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New analysis of ancient crystal grains embedded in rock from the back of Australia 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 contained in the crystals indicated that the hot molten rocks from which they came into contact with fresh water during the formation of the crystals, according to a study published Monday in the journal Nature Geoscience.
“By examining the age and oxygen isotopes in tiny crystals of the mineral zircon, we found unusually light isotopic signatures as far back as four billion years ago,” said lead study author Hamed Gamaleldien, assistant research fellow at School of Earth at Curtin University and Planetary Sciences in Australia and assistant professor at Khalifa University in the United Arab Emirates, in a news release. “Such light oxygen isotopes are usually the result of warm fresh water that alters rocks several kilometers below the Earth’s surface.”
Gamaleldien said evidence of the presence of fresh water could only be explained by the existence of dry land — a place where water would collect and seep into the continental crust.
“We have two important things here. We found the earliest evidence of fresh water and representative evidence of dry land above the sea,” he said.
The research shows that the Earth’s water cycle – where water moves between the land, the oceans and the atmosphere through evaporation and precipitation – was operating at that time.
This result, the authors said, means that the recipe for the origin of life existed less than 600 million years after the foundation of the Earth, long before the dinosaurs or even the earliest known microbial life. The earliest widely agreed-upon 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 … land masses and fresh water set the stage for life to flourish within a relatively short time frame — less than 600 million year after the planet came together,” study author Hugo Olierook, a senior research fellow at the Curtin School of Earth and Planetary Sciences, said in a statement.
“The findings are a significant step forward in our understanding of Earth’s early history and open doors for further exploration of the origins of life,” he said.
An early portal to Earth
The Hadean Eon, from 4.5 billion to 4 billion years ago, is the earliest chapter in Earth’s history and a dark geological age that is little understood because geologists do not have older rocks to study: The rocks are oldest known 4 billion years old. .
So how then do zircon crystals serve as a gateway into the planet’s earliest history? The tiny mineral grains are very hard and can be cemented into the younger rock. 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 so useful to geologists is that they incorporate a small amount of uranium into their structure and scientists can determine their age by measuring the radioactive decay of the uranium ions. The oldest material of terrestrial origin was zircon found in the Jack Hills formation dated to 4.4 billion years ago.
“(Zircon) is a unique mineral. It is very resistant and does not change (over time),” Gamaleldien said. “It is the only witness of the Hadean period.”
To reach their findings, the researchers extracted, dressed and polished 2,500 zircon grains — about the width of two to three strands of human hair — before dating 1,400 of them and measuring isotopes, or different versions, of oxygen within the zircons .
Salt water has a heavier isotope of oxygen, which is resistant to evaporation, while rainwater has a lighter isotope, Gamaleldien said. Two zircon crystals showed isotopic evidence of meteoric or freshwater water; one was 4 billion years old, and the other was 3.4 billion years old, he said.
The team ran 10,000 simulations of zircon composition using a computer model – how to mix hot molten rocks with seawater, rainwater or a combination of the two – and found that with just some fresh water they could match the light isotopic signature of their zircons to explain.
Conditions for the origin of life
Gamaleldien said it was impossible to know from their work if there would be large land masses, but there would be some dry land above sea level. In addition, fresh land and water, which could fall as rain, would have provided the necessary ingredients for the origin of life, he said.
Scientists have different theories about the origin of life on Earth. Some believe it was formed around deep ocean currents, but others suspect it occurred in shallow bodies of water on land. Gamaleldien said the new findings support the latter hypothesis, and the researchers would like to recover more zircon for geochemical analysis to further investigate.
John Valley, professor of geology at the University of Wisconsin-Madison, agreed that the conditions for life could exist on Earth that long ago. Valley was not involved in the new research but was one of the first scientists to use zircon to show that there were ancient oceans and cooler temperatures on Earth more than 4 billion years ago, challenging the view that Hadean Earth is a hellish orb with fiery seas of magma. .
However, he said that the fluid that 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 the today.
“The main story of the new paper is its conclusion that rainwater means the rocks were (on land) … rather than submarine,” Valley said. “This has always been considered as one possibility, but no new evidence has been presented that allows one to know this.”
Geochemist Beth Ann Bell, an assistant researcher at UCLA’s department of earth, planetary and space sciences, said the very light isotope values made a “strong case” for interactions between rock and fresh water during the Hadean period, suggesting that there was a certain amount of dry land. at that time. She was not involved in the study.
“Zircon is physically tough and will not weather well at the Earth’s surface,” Bell said in an email. “(It) routinely lives for billions of years in the crust and on the surface with its (intact) geochemical information.”
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