Water, water everywhere / Nor any drop to drink

Once upon a time, back in a period we call the Archean eon, around 3.5ish billion years ago, that line from The Rime of the Ancient Mariner may have accurately described the planet we call Earth. The mantle, that thick layer between the crust and the core, is a strange blend of solid and viscous liquid, and back in the earlier days of our planet, it was hotter than it is today, ranging from 1900-3000 degrees Kelvin as opposed to today’s much more balmy 1600-2600 degrees Kelvin. And that bonus heat could mean that the mantle didn’t have as much water as it does now, so maybe all that extra water it has now was on the surface of Earth, creating a global ocean.

In a new study in AGU Advances, a pair of researchers present findings that suggest that not only was there much more water on the planet’s surface, but that water could have changed the composition of our early atmosphere, reducing how much sunlight got reflected out into space. And those two factors would also have changed the global climate and what we think of as the habitat that first supported life.

Co-author Rebecca Fischer explains: It’s sometimes easy to forget that the deep interior of a planet is actually important to what’s going on with the surface. If the mantle can only hold so much water, it’s got to go somewhere else, so what’s going on thousands of kilometers below the surface can have pretty big implications.

The press release explains the work of co-author Junjie Dong, [who] developed a model to estimate the total amount of water that Earth’s mantle could potentially store based on its temperature. He incorporated existing data on how much water different mantle minerals can store and considered which of these 23 minerals would have occurred at different depths and times in Earth’s past. He and his co-authors then related those storage estimates to the volume of the surface ocean as Earth cooled.

These findings challenge the assumption that our global ocean size hasn’t changed much over geological time. At higher temperatures, minerals don’t hold on to water. Their structure is too “loose” so the water evaporates out. If it didn’t go into the mantle at those early temps, then it might have stayed on the surface. As the mantle cooled, the water was able to be trapped in the minerals, and the sea levels would have receded.

Now, there is a lot of uncertainty in these results. And that’s okay. It’s the first time the connection has been made between the temperature of the mantle minerals and their potential water content. Our global water budget and its fluctuations have been hard to quantify, and this research is a starting point. It’s also going to be useful for looking at other planets like Mars, which likely had a small ocean on its surface, and even at exoplanets and their potential water budget.

More Information

AGU press release

“Constraining the Volume of Earth’s Early Oceans With a Temperature‐Dependent Mantle Water Storage Capacity Model,” Junjie Dong et al., 2021 March 9, AGU Advances