The oldest crystals in the world show evidence of exposure to both fresh and salty water during their formation, a new study reports. This indicates that the very early Earth had both oceans and land on which rainwater could collect. Although hellish in many ways, it suggests a planet far more like the one we know today than anything else in the Solar System, and opportunities for life to evolve.
Most relics of the early Earth have long since been recycled through the mantle, leaving nothing to investigate. The Jack Hills in Western Australia hold zircon crystals up to 4.4 billion years old. They’re small, and bonded into considerably younger sedimentary rocks, but these are the oldest minerals on the planet, and offer crucial hints about the state of the Earth when they were formed.
Many zircons show evidence of having formed in water, and the type of oxygen within them reveals that water’s nature. The oceans contain water primarily formed with oxygen-16 atoms, but also with some oxygen-18. “When water evaporates, oxygen-16 evaporates more,” Dr Hugo Olierook of Curtin University told IFLScience, as it’s easier for the lighter molecules to escape. “It’s mostly controlled by temperatures, closer to poles it gets even lighter.” When the evaporated molecules fall as rain, the resulting lakes have reduced oxygen-18 abundance.
Billions of years later, Olierook is part of a team that has read the type of water in which the zircons formed from their isotope ratio. The vast majority of the Jack Hills zircons the team studied were either formed inside the Earth, without exposure to water at all, or under the ocean. However, a small proportion have isotope values consistent with forming in rainwater instead. Notably, all of these within the studied sample date to two periods of time: a narrow band around 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 formed under the oceans, while some supported freshwater lakes.
Image Courtesy of Simon Wilde
Previously, the oldest geological record of a water cycle came from 3.2 billion years ago. Geologists were confident the cycle started well before this, but couldn’t know how long before.
“Around the year 2000, the big theory was that 4 billion years ago the Earth was completely dry,” Olierook told IFLScience. “It was desolate landscape, the sky was orange, the ground 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 exciting enough to change the paradigm.” This caused a complete reversal, with the general assumption being the entire Earth at the time was probably covered by a global ocean, with at most small islands poking through.
However, the work Olierook and his colleagues have done demonstrates that, at the time, there must have been some land where freshwater lakes could form, otherwise any rain would have mixed with seawater without changing the isotopic ratio.
The fact that freshwater zircons are so rare may indicate such land was not particularly abundant, but that only tells the story at one spot on the globe. “Around 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 acknowledged ruefully, “Does bias our understanding of the early Earth.” Perhaps large dry areas existed at the time half a world away, and have been recycled through the mantle since.
The absence of freshwater zircons before 4.1-billion-years-ago, or between the two periods, also doesn’t prove land didn’t exist at those times. It may simply be missing from the limited sample the team had to work with.
Debate continues as to whether life emerged around hydrothermal vents at the bottom of the ocean, or in a “warm little pond”, as Darwin proposed. This work demonstrates both were present from very early on to make either option possible.
The study is published in Nature Geoscience.
