New research has revealed that the earth’s surface was bathed in fresh water about 4 billion years ago, compared to 500 million years ago.
A team of researchers from Australia and China used oxygen isotopes left in ancient minerals to determine when the first signs of fresh water could damage the skin of our newborn planet.
Jack Hills in Western Australia holds the oldest material left in the Earth’s crust. For 4.4 billion years, the first minerals were not changed by heat or pressure.
The dry, red, dusty landscape doesn’t receive much water today, but scientists have found evidence of Earth’s oldest rainfall in Hadean rock zircon crystals, marking a major shift in our understanding of the planet’s hydrological history.
“By examining the age and oxygen isotopes in small crystals of the mineral zircon, we found unusual light isotope signatures from four billion years ago,” said lead author Hamed Gamaleldien, a geologist at Curtin University in Australia.
Gamaleldien and colleagues used secondary ion mass spectrometry to analyze young zircon grains and determine which oxygen isotopes were present in the magma from which the crystals formed.
Jack Hills zircons have an “isotopically light” composition; they can only form under the mantle and are exposed to fresh water, specifically meteoric water that falls from the sky. Locked inside this crystal, it may be evidence of the Earth’s first crustal rain penetrating deep into the newly solidified crust.
“Light oxygen isotopes are usually the result of changes in hot, fresh water a few kilometers below the Earth’s surface,” said Gamaleldien.
“This evidence of fresh water inside the Earth challenges the theory that the Earth was covered by an ocean four billion years ago.”
Curtin University geoscientist and colleague Hugo Olierook said the research cuts across many fields of science.
“This discovery not only sheds light on the early history of the Earth but also provides evidence that life on land and in fresh water may have flourished in a relatively short period of time—less than 600 million years after the formation of our planet,” he said.
It was previously thought that the ship had sunk to the bottom of the ocean at the time. Some of the earliest known forms of terrestrial life are 3.48-billion-year-old microbial corals known as stromatolites, found about 800 km (about 500 miles) north of the Jack Hills in the Pilbara Craton.
But this new research shows that landmasses, bodies of fresh water, the water cycle, and even life on Earth originated much earlier than we thought.
It also reinforces the theory “Earth Cool,” as described by Wisconsin-Madison geoscientist John Valley, who, in a 2014 paper, called Hadean zircons the oldest material on Earth.
This theory suggests that shortly after the rock melted into the oceanic crust, the liquid Earth was cool enough to house liquid water, oceans, and the hydrosphere.
“The discovery is an important step forward in our understanding of Earth’s early history and opens the door to learning more about the origins of life,” Olierook said.
This research was published in Nature Geoscience.
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