An amazing fairyland of towering white spires--the tallest of which rises more than 180 feet to the height of an 18-story building--has been discovered nearly half a mile below the surface of the North Atlantic Ocean. The spires arise from a new type of hydrothermal vent field with distinctly different chemistry from any seen before. The researchers who discovered the field have named it Lost City.

The team, led by Deborah S. Kelley of the University of Washington, Donna K. Blackman of Scripps Institution of Oceanography, and Jeffrey A. Karson of Duke University, discovered the field last December during an ocean cruise aboard the research vessel Atlantis. The first scientific details of their finding were published this week [Nature, 412, 145 (2001)].

Most of the oceanic hydrothermal vent systems seen before are located at the youngest part of the seafloor, the mid-ocean ridges, where volcanic activity brings magma up from the center of the Earth. By contrast, the Lost City formation is on the flank of the mid-ocean ridge, growing up from rock that is 1.5 million years old. This type of rock, known as peridotite, is typically found much deeper in the oceanic crust than the basaltic rock of other hydrothermal vent systems. When seawater penetrates deep into the peridotite rock, it is heated to relatively modest temperatures of 40 to 75 šC, producing fluids that are rich in methane and hydrogen. That combination of high pH and modest temperature is quite
different from the fluids that emerge from "conventional" hydrothermal vents, which sometimes reach temperatures as high as 400 šC.

The spires that form when the Lost City fluids come in contact with the colder water of the ocean floor are the tallest chimneys of their kind ever observed. They are composed entirely of calcium carbonate and magnesium hydroxide minerals, the researchers find. That contrasts with the iron- and sulfur-rich minerals that form at "classic" hydrothermal vents.

Both types of hydrothermal systems support a rich variety of microbial life, although the researchers see many fewer crabs and other large animals at Lost City than are sometimes found at other hydrothermal vents. And they have only begun to culture and examine the microbes they collected at Lost City.

Although Lost City is the first hydrothermal field of its kind to be seen, the researchers speculate that such fields may not be rare at all, as many parts of the ocean would appear to be regions where they could form, and so much of the ocean floor remains unexplored.

"We thought that we had seen the entire spectrum of hydrothermal activity on the seafloor, but this major discovery reminds us that the ocean still has much to reveal," says Margaret Leinin, assistant director for geosciences at the NSF, which funded the research.

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