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A new analysis of ancient crystal grains embedded in rocks from the Australian outback suggests that Earth had dry land and fresh water about 4 billion years ago – a time when scientists thought the planet was completely covered in ocean.
Chemical clues in the crystals revealed that the hot, molten rocks that formed them came into contact with fresh water during the crystals’ formation, according to a study published Monday in the journal Nature Geoscience.
“By examining the age and oxygen isotopes in small crystals of the mineral zircon, we have found unusually light isotope signatures as early as four billion years ago,” said lead researcher Hamed Gamaleldien, adjunct researcher at Curtin University’s School of Earth and Planetary Sciences in Australia and assistant professor at Khalifa University in the United Arab Emirates, in a press release. “Such light oxygen isotopes are usually the result of hot, fresh water altering rocks several kilometers below the Earth’s surface.”
Gamaleldien said the evidence of the presence of fresh water could only be explained by the existence of dry land – where water would collect and seep into the continental crust.
“We have two important things here. We have discovered the earliest evidence of fresh water and representative evidence of dry land over the sea,” he added.
The research indicates that Earth’s water cycle – where water moves between land, oceans and atmosphere through evaporation and precipitation – was operating at the time.
This finding, according to the authors, means that the recipe for the origin of life existed less than 600 million years after the formation of the Earth, long before the dinosaurs or even the earliest known microbial life. The earliest generally accepted evidence of life — and fresh water — comes from stromatolites, fossilized microbes that formed mounds in hot springs 3.5 billion years ago, Gamaleldien said.
“This discovery not only sheds light on Earth’s early history, but also suggests that landmasses and freshwater paved the way for life to flourish within a relatively short timeframe – less than 600 million years after the planet formed,” says co-author Hugo Olierrook. a senior research fellow at Curtin’s School of Earth and Planetary Sciences, in a statement.
“The findings mark an important step forward in our understanding of Earth’s early history and open doors for further research into the origins of life,” he added.
A portal to early Earth
The Hadean Eon, from 4.5 billion to 4 billion years ago, is the earliest chapter in Earth’s history and a geologically dark epoch that is little understood because geologists simply don’t have rocks that old to study: the oldest known rocks are 4 billion years old. .
So how then do zircon crystals act as a gateway to the planet’s earliest history? The small mineral grains are extremely tough and can be cemented into younger rocks. The zircons in the study were found in 3.1 billion-year-old orange sandstone from the Jack Hills Formation, an outcrop of weathered rock in Western Australia.
What makes zircons particularly useful to geologists is that they incorporate some uranium into their structure and scientists can determine their age by measuring the radioactive decay of uranium ions. The oldest material of terrestrial origin was zircon, found in the Jack Hills Formation, dating to 4.4 billion years ago.
“(Zircon) is a unique mineral. It is very resistant and does not change (over time),” said Gamaleldien. “It is the only witness to the Hadean period.”
To arrive at their findings, researchers extracted, mounted and polished 2,500 zircon grains – about the width of two or three strands of human hair – before dating 1,400 of them and measuring different isotopes, or versions, of oxygen in the zircons.
Salt water contains heavier oxygen isotopes, which resist evaporation, while rainwater contains lighter isotopes, Gamaleldien said. Two zircon crystals showed isotopic evidence of meteoric or fresh water; one was 4 billion years old, while the other was 3.4 billion years old, he said.
The team ran 10,000 simulations of zircon’s composition using a computer model — how hot molten rock mixed with seawater, rainwater, or a combination of both — and found that only with some fresh water could they determine the light isotopic signature of their zircon to declare.
Conditions for the origin of life
Gamaleldien said it was impossible to know from their work whether there would have been large landmasses, but there would have been some dry land above sea level. Furthermore, land and fresh water, which would likely have fallen as rain, would have provided the essential ingredients for life to emerge, he said.
Scientists have several theories about the origin of life on Earth. Some believe it originated around deep ocean vents, but others suspect it originated in shallow bodies of water on land. Gamaleldien said the new findings provide support for the latter hypothesis, and the researchers want to recover more zircons for geochemical analysis to investigate further.
John Valley, a professor of geosciences at the University of Wisconsin-Madison, agreed that conditions for life could have existed on Earth so long ago. Valley was not involved in the new research, but was one of the first scientists to use zircons to show that Earth had ancient oceans and cooler temperatures more than 4 billion years ago, challenging the view that the Hadean Earth was an infernal sphere was with fiery seas of magma. .
However, he said that the fluid the zircon precursor came into contact with could have been rainwater or seawater and that the computer model used by the study authors assumed that the isotopic composition of the Hadean ocean was identical to that of today’s oceans.
“The main novelty of the new paper is the conclusion that rainwater means the rocks were (on land)… rather than under water,” Valley said. “This has always been considered one possibility, but no new evidence has been presented that would allow us to know this.”
Geochemist Beth Ann Bell, an assistant research scientist in UCLA’s department of earth, planetary and space sciences, said the very light isotope values ”made a strong argument” for rock-freshwater interactions during the Hadean period, which involved some amount of dry country implied. in that time. She was not involved in the investigation.
“Zircon is physically strong and will not weather at the Earth’s surface,” Bell said in an email. “(It) routinely survives for billions of years in the crust and at the surface with its geochemical information (intact).”
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