February 10, 2003
Volcanic seamounts siphon ocean water through
the seafloor
Researchers trace flow over long distances
By Tim Stephens
Researchers have discovered a pair of seamounts on the ocean floor that
serve as inflow and outflow points for a vast plumbing system that circulates
water through the seafloor. The seamounts are separated by more than
30 miles (52 kilometers).
"One big underwater volcano is sucking in seawater, and the water
flows north through the rocks of the seafloor and comes out through
another seamount," said Andrew Fisher, an associate professor of
Earth sciences at UCSC.
Scientists have known for decades that enormous quantities of ocean
water circulate through the seafloor, flowing through the porous volcanic
rock of the upper oceanic crust.
In the process, the water extracts large amounts of heat from the crust,
which is warmed by the constant flow of heat out of the Earth's interior.
The mystery has been how the water gets in and out of the crust, most
of which is covered by a thick layer of relatively impermeable sediments.
"The problem is that most of the seafloor doesn't have much exposed
rock that would be permeable to water," Fisher said.
There is exposed rock on seamounts, underwater volcanoes that rise
up through the sediment layer. The new discovery shows that water can
travel long distances through the basaltic rock of the seafloor from
one seamount to another. The findings have implications for understanding
heat flow through the crust, the chemistry of ocean water, the microbial
communities that live within the ocean floor, and the characteristics
of subduction zones where oceanic crust dives beneath the continental
plates.
Fisher and a team of collaborators from various institutions described
their findings in a paper published in the February 6 issue of the journal
Nature. Fisher's coauthors include Michael Hutnak and Abdellah
Cherkaoui of UC Santa Cruz; Earl Davis and Robert Macdonald of the Geological
Survey of Canada; Volkhard Spiess, Lars Zühlsdorff, and Heiner
Villinger of the University of Bremen, Germany; Lizet Christiansen of
Johns Hopkins University; K. Michelle Edwards and Keir Becker of the
University of Miami; Michael Mottl of the University of Hawaii; and
C. Geoff Wheat of the University of Alaska.
The researchers examined an area of the seafloor in the northeast Pacific,
about 120 miles (200 kilometers) west of Vancouver Island. Further west
is the Juan de Fuca Ridge, where two plates of the oceanic lithosphere
are spreading apart. To the east, the Juan de Fuca Plate plunges beneath
the edge of the North American Plate.
"We picked this spot on the ridge flank, between the spreading
center and the subduction zone, because the sediment cover there is
unusually thick and only a few seamounts are exposed. We knew from previous
studies that warm water was coming out of one seamount, and we wanted
to find where it was going in," Fisher said.
The researchers used a heat-flow probe to map out the temperature patterns
within the seafloor. The results showed that cold water is flowing into
the seafloor through a large seamount known as Grizzly Bare. The outflow
point is a smaller seamount to the north called Baby Bare. Water samples
collected from the seafloor between the two seamounts showed progressive
changes in the chemistry of the water as it flows northward.
"The surprising thing was that the water goes in 52 kilometers
from where it comes out. This shows that the circulation system is connected
over very large distances," Fisher said.
The researchers used the change in water chemistry to calculate the
rate of flow. Their best estimate is that it takes about 40 to 400 years
for water to flow from one seamount to the other.
Now the challenge is to understand how the system works and what causes
water to go in at one point and come out at another. Heat flow from
inside the Earth appears to be the driving force, Fisher said. Warm
water rising out of Baby Bare creates a suction effect that pulls cold
water in at Grizzly Bare. A computer simulation showed that the temperature
difference is enough to keep the flow going.
"It's a hydrothermal siphon, like siphoning water through a hose--once
it gets going it can maintain itself. The question is, What gets it
started?" Fisher said.
One possibility is that discharge is favored at smaller seamounts,
where escaping water would lose less heat than it would if it were spread
out over a larger area. The initial push could also come from an extreme
event, such as an earthquake or a major storm, that causes a change
in pressure over one area.
"The driving forces are small, so small perturbations may be enough
to get it started," Fisher said.
Fisher and his collaborators have discovered similar systems on the
floor of the Pacific near Costa Rica. Identifying discharge sites could
be useful to scientists studying the microbial communities recently
discovered living in the rocks of the oceanic crust, he said.
"These sites offer little windows into the subsurface, where we
can sample the chemistry of the water and study any critters that get
blown out along with it," Fisher said.
In addition, understanding the circulation of water through the oceanic
crust may shed light on the behavior of subduction zones. When water
gets squeezed out of a plate during subduction, it may lubricate the
main fault and affect where earthquakes occur. Water that gets subducted
deep into the mantle, on the other hand, may contribute to the explosive
volcanism seen, for example, at Mount St. Helens and at Costa Rica's
Arenal volcano.
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