The world’s oldest crystals show evidence of exposure to fresh and salt water during their formation, a new study shows. This suggests that the very early Earth had oceans and land on which rainwater could collect. Although hellish in many ways, it indicates the existence of a planet that resembles the planet we know today more than anything else in the solar system, and indicates opportunities for life to evolve.
Most traces of the early Earth have long since been recycled through the mantle, leaving nothing to investigate. The Jack Hills Mountains in Western Australia contain zircon crystals up to 4.4 billion years old. They are small in size, embedded in younger sedimentary rocks, but they are the oldest minerals on the planet, and provide important hints about the state of the Earth when they formed.
Many zircons show evidence of having formed in water, and the type of oxygen within them reveals the nature of the water. The oceans contain water composed primarily of oxygen-16 atoms, but also with some oxygen-18 atoms. “When water evaporates, more oxygen evaporates.” Dr. Hugo Ollerock It’s easier for lighter molecules to escape, Curtin University’s told IFLScience. “Temperatures are mostly controlled, and the closer you get to the poles it gets lighter.” When evaporated particles fall as rain, the resulting lakes reduce the availability of oxygen18.
Billions of years later, Ollerok was part of a team that read the type of water in which zircon is formed from its isotope ratio. The vast majority of the Jack Hills zircons that the team studied either formed within the Earth, with no exposure to water at all, or under the ocean. However, a small proportion have isotopic values consistent with formation in rainwater instead. It is worth noting that all of these elements within the studied sample date back to two time periods: a narrow range about 3.4 billion years ago, and from 3.9 to 4.02 billion years ago.
Today the Jack Hills are rocky and dry, but on the beginning of the Earth most of them formed under oceans, while some supported freshwater lakes.
Image courtesy of Simon Wilde
Previously, the oldest geological record of the water cycle came from 3.2 billion years ago. Geologists were confident that the cycle began long before that, but they could not know how long before that.
“Back around 2000, the big theory was that 4 billion years ago the Earth was completely dry,” Ollerok told IFLScience. “The landscape was desolate, the sky was orange, and the land was brown. Then in 2001 evidence of water more than 4 billion years ago was found. We didn’t know the composition, but it was dramatic enough to change the paradigm.” This caused a reversal. Complete, as the general assumption was that the entire Earth at that time was probably covered by a global ocean, with at most small islands visible.
However, work by Ollerok and his colleagues shows that at that time, there must have been some land where freshwater lakes could form, otherwise any rain would have mixed with seawater without changing the isotope ratio.
The fact that zircon is rare in freshwater may suggest that this land was not particularly abundant, but that only tells the story for one place on the globe. “About 5 to 10 percent of the zircons found at the Jack Hills are 4 billion years old or older. The next highest percentage elsewhere is one in 10,000,” Ollerok told IFLScience. This “biases our understanding of the early Earth,” he lamented. There may have been large dry areas at that time half a world away, and they have been recycled through the mantle ever since.
The absence of zircon in fresh water 4.1 billion years ago, or between the two periods, also does not prove that the Earth did not exist at those times. It may simply be missing from the limited sample the team had to work with.
Debate continues over whether life arose around hydrothermal vents on the ocean floor, or in a “little warm pool,” as Darwin suggested. This work shows that both were present from too early to make either option possible.
The study is published in Natural earth sciences.