Researchers believe that land would be scarce about three to four billion years ago.
Alec Brenner / Harvard University
By Paul Voosen
Over the centuries, sea levels have risen and fallen with temperatures – but the total surface water of the earth has always been accepted. There is evidence that the oceans of the planet had almost twice as much water about three to four billion years ago – enough to sink today’s continents below the summit of Mount Everest. The flood could have fueled the engine of plate tectonics and made life more difficult to start on land.
It is suspected that rocks in the current mantle, the thick layer of rock beneath the crust, sequester an ocean’s water or more in their mineral structures. But early in Earth’s history, the mantle, warmed by radioactivity, was four times warmer. Recent work with hydraulic presses has shown that many minerals cannot contain as much hydrogen and oxygen at mantle temperatures and pressures. “This suggests that the water had to be somewhere else,” says Junjie Dong, a graduate student in mineral physics at Harvard University, who led a model based on laboratory experiments, which were conducted today in AGU advances. “And the most likely reservoir is the surface.”
The paper makes intuitive sense, says Michael Walter, an experimental petologist at the Carnegie Institution for Science. “It’s a simple idea that can have important consequences.”
Two minerals found deep in the mantle store much of its water today: wadsleyite and ringwoodite, high-pressure variants of the volcanic mineral olivine. Rocks rich in minerals make up 7% of the planet’s mass, and although only 2% of their weight is water today, it’s a bit, ‘says Steven Jacobsen, an experimental mineralogist at Northwestern University.
Jacobsen and others created these mantle minerals by squeezing rock powders into tens of thousands of atmospheres and heating them to 1600 ° C or more. Dong’s team compiled the experiments to show that wadsleyite and ringwoodite fracture hold less water at higher temperatures. In addition, the team predicts that, while the mantle has cooled, these minerals themselves would become more abundant, contributing to their ability to absorb water as the earth ages.
The experiments are not the only ones representing a water-bound planet. “There’s pretty clear geological evidence,” said Benjamin Johnson, a geochemist at Iowa State University. Titanium concentrations in four billion zircon crystals from Western Australia indicate that it formed underwater. And some of the oldest known rocks on earth, 3 billion year old formations in Australia and Greenland, are cushion basalts, spherical rocks that only form when magma cools underwater.
Work by Johnson and Boswell Wing, a geobiologist at the University of Colorado, Boulder, provides more evidence. Samples of a 3.24 billion-year-old stretch of coast left on the Australian mainland were much richer in a heavy oxygen isotope than the modern oceans. Because water loses this heavy oxygen when rain reacts with the continental crust to form clay, the abundance in the ancient ocean suggests that the continents were still scarce at that point, Johnson and Wing concluded in a 2020. Natural Sciences study. The finding does not necessarily mean that the oceans were larger, Johnson says, but: “It is easier to have continents under water when the oceans are larger.”
Although the greater ocean would make it harder for the continents to stick their necks out, this may explain why they appear to be moving early in Earth’s history, says Rebecca Fischer, an experimental petologist at Harvard and co-author of the AGU advances study. Larger oceans could help kick off plate tectonics as water penetrated through fractures and weakened the crust, creating subduction zones where one crust slipped beneath the other. And once an underground dive began, the drier, inherently stronger mantle would help bend the plate, ensuring the dive continues, says Jun Korenaga, a geophysicist at Yale University. “If you can’t bend plates, you can have no plate tectonics.”
Thomas Carell, a biochemist at the Ludwig Maximilian University of Munich, says the evidence for larger oceans challenges scenarios for how life on earth began. Some researchers believe that it started at nutrient-rich hydrothermal vents in the ocean, while others prefer shallow ponds on dry soil, which would have evaporated frequently, creating a concentrated chemical bath.
A larger ocean exacerbates the biggest strike against the underwater scenario: that the ocean would have diluted any emerging biomolecules to insignificant. But drowning most of the soil also complicates the thin dam scenario. Carell, an advocate for the dam, says in light of the new article, he is now considering another birthplace for life: sheltered, watery bags in oceanic rocks that have broken the surface in volcanic seas. “Maybe we had little caves in which it all happened,” he says.
The ancient water world is also a reminder of how conditional the evolution of the earth is. The planet was probably dried up until water-rich asteroids bombed it shortly after its birth. If the asteroids had deposited twice as much water or the current mantle had less appetite for water, the continents, so essential to the planet’s life and climate, would never have formed. “It’s a very fine system, the earth,” Dong says. “Too much water, or too little, and it will not work.”