Was primordial Earth just a huge orb of water with no land, like Waterworld?

It’s the sort of thing you could only imagine in a post-apocalyptic movie. Earth is nothing but ocean, with cities that were once powerful no more than drowned wreckage, and the only way to survive was to keep traversing the endless seas.

Kevin Costner’s Mariner was stuck in his trimaran after the polar ice cap melted and the sea submerged everything. If Waterworld was not a vision of the dystopian future imagined in the film but instead took the Mariner’s boat back to our planet’s distant past some 3 billion years ago, he would have found that he had to stay afloat because continents might not have even existed. Scientists at the University of Colorado Boulder recently discovered something that suggests Earth may have been devoid of land when it was still a nascent planet.

“Earth’s water cycle may have gone through two separate phases…before and after the emergence of the continents,” said CU Boulder associate professor Boswell Wing, who coauthored a study recently published in Nature Geoscience.

Before any science could actually happen, Wing and his colleagues ventured out to the inhospitable Panorama district deep in the Northwestern Australian outback. As unbelievable as it is, this scorching and desolate place was once submerged in water, with the dried-up hydrothermal vents (where life could have emerged) to prove it. Those vents are hotbeds of life on Earth and possibly even alien moons like Enceladus and Europa. The team wanted to get to a 3.2-billion-year-old overturned piece of ocean crust that could tell them how likely it was that Earth was once a cosmic ocean.

“There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction,” said Benjamin Johnson, who was part of that research team while he was a postdoc in Wing’s Lab at CU Boulder.

Hydrothermal chemistry revealed what early Earth may have looked like. Oceanic crust that was altered by the chemical composition of ocean water and marine sediment hides clues frozen in time. The team sampled over a hundred rocks to analyze them back in the lab. They were seeking two different oxygen isotopes (variants of an element whose nuclei have the same amount of protons but different numbers of neutrons), Oxygen-18 and the slightly lighter Oxygen-16.

Sure enough, their analysis found that the ratio of both isotopes showed slightly higher levels of Oxygen-18 atoms than you’d find in today’s oceans. The difference is deceptively small. Just a tiny shift in the level of one of these isotopes can mean a massive shift in how we think of our prehistoric planet. Oxygen-18 and other heavier oxygen isotopes are pretty much eaten up by the clay in soil which makes up land masses.

An excess of Oxygen-18 could mean that there simply weren’t any enormous hunks of soil to devour those isotopes. It doesn't necessarily mean there was no land to be seen, since there could have been the beginnings of what would turn into continents, but that remains a possibility.

So there really might have been a Waterworld era on Earth — just no Mariner.

(via University of Colorado Boulder/Nature Geoscience)