Two pieces of evidence tipped researchers off to the location of the hidden vents. First, the ocean is stratified with layers of lighter water sitting on top of layers of denser water. Second, when a seafloor vent erupts, it spews gases rich in rare helium-3, an isotope found in earth's mantle and in the magma bubbling below the vent. As helium-3 disperses through the ocean, it mixes into a density layer and stays there, forming a plume that can stretch over thousands of kilometers.
The research team was analyzing ocean-helium measurements to study how the deep ocean exchanges dissolved gases with the atmosphere when they came across a helium plume that looked out of place. It was in a southern portion of the Pacific Ocean, below a large and well-known helium plume coming off the East Pacific Rise, one of the best-studied vent regions on earth. But this mystery plume appeared too deep to have the same source.
Suspecting that it was coming from the Pacific Antarctic Ridge instead, the researchers compiled a detailed map of ocean-density layers in that region, using some 25,000 salinity, temperature and depth measurements. After locating the helium plume along a single density layer, they compared the layer to topographic maps of the Pacific Antarctic Ridge to figure out where the plume would intersect.
The sites they identified cover 340 miles of ridge line. This chain of volcanic mountains lies about three miles below the ocean surface, and its mile-high peaks are cut by steep canyons and fracture zones created as the sea floor spreads apart.
"They haven't found vents, but they've narrowed the places to look by quite a bit," said Edward Baker, a vent expert at the National Oceanic and Atmospheric Administration.
"You don't have to land right on top of a vent to know it's there. You get a rich mineral soup coming out of these smokers—methane, iron, manganese, sulphur and many other minerals. Once you get within a few tens of kilometers, you can detect these other tracers," said Robert Newton, a Lamont oceanographer and study co-author.
Since the discovery of the first hydrothermal vents in the late 1970s, scientists have searched for far-flung sites, in the hunt for new species and adaptive patterns that can shed light on how species evolved in different spots. Cindy Van Dover, a deep sea biologist and director of the Duke University Marine Laboratory, says she expects that new species will be found on the Pacific Antarctic Ridge, and that this region may hold important clues about how creatures vary between the Indian and Pacific Oceans, on either end.
Citation: Winckler et al., Mantle helium reveals Southern Ocean hydrothermal venting, Geophys. Res. Lett., March 2010, 37, L05601; doi:10.1029/2009GL042093
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