There is an ocean at the core of the Earth – but not in the way you think

There is at least as much water under our feet as in all the oceans on earth combined, at a depth of hundreds of kilometers. The water of this huge “ocean” does not slosh back and forth in liquid form, but is trapped in the stone – a sea of ​​stone, as it were.

The message is not fresh. As early as 2014, a study was published in the scientific journal “Nature” that demonstrated the presence of water at these depths for the first time. The researchers then succeeded for the first time in finding hydrous ringwoodite in a small diamond from the transition zone between the upper and lower mantle at a depth of 410 to 660 kilometers. Such inclusions in diamond arise where the crystal was formed; they can use it to preserve minerals from deep within the earth.

Ringwoodite, a densely packed mineral previously known only from meteorites, can store large amounts of water. However, the diamond studied was too small to study its chemical composition. Since the environment from which the diamond originated could not be determined, the results were not representative of the average mantle.

However, last September, an international research team led by Tingting Gu finally confirmed the finding. The new study analyzed a much larger diamond from Botswana, which came from 410 miles (660 kilometers) deep — directly from the contact zone between the transition zone and the lower mantle. Diamonds of this depth are extremely rare, even for those already rare “super deep” gems, which make up only one percent of diamonds. Ringwoodite is the predominant mineral in this contact zone.

The research team examined both the water content and the chemical composition of the diamond using laser-assisted Raman spectroscopy and FTIR spectrometry. In fact, the diamond contains ringwoodite inclusions, which in turn have a high water content. The chemical composition revealed the diamond’s origin: it came from an average piece of mantle, suggesting that the water inclusions are representative of this entire layer. “With this study, we have shown that the transition zone is not a dry sponge, but stores a lot of water,” explains geologist Frank Brenker from the Institute of Geosciences at the Goethe University of Frankfurt, who was involved in the study.

This rock layer of ringwoodite and wadsleyite could store enormous amounts of water: up to six times as much as in the oceans on the Earth’s surface. The Earth’s water cycle, which also includes subsurface water deposits, is thus considerably more extensive than previously assumed. The fact that water reaches such depths and the transition zone is due to plate tectonics. Earth’s plates sink again and again, also transporting deep-sea sediments into the Earth’s interior. These sediments contain large amounts of water and carbon dioxide (CO₂).

The movement of the plates into the Earth’s interior is slowed or stopped by the conversion of the mineral olivine, which predominates in the upper mantle, in deeper layers (see info box). “Subductive plates often struggle to fully penetrate the transition zone. That is why there is an entire graveyard of such plates under Europe in this zone,’ says Brenker.

The braking effect of the rock transition zone also applies in the opposite direction, ie from bottom to top. It may happen that so-called mantle plumes – which are rising streams of hot rock from the deep mantle – are held back at the bottom of the transition zone. In addition, aquifer mantle plumes are already beginning to melt in the transition zone, and not just before the surface. The high water content makes the mantle softer and more dynamic. This in turn has an effect on plate tectonics – the result is that the plates move more strongly and thus dip into the transition zone more often. One can speak of a cycle of rocks. (i.e)