The world’s oldest crystals show evidence of exposure to fresh and salt water during their formation, a new study reports. This indicates that the very early Earth had both oceans and land where rainwater could collect. While 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 possibilities for life to develop.
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 merged into much younger sedimentary rocks, but these are the oldest minerals on the planet and provide key clues about the state of the Earth when they formed.
Many zircons show evidence of being formed in water, and the type of oxygen in them reveals the nature of the water. Oceans contain water primarily formed with oxygen-16 atoms, but also some oxygen-18. «When water evaporates, oxygen-16 evaporates more,» Curtin University’s Dr Hugo Olierook told IFLScience, as it’s easier for lighter molecules to escape. «It’s mostly controlled by temperature, it gets even lighter closer to the poles.» When the vaporized molecules fall as rain, the resulting lakes have a reduced amount of oxygen-18.
Billions of years later, Olierook is part of a team that used isotope ratios to read the type of water in which the zircons formed. The vast majority of the Jack Hills zircons the team studied were formed inside the Earth, without any exposure to water, or beneath the ocean. However, a small fraction has isotopic values consistent with formation in rainwater. Notably, all of these within the studied sample date from two time periods: a narrow range of about 3.4 billion years ago and from 3.9-4.02 billion years ago.
Today, the Jack Hills are a rocky and dry region, but in the early Earth, most of them formed beneath the ocean, while some supported freshwater lakes.
Image courtesy of Simon Wilde
Previously, the oldest geological record of the water cycle dates back to 3.2 billion years ago. Geologists were convinced that the cycle had started long before that, but they couldn’t know how much earlier.
«Around 2000, the big theory was 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 was found of the existence of water more than 4 billion years ago. We didn’t know the lineup, 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 it.
However, the work done by Olierook and his colleagues shows that there must have been some land at the time on which freshwater lakes could have formed, otherwise the rain would have mixed with seawater without changing the isotope ratios.
The fact that freshwater zircons are so rare may mean that such land was not particularly rich, 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 admitted, «hinders our understanding of the early Earth.» Perhaps there were large dry areas halfway around the world at that time and they have since been recycled through the mantle.
The absence of freshwater zircons before 4.1 billion years, or between those two periods, also does not prove that land did not exist at that time. Maybe it’s just 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 suggested. This paper shows that both were present from very early on to make both options possible.
The study was published in the journal Nature Geoscience.
