Leaky seas

From New Scientist, 11 September 1999

Within a billion years, our planet could be as dry and barren as Mars, claim geologists in Tokyo. They have calculated that the oceans are leaking water into the Earth's mantle five times as fast as it is being replenished.

WITHIN a billion years, our planet could be as dry and barren as Mars, claim geologists in Tokyo. They have calculated that the oceans are leaking water into the Earth's mantle five times as fast as it is being replenished.

Geoscientists believe that a huge reservoir of water is bound up in minerals in the transition zone between the upper and lower mantles, about 400 kilometres below the Earth's surface (New Scientist, 30 August 1997, p 22). Water enters the mantle at subduction zones, where oceanic crustal plates dive under continental plates. It returns to the surface at volcanic hot spots and mid-ocean ridges, where molten rock from the upper mantle is pushed up through the Earth's crust.

Most researchers have assumed that these flows are roughly in balance. But when Shigenori Maruyama and his colleagues at the Tokyo Institute of Technology tried to provide some hard numbers, they came to a very different conclusion. Each year, they say, about 1.12 billion tonnes of ocean water seeps into the mantle's transition zone. Yet they can only account for 0.23 billion tonnes moving in the opposite direction. "The world's oceans will dry up within a billion years," says Maruyama. "Earth's surface will look very much like the surface of Mars, where a similar process seems to have taken place."

Maruyama bases his calculations on estimates of the volume of rock being subducted and the volume leaving the mantle, and experiments showing how much water is absorbed by the minerals, primarily lawsonite, formed in subduction zones at about 100 kilometres below the surface.

As they travel deeper, these minerals become unstable and release the water into hydrous dense silicates, which enter the transition zone. But this happens only if the temperature increases relatively slowly with depth--otherwise the water would be released at a shallower depth and return to the surface. "In the early part of Earth's history the temperature gradient in the subduction zones was far too high," says Maruyama. "But around 750 million years ago the subduction zones cooled to the point where the process could begin."

Since then, Maruyama estimates, the leakage will have caused sea level to drop by around 600 metres. This trend would largely be obscured in the geological record by shorter-term variations in sea level.

Maruyama will present his findings at a meeting of the American Geophysical Union in San Francisco in December. But his team's work is already making waves. "The general idea appears quite plausible," says Raymond Jeanloz of the University of California at Berkeley. The difficulty, he says, is being sure you've accounted for all the mantle's inputs and outputs.

Maruyama believes that his figures for water loss from the oceans are conservative. But he admits that there are uncertainties about the exact amount of water emerging from mid-ocean ridges.

Even if Maruyama's calculations are spot on, however, the process will not counter the short-term problem of sea level rises caused by global warming. And a billion years from now, the Earth will probably have bigger problems than leaky oceans. By that time the Sun will be expanding, making life uncomfortably hot for whoever--or whatever--is still living on the planet.

Peter Hadfield, Tokyo