The world’s oldest crystals show evidence of exposure to both fresh and salt water during their formation, a new study reports. This shows that the very early Earth had both oceans and 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 possibilities for life to develop.
Most relics from the early Earth have long since been recycled through the mantle, leaving nothing to explore. Jack Hills in Western Australia contains zircon crystals up to 4.4 billion years old. They are small and bound in significantly younger sedimentary rocks, but they are the oldest minerals on the planet and offer crucial clues about the state of Earth when they formed.
Many zircons show evidence that they formed in water, and the type of oxygen in them reveals the nature of the water. The oceans contain water formed mostly of oxygen-16 atoms, but also with some oxygen-18. “When water evaporates, oxygen-16 evaporates more,” Curtin University’s Dr Hugo Olierook told IFLScience, because it’s easier for lighter molecules to escape. “It’s controlled mostly by temperatures, closer to the poles it gets even lighter.” When the vaporized molecules fall as rain, the resulting lakes reduce the abundance of oxygen-18.
Billions of years later, Olierook is part of a team that has deduced the type of water in which the zircons formed from their isotopic ratio. Most of the Jack Hills zircons the team studied were either formed inside the Earth, without any exposure to water, or under the ocean. However, a small fraction have isotopic values consistent with formation in rainwater. It should be noted that all of them in the studied sample date from two time periods: a narrow band about 3.4 billion years ago and from 3.9-4.02 billion years ago.
Today, the Jack Hills are a rocky and arid region, but on early Earth, most formed under oceans, while some supported freshwater lakes.
Image courtesy of Simon Wilde
Before that, the oldest geological record of a water cycle was 3.2 billion years ago. Geologists were confident that the cycle had begun long before that, but they could not know how long before that.
“Around 2000, the big theory was that 4 billion years ago the Earth was completely dry,” Oliruk told IFLScience. “It was a desolate landscape, the sky was orange, the ground was brown. Then in 2001, evidence of water dating back more than 4 billion years was discovered. We didn’t know the composition, but it was exciting enough to change the paradigm.” This led to a complete reversal, with the general assumption being that the entire Earth at the time was probably covered by a global ocean, with at most small islands piercing through it.
However, the work that Olierook and his colleagues have done demonstrates that at that time there must have been land where freshwater lakes could have formed, otherwise any rain would have mixed with seawater without changing the isotopic ratio .
The fact that freshwater zircons are so rare may mean that such land was not particularly abundant, but it 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,” Oliruk told IFLScience. “The next highest rate anywhere else is one in 10,000.” This, he admitted ruefully, “distorts our understanding of the early Earth.” Perhaps large dry areas existed half a world away at that time and have since been recycled through the mantle.
The absence of freshwater zircons before 4.1 billion years or between the two periods also does not prove that the earth did not exist at that time. It could simply be missing from the limited sample the team had to work with.
Debate continues as to whether life arose around hydrothermal vents on the ocean floor or in a “warm little pond” as Darwin proposed. This work shows that both were present from very early on to make either option possible.
The research is published in Nature Geoscience.
