Is there an immense ocean, far beneath the surface of the Earth, that replenishes the oceans above? Recent news items describe a deep reservoir containing as much as three-quarters of the Earth’s water supply. Most of these news stories are careful to note that this isn’t some great sloshing underground pool, and you won’t find any fish living there. Rather, the water is “bound up” in mineral deposits and released when these minerals are put under immense pressure. Some news stories compare the minerals to sponges, which is not something you usually associate with rocks. (Here are a couple of examples of the news items: Daily Digest News, The Guardian.)
What’s really going on here? Last March, a research paper in the journal Nature reported the discovery of a tiny crystal of a mineral called ringwoodite, encased in a diamond that was plucked out of shallow river gravel by artisan miners near Juína, Brazil. A research team led by Graham Pearson (University of Alberta, Canada) found the ringwoodite crystal (60 microns, less than the thickness of a sheet of paper) inside a dirty-looking brown diamond that they had bought for about $20 (US), according to an article in Sci-News. (Here’s the article, with a photo of the diamond and ringwoodite crystal.).
This month, a paper in Science used this discovery, along with geological measurements, lab studies, and models, as evidence to support an idea that geologists had been looking at for many years. (Here are a couple of press releases, from the University of New Mexico and Northwestern University, where the two lead authors are located.)
Scientists suspect that ringwoodite may be common in a “transition zone” that lies 410 to 660 km (250–410 miles) below the Earth’s surface, based on seismic measurements, which track changes in the speed and direction of earthquake vibrations. Ringwoodite has been made in the lab and found in meteorites, but this was the first time that anyone had found a naturally occurring crystal from inside the Earth.
Why do we care about this elusive mineral? Because it’s a high-pressure form of another mineral called olivine. As the name suggests, many forms of olivine are dark green (it can range from yellow to black). The dark-green sands on Papakolea Beach, Hawaii are mostly olivine, and this mineral is common all over the world. At very high pressures, like those you would find more than 660 km beneath the Earth’s surface, olivine transforms into another mineral called perovskite. The presence of one form or another affects the way that earthquake vibrations travel through the Earth. These two minerals have been studied intensively, and geologists use seismic signals as clues to help them map mineral forms and geological activity far beneath the surface, where it’s hard to get the information any other way.
Ringwoodite is intermediate between olivine and perovskite. Because of the way its crystals form, they can contain as much as 3% by weight of something called hydroxyl radicals (OH•). (The crystal reported in the Nature paper had 1.5%.) This “radical” has nothing to do with political activism or unorthodox beliefs. Used in a chemistry context, the term refers to a water molecule (HOH, more commonly written H2O) that has one hydrogen atom stripped away, leaving behind a spare electron that it can share with something else, in this case, the elements in ringwoodite’s crystal structure.
Olivine doesn’t contain hydroxyl radicals. When olivine is put under pressure, water from the surrounding environment can be converted into OH•, forced inside, and incorporated into the framework of atoms, forming ringwoodite. The more OH• trapped in the framework, the faster the sound waves travel through it, which is why geologists had suspected that there was a water-containing mineral intermediate between olivine and perovskite.
Put ringwoodite under even more pressure, and it converts into perovskite. The hydroxyl radicals are forced back outside again, where they recombine with hydrogen (which is just about everywhere) to form liquid water. This water causes the perovskite to melt a little, in much the same way that sprinkling salt on ice causes the ice to melt. The scientists who published the paper in Science had actually seen this melting behavior in the lab, when they used a device called a diamond anvil cell to put immense pressure on a ringwoodite crystal and convert it to perovskite.
Geologists had seen seismic waves suddenly slowing down in regions where they had other clues that one rock layer was sinking down below another. A layer of partially melted perovskite that was taking a dive would slow down a seismic wave like this.
Previous examination of ancient minerals called basalts, taken from mid-ocean ridges, suggested that Earth’s upper mantle has a water content of 0.005 to 0.02% water by weight. Lab and modeling studies show that ringwoodite and a related mineral, wadsleyite, can hold between 1 and 3% water by weight. Seismic evidence suggests that the water content below 660 km is much less. Thus, if these studies are right, the transition zone is where most of the water is. If this zone extends all over the world, and its average water content is 1% (a conservative estimate that needs to be verified), this translates into nearly three times as much water as the oceans contain, which is where the news reports got their three-quarters number (three-quarters in the transition zone, one-quarter in the oceans).
Geologists have been working for decades to create a model that “balances the books” on where water comes from and where it goes — a “whole-Earth water cycle”. They had long suspected that there was a subterranean source that acts as a buffer zone to keep the amount of water in the oceans fairly constant. This latest evidence provides more clues to help them fill in the missing pieces.