You might ask “Ocean? Surely it’s still there where it’s always been?”
For the last 500 million years that has been more or less true, though sea levels have varied substantially, but in general things have been as always. But before that? A news bite from PhysOrg suggests evidence that a lot of seawater has ended up in the mantle. Part of the ocean has drained away. This would have had dramatic effects on the available land-levels and the potential for Life to benefit from shallow water – much of the deep ocean is desert, feed only by what is produced in the continental shallows. It may be no coincidence that the first macroscopic life, seaweed/macroalgae, appeared some billion years ago.
Looking back in Net-Time there’s also this curious research by some Japanese geoscientists…
…a BBC news-bite from Sept 8, 1999. But still pretty much on the money. Shigenori Maruyama and colleagues estimated that sea-levels had dropped by 600 metres in the last 0.75 Gyr and the oceans would be gone in another billion. More or less in line with the new research that suggests half Earth’s water has drained into the mantle since the ocean formed. While that might sound like a lot it’s a depth of 5.3 kilometres of water, when averaged over the Earth’s surface. Just 0.25% of Earth’s total volume and the mantle’s total volume is 83%. Thus a drop of water in a bucket of lava…
Addendum
Eldridge Moores, a Professor Emeritus of Geoscience, has suggested for some years that sea-levels have declined over geological time, though due to a different process. By his reckoning the oceanic crust was thicker and thus isostasy meant the average ocean depth was shallower – meaning all that water covered the continents too. Only mountain peaks poked above the waves. Then, roughly as Rhodinia began forming, the thicker crust gave way to thinner oceanic crust and that super-continent of the day rose from the waves over the next few hundred million years.