Ancient crystals suggest freshwater on Earth 4 billion years ago
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A zircon grain (~250 μm by 70 μm, approximately the width of a human hair). Photo: Hugo Olierook/Curtin University
Ancient grains of zircon crystals contain evidence suggesting dry land and freshwater may have existed on Earth 4 billion years ago, scientists report.
Why it matters: Some scientists consider freshwater and dry land to be prerequisites for the emergence of life on Earth — which fossil evidence indicates happened about 3.5 billion years ago.
- The new finding suggests those ingredients were present on Earth less than 600 million years after the planet formed.
How it works: Zircon crystals are incredibly tough material that can withstand weathering, deformation, and other intense processes, says study co-author Hugo Olierook, a senior research fellow at Curtin University in Perth, Australia.
- In their new study, Olierook and his collaborators analyzed zircon grains embedded in 3.1 billion-year-old sandstone from Australia's Jack Hills.
What they found: Analyzing thousands of the tiny crystals, the team found unusually light oxygen isotopes, or forms of the element, in two of them. The ratio of light to heavy oxygen isotopes detected suggests the zircons' precursor magma once interacted with freshwater deep below Earth's surface.
- One crystal was 4 billion years old and the other was 3.4 billion years old.
- The team then ran computer simulations of molten rocks interacting with freshwater, saltwater or both and found the presence of light isotopes in the zircons requires the rocks to have encountered freshwater, the team reports in Nature Geoscience.
- It is a small percentage of rocks that appear to have interacted with freshwater though, Olierook says. "That probably tells us that whilst there was probably freshwater on Earth, it was fairly sporadic and fairly isolated."
The intrigue: Olierook says the biggest finding, though, is that if "you have freshwater on Earth, you must also have land on Earth." Land is important because periods of wetness and dryness may have been necessary for the emergence of life. (Other scientists think life may have emerged deep in the ocean around hydrothermal vents.)
- Lighter isotopes of water evaporate more easily, leaving saltwater behind. If that water fell back into the ocean, the lighter isotopes wouldn't be detected. But because they are, that indicates the water rained down on land.
- The team says that may be what happened, allowing the water to seep below the Earth's surface and interact with the molten rock, trapping lighter oxygen isotopes in the zircons and marking the start of Earth's critical water cycle.
Yes, but: The authors acknowledge the fluid could have been seawater. "We're not 100% sure," Olierook says.
- The appearance of freshwater 4 billion years ago could also be "a blip," he says. Maybe it took a while for the water cycle on Earth to get started and life didn't emerge right away.
- Or, maybe it did and it was all bombarded by meteorites, he said.
