A reservoir of water three times the volume of the world's oceans has been found about a tenth of the way to the centre of the Earth, about 600 km below the surface. It could explain where the oceans came from.
Steven Jacobsen of Northwestern University in Evanston, Illinois, and colleagues used seismometers to study the seismic waves from more than 500 earthquakes. By measuring the speed of the waves at different depths, the team could determine which types of rocks they passed through. The water layer revealed itself because the waves slowed down – it takes them longer to get through hydrated rock than through dry rock (Science, doi.org/s66).
The finding complements a recent study by Graham Pearson of the University of Alberta in Edmonton, Canada. He studied a diamond from the mantle's transition zone, that was carried to the surface via a volcano, and found that it held the water-bearing blue mineral ringwoodite (Nature, doi.org/s6h). Taken together, the two findings suggest that the water found by Jacobsen's team is bound in ringwoodite.
The transition zone is the lowest part of the upper mantle, and lies from 410 to 660 km below the Earth' surface (see figure). At the top of the transition zone there is a sudden increase in density from about 3.5 to 3.7 g/cm3. This boundary is like the Moho (the boundary between the crust and the mantle) in that it reflects seismic waves, but is unlike the Moho in that it is caused by a change in mineral phase rather than a change in chemical composition. The mineral olivine, which is stable in the upper mantle, is no longer stable at the pressures in the transition zone and converts to denser phases. In the top half of the transition zone olivine converts to β-spinel, and in the bottom half β-spinel converts into the true spinel structure of ringwoodite.
It had previously been known that the transition zone is capable of holding a great deal of water, up to 2 per cent by weight. In contrast, the rock of the upper mantle above the transition zone (peridotite) can only hold about 0.1 per cent by weight of water. Because the transition zone occupies 10 per cent of the volume of the Earth, it could potentially hold up to five times the volume of the Earth’s oceans. Jacobsen’s results suggest that 60 per cent of this potential capacity – 3 times the volume of the Earth’s oceans – is taken up.
The reservoir throws light on the origin of Earth's water. Some geologists think it arrived in comets, but the new finding suggests the oceans oozed out of Earth's interior.
"It's good evidence Earth's water came from within," says Jacobsen.
This story is based on an article in New Scientist. The information on the properties of the transition zone is taken from “Earth. An Introduction to Physical Geology”, by E.J. Tarbuck & F.K. Lutgens (9th edition, 2008, p. 332, Pearson).