Mar 10, 2021 08:31 PM
https://www.sciencemag.org/news/2021/03/...ater-world
EXCERPTS: Across the ages, sea levels have risen and fallen with temperatures—but Earth’s total surface water was always assumed to be constant. Now, evidence is mounting that some 3 billion to 4 billion years ago, the planet’s oceans held nearly twice as much water—enough to submerge today’s continents above the peak of Mount Everest. The flood could have primed the engine of plate tectonics and made it more difficult for life to start on land.
Rocks in today’s mantle, the thick layer of rock beneath the crust, are thought to sequester an ocean’s worth of water or more in their mineral structures. But early in Earth’s history, the mantle, warmed by radioactivity, was four times hotter. Recent work using hydraulic presses has shown that many minerals would be unable to hold as much hydrogen and oxygen at mantle temperatures and pressures. “That suggests the water must have been somewhere else,” says Junjie Dong, a graduate student in mineral physics at Harvard University who led a model, based on those lab experiments, that was published today in AGU Advances. “And the most likely reservoir is the surface.”
The paper makes intuitive sense, says Michael Walter, an experimental petrologist at the Carnegie Institution for Science. “It’s a simple idea that could have important implications.”
[...] The experiments aren’t alone in suggesting a water-bound planet. “There’s pretty clear geological evidence,” too, says Benjamin Johnson, a geochemist at Iowa State University. Titanium concentrations in 4-billion-year-old zircon crystals from Western Australia suggest they formed underwater. And some of the oldest known rocks on Earth, 3-billion-year-old formations in Australia and Greenland, are pillow basalts, bulbous rocks that only form as magma cools underwater.
[Work by Johnson and Boswell Wing, a geobiologist at the University of Colorado, Boulder, offers more evidence. Samples from a 3.24-billion-year-old chunk of oceanic crust left on Australia’s mainland were far richer in a heavy oxygen isotope than the present-day oceans. Because water loses this heavy oxygen when rain reacts with the continental crust to form clays, its abundance in the ancient ocean suggests the continents had barely emerged by that point, Johnson and Wing concluded in a 2020 Nature Geoscience study. The finding doesn’t necessarily mean the oceans were larger, Johnson notes, but, “It is easier to have submerged continents if the oceans are bigger.” (MORE - details)
EXCERPTS: Across the ages, sea levels have risen and fallen with temperatures—but Earth’s total surface water was always assumed to be constant. Now, evidence is mounting that some 3 billion to 4 billion years ago, the planet’s oceans held nearly twice as much water—enough to submerge today’s continents above the peak of Mount Everest. The flood could have primed the engine of plate tectonics and made it more difficult for life to start on land.
Rocks in today’s mantle, the thick layer of rock beneath the crust, are thought to sequester an ocean’s worth of water or more in their mineral structures. But early in Earth’s history, the mantle, warmed by radioactivity, was four times hotter. Recent work using hydraulic presses has shown that many minerals would be unable to hold as much hydrogen and oxygen at mantle temperatures and pressures. “That suggests the water must have been somewhere else,” says Junjie Dong, a graduate student in mineral physics at Harvard University who led a model, based on those lab experiments, that was published today in AGU Advances. “And the most likely reservoir is the surface.”
The paper makes intuitive sense, says Michael Walter, an experimental petrologist at the Carnegie Institution for Science. “It’s a simple idea that could have important implications.”
[...] The experiments aren’t alone in suggesting a water-bound planet. “There’s pretty clear geological evidence,” too, says Benjamin Johnson, a geochemist at Iowa State University. Titanium concentrations in 4-billion-year-old zircon crystals from Western Australia suggest they formed underwater. And some of the oldest known rocks on Earth, 3-billion-year-old formations in Australia and Greenland, are pillow basalts, bulbous rocks that only form as magma cools underwater.
[Work by Johnson and Boswell Wing, a geobiologist at the University of Colorado, Boulder, offers more evidence. Samples from a 3.24-billion-year-old chunk of oceanic crust left on Australia’s mainland were far richer in a heavy oxygen isotope than the present-day oceans. Because water loses this heavy oxygen when rain reacts with the continental crust to form clays, its abundance in the ancient ocean suggests the continents had barely emerged by that point, Johnson and Wing concluded in a 2020 Nature Geoscience study. The finding doesn’t necessarily mean the oceans were larger, Johnson notes, but, “It is easier to have submerged continents if the oceans are bigger.” (MORE - details)