Lost City and the Search For Life

Abstract:

Since their discovery in 1979, high-temperature hot springs fueled by submarine volcanoes have served as models in the search for life elsewhere in the solar system. The early recognition of hot microbial biospheres associated with active volcanism along global mid-ocean ridges fundamentally shifted concepts of how planets and life co-evolve. These metal-rich, acidic systems were thought to represent the most extreme conditions on Earth, hosting novel life based on chemosynthesis. In 2000, this paradigm was profoundly impacted by the discovery of the Lost City hydrothermal field, characterized by conditions never before seen. Here, 60 m-tall limestone chimneys vent pH 10-11, metal-poor, 90°C fluids rich in hydrogen and abiotically-produced methane and formate. The fluid chemistry is driven, not by volcanic heat, but by fluid-rock reactions in underlying ultramafic basement at up to ~ 200°C. These peridotite-hosted biotopes differ significantly from volcanic-hosted vent systems in which carbon dioxide is a dominant volatile species. Instead, serpentinzation reactions yield high hydrogen and low molecular-weight hydrocarbons that result in energy-rich habitats with cell concentrations that reach 10⁹cells/gram carbonate. The absence of volcanism, and on-going serpentinization reactions result in a remarkably stable system with venting active for >150,000 years.

With the discovery of Lost City, it is clear that high temperature volcanic activity is not a prerequisite for life. Exothermic serpentinization reactions occur under a wide-range of temperatures and can result in up to a 40% volume of expansion, which may both close and open fractures. Hence, Lost City may serve as a model in the search for life on other planets and moons with rocky, undifferentiated terranes and overlying oceans. Within such systems, interior cooling, coupled with tidally-induced heating may induce hydrothermal flow, and perhaps life-sustaining environments in the absence of volcanism.