Over time, sea levels have risen and fallen but Earth’s total surface water was assumed to be constant. New evidence indicates that some 3 billion to 4 billion years ago, the planet’s oceans held nearly twice as much water. This could have initiated the mechanism of plate tectonics and made it more difficult for life to start on land.
Rocks in the Earth’s mantle are believed to be saturated with an ocean’s worth of water or more in their mineral structures. But early in Earth’s history, the mantle was four times hotter. Recent studies have shown that many minerals would be unable to hold as much hydrogen and oxygen at mantle temperatures and pressures thus it most likely that the water was held on the surface.
Two minerals found deep in the mantle store much of its water today. They are wadsleyite and ringwoodite. Rocks rich in these minerals make up 7% of the planet’s mass and although only 2% of their mass is water today it adds up to a lot. The team predicted that as the mantle cooled, these minerals became more abundant, adding to their ability to soak up more water over time.
Larger oceans could have initiated plate tectonics as water penetrated fractures and weakened the crust, creating subduction zones where one slab of crust slipped below another. Once a subducting plate began to dive, the dryer, stronger mantle would have helped bend the slab, ensuring its plunge would continue.
Voosen, P. (2021). Ancient Earth was a water world. Science, 371(6534), 1088-1089. https://www.sciencemag.org/news/2021/03/ancient-earth-was-water-world