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March 3, 2020 03:30 am

Earth May Have Been a 'Water World' 3 Billion Years Ago, Scientists Find

An anonymous reader quotes a report from The Guardian: Scientists have found evidence that Earth was covered by a global ocean that turned the planet into a "water world" more than 3 billion years ago. Telltale chemical signatures were spotted in an ancient chunk of ocean crust which point to a planet once devoid of continents, the largest landmasses on Earth. If the findings are confirmed by future work, they will help researchers to refine their theories on where and how the first single-celled life emerged on Earth, and what other worlds may be habitable. "An early Earth without emergent continents may have resembled a 'water world', providing an important environmental constraint on the origin and evolution of life on Earth, as well as its possible existence elsewhere," the scientists write in Nature Geoscience. Their work centered on a geological site called the Panorama district in north-western Australia's outback, where a 3.2 billion-year-old slab of ocean floor has been turned on its side. Locked inside the ancient crust are chemical clues about the seawater that covered Earth at the time. The scientists focused on different types of oxygen that seawater had carried into the crust. In particular, they analyzed the relative amounts of two isotopes, oxygen-16 and the ever-so-slightly-heavier oxygen-18, in more than 100 samples of the stone. They found that seawater contained more oxygen-18 when the crust was formed 3.2 billion years ago. The most likely explanation, they believe, is that Earth had no continents at the time, because when these form, the clays they contain absorb the ocean's heavy oxygen isotopes. The Earth wasn't entirely landless, however. "The scientists suspect that small 'microcontinents' may have poked out of the ocean here and there," the report adds. "But they do not think the planet hosted vast soil-rich continents like those that dominate Earth today."

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