The fascinating biogeochemistry of the Endeavor hydrothermal venting systems depends largely on unseen equilibria and reactions occurring beneath the seafloor. These interactions determine the environmental conditions and resources that host unique biological communities dependent on abiotically derived energy sources.
Geology: Hydrothermal flow is most simply explained by the single pass model (Lowell 2004). Seawater first enters the crust in the relatively shallow recharge zone, where it percolates downwards through cracks in the rocks. Between 1-2 kilometers, immediately above the melt lens, it enters the reaction zone (Von Damm 2004). The fluid then gains heat until its buoyancy forces it to rise up through the discharge zone. The vent fluid exits the crust in areas of either focused or diffuse flow, and may be altered by the basalts and gabbroic material it moves through [see figure 1]. Five high temperature (300-400 degrees C), focused flow venting fields have been identified on the Endeavor axial valley (Sasquatch, Salty Dawg, High Rise, Main Endeavor, and Mothra). These fields are around 0.5km long and spaced 2-3km apart on the ridge axis along major faults. Regions of low-temperature, diffuse flow lie between and around the fields (Van Ark 2007).
Chemistry: At the Endeavor fields, a chlorinity gradient exists from 505 mmol kg-1 at vents in the northeast of the valley to 255 mmol kg-1 in the southwest, 94 to 47% of normal seawater chloride values. This chloride depletion in hydrothermal fluids is best explained by supercritical phase separation in the recharge zone, with the loss of chloride rich brine over the melt lens (Butterfield 1994). Chloride concentration serves as a useful tracer of many other elements and gases, which tend to vary predictably with chloride. Cation uptake in the reaction zone is primarily controlled by fluid chloride concentrations, so removal of chloride in the brine layer results in enrichment of many cations in the fluid leaving the vent, such as calcium, potassium, sulfur, copper, and zinc (Alt 1995). Many of these cations serve as catalysts for abiotic and biotic chemosynthetic reactions. In regions of focused flow, metal sulfides precipitate to form the familiar black smoker chimneys, while deposits of anhydrite and sulfur ores precipitate out of solution after mixing with seawater (Delaney 1992; Lowell 2004). In areas of diffuse flow with subseafloor mixing, these compounds are deposited along the flow path.
Biology: Thermal and chemical gradients in the chimney structures and diffuse flow areas provide varied and unique microbial habitats. Metals that are enriched in vent fluid contribute to the formation of small organic molecules necessary for life at vent systems. Such simple molecules include ammonia, organic carbons, organic sulfur compounds, peptides, sulfate, nitrate, and simple alcohols (Luther 2004). These starting materials can be used by vent microbes, either as energy sources or to synthesize more complex organics (Kelley 2002). Such bacteria and archaea create microenvironments in biofilms and mats, providing a base and food source for the development of more complex macrofaunal assemblages, including spider crabs, polychaete worms, tube worms, and limpets (Glickson 2007). This complex and unique ecosystem is entirely dependent on chemosynthesis, unlike the oceanic and terrestrial environments found worldwide.
Fig. 1 (Lowell 2004)
References (ACS format)
Alt, J.C. Subseafloor processes in mid-ocean ridge hydrothermal systems. In Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions; Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S., Thomson, R.E., Eds.; Geophysical Monograph 91; American Geophysical Union: Washington, DC, 1995; 85- 114.
Butterfield, D.A. et al; Gradients in the composition of hydrothermal fluids from the Endeavor segment vent field: Phase separation and brine loss. Journ. of Geophys. Rsch 1994, Vol 99, nB5, 9561-9583.
Delaney, J.R. et al; Geology of a vigorous hydrothermal system on the Endeavor Segment, Jaun de Fuca Ridge. Journ. of Geophys. Rsch 1992, Vol. 97, nB16, 19663-19682.
Glickson, D.A., Kelley, D.S., Delaney, J.R.; Geology and hydrothermal evolution of the Mothra Hydrothermal field, Endeavor Segment, Juan de Fuca Ridge. Geochem. Geophys. Geosys. 2007, Vol. 8, n6.
Kelley S. D., Baross A. J., Delaney, J. R.; Volcanoes, fluids, and life at mid-ocean ridge spreading centers. Annual Rev. Earth Planet. Sci. 2002, 385-349.
Lowell, R.P., Germonaovich, C.R.; Hydrothermal processes at mid-ocean ridges: Results from scale analysis and single-pass models. In Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans; German, C.R., Lin, J., Parson, L.M., Eds.; Geophysical Monograph 148; American Geophysical Union: Washington, DC, 2004; 219-244.
Luther, G. W.; Activation of diatomic and triatomic molecules for the synthesis of organic compounds: Metal catalysis at the subseafloor biosphere. In The Subseafloor Biosphere at Mid-Ocean Ridges; Wilcock, W. S. D. et al, Eds.; Geophysical Monograph 144; American Geophysical Union: Washington, DC, 2004; 191-198.
Van Ark, E.M. et al; Seismic structure of the Endeavor Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity. Journ. of Geophys. Rsch. 2007, Vol. 112.
Von Damm, K.L. Controls on the chemistry and temporal variability of seafloor hydrothermal fluids. In Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions; Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S., Thomson, R.E., Eds.; Geophysical Monograph 91; American Geophysical Union: Washington, DC, 1995; 222-247.
