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A new analysis of ancient crystal grains embedded in rocks in the Australian outback shows that Earth had dry land and fresh water about 4 billion years ago; At that time, scientists thought the planet was completely covered by ocean.
Chemical clues contained in the crystals revealed that the hot, molten rocks from which they originated came into contact with freshwater during crystal formation, according to a study published Monday in the journal Nature Geoscience.
“By examining age and oxygen isotopes in small crystals in the mineral zircon, we found unusual light isotope signatures dating back as far as four billion years,” said Hamed Gamaleldien, an associate research fellow at Curtin University’s Earth School and lead author of the study. He is an assistant professor of Planetary Sciences in Australia and Khalifa University in the United Arab Emirates, he said in a news release. “Such light oxygen isotopes are typically the result of hot, fresh water altering rocks several kilometers below the Earth’s surface.”
Evidence for the existence of freshwater can only be explained by the presence of dry land, where water collects and seeps into the continental crust, Gamaleldien said.
“We have two important things here. We discovered the oldest evidence of fresh water and evidence representing dry land over the sea,” he added.
The research shows that Earth’s water cycle, in which water moves between land, oceans and the atmosphere through evaporation and precipitation, was operating at that time.
This finding means that the description of the origin of life existed less than 600 million years after the formation of the Earth, long before dinosaurs or even the oldest known microbial life, the authors said. The oldest widely accepted evidence for the existence 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 shows that landmasses and fresh water laid the groundwork for the development of life in a relatively short period of time – less than 600 million years after the planet formed,” said study co-author Hugo Olierook. “It shows,” he said. a senior research fellow in Curtin’s School of Earth and Planetary Sciences, said in a statement.
“The findings mark a significant step forward in our understanding of Earth’s early history and open the door to further research into the origins of life,” he added.
A portal to the early Earth
The Hadean Eon, 4.5 billion to 4 billion years ago, is the oldest episode in Earth’s history, a geological dark age that is little understood because geologists do not have rocks old enough to study: The oldest known rocks are 4 billion years old.
So how do zircon crystals serve as a gateway to the planet’s oldest history? The tiny mineral grains are particularly hard 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 precisely determine the age of zircons by measuring the radioactive decay of uranium ions. The oldest material of terrestrial origin was zircon found in the Jack Hills formation and dated to 4.4 billion years ago.
“(Zircon) is a unique mineral. It is quite durable and does not change (over time),” Gamaleldien said. “This is the only witness to the Hadean period.”
To arrive at the findings, the researchers extracted, combined and polished 2,500 zircon grains, about the width of two or three human hairs, then dated 1,400 of them and measured different isotopes, or versions, of oxygen within the zircons.
Gamaleldien said salt water contains heavier oxygen isotopes that are resistant to evaporation, while rainwater contains lighter isotopes. Two zircon crystals showed isotopic evidence of meteoric or fresh water; He said that one was 4 billion years old and the other was 3.4 billion years old.
The team ran 10,000 simulations of zircon composition (how hot molten rock mixes with seawater, rainwater, or a combination of both) using a computer model and found that they were able to explain the light isotope signature of their zircons with just a hint of freshwater.
Conditions for the origin of life
Gamaleldien said it’s impossible to know from his studies whether there are large land masses, but there may be some dry land above sea level. Moreover, land and fresh water, which would likely fall as rain, could provide the materials necessary for the origin of life, he said.
Scientists have different theories about the origin of life on Earth. Some believe it occurs around deep ocean vents, but others suspect it occurs in shallow bodies of water on land. Gamaleldien said the new findings support the second hypothesis, and the researchers want to obtain more zircons for geochemical analysis for further research.
John Valley, a geology professor at the University of Wisconsin-Madison, agrees that life conditions could have existed on Earth a long time ago. Valley was not involved in the new research, but he was among the first scientists to use zircon to show that Earth had ancient oceans and cooler temperatures more than 4 billion years ago, challenging the view that the Hadean Earth was a hellish sphere with seas of fiery magma. .
But the liquid the zircon precursor came into contact with could have been rainwater or seawater, he said, and the computer model the study authors used assumed the isotopic composition of the Hadean ocean was the same as that of today’s oceans.
“The main innovation of the new paper is that it concludes that rainwater means the rocks are (on land) rather than submarine,” Valley said. “This has always been considered a possibility, but no new evidence has been presented that would allow us to know.”
Geochemist Beth Ann Bell, an assistant researcher in UCLA’s department of earth, planetary and space sciences, said the very light isotope values ”make a strong case” for interactions between rock and freshwater during the Hadean period, implying some dry land. In that case. He was not involved in the research.
“Zircon is physically solid and does not erode 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 (intact) geochemical knowledge.”
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