Giant underground “ocean” discovered beneath us

Graham Pearson, a scientist at University of Atlanta, holds the rough diamond containing the first terrestrial sample of ringwoodite ever found. Image: Richard Siemens.

A new study by American researchers concluded that in the mantle layer that lies between the crust and the core, there are vast reserves of water, probably three times the total volume of water of all oceans on the surface of our planet.

This “reservoir” of water, which seems to be stable over time, lies at a depth of up to 700 kilometers beneath the planet’s surface, in the intermediate zone between the upper and the lower mantle.

The researchers, led by geophysicist Steven Jacobsen of the Northwestern University and seismologist Brandon Schmandt of the University of New Mexico, estimate that water is “locked” (molecularly trapped) in a mineral called ringwoodite.

This is a relatively rare crystalline structure with a rock formed from olivine under very high pressures and temperatures.

The existence of such large quantities of ground water sheds new light on the hydrological cycle of the Earth and the origin of the seas and oceans, since they have more or less maintained their size over millions of years. Some geologists believe that water on Earth came from space or, in particular, from icy comets that fell on our planet. But the new discovery reinforces the alternative possibility that the oceans gradually emerged from the depth of the early Earth, thanks to the continuous recycling of tectonic plates. “The indications show that the Earth’s water came from inside,” said Jacobsen.

The U.S. researchers used more than 2,000 seismographs to study seismic waves that provoked over 500 earthquakes in the USA. By measuring the propagation velocity of underground seismic waves at different depths, the scientists identified the density and therefore the type of rocks that were passed by the waves.

The ‘well hidden’ water was discovered because its presence slowed down the seismic waves in the corresponding layers (a seismic wave takes longer to penetrate a hydrated rock than an anhydrous one).

The seismic observations are supplemented by laboratory experiments, from which the researchers concluded that hydrated ringwoodite at depths of around 700 km acts like a “sponge” that is gradually melting under high pressure and temperature and traps the previously absorbed water in the transition zone. “When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit. This is called dehydration melting,” said Brandon Schmandt.

Although the study concerns only the territory of the U.S., the researchers believe that something similar is happening around the world and are already planning to expand the study to other countries. The study is published in the Journal Science.

Sources

• Story: The Mind Unleashed
• Image: University Of Alberta