A Lost City of the Deep

A new hydrothermal vent field, which scientists have dubbed "The Lost City," was discovered Dec. 5 on an undersea mountain in the Atlantic Ocean by a team of researchers from Duke, the Scripps Institution of Oceanography, the University of Washington and other institutions conducting the National Science Foundation (NSF)-supported expedition. The unexpected discovery occurred at 30 degrees north on the Mid-Atlantic Ridge during an oceanographic cruise aboard the research vessel Atlantis. "We thought that we had seen the entire spectrum of hydrothermal activity on the sea floor, but this major discovery reminds us that the ocean still has much to reveal," said Margaret Leinen, NSF assistant director for geosciences. "If this vent field was on land, it would be a national park," said Duke structural geologist Jeff Karson, a co-principal investigator who, along with University of Washington geologist Deborah Kelley, dived in the submersible Alvin to the site. "These structures, which tower 180 feet above the sea floor, are the largest hydrothermal chimneys of their kind ever observed," added Kelley, a second co-principal investigator. Perhaps most surprising is that the venting structures are composed of carbonate minerals and silica, in contrast to most other mid-ocean ridge hot spring deposits which are formed by iron and sulfur-based minerals. The low-temperature hydrothermal fluids may have unusual chemistries because they emanate from mantle rocks. Nothing like this submarine hydrothermal field has ever been previously observed, say the scientists. These events are unique, they believe, because they rest on 1 million-year-old ocean crust formed tens of kilometers beneath the sea floor, and because of their incredible size. Dense macrofaunal communities such as clams, shrimps, mussels and tube worms, which typify most other mid-ocean ridge hydrothermal environments, appear to be absent in this field. The Lost City Field was discovered unexpectedly while scientists were studying geological and hydrothermal processes that built an unusually tall, 12,000-foot mountain at this site. In this area, deep mantle rocks called serpentinized peridotites, and rocks crystallized in sub-sea floor magma chambers, have been uplifted several miles from beneath the sea floor along large faults that expose them at the surface of the mountain. "As so often happens, we were pursuing one set of questions concerning building of the mountain and we stumbled onto a very important new discovery," said Donna Blackman, a geophysicist from the Scripps Institution of Oceanography and chief scientist of the expedition. She added that "the venting towers are very spectacular and, although they bring up a whole new set of questions, we will learn about the evolution of the mountain itself as we study the vents carefully in the future." Observations using the submersible Alvin and deep-towed vehicle Argo, operated by Woods Hole Oceanographic Institution, show that the field hosts numerous active and inactive hydrothermal vents. The steep-sided, 180-foot-tall deposits are composed of multiple spires that reach 30 feet in width at their tops. They are commonly capped by white, feathery hydrothermal precipitates. The tops and sides of the massive edifices are awash in fluids that reach temperatures up to 160 degrees. From the sides of the structures, abundant arrays of delicate, white flanges emerge. Similar to cave deposits, complex, intergrown stalagmites rise several meters above the flange roofs. Underneath the flanges, trapped pools of warm fluid support dense mats of microbial communities that wave within the rising fluids. Down slope, hundreds of overlapping flanges form hydrothermal deposits reminiscent of hot spring deposits in Yellowstone National Park. During the Alvin dive, expedition leader Patrick Hickey collected rocks, fluids and biological samples for shore-based analyses. "These vents are way outside an area where we expected to find them ... (and) substantially beyond any area you could have thought they could exist," said Karson, a geology professor at the Nicholas School of the Environment's Division of Earth and Ocean Sciences, during an interview aboard the Atlantis, which acts as the research submarine Alvin's launching platform. "We don't know their heat source. We don't know what their plumbing system is like. What makes them really special is their location: in very different rocks. "We know so little about that part of our planet that's underwater," Karson continued. "We've explored so little of it. So it shouldn't be too surprising to find new and unexpected things down there. We know a lot about the general aspects. But when it comes down to the fine scale features that exist on the sea floor and ocean crust, we're really just at the earliest stages of understanding that." Three Duke graduate students on the research team also participated, in various ways, in documenting the Lost City findings. "It was so neat to be on a cruise where such an exciting observation was made," said Heather Hanna, a second-year graduate student in geochemistry who helped process samples of the delicate stone vent structures that Alvin returned to Atlantis' deck. After the University of Washington's Kelley discovered small unidentified worm-like and urchin-like animals in the scalloped matrices of some of those samples, Hanna found herself in the unfamiliar role of preserving biological samples as well. "Normally when I deal with biological samples, they have been fossilized," Hanna said. "The vent samples gave me something new and different, kind of a change of pace." Expedition scientists first discovered the strange and beautiful formations in the Lost City vent field while photo-surveying parts of the underwater mountain using an unmanned towed submersible vehicle called Argo II. The night of that dramatic discovery, Hanna and fellow Duke graduate students Nick Bacher and Peter Rivizzigno were all standing watches in the control van where Argo video and still images are monitored. "It felt cool to be the first person to see them," Rivizzigno said. "I was working the screen when they came up. Pretty much all in there saw it together." It then fell to Rivizzigno and Bacher to perform the eye-straining chore of melding different overlapping Argo digital underwater photographs of parts of Lost City vent structures into complete panoramic images, called "mosaics." They said that was comparatively easy compared to other Argo views of the underwater mountain's complicated terrain they have pieced together in recent weeks for Karson and other expedition scientists. Rivizzigno, a structural geology master's student studying under Karson, demonstrated how the pair had to use two different computerized imaging programs to make it all work. "For this one to work, all images have to be the same size and shape," he said. "This one can handle images of any size and shape. I just sit here and look for features common to both images. It hurts your eyes after a while. Some of the images, like pillow lavas, all look the same." But the eye-straining work can be rewarding. "By studying such environments, we may learn about ancient hydrothermal systems and the life that they support," suggested Kelley.