Ancient ocean crust suggests Earth was once almost entirely covered in water

Today, oceans cover more than two-thirds of the Earth's surface, but 3.2 billion years ago, our planet may have had no land at all. 

"We're picturing basically a Waterworld kind of Kevin Costner-type situation here where most of the planet was covered totally in water," said Benjamin Johnson, an assistant professor of geological sciences at Iowa State University.

The 1995 movie Waterworld was set in a dystopian future when the ice caps have melted leaving no land in sight, which — to be clear — is not a likely scenario for the future. However, in our distant past, geologic processes may have made a waterworld possible.

There might have been little sort of micro-continents or tiny volcanic islands, but nothing like the world today where we have expansive areas of continental crust and soils and ecosystems.- Benjamin Johnson, Iowa State University

Johnson's recent study looked at ancient ocean crust, now exposed in the remote Australian outback. It holds evidence that suggests any bits of land that may have been protruding from the vast oceans covering Earth 3.2 billion years ago, would have been sparse.

"There might have been little sort of micro-continents or tiny volcanic islands, but nothing like the world today where we have expansive areas of continental crust and soils and ecosystems," Johnson told Quirks & Quarks host Bob McDonald.

Conundrum in sedimentary limestone rocks

Johnson's work was aimed at resolving a geological paradox. Scientists had been investigating Earth's distant geological past in sedimentary limestone rocks, mainly composed of ancient marine organisms, but the story the rocks were telling didn't add up.

"They suggest that either the oceans were really, really really hot, maybe 80 degrees, 90 degrees Celsius, or there was something really really different about the water cycle on Earth — how water moves between the oceans and the rocks that are present on the planet," said Johnson.

To figure out what the water cycle might have been like in Earth's distant past, they went looking for more samples of old rocks to see what more detail they could reveal about conditions billions of years ago. 

Looking for clues in ancient ocean crust 

Johnson said they decided to look for answers in hydrothermal vents that were once embedded in the ancient ocean crust "where seawater percolates down through the crust, it's heated up, comes back out as hot billowing fluid coming out of the ocean crust."

These rocks can be found in a region in Australia's northwestern outback called Panorama. It preserves a piece of ancient ocean crust sticking out perpendicularly from the ground. 

"You can really see the whole profile of the ocean crust from this time exposed at this place," said Johnson.

What Johnson was after was a signature of the chemistry of the ancient ocean. Vast amounts of water circulated through these hydrothermal vents, leaving a characteristic fingerprint in the rock. 

If there are no continents above sea level, no weathering, no soils, no clays, all that heavy oxygen-18 that would otherwise be in the continents stays in the ocean. And that's exactly what we see at Panorama.- Benjamin Johnson, Iowa State University

He said it's a lot like studying leftover coffee grounds in a stovetop espresso maker to figure out what the water that interacted with it was like.

"It's a great analogy because it's hot water going through solid material, coffee, that changes the composition of the coffee and the water."

The team figured out these rocks had much more of the heavy oxygen-18 isotope than oceans do today, and that turned out to be critical information. 

Making sense of the geological record

Oxygen comes in two stable versions or isotopes, the heavier oxygen-18 and the lighter oxygen-16.

When rain or snow falls on continents, clays and soils take up oxygen-18 more than oxygen-16, leaving the water that returns to the ocean depleted in the heavier isotope.

"Honestly, I was very surprised by the value of the oxygen isotopes that we estimated from this technique," he said.

There are a couple of potential explanations for why the oxygen isotope composition was much higher 3.2 billion years ago than today, but the simplest explanation, according to Johnson, is "there were no continents above sea level."

"If there are no continents above sea level, no weathering, no soils, no clays, all that heavy oxygen-18 that would otherwise be in the continents stays in the ocean. And that's exactly what we see at Panorama," he said.

Produced and written by Sonya Buyting