Megaplume, a really big undersea find

Megaplume, a really big undersea find – submarine geophysics

Gina Maranto

A few months ago oceanographers Edward Baker and Gary Massoth were out on the research ship Dicoverer looking for plumes, columns of water that rise like chimney smoke from vents in the sea floor. By sampling plumes, which are loaded with dissolved minerals and trace gases, Baker and Massoth, both of the Pacific Marine Environmental lab in Seattle, aimed to determine what role hydrothermal vents play in the chemistry of the ocean.

What they stumbled across, 260 miles off Oregon in about 7,200 feet of water over the Juan de Fuca ridge, was a phenomenon no one had seen before. Most vents pump out a few quarts of water per second and yield plumes no more than several hundredths of a degree warmer than the ambient sea water. The plumes stretch a few hundred, at most a couple of thousand, feet above the ocean floor. This time, however, Sleuth, a submersible that’s towed behind Discoverer and measures changes in temperature and salinity, among other things, limned a vast oval plume twelve miles in diameter. It encompassed some 24 cubic miles of water that was as much as a quarter of a degree warmer than the surrounding sea water. ”A temperature difference like that might not seem like much,” says geophysicist Stephen Hammond, who helped analyze data from the plume, ”but the heat in this plume represents ten to twenty billion kilowatt hours of energy,” two to four times the yearly output of the Bonneville Dam.

Baker tracked the plume for six days and concluded that it was probably newly formed, since the water in it contained substances like anhydrite that rapidly dissolve from hydrothermal fluids when they come in contact with colder water. Two months later, when researchers revisited the site, the plume had disappeared.

Scientists don’t know how the mega plume was generated. Smaller plumes are formed when cold water percolates through fissures in the hot rock that’s constantly created at the mid-oceanic ridges; a computer model shows that the megaplume would have required a crack in the crust a foot wide, and half a mile to a mile long, emitting 570 F. water for a few days.

Hammond suspects that oversized plumes may periodically blossom in all ocean basins but haven’t been spotted because they’re so short-lived. If they occur commonly and let loose quantities of gases and minerals comparable in size to those deposited in the oceans by normal plumes, the weathering of the landmasses, river runoff, and atmospheric exchange, a lot of ideas about the role of hydrothermal venting will have to be revised. Beyond that, the release of large amounts of carbon dioxide from the plumes could change assumptions about the warming of the earth as a result of an increase of carbon dioxide in the atmosphere. With man-made CO creating a greenhouse effect of potentially disastrous proportions, knowledge of all the natural sources will help scientists predict more precisely how warm the world will get.

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