THE LOST CITY HYDROTHERMAL FIELD: A NOVEL ULTRAMAFIC-HOSTED SUBMARINE SYSTEM

The Lost City Hydrothermal Field (LCHF), which is hosted on > 1-2 my old serpentinized mantle, represents a new class of hydrothermal system that has been venting for at least 30,000 years. It is located near the top of the Atlantis Massif, an oceanic core complex at 30N on the Mid-Atlantic Ridge. Extreme crustal attenuation in this area has resulted in exposure of variably altered and deformed mantle rocks, with lesser gabbroic material along a steep escarpment that rises >5000 m over a lateral distance of < 30 km. To the south, the field is bounded by the Atlantis Transform, and to the east by the Mid-Atlantic Ridge. It rests on a down-dropped, serpentinite block at a water depth of ~800 m and it extends for nearly 400 m in length. High precision sonar mapping with the underwater autonomous vehicle ABE, has resulted in a < 5 m resolution bathymetric map of the entire field and adjacent terrain. These data, coupled with Alvin observations, show that an east-west, 200 m long lineament that is intersected by a well-developed ridge, and gently west-dipping faults control flow throughout much of the field and its formation. The LCHF is an astounding, intensely beautiful area that hosts numerous composite carbonate towers that vent fluids at temperatures up to 90°C with pH’s up to 11.0. Many structures contain an array of delicate flanges, multiple pinnacles, and beehive deposits. The structure Poseidon dominates the field: it is over 60 m in height, >40 m in length and hosts multiple active and inactive towers and smaller pinnacles. Perpendicular growths of carbonate form parasitic flanges that trap highly reflective pools of hydrothermal fluid. Associated stalagmite-like growths reach 3 stories in height. Mixing of high pH hydrothermal fluids, which are low in metals, magnesium, and silica, with seawater results in precipitation of seawater-derived carbonate and formation of deposits rich in aragonite with lesser brucite. Flow is driven by exothermic serpentinization reactions at depth.

The diffusely venting alkaline fluids are enriched in methane, hydrogen, and hydrocarbons and they support dense growths of filamentous bacteria on the exteriors of chimneys and biofilm development within the walls of the structures. This highly reduced, high pH ecosystem may represent our closest analogue to early Earth hydrothermal systems.