Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.b62a..03k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #B62A-03
Other
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 3035 Midocean Ridge Processes, 8135 Hydrothermal Systems (8424)
Scientific paper
The recent discovery of the peridotite-hosted Lost City Hydrothermal Field (LCHF) raises the possibility that such systems are prevalent not only on Earth, but that similar systems may have existed, or currently exist, elsewhere in the solar system. The LCHF, which rests atop the Atlantis massif at 30N on the Mid-Atlantic Ridge, is unlike any previously known hydrothermal field: 1) it is located on 1.5 my-old crust, nearly 15 km west of the spreading axis; 2) it hosts at least 30 active and inactive carbonate-brucite chimneys that tower up to 60 m above the seafloor; 3) the venting pinnacles appear to be the surface expression of warm (40-75C), high pH (9-10) fluids emanating from fault zones that tap a region of active serpentinization in the underlying peridotites; and 5) hydrothermal flow is facilitated by exothermic serpentinization reactions at depth. The diffusely venting fluids support dense and diverse communities of mesophilic to hyperthermophilic organisms that may include sulfur-, methane- and hydrogen-oxidizers. The Lost City Field may represent our closest analogue to hydrothermal systems operative during early Earth where ultramafic rocks were predominant. The reducing conditions associated with serpentinization of ultramafic material may be similar to those present in the Hadean ocean (4.5-3.9 Gyr) and it has been suggested that such high-pH systems were a requirement for the emergence of life on the seafloor. Model calculations based on thermodynamic considerations and experimental studies suggest that synthesis of numerous organic compounds is favored during mixing of warm, serpentinite-derived, high-pH, reducing fluids with cool, oxygenated seawater. Dissolved hydrogen, present in hydrothermal fluids due to reaction of olivine and other iron-bearing minerals with fluids, provides the reduction potential and the thermodynamic drive for organic synthesis. Significant quantities of methane and hydrogen produced during serpentinization reactions form critical nutrients for microbial communities within submarine systems. Many of the carbonate-veined serpentinites (ophicalcites) and breccias that underlie the LCVF are similar to those known from ancient ophiolites, including Archean (>3000 m.y. old) examples. These types of assemblages may represent a linkage to hydrothermal and possibly biological activity at the time of the oldest known life on Earth. The warm, organic- and volatile-enriched environment present within the porous interior of ancient hydrothermal deposits may have been extremely suitable habitats for the emergence of thermophilic or hyperthermophilic anaerobic organisms capable of utilizing methane and hydrogen. The LCVF may provide new insights into the search for life elsewhere in the solar system. This hydrothermal field highlights the fact that volcanic heat is not a requirement for fluid flow, but that a large component of energy to drive flow may come directly from exothermic reactions as fluids interact with ultramafic material. It also shows that hydrothermal systems, and the life that they support, can exist far away from major spreading centers. Collectively, these observations indicate that water-bearing planets, chondritic in composition, that have experienced tectonic processes are potential sites for Lost City type systems and life.
Baross John A.
Frueh-Green G. L.
Karson Jeffrey A.
Kelley Deborah S.
Schrenk Matthew O.
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