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  Geotimes - May 2007 - Deep Earth may hold an ocean

Deep Earth may hold an ocean

Earth’s deep interior, more than 1,000 kilometers below the surface in the mantle, could prove to be a watery place. That’s the conclusion researchers drew from an anomaly uncovered by the first global map of Earth’s lower mantle, using a new type of seismic analysis.

Researchers have suspected that water could exist deep within Earth, transported there when water-rich minerals in the cold crust of a tectonic plate are drawn down, or subducted, into the mantle. The extreme temperatures and pressures within the mantle destabilize the water, which then moves back toward the surface and eventually escapes through volcanism. Water under such unstable conditions, however, creates a distinct seismic signature — a feature that Jesse Lawrence of the University of California at San Diego and colleagues noticed in new 3-D seismic maps of Earth’s mantle. These seismic anomalies apparent on the map provide the first evidence that water can reach the lower mantle, the team reported in a study awaiting publication by the American Geophysical Union.

Mapping the seismic anomaly was possible because as seismic waves travel through a material, they lose energy. Waves of different frequencies weaken, or “dampen,” to different degrees. The process resembles, for example, music heard through a wall, Lawrence says. “You’ll hear the low bass tones fine, but not the high-pitched sounds,” he says. Similar dampening occurs to seismic waves traveling through Earth. And if water gets in the way, waves dampen even further.

To get a picture of dampening on a global scale, the team collected dampening information for 80,000 seismic wave measurements from 898 earthquakes that occurred around the world between 1990 and 2002. They combined these measurements with existing maps of seismic wave speeds that were previously used to map Earth’s interior, but that could not distinguish whether an anomaly was caused by temperature or water.

The resulting map turned up unusually high levels of wave dampening at depths of about 700 to 1,400 kilometers, and spanning a region west of the Western Pacific subduction zones, from the North Pole to the equator. The largest wave dampening occurred in the mantle just northwest of Beijing, which the researchers dubbed the “Beijing anomaly.”

The presence of water is the best explanation for the Beijing anomaly, the researchers say. A cold slab of Earth’s crust sinking deep into the mantle could act “like a pipeline,” the team reported, for the water-bearing minerals to reach such depths where it could become unstable and release water that diffuses into surrounding rock.

The research contributes new information to an argument that has been ongoing for at least 20 years, regarding how deep and by what means water is transported into Earth’s mantle, says David Bercovici, a geophysicist at Yale University. If the team’s research is correct, it provides direct evidence that slabs of earth are dragging water into the lower mantle, and that rock phases are stable not only several hundred kilometers down, but several thousand kilometers down.

Exactly how much water exists in the mantle is uncertain, however, and depends on the type of rocks that compose the mantle and how much water they can hold. That amount, the team reports, could range from one to five times as much as in Earth’s oceans, the team reports.

The actual amount of water in the mantle is also difficult to determine because of the complex, and not entirely understood, process by which water circulates through Earth. Details of mantle circulation and its interaction with the atmosphere is “still a very big problem,” Bercovici says, including questions about how much water is actually carried down, how it is stored, and how much and by what method water returns to the surface. “The easiest answer is that nothing is happening, and no water is going down or coming up,” he says. “But this study, if true, gives direct evidence” that contradicts that, and tells a story that is “much more interesting” than previously thought.

Kathryn Hansen

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