(Source: Japan Coast Guard)
February 7, 2012
Hydrographic and Oceanographic Department of Japan
Japan Agency for Marine-Earth Science and Technology
Fukada Geological Institute
University of Iowa
University of Texas at Dallas
University of Hawaii
University of Rhode Island
An US-Japan science team has discovered an ecosystem that feeds on mantle material (i.e., serpentinized peridotite, or serpentinite) in the southern Mariana forearc. Abundant chemosynthetic biological communities, principally vesicomyid clams, associated with serpentinites (altered samples of Earth's upper mantle), were discovered during a submersible Shinkai 6500 dive in the southern Mariana forearc in September 2010. The ecosystem is located on the deep seafloor (approximately 5620 m) of the inner trench slope of the Mariana Trench near the Challenger Deep, the deepest spot on Earth's solid surface.
This is the first description of vesicomyid clams anywhere in the Mariana forearc, and the first live examples described from a low-temperature serpentinite-hosted hydrothermal system from either convergent or divergent plate margins, extending our knowledge about the biogeography of these clams. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.
This study will be published online in the Proceedings of the National Academy of Sciences of USA on the week of February 6, 2012.
The Mariana arc-trench system is a convergent plate margin where the mantle of the overriding Philippine Sea Plate interacts with fluids released by the subducting Pacific Plate. Forearc is a broad region between the trench axis and the associated volcanic arc.
A number of serpentinite mud volcanoes exist in the Mariana forearc north of Guam, however none is known from the south of Guam. Instead, serpentinized peridotite crops out extensively in the inner trench slope there. The Mariana forearc south of Guam faces the Challenger Deep (Fig. 1).
Cold seeps and hydrothermal vents associated with serpentinized mantle have been known for 15 years. In 1997, a high pH cold seep associated with Bathymodiolus mussel communities was discovered from the South Chamorro Seamount (a serpentinite mud volcano) east of Guam in the Mariana forearc. In 2001, a serpentinite-hosted, low-temperature system, named the Lost City hydrothermal field, was discovered along the Mid-Atlantic Ridge. There, carbonate chimneys are produced by highly reducing, high pH vent fluids. The discovery of the Lost City hydrothermal field has stimulated interest in the role of serpentinization of peridotite in generating H2- and CH4-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments, because previously all of the known hydrothermal vents along mid-ocean spreading ridges (divergent plate margins) were high-temperature, sulfide-rich, and low pH (acidic environments).
The YK10-12 cruise in September 2010 was conducted to study mantle and lower crust exposed in the Mariana forearc south of Guam with the submersible Shinkai 6500 (Fig. 1). Abundant chemosynthetic biological communities, principally consisting of vesicomyid clams, associated with serpentinized peridotite were discovered during Shinkai 6500 dive #1234 in the inner trench slope of the Mariana Trench, about 80 km northeast of the Challenger Deep. Although no active fluid venting was observed during the dive, the clustering of biological activity about 5620 m deep suggests that the fluids responsible for nourishing the clams mainly vented along this horizon (Fig. 2). The scientist team therefore named this field the "Shinkai Seep Field (SSF)". Dive #1234 collected more than 30 live vesicomyid clams, along with serpentinized peridotite, subordinate gabbro and a fragment of a white to ivory colored potential vent chimney. Rocks exposed around this community indicate that this section of the dive was on serpentinized lithospheric mantle.
This is the first description of vesicomyid clams anywhere in the Mariana forearc. Although vesicomyid clams are among the dominant invertebrates of chemosynthesis-based communities found principally at methane cold seeps derived from sediment diagenesis (such as at the Nankai Trough off southwest Japan) and high-temperature hydrothermal vents (such as at the Galapagos Rift), there have been no live examples from a serpentinite-hosted hydrothermal system including serpentinite mud volcanoes. The SSF vesicomyid clam community is the first live example described from a low-temperature serpentinite-hosted hydrothermal system from either convergent or divergent plate margins.
On the basis of shell morphology, the vesicomyid clams are likely a new species (Fig. 3). Based on DNA sequencing, this species appears to be more closely related genetically to the vesicomyid clam described from the high-temperature serpentinite and gabbro-hosted Logatchev hydrothermal vent field (Fig. 3). This discovery indicate that species closely akin can distribute separately, extending our knowledge about the biogeography of these clams.
Scanning electron microscope (SEM) observation and X-ray diffraction (XRD) analyses of the potential chimney fragment indicate that it consists mostly of brucite (Mg(OH)2) and acicular aragonite (CaCO3). Brucite chimneys are also known in serpentinite mud volcanoes from the Mariana forearc north of Guam, consistent with observations at the SSF.
In the mantle below the SSF, serpentinization of olivine-rich peridotite produces H2-rich alkaline fluids, magnetite, and Fe-Ni alloys:
Olivine + H2O = Serpentine + Brucite + Magnetite（and Fe-Ni alloy）+ H2
Fischer-Tropsch-type reactions between this H2 and CO2 in the fluid (using magnetite and Fe-Ni alloys as catalysts) result in abiotic formation of methane (CH4):
CO2 + 4H2 = CH4 + 2H2O
Then, anaerobic oxidation of CH4 generates hydrogen sulfide (H2S):
CH4 + SO42- = H2S + H2O + CO2-
The sulfide-oxidizing bacteria that are symbiotic with chemosynthetic biological communities oxidize this H2S, obtaining energy and producing organic matter through Calvin-Benson cycle. Chemosynthetic biological communities finally feed on this organic matter:
H2S + 2O2 = SO42- +2H+
These four reactions in serpentinite-hosted hydrothermal systems may have played important roles in the emergence of life on the Earth, and may have astrobiological implications for the presence of life on other planets and solar systems. It has been considered that serpentinite-hosted hydrothermal systems can not sustain high-biomass communities, because the Lost City hydrothermal field has only a small-biomass communities. Our discovery therefore indicates that these serpentinite-hosted low-temperature systems can sustain high-biomass communities.
We infer that the SSF represents a serpentinite-hosted ecosystem supported by fault-controlled fluid pathways connected to the decollement within a convergent margin setting. The serpentinization process beneath the SSF is likely controlled by persistent fluid flow from the subducting slab, although the bathymetry around the SSF indicates that it is not associated with a discernible serpentinite mud volcano. The deep geology of the southern Mariana forearc is dominated by peridotite and is heavily faulted, suggesting that more SSF-type seeps exist along the forearc facing the Challenger Deep. Similar vents may also exist in other convergent margins like the Tonga forearc where extensive peridotite exposures in the inner trench wall also are known.
Our discovery supports the prediction that serpentinite-hosted vents are widespread on the ocean floor. Therefore, this discovery could help us better understand how the full range of seafloor hydrothermal activity contributes to the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.
Fig. 1. (Left) Index map showing the location of the Mariana Trench and the Shinkai Seep Field. The Shinkai Seep Field is located in the inner trench slope of the Mariana Trench, about 80km northeast of the Challenger Deep, Earth's deepest location.
(Right) Detailed bathymetric map of the Shinkai Seep Field, showing the track of the Shinkai 6500 dive #1234.
Fig. 2. (Lef) View of the Shinkai Seep Field, photograph taken by Shinkai 6500, field of view ~10m. Note abundant chemosynthetic biological communities, principally vesicomyid clams associated with serpentinized peridotite.
(Middle) View of the Shinkai Seep Field, photograph taken by Shinkai 6500, field of view ~3m. Note vesicomyid clams and galatheid crabs.
(Right) Typical example of serpentinized peridotite (cut surface) collected during the discovery dive. White veins mark fluid pathways. Note 10 cm scale bar.
Fig. 3. Specimen photograph of the Shinkai Seep Field vesicomyid clam (exterior of left valve and dorsal view). This species is akin to the vesicomyid clam described from the Logatchev hydrothermal vent field.