"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 the University of California, Santa Cruz.
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.