On Dec 4 last year, a team of researchers aboard the vessel Atlantis were stunned to find a “giant hydrothermal vent system” in the mid-Atlantic (read more in Geotimes, February 2001). The scientists were so impressed by the 400-meter field of columns and pinnacles that they called it the Lost City. One of its vents, at 60 meters high, is the tallest hydrothermal chimney ever found and all of the vents rise -to the surprise of scientists- out of volcanically inactive oceanic crust. They are also made of carbonate and magnesium hydroxide, not sulfide like the black smokers of most volcanic vent systems.
In the July 12 Nature, the scientists who discovered the Lost City elaborate on their find, explaining that its geology and chemistry are unlike other known hydrothermal vent systems. They add that its resident microbial population could represent the earliest life that thrived in Earth’s oceans.
Deborah Kelley from the University of Washington, Donna Blackman from the Scripps Institution of Oceanography and Jeff Karson of Duke University led a team of more than half a dozen scientists from the University of Washington and the Institute for Mineralogy and Petrology in Zurich, Switzerland. They report that the Lost City, located near the Mid-Ocean Ridge, is about 700 to 800 meters down on an old portion of crust called the Atlantis massif. ArgoII, a remotely operated vehicle, and Alvin, a manned submersible, collected sediment samples and snapped pictures of the vents. These data indicate that the Lost City is indeed an undersea metropolis. Its skyscrapers are delicate towers of calcite, aragonite and brucite with “dendritic growth ornament[ing] the edges,” the authors write.
Water chemistry data show that the seafloor chimneys spew out calcium-saturated water with relatively low temperature (25 to 75 degrees Celsius) and high pH (9.0 to 9.8). When this slurry mixes with seawater, it precipitates the tall, carbonate columns. The authors found that the effluent of these columns is “in marked contrast to vent fluids collected from basalt-hosted environments,” which typically spawn sulphurous hydrothermal vents. They explain that serpentinized ultramafic rocks, igneous rocks formed at great depth, underlie the Lost City. Kelley and her colleagues believe that exothermic reactions replacing plutonic minerals with serpentine create the Lost City’s unique hydrothermal environment.
Like the sulphurous, black smoker environments commonly found at spreading ridges, the Lost City hosts extensive microbial communities. The submersibles discovered both Archea and Eubacteria prokaryotes living as white to gray colored filaments and biofilms on the vents. These extremophiles support small populations of sea urchins, crabs, corals and sponges, and, the authors say, may recreate the environment of the ancient Hadean Ocean, 3.8 to 4.5 billion years ago. Kelley and her colleagues theorize that the reduced carbon compounds, high pH and warm water of the Lost City vents could have been hospitable conditions for “the most ancient of life-forms on Earth.”
They add that the Lost City “is a reminder of the discoveries remaining to be made on the seafloor, which may hold important clues to the origin and diversity of life.”