Research cruises off the Pacific Northwest have exposed a "hydrothermal siphon" that sucks seawater into a dead volcano and spits it out more than 50 kilometers away. The study shows that mounds of old volcanic rock, called seamounts, are the drains and spigots for a vast plumbing system that pumps hot, mineral-rich fluids under the sea floor.
Earth's interior heats the sea, but scientists haven't been sure where most of the warmth emerges. Some escapes via volcanoes and hot hydrothermal vents at midocean ridges. Elsewhere, a thick blanket of mud insulates the ocean floor, dotted by seamounts. Because volcanic rocks are porous, some oceanographers speculated that cold seawater percolates into seamounts, warms up in the crust, and emerges at other seamounts as mineral-rich fluids. However, no one knew whether fluids could travel between widely separated seamounts without oozing through the sediments above.
The new study, reported in the 6 February issue of Nature, shows that hydrothermal plumbing is quite capable. A team led by hydrogeologist Andrew Fisher of the University of California, Santa Cruz, explored the sediment-covered flanks of the Juan de Fuca Ridge about 200 kilometers west of Washington state. Using drilling, seismic surveys, and heat-flow probes, the team found a direct flow between two seamounts, called Grizzly Bare and Baby Bare. Sea-floor temperatures near Grizzly Bare indicated that the seamount absorbs cold water. Then, chemical analysis of water samples collected from boreholes showed that fluids flows northeast before emerging at Baby Bare, 52 kilometers away. "People had suggested that fluids could travel long distances, but no one had defined a flow path," Fisher notes.
Carbon-14 dating of the fluids suggests that they take 40 to 400 years to travel through the porous rocks. Along the way, they pick up minerals that play a key role in sustaining deep-sea life. The warm fluids vented at Baby Bare support clams and other organisms that live nearly 3 kilometers deep.
The study suggests that layers of sediments perhaps 10 to 20 meters thick can seal the sea floor and make seamounts the most important conduits for heat and fluid flow--especially on the sloping flank of a midocean ridge, says oceanographer John Sclater of the Scripps Institution of Oceanography in La Jolla, California. "It's becoming physically very simple," Sclater says.