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A new analysis of ancient crystal grains embedded in rock in the Australian outback suggests Earth had dry land and fresh water around 4 billion years ago – a time when scientists thought that the planet was entirely covered by ocean.
Chemical clues in the crystals revealed that the hot, molten rocks from which they came came into contact with fresh water during the formation of the crystals, according to a study published Monday the journal Nature Geoscience.
“By examining the age and oxygen isotopes in tiny crystals of the mineral zircon, we found unusually light isotopic signatures from four billion years ago,” said the study’s lead author, Hamed Gamaleldien, Assistant Researcher at the Earth School and Curtin University. Planetary Sciences in Australia and assistant professor at Khalifa University in the United Arab Emirates, in a press release. “These light oxygen isotopes are typically the result of warm, fresh water weathering rocks many kilometers below the Earth’s surface.”
Gamaleldien stated that the presence of fresh water could only be explained by the existence of arid lands, where water would accumulate and seep into the continental crust.
“We have two important things here. We discovered the first evidence of fresh water and representative evidence of dry land above the sea,” he added.
Research indicates that Earth’s water cycle – when water moves between the land, oceans and atmosphere through evaporation and precipitation – was operating at that time.
This discovery, the authors say, means that the recipe for the origins of life existed less than 600 million years after Earth formed, long before dinosaurs or even the first known microbial life. The first widely recognized 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 the early history of Earth, but also suggests that land masses and fresh water paved the way for life to flourish in a relatively short time, less 600 million years after the formation of the planet,” said study co-author Hugo Olierook. senior research fellow at Curtin’s School of Earth and Planetary Sciences, in a statement.
“These discoveries mark a significant advance in our understanding of Earth’s early history and open the door to deeper exploration of the origins of life,” he added.
The Hadean Eon, 4.5 to 4 billion years ago, constitutes the first chapter in Earth’s history and a dark geological age that is little understood because geologists simply don’t have rocks as old to study: the oldest known rocks are 4 billion years old. .
So how do zircon crystals act as a portal to the planet’s early history? The tiny mineral grains are particularly strong and can be cemented into younger rocks. The zircons studied were discovered 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 a zircon found in the Jack Hills Formation and dated to 4.4 billion years ago.
“(Zircon) is a unique mineral. It is very durable and does not deteriorate (over time),” Gamaleldien said. “It is the only witness to the Hadean period.”
To arrive at their findings, the researchers extracted, mounted and polished 2,500 grains of zircon – about the width of two or three strands of human hair – before dating 1,400 and measuring different isotopes, or versions, of the oxygen inside the zircons.
Salt water contains heavier isotopes of oxygen, which resist evaporation, while rainwater contains lighter isotopes, Gamaleldien explained. Two zircon crystals showed isotopic traces 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 composition using a computer model (how molten rock mixes with seawater, rainwater, or a combination of both) and discovered that only with a little fresh water could they explain the light isotopic signature of their zircons.
Gamaleldien said it was impossible to know from their work whether there would have been large land masses, but there would have been dry land above sea level. Additionally, the land and fresh water, which would likely have fallen as rain, would have provided the essential ingredients for the origin of life, he said.
Scientists have different theories about the origins of life on Earth. Some think it formed around deep ocean vents, but others think it formed in shallow bodies of water on land. Gamaleldien said the new findings support the latter hypothesis and that researchers want to recover more zircons for geochemical analysis to further their research.
John Valley, a professor of geosciences at the University of Wisconsin-Madison, agreed that the conditions necessary for life could have existed on Earth so long ago. Valley was not involved in the new research, but was among the first scientists to use zircons to show that Earth had ancient oceans and colder temperatures more than 4 billion years ago, bringing into question questions the idea that Hadean Earth was a hellish orb with seas of flaming magma. .
However, he added that the fluid with which the zircon precursor came into contact 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 concluding that rainwater means the rocks were (on land) … rather than underwater,” Valley said. “This has always been considered a possibility, but no new evidence has been presented that would indicate 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 ”make a strong case” for interactions between the rock and fresh water during the Hadean period, which implied a certain amount of dry land. at this moment. She did not participate in the study.
“Zircon is physically strong and does not weather on the Earth’s surface,” Bell said in an email. “(It) consistently survives billions of years in the crust and on the surface because of its (intact) geochemical information.”