The world’s oldest crystals show evidence of exposure to both fresh and salt water during their formation, a new study says. This suggests that the very early Earth had oceans as well as land on which rainwater could collect. Although hellish in many ways, it suggests a planet much more like the one we know today than anything else in the Solar System, and the opportunity for life to evolve.
Most of the relics of early Earth have long since been recycled through the mantle, leaving nothing to explore. Jack Hills in Western Australia preserves zircon crystals up to 4.4 billion years old. They are small and embedded in much younger sedimentary rocks, but they are the oldest minerals on the planet and offer crucial clues about the state of the Earth when it formed.
Many zircons show signs of having formed in water, and the type of oxygen in them reveals the nature of the water. Oceans contain water primarily made up of oxygen-16 atoms, but also oxygen-18. “When water evaporates, oxygen-16 evaporates more,” Curtin University’s Dr Hugo Olierook told IFLScience, because lighter molecules escape more easily. “It’s mostly driven by temperatures, it’s even lighter closer to the poles.” When the vaporized molecules fall as rain, the resulting lakes have a reduced abundance of oxygen-18.
Billions of years later, Olierook is part of a team that read the type of water in which the zircons formed from their isotopic ratio. The vast majority of the Jack Hills zircons the team examined were formed either inside the Earth without any exposure to water or under the ocean. However, a small fraction has isotopic values consistent with formation in rainwater. It is noteworthy that all of them in the studied sample come from two time periods: a narrow band 3.4 billion years ago and 3.9-4.02 billion years ago.
Today, the Jack Hills are a rocky and dry region, but on early Earth, most formed under oceans, while some supported freshwater lakes.
Image courtesy of Simon Wild
Previously, the oldest geological record of the water cycle came from 3.2 billion years ago. Geologists were convinced that the cycle had started long before, but they could not know how long before.
“Around the year 2000, there was a big theory that 4 billion years ago the Earth was completely dry,” Olierook told IFLScience. “It was a desolate landscape, the sky was orange, the ground brown. Then in 2001, evidence of water more than 4 billion years ago was found. We didn’t know the composition, but it was exciting enough to change the paradigm.” This caused a complete reversal, with the general assumption that the entire Earth was probably covered by a global ocean at the time, with at most small islands poking through.
However, the work Olierook and his colleagues have done demonstrates that there must have been some land at the time where freshwater lakes could have formed, otherwise any rain would have mixed with seawater without changing the isotope ratio.
The fact that freshwater zircons are so rare may indicate that such land was not very abundant, but that only tells the story of one place on the globe. “About 5-10 percent of the Jack Hills zircons are 4 billion years old or older,” Olierook told IFLScience. “The next highest proportion anywhere else is one in 10,000.” This, he ruefully acknowledged, “distorts our understanding of the early Earth.” Perhaps there were large dry regions half a world away at the time and they have since been recycled through the mantle.
The absence of freshwater zircons from 4.1 billion years ago or between these two periods also does not prove that earth did not exist at that time. In the limited sample the team had to work with, it may simply be missing.
Debate continues as to whether life emerged around hydrothermal vents on the ocean floor or in a “warm little pool” as Darwin suggested. This work shows that both possibilities were present from the very beginning to enable both possibilities.
The study is published in Nature Geoscience.