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鈥檚 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鈥檚 surface
(快猫短视频, 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鈥檚 crust (see Diagram).
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鈥檚 transition zone. Yet they can only account for 0.23 billion tonnes
moving in the opposite direction. 鈥淭he world鈥檚 oceans will dry up within a
billion years,鈥� says Maruyama. 鈥淓arth鈥檚 surface will look very much like the
surface of Mars, where a similar process seems to have taken place.鈥�
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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鈥攐therwise
the water would be released at a shallower depth and return to the surface. 鈥淚n
the early part of Earth鈥檚 history the temperature gradient in the subduction
zones was far too high,鈥� says Maruyama. 鈥淏ut around 750 million years ago the
subduction zones cooled to the point where the process could begin.鈥�
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鈥檚 work is already making waves.
鈥淭he general idea appears quite plausible,鈥� says Raymond Jeanloz of the
University of California at Berkeley. The difficulty, he says, is being sure
you鈥檝e accounted for all the mantle鈥檚 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鈥檚 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鈥攐r whatever鈥攊s still living on the planet.