A reservoir of water three times the volume of all the oceans on Earth has been discovered beneath the surface of our planet, according to an international study, the findings of which were published in the journal Nature Geoscience.
The ‘ocean’ was found between the transition zone of the Earth's upper and lower mantle as part of research carried out by a German-Italian-American team. The study lends evidence to support the theory that ocean water accompanies subducting slabs and thus enters the transition zone (TZ). In other words, our planet's water cycle includes the Earth's interior.
The TZ - a boundary layer that separates the Earth’s upper and lower mantles, is located at a depth of 410 to 660 kilometers. The research team analyzed a rare diamond from Botswana, Africa that was formed at the boundary between the transition zone and the lower mantle.
Using Raman spectroscopy and FTIR spectrometry, the research revealed that the stone contained numerous ringwoodite inclusions. The olivine mineral ringwoodite only forms under extreme pressure deep in the mantle.
Furthermore, olivine, which constitutes around 70 percent of the Earth’s upper mantle and is also called peridot, can morph into a variety of forms depending on the depth at which it is found. The ringwoodite is notable for being able to contain water in the form of hydroxide ions (oxygen and hydrogen atoms bound together) within its structure.
“These mineral transformations greatly hinder the movements of rock in the mantle,” explains Prof. Frank Brenker from the Institute for Geosciences at Goethe University in Frankfurt, who was part of the study.
Up until the current research, nothing was known about the long-term effects of “sucking” material into the transition zone on its geochemical composition. Whether larger quantities of water existed there was also a mystery.
“The subducting slabs also carry deep-sea sediments piggy-back into the Earth’s interior. These sediments can hold large quantities of water and CO2. But until now it was unclear just how much enters the transition zone in the form of more stable, hydrous minerals and carbonates – and it was therefore also unclear whether large quantities of water really are stored there,” Brenker explained.
Furthermore, the research group determined that the chemical composition of the diamond they analyzed was almost exactly the same as that of virtually every fragment of mantle rock found in basalts across our planet.
"In this study, we have demonstrated that the transition zone is not a dry sponge, but holds considerable quantities of water… This also brings us one step closer to Jules Verne's idea of an ocean inside the Earth," Prof. Frank Brenker concluded.
Hydrous ringwoodite was first detected in a Brazil diamond from the transition zone as early as 2014 in research that Brenker also participated in. However, the precise chemical composition of the stone was not determined due to its small size. The inclusions in the 1.5-centimeter diamond from Botswana were large enough to supply final confirmation of the preliminary results from 2014.