Giant swirling plumes of warm seawater that form above volcanic vents in the deep sea can persist for a year, according to oceanographers who for the first time have tracked one such plume. The results, reported in today's Science, suggest that these gentle whirlpools may act as "floating oases" of important minerals and tiny life-forms from the abyss.
At the deep-sea ridges where oceanic crust is born, scalding fluids support bacteria, tubeworms, and clams. If a large amount of magma oozes up toward the seafloor, a year's worth of hot fluid can be released in a few hours. Researchers first saw the aftermath of such an event in 1986 above the Juan de Fuca Ridge, off the Oregon coast. Shaped like a lens, the vast rotating plume was 20 kilometers across, 600 meters thick, and about 0.25 degrees Celsius warmer than the ambient deep-sea temperature of 2 degrees. However, no one knew how long the plumes lasted.
In early 1996, a team from the National Oceanic and Atmospheric Administration (NOAA) found a smaller plume above the Gorda Ridge, 200 kilometers west of southern Oregon. They deployed an instrument designed to float 2 kilometers deep. During its programmed 60-day lifetime, the buoy recorded sonar beeps from fixed sound sources. This provided twice daily positions, which revealed that the float was swirling clockwise with the plume at a tenth of a kilometer per hour. "That may seem slow, but in the deep ocean that's really cooking," says oceanographer John Lupton of NOAA's Pacific Marine Environmental Laboratory in Newport, Oregon. Shipboard measurements by Lupton's team showed that levels of iron, manganese, and other tracers in the plume held steady during that period. Because the particles showed no signs of diffusing, the researchers projected that the plume would stay intact for at least a year.
An armada of floats would be an excellent way to probe the influence of plumes on the ocean, says marine geologist John Delaney of the University of Washington, Seattle. "We have no idea how much energy from hydrothermal vents is released in these intermittent pulses," Delaney says. The largest plumes could replenish minerals in the ocean's midwaters, because they rise a kilometer above the vents. They also may help the larvae of deep-sea organisms disperse from hydrothermal vents, he adds.