Mothra Hydrothermal Vent Field

The Mothra Hydrothermal Vent Field (Mothra) is situated at the south end of the Endeavour Segment of the Juan de Fuca Ridge.  Mothra was discovered in 1996 following the initial detection of water column temperature anomalies in 1986 (Kadko et al., 1990; Yoerger et al., 2000; Delaney et al., 2001; Kelley et al., 2001b).  Located in 2200 m of water, the Mothra field has various terrain features such as pillow lava flows, lobate flows and basalt talus material (Figure 1).  Situated in an axial valley, Mothra contains six active hydrothermal vent clusters (Cauldron, Twin Peaks, Faulty Towers, Crab Basin, Stonehenge and Cuchalainn) which range from 40 m to 200 m from one another. This spacing indicates multiple conduits of active up flow beneath the field substrate. Each cluster contains both active and inactive vents and average height ranges from 7 m to 14 m for the active chimneys and 1 m to 4 m for the inactive chimneys (Figure 2).  At each cluster the close grouping of active and extinct chimneys indicate that shallow subsurface flow is complex and transient. Two subsurface processes that facilitate the hydrothermal flow are tectonic movement and the tapping of magmatic heat.

Mothra hydrothermal vent chimneys are taller, with steep sides and generally more slender than chimneys found at other Endeavour sites.  Vent chimneys at Mothra have fewer flanges and the chimney walls are more porous than the chimneys at the other sites. The smaller diameter vents can clog over time due to accumulation of minerals on the conduit walls, causing fluids to be rerouted to other structures. During the Edifice Rex Sulfide Recovery Expedition four large segments of chimneys were recovered from the central portion of the Mothra Vent Field (Yoerger et al., 2000; Delaney et al., 2001; Kelley et. al., 2001b).  Most chimneys vent diffuse flow range from 30 degrees C to 220 degrees C; however rare black smokers do occur that exceed 300 ̊C, which are more common at other vent fields of the Endeavour Segment (Delaney et al., 1992; Lilley et al., 2003).  It is believed that the hydrothermal vent structures were more widely distributed over the Mothra Vent Field at one time, as is evidenced by the distribution of extinct sulfide chimneys and sulfide debris between the active vent fields. 

            The water discharging from the Mothra vents have salinities that are higher in concentration than the ambient seawater.  The western boundary fault system is the source of the upward migration of fluids that maintain the long-lived hydrothermal vent structures.  The CO2 concentration in the vent fluids are the lowest at Mothra compared with the other hydrothermal sites (Proskurowski et al., 2004).  A measurement of the total heat emitted from a vent known as a heat content anomaly was found to be the lowest at the Mothra sites (Kellogg et al., 2006). The chimneys of the Mothra vents are mainly "sulfide minerals such as sphalerite, wurtzite, pyrite (+/- chalcopyrite) with smaller amounts of amorphous silica clays and sulfate-bearing minerals" (Kristall et al., 2006).

            Biological organisms found on and around Mothra vent structures include dense colonies of many species of macrofauna such as spider crabs, tube worms, limpets, and polychaete worms.  At the base of the vents where fluid discharge occurs from cracks in the basalt substrate, bushes of tubeworms and bacterial mats can be found.  The macrofaunal communities that were observed at Mothra are also present at the other Endeavour Segments. The vent system can support not only larger organisms, but can also sustain a diverse selection of microorganisms comprising biofilms and cell clusters. Among the cells present in these bacterial mats are filamentous and rod-shaped bacteria. Using the structure Finn as an example of cell spatial distribution, cell abundance is the greatest on the exterior chimney walls. These organisms are hypothermophilic, may contribute to hydrogen oxidation and sulfur reduction, and serve in symbiotic relationships with the surrounding macrofauna. Bacterial cells in this region withstand temperatures of 150 degrees C to 302 degrees C (Schrenk et al., 2003). The presence of thermal and chemical gradients drives the distribution of bacteria as one moves from the outer chimney walls toward the interior. Two types of bacteria, eubacteria and archaea, are found on the outer wall region.  Archaea dominates as one approaches the interior and inner conduit of the chimney.

Glickson et al., 2007

Figure 1.  This figure represents the geology of the Mothra hydrothermal vent field.  Shown are the individual vent clusters (Cauldron, Twin Peaks, Faulty Towers, Crab Basin, Cuchalainn and Stonehenge) that contain active and extinct chimneys.  The major basaltic flow morphologies include lobate, pillow, ropy sheet, and chaotic flows, in addition to basalt talus and collapse terrain. The circled letters represent photographs of geologic features that appear in the original manuscript.

Glickson et al., 2007

Figure 2.  This figure is a mosaic of the Twin Peaks hydrothermal cluster.  Shown are both active and inactive vent chimneys.  Two active vent chimneys in this mosaic are Climber and Flying Buttress that are approximately 8 m in height.  The smaller extinct structures surround the active vent structures.

Ocean 411

School OF Oceanography University of Washington

Where We Went

Information about places we are going

placeholderNEPTUNE Canada
Canadian NEPTUNE cabled observatory

 

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ODP 889 Survey area

 

 

placeholderBarkely Canyon
Barkely Canyon Node Survey area

 

 

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Information about the Endeavor Segment