresh water and exposed continental crust are two of the ingredients some scientists posit were necessary for life to begin.
Currently, the earliest evidence of life may be 3.5-billion-year-old stromatolites from Australia. The new work shows “we have the same conditions around 4 billion years ago,” said study coauthor Hamed Gamaleldien, a geochemist at Khalifa University in the United Arab Emirates. “So we pushed back the line 500 million years.”
Looking at the Young Earth
Little remains of Hadean Earth. Most rocks from that time have been worn away or subducted deep under the crust. Some of the only remaining minerals from the Hadean are crystals of zircon embedded in younger rocks.
Zircon is tough and resistant to chemical alteration, and it contains small bits of uranium, which allow scientists to date it. Zircon also contains oxygen, which was the key to the new discovery.
Oxygen has three stable isotopes: oxygen-16, oxygen-17, and oxygen-18. The ratio of light oxygen (16O) to heavy oxygen (18O) is influenced by Earth processes such as evaporation and condensation, and deviations from the standard ratio have long been used by paleoclimatologists as a proxy for temperature. The deviation has its own measure: δ18O.
The δ18O in some Australian zircons has been used in new research. The planet likely had a solid crust at that time, as well as liquid water oceans, as opposed to the globe-spanning magma oceans previously imagined.
In the new study, Gamaleldien and his colleagues took samples from Australia’s Jack Hills, a region known to contain the oldest zircon grains on Earth. They dated some of the grains they collected to 3.4 billion years ago and others to around 4 billion years ago, before the end of the Hadean.
“We do not know if the conditions for the origin of life on the Earth were optimal for its emergence or just good enough for it to happen.”
The researchers then determined δ18O in the zircons and found that samples from both time periods contained a higher ratio of 16O relative to 18O.
That’s a strong signal that these zircons formed by interacting with fresh water, Gamaleldien said. Fresh water, which forms as vapor as seawater evaporates, is more enriched with 16O and retains that ratio as it falls to land. The δ18O in the zircons studied by Gamaleldien follows this pattern and is therefore closer to what researchers would expect to find in rocks that had interacted with fresh water, not seawater. And if rocks were interacting with fresh water, it must mean there was dry land sticking up above the ocean, hinting that Earth had continental crust 4 billion years ago.
The indication of a continental crust is not the only implication of their research, Gamaleldien said. One theory of life’s origins suggests it began in shallow pools of water on land, where prebiotic ingredients could collect. The presence of continental crust and an evaporation-precipitation cycle 4 billion years ago means the stage could have been set for life to form just 500 million years after Earth’s formation.
The study does not weigh in on whether life was, in fact, present at that time. “We do not know if the conditions for the origin of life on the Earth were optimal for its emergence or just good enough for it to happen,” said Stephen Mojzsis, a geologist at the Hungarian Academy of Sciences not affiliated with the new research. This research “pushes us a little bit more towards the optimal side.”
The Elusive Emergence of Fresh Water
The case for fresh water 4 billion years ago isn’t closed, however. There are other ways to get the oxygen ratios found by the researchers, according to Ilya Bindeman, a geochemist at the University of Oregon who wasn’t affiliated with the study.
One assumption the researchers make is that the background oxygen isotopic ratio of seawater hasn’t changed over Earth’s history. A number of studies suggest that’s the case, but still, said Bindeman, it’s not settled. Finding other isotopic ratios could alter scientists’ interpretations of zircon crystals such as these. Alternatively, zircons with similarly light oxygen isotopic ratios could also form from seawater interacting with high-temperature rocks, he said.
In any case, both Bindeman and Mojzsis welcomed the new samples of 4-billion-year-old zircon grains the authors found and analyzed. These fragments from Earth’s early days are not easy to come by and represent some of the only remnants of a rapidly evolving planet.
—Nathaniel Scharping (@nathanielscharp), Science Writer
Citation: Scharping, N. (2024), Four-billion-year-old zircons may contain our earliest evidence of fresh water, Eos, 105, https://doi.org/10.1029/2024EO240297. Published on 16 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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