Venting concerns: exploring and protecting deep-sea communities

Science News,  Oct 7, 2006  by Janet Raloff

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Researchers cruising the South Pacific between Tonga and Fiji study huge snails that, aided by an abundance of bacteria housed in their gills, feed off plumes of metal-rich compounds at active hydrothermal vents. Scientists working off the California coast use chemical-sniffing probes, robotically driven subs, and seafloor-tethered temperature sensors to watch flows of lava pave over a once-thriving ecosystem at hydrothermal vents several kilometers below the ocean's surface. And in waters off Papua New Guinea, a mining company analyzes metal deposits around inactive, underwater volcanoes that contain, on average, 10 times as much copper as typical ores on land do.

These stuoles exemplify the breadth of research under way at one of Earth's last great frontiers, the geologically active ocean bottom. Sites include hydrothermal vents on the 65,000-km-long ridge that meanders through all the world's oceans and at deeply submerged volcanoes in the tropical west Pacific. Only 35 years ago, scientists didn't know that geologically active sites existed underwater. Now, they have direct evidence of some 300 such spots and suspect that another 700 or so await discovery. The researchers are also uncovering signs of past geological activity at many sites.

As recently as 1990, the cost and difficulty of getting to the few then-recognized sites limited visits by scientists. Today, however, submerged volcanoes and other deep hydrothermal vents have become stages for intensive research activity. Every visit to a vent site, even a repeat visit, brings new discoveries, says Cindy Van Dover, director of Duke University's Marine Laboratory in Beaufort, N.C.

"Some places are so popular that you get ships [above them] stacked up on top of one another" notes Van Dover. If researchers aren't careful, she says, they'll interfere with each other's work or harm the vent ecosystems.

Such concerns triggered scientists to issue, earlier this year, a code of conduct for hydrothermal-vent research. In July, underwater volcanologist Colin Devey of the University of Kiel in Germany described the code at the Euroseience Open Forum in Munich. His university is the current home of InterRidge, the loosely affiliated group of hydrothermal-vent researchers that developed the code.

Explains Devey, "On a highway with only two vehicles, having rules of the road is fairly unimportant. But every time another vehicle comes along, it becomes increasingly important to know such things as which side of the road to drive on and who has the right of way." With scientific and mining-exploration visits to vent sites increasing, he says, "we realized we needed rules to deal with that traffic."

The new code also serves as a self-policing protocol for investigators who conduct their research largely out of sight, usually kilometers below the sea's surface.

HOT SPOTS The geologically active sites where these scientists work represent two extremes of the global tectonic system. The best characterized of these hydrothermal vents occur at various intervals along a tectonic plate's new-forming edge, known as a spreading zone. Earth's mid-ocean ridge is one near-continuous zone of spreading seafloor.

As two plates pull apart, breaches occasionally permit magma to break through the ridge's crest, where the molten rock heats zones of rock and water (SN: 4/1/06, p. 202). A spreading zone can be considered a "linear volcano" with vent holes occurring at various points along its meandering crest, explains Baker, an oceanographer at the National Oceanic and Atmospheric Administration's Seattle lab.

Last December, Baker and his colleagues traveled to a previously uncharted 400-km span of the mid-ocean ridge in the Pacific, just north of the Galapagos Islands. Their sophisticated equipment turned up direct evidence of three new hydrothermal vents in that spreading zone and indirect evidence--hydrothermal plumes--of several more.

On the opposite end of a tectonic plate from its spreading zone may be a subduction zone, a span where two plates collide and one plate is forced beneath the other. Weak spots emerge near the edge of the disappearing plate, permitting magma to punch through and form conventional, conical volcanoes that happen to be underwater. These vulnerable areas are known as back-arc basins (SN: 6/10/06, p. 365).

Both types of hydrothermal vents--at seafloor ridges and at back arcs--spew hot water that's oxygen-deprived and metal-rich, and they host a wide variety of exotic marine life. Researchers marvel at the diversity of unusual animals populating these dark and seemingly inhospitable zones, some of the organisms surviving near water as hot as molten lead. Because animals at these sites derive their energy solely from the microbial transformation of chemicals spewed by the geological activity, their ecosystems are referred to as chemosynthetic.

In the decade after the first hydrothermal vents were discovered in 1977, notes Baker, the high costs of visiting such areas limited scientists' choices. Researchers tended to go to ridge sites where they were virtually guaranteed to find vents, so they focused on similar sites and found similar things.