An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 degrees N

Evidence is growing that hydrothermal venting occurs not only along mid-oce an ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and t he Atlantis fracture zone. The vent field-named 'Lost City'-is distinctly d ifferent from all other known sea-floor hydrothermal fields in that it is l ocated on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominate d by steep-sided carbonate chimneys, rather than the sulphide structures ty pical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting den se microbial communities that include anaerobic thermophiles. Because the g eological characteristics of the Atlantis massif are similar to numerous ar eas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these re sults indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.