Hidden ocean 400 miles below the planet surface

By Henry Fountain / New York Times News Service

Published Jun 17, 2014 at 12:01AM

An analysis of seismic waves passing through the deep earth appears to confirm what laboratory experiments have suggested was possible: that an ocean of water is tied up in the mantle, 400 miles below the surface.

Any voyager to the center of the earth could leave the wet suit behind, however. The water is not liquid, but rather bound in minerals that exist at the extreme pressures found at such depths.

The finding suggests that processes that occur in the shallower mantle and that cause volcanoes and related activity at the surface are also occurring farther down.

“It’s a new view of the structure of this part of the earth,” said Brandon Schmandt, a geophysicist at the University of New Mexico and an author of a recent paper in the journal Science describing the research.

The work also adds credence to the idea that the earth’s water accumulated in the interior during the planet’s formation, rather than arriving later through the bombardment of icy comets. In this view, water bound up in minerals in the mantle, the 1,800-mile-thick layer between the thin crust and the hot metallic core, degassed over time and reached the surface.

The scientists studied a part of the mantle called the transition zone, from about 300 to 440 miles deep. The ability of this zone to contain water — and apparently to retain a lot of it — “may have something to do with stabilizing or buffering the size of the oceans,” said Steven D. Jacobsen, a mineralogist at Northwestern University and another author of the paper. “It may be fortunate that the earth’s interior can act like a sponge.”

Jacobsen synthesizes deep-earth minerals by replicating the extreme pressures that exist hundreds of miles below the surface. Over the years when he has made a blue mineral called ringwoodite, which is ubiquitous in the transition zone, it has formed with water. But that was just lab work, Jacobsen said, adding, “We haven’t known whether it could happen down there.”

Schmandt analyzed seismic data from the USArray project, in which 400 mobile seismometers have been deployed across the United States to create high-resolution images of the mantle. The analysis showed signs of melting in the transition zone, in areas where convection was causing the mantle to flow downward.

Melting of the mantle occurs close to the surface, creating the magma that is responsible for volcanic hot spots around the world. The process is called dehydration melting, because as parts of the mantle slide deeper at places where the earth’s tectonic plates meet, the increasing pressure causes minerals in the mantle to release their water, lowering the melting temperature.

But finding evidence of melting much farther down, as Schmandt did, was difficult to explain “unless you invoke water,” Jacobsen said.

Their work suggests that what happens deep in the mantle is similar to what happens close to the surface, the researchers said. Ringwoodite and other minerals in the transition zone must contain water, which they release as they move deeper and the pressure increases. This leads to the melting that Schmandt found.

But it’s not as if there are huge chambers of magma hundreds of miles below the surface, Jacobsen said. The melting occurs on the boundaries of the mineral grains. “You end up having a mushy rock,” he said.

The work is all part of trying to understand how the dynamics of the deep earth affect what happens at the surface.

“We’re trying to connect the rock cycles — i.e., plate tectonics — with water cycles,” Jacobsen said. “The more we look, the deeper it goes.”