  By Ker
Than
LiveScience Staff Writer
posted: 28 February 2007
01:28 pm
ET
Scientists scanning the deep interior of Earth have found evidence of a vast water
reservoir beneath eastern Asia that is at least the volume of the Arctic
Ocean.
The discovery marks the first time such a large body of water
has found in the planet’s deep
mantle.
The finding, made by Michael Wysession, a seismologist at
Washington State University in St. Louis, and his former graduate student Jesse
Lawrence, now at the University of California, San Diego, will be detailed in a
forthcoming monograph to be published by the American Geophysical Union.
Looking down deep
The pair analyzed more than 600,000 seismograms—records of waves
generated by earthquakes
traveling through the Earth—collected from instruments scattered around the
planet.
They noticed a region beneath Asia where seismic waves appeared
to dampen, or “attenuate,” and also slow down slightly. “Water slows the speed
of waves a little,” Wysession explained. “Lots of damping and a little slowing
match the predictions for water very well.”
Previous predictions calculated that if a cold slab of the ocean floor were to sink thousands
of miles into the Earth’s mantle, the hot temperatures would cause water stored
inside the rock
to evaporate out.
“That is exactly what we show here,” Wysession said. “Water
inside the rock goes down with the sinking slab and it’s quite cold, but it
heats up the deeper it goes, and the rock eventually becomes unstable and loses
its water.”
The water then rises up into the overlying region, which becomes
saturated with water [image].
“It would still look like solid rock to you,” Wysession told
LiveScience. “You would have to put it in the lab to find the water in
it.”
Although they appear solid, the composition of some ocean floor
rocks is up to 15 percent water. “The water molecules are actually stuck in the
mineral structure of the rock,” Wysession explained. “As you heat this up, it
eventually dehydrates. It’s like taking clay and firing it to get all the water
out.”
The researchers estimate that up to 0.1 percent of the rock
sinking down into the Earth’s mantle in that part of the world is water, which
works out to about an Arctic Ocean’s worth of water.
“That’s a real back of the envelope type calculation,” Wysession
said. “That’s the best that we can do at this point.”
The Beijing anomaly
Wysession has dubbed the new underground feature the “Beijing
anomaly,” because seismic wave attenuation was found to be highest beneath the
Chinese capital city. Wysession first used the moniker during a presentation of
his work at the University of Beijing.
“They thought it was very, very interesting,” Wysession said.
“China is under greater seismic
risk than just about any country in the world, so they are very interested
in seismology.”
Water covers 70 percent of Earth’s surface and one of its many
functions is to act like a lubricant for the movement of continental plates.
“Look at our sister planet, Venus,” Wysession said. “It is very hot
and dry inside Venus, and Venus has no plate tectonics. All the water probably
boiled off, and without water, there are no plates. The system is locked up,
like a rusty Tin Man with no oil.”
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