The remarkable hydrothermal vent structures serendipitously discovered last December in the mid-Atlantic Ocean, including a massive 18-story vent taller than any seen before, are formed in a very different way than ocean-floor vents studied since the 1970s, according to findings published July 12 in the journal Nature. The circulation of fluids that forms this new class of hydrothermal vents apparently is driven by heat generated when seawater reacts with mantle rocks, not by volcanic heat.
No one has previously seen a field quite like this but Deborah Kelley, a University of Washington oceanographer and lead author of the Nature paper, says this kind of vent may be common on the seafloor. If so, scientists may have underestimated the extent of hydrothermal venting, the amount of heat and chemicals pouring into the world's oceans and the abundance of life that thrives in such conditions.
"Rarely does something like this come along that drives home how much we still have to learn about our own planet," Kelley says. "We need to shed our biases in some sense about what we think we already know."
The Lost City Field, named partly because it sits on the seafloor mountain Atlantis Massif, was discovered Dec. 4. The expedition was funded by the National Science Foundation and led by Scripps Institution of Oceanography's Donna Blackman, UW's Kelley and Duke University's Jeffrey Karson. Blackman and Karson are among the paper's co-authors.
Lost City is like other hydrothermal vent systems where seawater circulates beneath the seafloor gaining heat and chemicals until there is enough heat for the fluids to rise buoyantly and vent back into the ocean. As the warm fluids mix with cold seawater the chemicals separate from the vent fluids and solidify, sometimes piling up into impressive mounds, spires and chimneys of minerals.
It was immediately clear, however, that the Lost City Field was unlike other hydrothermal vent systems in a number of ways. First, there was the height attained by some of the structures – the mighty 180-foot vent scientists named Poseidon compares to previously studied vents that mostly reach 80 feet or less. The new vents are nearly 100 percent carbonate, the same material as limestone in caves, and range in color from a beautiful clean white to cream or gray, in contrast to black smoker vents that are a darkly mottled mix of sulfide minerals. And perhaps the Lost City's most distinctive feature is that it is sitting on 1.5 million-year-old crust formed from mantle material.
"We did not realize that hydrothermal activity of this sort could be taking place on seafloor generated millions of years ago," says Margaret Leinen, assistant director for geosciences at the National Science Foundation.
Most previously known vents form along the youngest part of spreading "centers," areas where tectonic forces pull apart the seafloor and magma flows up into the space sometimes during volcanic eruption. Heat from the underlying magma chambers drives hydrothermal vent circulation and generates water temperatures as high as 400°C.
Lost City is in a part of the ocean where magma chambers are present only rarely and volcanic eruptions happen perhaps every 5,000 to 20,000 years, compared to fast-spreading centers where eruptions may occur every five to 10 years. In the area of the Lost City, spreading and faulting during the last 1 million to 1.5 million years has stripped the mountain down to the underlying mantle rocks. Hydrothermal circulation appears to be driven by seawater that permeates into the deeply fractured surface and transforms olivine in the mantle rocks into a new mineral, serpentine, in a process called serpentinization.
The heat generated during serpentinization appears to drive hydrothermal circulation at the Lost City, Kelley says. The process produces low temperature fluids of 40 to 75°C that are rich in methane and hydrogen.
Papers published in the early 1990s noted that methane-hydrogen signatures were common over slow- or ultra-slow-spreading centers like the Mid-Atlantic Ridge, where Lost City is. That led scientists to believe that venting was occurring, but there had been no example like the Lost City Field before now, Kelley says.
If the Nature paper is right about the forces driving hydrothermal circulation at the Lost City Field, Kelley says it's easy to imagine there could be many more such systems. Within a mere 50-mile radius of the Atlantis Massif are three similar mountains subject to the same fracturing, the same intrusion of seawater and perhaps the same reactions with mantle material. And those four represent only a tiny fraction of the potential sites along the 6,200 mile Mid-Atlantic Ridge, as well as the Indian ridges and the Arctic Ridge, also considered slow- and ultraslow-spreading centers.
Although large animals that typify other vent environments appear to be rare at Lost City, microbial life seems to thrive there. The microbial samples collected at Lost City show a community that is diverse and so dense in places that magnification reveals rocks so covered with microorganisms that one can't see the minerals, Kelley says. "These environments may host a significant and important amount of microbial life, if these systems prove to be common and operate for long periods on old ocean crust."
Other authors of the paper are Gretchen Fruh-Green of the Institute for Mineralogy and Petrology in Zurich; Pete Rivizzigno of Duke; David Butterfield, Marvin Lilley, Eric Olson, Mathew Schrenk, Kevin Roe and Geoff Lebon, all from the University of Washington or affiliated with the National Ocean and Atmospheric Administration; and the shipboard party on the expedition last December.
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