South Chamorro Seamount Research Articles

Serpentinite‐hosted chemosynthetic community of South Chamorro Seamount, Mariana Forearc

AbstractDeep‐sea chemosynthetic ecosystems are ‘oases’ of life powered by reducing geofluids, of which serpentinite‐hosted seeps are among the least studied. South Chamorro Seamount, a serpentine mud volcano on the Mariana Arc, has been known to host chemosynthesis‐based assemblages since 1996, but no detailed information on the fauna was published. Here, we revisited South Chamorro to characterise its biodiversity. We located two regions of chemosynthetic communities dominated by bathymodioline mussels, vesicomyid clams, and chaetopterid parchment worms: one on the northwestern flank (‘Fryer Site’) and one on the southern summit (‘Summit Site’). We sampled a total of 20 species including 13 molluscs, five annelids, and two crustaceans – all present on the more active Summit Site but only a subset being found at Fryer Site. A mussel bed surrounding the Fryer Site was drilled by the Ocean Drilling Program in 2001 resulting in six holes, the deepest being 266 m (Hole 1200C). Cuttings ~50 cm deep still cover an approximately 60 m radius around Hole 1200C even 22 years later, and there is no sign of recovery. Low geofluid supply in serpentinite‐hosted seamounts may not allow decadal recovery of animal colonies, unlike a previous drilling site in an Okinawa Trough vent.

Unexpected discovery of a serpentinite‐hosted chemosynthetic ecosystem on Asùt Tesoru Seamount, Mariana Forearc

AbstractChemosynthetic ecosystems powered by microbial primary production are rare ‘hot spots’ of biological activity in the deep‐sea characterized by dense aggregations of specially adapted animal species. Among settings where such systems have been found, serpentinite‐hosted seep systems supported by alkaline geofluid are particularly understudied with just a few known sites worldwide. Mariana Forearc hosts the world's only series of active serpentinite mud volcanoes, but seep communities have only been reported from South Chamorro Seamount where large bathymodioline mussels dominate. Here, we report the discovery of a serpentinite‐hosted seep on the conical summit of Asùt Tesoru Seamount, Mariana Forearc. Named the ‘Big Blue Seep’, this field features white, likely carbonate crusts inhabited by animals, under which fluid seepage could be seen. We confirm 16 animal species, including typical seep‐associated fauna such as Desbruyeresia gastropods and Acharax awning‐clams. This is surprising as previous research expeditions did not notice any sign of chemosynthesis‐based ecosystems on this seamount, although the community is indeed difficult to spot due to the lack of large‐bodied epifauna such as mussels. The Big Blue Seep is adjacent to three drill holes made by the International Ocean Discovery Program expedition 366 (Holes U1496A‐C), which may have impact on seepage. Our findings represent the second chemosynthesis‐based ecosystem associated with serpentinite mud volcanism, suggestive of further such communities on other Mariana Forearc mud volcanoes.

Phylogenetic constraint and phenotypic plasticity in the shell microstructure of vent and seep pectinodontid limpets

Pectinodontid limpets of the genus Bathyacmaea are endemic to hot vents and cold seeps and exhibit greatly variable shell and radular macro-morphologies, rendering reliable species-level identification challenging. Here, we analyzed shell microstructures of western Pacific vent/seep Bathyacmaea limpets using scanning electron microscopy and Raman spectrophotometry to test its usefulness in providing phylogenetic signals. Bathyacmaea shells comprised of two forms of calcitic microstructure including irregular spherulitic prismatic type-A (ISP type-A) and semi-foliated (SF), as well as the aragonitic crossed lamellar (CL) microstructure. Despite marked differences in macroscopic shell morphologies once leading them to be classified into different species or even genera, six morphotypes of Bathyacmaea nipponica from different chemosynthetic localities and substrates shared an outermost ISP-A layer and alternating layers of SF and CL structures in their outer and inner shell layers. A genetically divergent lineage recovered from the South Chamorro Seamount, however, differed in having a simple three-layered shell composition consisting of ISP-A, SF, and CL structures, in that order, from the outside, and an unusually thin inner shell layer consisting of only CL structure. Moreover, the ratio of aragonite and calcite varied with habitat conditions, with calcite dominating in vents and aragonite dominating in seeps. These results suggest that the shell microstructure of pectinodontids is under phylogenetic constraints and provides useful taxonomic signals, while the mineral polymorphism in aragonite/calcite ratio varies according to environmental factors. Furthermore, microstructures of two ‘species’ from Cretaceous seeps confirmed the same patterns in fossil lineages.

Fluid transport and reaction processes within a serpentinite mud volcano: South Chamorro Seamount

Natural fluids with a pH (25 °C) up to 12.3 were collected from a sub-seafloor borehole observatory (Ocean Drilling Program (ODP) Hole 1200C) on South Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc. We used systematic differences in the chemical compositions of pore waters from drilling operations during ODP Leg 195 and borehole fluids collected subsequently from Hole 1200C to define two endmember solutions, one of which was a sulfate-rich fluid with a methane concentration of 50 mM that ascends from the subduction channel and the other was a low-sulfate fluid. The sequence of sample collection and fluid compositions constrain subsurface hydrologic conditions. Deep-sourced, sulfate- and methane-rich, sterile fluids from the subduction channel can reach the seafloor unchanged within the central conduit, whereas other fluid pathways likely intersect the pelagic sediment that underlies the serpentinite mud volcano, providing potentially suitable conditions and inoculum for microbial anaerobic oxidation of methane (AOM). These AOM-affected, low-sulfate fluids also make it to the seafloor where they discharge. The source of the sulfate- and methane-rich fluid in the subduction channel is attributed to abiotic methane production fueled by hydrogen production from serpentinization and carbonate dissolution. This methane production includes a mechanism to raise the pH above values from serpentinization alone. Results from South Chamorro Seamount represent an end member along a transect defined by the distance from the trench. Results from this site are applied to other serpentinite mud volcanoes along this transect to speculate on likely chemical conditions within shallower and cooler portions of the subduction channel.

Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region

Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24-40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich (similar to 42 wt%), but have low contents in Al2O3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%-25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount (mostly >25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements (Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.

Blueschist from the Mariana forearc records long-lived residence of material in the subduction channel

From ca. 50 Ma to present, the western Pacific plate has been subducting under the Philippine Sea plate, forming the oceanic Izu-Bonin-Mariana (IBM) subduction system. It is the only known location where subduction zone products are presently being transported to the surface by serpentinite-mud volcanoes. A large serpentine mud “volcano” forms the South Chamorro Seamount and was successfully drilled by ODP during Leg 195. This returned mostly partially serpentinized harzburgites enclosed in serpentinite muds. In addition, limited numbers of small (1 mm–1 cm) fragments of rare blueschists were also discovered. U–Pb dating of zircon and rutile from one of these blueschist clasts give ages of 51.1 ± 1.2 Ma and 47.5 ± 2.0 Ma, respectively. These are interpreted to date prograde high-pressure metamorphism. Mineral equilibria modelling of the blueschist clast suggests the mineral assemblage formed at conditions of ∼1.6 GPa and ∼590°C. We interpret that this high-pressure assemblage formed at a depth of ∼50 km within the subduction channel and was subsequently exhumed and entrained into the South Chamorro serpentinite volcano system at depths of ∼27 km. Consequently, we propose that the material erupted from the South Chamarro Seamount may be sampling far greater depths within the Mariana subduction system than previously thought. The apparent thermal gradient implied by the pressure–temperature modelling (∼370°C/GPa) is slightly warmer than that predicted by typical subduction channel numerical models and other blueschists worldwide. The age of the blueschist suggests it formed during the arc initiation stages of the proto-Izu-Bonin-Mariana arc, with the P–T conditions recording thermally elevated conditions during initial stages of western Pacific plate subduction. This indicates the blueschist had prolonged residence time in the stable forearc as the system underwent east-directed rollback. The Mariana blueschist shows that subduction products can remain entrained in subduction channels for many millions of years prior to exhumation.

Cool, alkaline serpentinite formation fluid regime with scarce microbial habitability and possible abiotic synthesis beneath the South Chamorro Seamount

South Chamorro Seamount (SCS) is a blueschist-bearing serpentinite mud volcano in the Mariana forearc. Previous scientific drilling conducted at SCS revealed highly alkaline, sulfate-rich formation fluids resulting from slab-derived fluid upwelling combined with serpentinization both beneath and within the seamount. In the present study, a time-series of ROV dives spanning 1000 days was conducted to collect discharging alkaline fluids from the cased Ocean Drilling Program (ODP) Hole 1200C (hereafter the CORK fluid). The CORK fluids were analyzed for chemical compositions (including dissolved gas) and microbial community composition/function. Compared to the ODP porewater, the CORK fluids were generally identical in concentration of major ions, with the exception of significant sulfate depletion and enrichment in sulfide, alkalinity, and methane. Microbiological analyses of the CORK fluids revealed little biomass and functional activity, despite habitable temperature conditions. The post-drilling sulfate depletion is likely attributable to sulfate reduction coupled with oxidation of methane (and hydrogen), probably triggered by the drilling and casing operations. Multiple lines of evidence suggest that abiotic organic synthesis associated with serpentinization is the most plausible source of the abundant methane in the CORK fluid. The SCS formation fluid regime presented here may represent the first example on Earth where abiotic syntheses are conspicuous with little biotic processes, despite a condition with sufficient bioavailable energy potentials and temperatures within the habitable range.

First seep-dwelling Desbruyeresia (Gastropoda: Abyssochrysoidea) species discovered from a serpentinite-hosted seep in the Southeastern Mariana Forearc

ABSTRACTThe South Chamorro Seamount (13°47'N, 146°00'E) on the southeastern Mariana Forearc is one of a series of serpentine mud volcanoes in the area, and hosts an active fluid seepage area on its summit. During a recent research cruise three specimens of a small rissoid-like gastropod were collected with chemoautotrophic bivalves. Subsequent morphological and molecular investigations revealed it to be a novel species of Desbruyeresia (Abyssochrysoidea: Provannidae), described herein as Desbruyeresia chamorrensis n. sp. Although all known specimens had a heavily corroded apex, so the protoconch could not be observed, its radular characteristics are very similar to those of other Desbruyeresia species. Phylogenetic reconstruction using the cytochrome c oxidase subunit I (COI) gene also supports its placement within Desbruyeresia. The new species is distinguished from all other described congeners by having more numerous (17–20) axial ribs on the teleoconch, and a broad shell for the genus (shell width to height ratio 0.6–0.65). Living Desbruyeresia were previously only known from hydrothermal vents and this is the first species known to inhabit seeps or serpentinite-hosted chemosynthetic habitats.http://zoobank.org/urn:lsid:zoobank.org:act:3BDA021E-45ED-439A-9D5D-402BBB29293E

The fate of subducted oceanic slabs in the shallow mantle: Insights from boron isotopes and light element composition of metasomatized blueschists from the Mariana forearc

Serpentine muds from South Chamorro Seamount (SCS), drilled during ODP Leg 195 at Site 1200 contain metamafic clasts that experienced blueschist-facies metamorphism (including the critical mineral assemblage pumpellyite – Na-amphibole – epidote). These schists represent fragments from the actual slab–mantle interface at ~ 27 km depth. Their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the Mariana forearc, a region of mélange formation as it can also be observed in onland exposures such as the Catalina Schist. As the Mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer during the subduction processes at an active convergent margin. This study presents the first data of detailed B isotope ( δ 11B) and light element variations in blueschist-facies minerals from the Mariana arc system. The primary foci are B and Li concentrations and δ 11B values analyzed by SIMS and ToF-SIMS techniques. Minerals such as (Na-rich) amphibole, phengite and chlorite are found to be strongly enriched in Li (up to 70 μg/g), Be (up to 8 μg/g) and B (up to 35 μg/g) and with δ 11B values of − 6 ± 4‰. These new data are consistent with isotopically heavy B being released into the Mariana forearc mantle wedge (serpentinization of dry mantle peridotite after interaction with B-rich slab-released high pH fluids) and confirm models of significant B-loss and B isotope fractionation during forearc (shallow) slab dehydration. The elevated Li, Be and B concentrations in minerals that comprise the bulk of the rocks, namely, amphibole, phengite, and chlorite bear a strong potential to further transport Li and B as well as the isotopically light component of B to greater depths in the mantle, where ongoing metamorphism is responsible for further isotope and elemental fractionation and the formation of distinct mantle reservoirs, e.g. volcanic arc and oceanic intra-plate (OIB) magmas.

Microbiology of Seamounts: Common Patterns Observed in Community Structure

Much interest has been generated by the discoveries of biodiversity associated with seamounts. The volcanically active portion of these undersea mountains hosts a remarkably diverse range of unusual microbial habitats, from black smokers rich in sulfur to cooler, diffuse, iron-rich hydrothermal vents. As such, seamounts potentially represent hotspots of microbial diversity, yet our understanding of the microbiology of seamounts is still in its infancy. Here, we discuss recent work on the detection of seamount microbial communities and the observation that specific community groups may be indicative of specific geochemical scenarios, such as iron and sulfur cycling. These observations are based on the metabolisms predicted by phylogenetic characteristics exhibited by the dominant populations found within these microbial communities as compared to the closest related isolate found in culture. Therefore, these studies combine the use of both cultivation-dependent and -independent analyses. Cultivation-independent studies were primarily completed using cloning and sequencing techniques targeting small subunit ribosomal gene (SSU rDNA) biomarkers along with similar biomolecular tools like terminal-restriction fragment length polymorphism (T-RFLP) and quantitative polymerase chain reaction (Q-PCR), which allow for the determination of phylotypes (analogous to species). We discuss the notion of Zetaproteobacteria and/or Epsilonproteobacteria being the most common members of hydrothermal habitats associated with seamounts exhibiting volcanic activity. Another noneruptive seamount scenario is also examined, for example, South Chamorro Seamount, an active forearc serpentinite mud volcano. iNtroductioN Microbial life is remarkable for its resilience to extremes of temperature, pH, and pressure, as well its ability to persist and thrive using an amazing number of organic or inorganic food sources. Nowhere are these traits more evident than in the deep ocean. Much of the deep seafloor consists of cold, relatively static sedimentary environments where heterotrophic microbes exist in significant numbers, but conditions remain relatively unchanged over long periods of time. Mid-ocean ridges associated with diverging plate boundaries are often sites of high-temperature hydrothermal venting and host many chemoautotrophic microbes; however, these systems are more homogeneous with respect to their physical and chemical properties than vent systems associated with volcanic arcs and backarcs (Takai et al., b y d aV i d e M e r s o N a N d c r a i g l . M o y e r common Patterns observed in community structure Th is aticle as been pulished in O ceagraphy, Vlum e 3, N um er 1, a qurterly jornal of Th e o ceagraphy soiety. © 2010 by Th e o ceagraphy soiety. a ll rhts rerved. Perm ision is gnted to cpy his aticle or se in teching nd rearch. replication, sstem m tic repruction, or coective redirbution of ny prtion of his aticle by phocopy m acine, repsting, or oher m eans is perm ited only w ith he aproval of Th e o ceagraphy soiety. send ll corrondence o: ifo@ tos.org or Th e o ceagraphy soiety, Po ox 131, rokville, M d 208-1931, u sa .

Strontium isotopic relations among pore fluids, serpentine matrix, and harzburgite clasts, South Chamorro Seamount, Mariana forearc

Subduction of the Pacific plate beneath the Mariana forearc releases fluids to the overlying mantle wedge that ascend, producing serpentinite “mud” that discharges on the ocean floor. As part of Leg 195 of the Ocean Drilling Program cores were obtained from drill-holes into the mud volcanoes. We report the isotopic composition of Sr in water squeezed from intervals of the cores, in the serpentinite mud, in leaches of the serpentinite mud, and in entrained small harzburgitic clasts. Except in the upper few meters below the seawater–mud interface, where pore water approaches seawater Sr concentration and isotopic ratio, Sr concentration and isotopic composition remain constant at 3–6 µmol/kg and ~ 0.7054. Because the elemental chemistry of the pore water is unlike seawater, this isotopic composition reflects fluids derived from the subducted slab, probably modified by reaction with mantle material during ascent. Higher Sr isotopic ratios, up to 0.7087, – but not with higher Sr concentrations in pore water – occur superimposed on an advection profile at 13–16 mbsf surrounding a thin layer of foraminiferal sand. Since the upward seepage velocity of slab fluids in the mud volcano vents is a few cm/yr, exchange of Sr between these carbonates and the rising fluids must have occurred within a maximum of a few hundred years, essentially instantaneously given the millions, or tens of millions, of years the mud volcanoes have been in existence. In contrast, the strontium isotopic compositions of leached serpentinite mud, and of small harzburgite clasts entrained in the mud, are always significantly greater than that of the pore water. In small harzburgite clasts the ratio reaches 0.7088, almost as high as the seawater value of 0.7092 and much higher than the value of typical mantle-derived strontium of ~ 0.704. The serpentinite muds and harzburgite clasts clearly equilibrated with seawater Sr when they were initially deposited at the surface of the seamount, but following burial they have not fully equilibrated with strontium in the pore water now discharging through the vents. These variations in the strontium isotopic composition of solids and pore waters are more consistent with episodic expulsion of fluids in the subduction zone than steady state flow. Whereas strontium in carbonates equilibrates isotopically within a few hundred years, strontium in buried harzburgite clasts does not equilibrate in the same time, assuming steady state rates of upward fluid flow. By inference, the harzburgite clasts and associated serpentinite mud must have been near the seafloor, unburied, for a yet undetermined but much longer period of time to have equilibrated from ~ 0.704 to 0.709 prior to subsequent burial. It may be possible to characterize at least the periodicity of fluid release in the mud volcano setting by investigating the zonation of strontium isotopic composition of hartzburgite clasts throughout the 60-meter deep composite cores.

Borehole observations of fluid flow from South Chamorro Seamount, an active serpentinite mud volcano in the Mariana forearc

A sealed borehole observatory (CORK) was deployed on South Chamorro Seamount, an active serpentinite mud volcano in the Mariana forearc to explore subduction-related processes on a non-accretionary, convergent plate margin. Formation fluid was overpressured relative to ambient hydrostatic conditions. Fluid flowed from the borehole at ~ 0.2 L/s when the observatory was opened to recover instruments 2 yr after it was installed. The chemical composition of the formation fluid is similar to that extrapolated from trends in pore water data collected during Ocean Drilling Program Leg 195 when the observatory was established. Reduced sulfur is present in this highly-alkaline (pH 12.4) formation fluid, indicative of microbial activity at or below the depth of the screened casing, 149–202 m below the seafloor. Discharge from the open borehole continued for 37 days, until the observatory was resealed. This discharge requires significant permeability at depth (> 6 × 10 − 14 m 2). Zones of high permeability may be associated with the formation of headwall scarps, consistent with numerous slumps on the southeastern flank of the seamount, and likely shape a geochemical environment suitable for an active microbial community.

PETROGENESIS OF MANTLE PERIDOTITES FROM THE IZU-BONIN-MARIANA (IBM) FOREARC

Serpentinised spinel harzburgites to orthopyroxene-rich spinel dunites recovered during the Ocean Drilling Program (ODP) Leg 195 on top of the South Chamorro Seamount (southern sector of the Mariana forearc, West Pacific Ocean), along with additional spinel harzburgites from Conical and Torishima Seamounts (northern Mariana and Izu-Bonin forearc, respectively), previously collected during the ODP Leg 125, have been investigated to shed light on the nature and evolution of forearc mantle in the intra-oceanic supra-subduction environment. All the samples show a marked heterogeneity in terms of petrographic, mineralogical and geochemical features that suggests a complex, multistage evolution involving, at variable extent, partial melting, reactive porous flow melt migration and subsolidus metamorphic re-equilibration under decreasing T and open system conditions. Geochemical evidence of the interaction between peridotites and various melts/fluids is the ubiquitous enrichment in highly incompatible elements, such as Large Ion Lithophile Elements (LILE). As for the high-T evolution of these peridotites, a three-stages-model is proposed, involving: 1) a former depletion event, during which the IBM forearc peridotites experienced 20-25% polybaric fractional melting during adiabatic upwelling; 2) a second depletion event characterised by a marked impoverishment in modal orthopyroxene, related to the upraise migration of ultra-depleted melts; 3) a late interaction between a relatively small volume of residual melts and the refractory mantle sequence. Oxidation state of the mantle minerals meanly decreases from north (Torishima Seamount) to south (South Chamorro), according to significant different contributions coming from the subducted Pacific Plate. In particular, the absence of a marked oxidation in South Chamorro peridotites suggests that the percolating melts during Stage 2 had not significant slab-derived component. This observation lead us to conclude that a thermal anomaly in the western Pacific mantle rather than the injection of hydrous components must be the “engine” determining the extreme depletion of the oceanic forearc peridotites and the arc formation. In this frame, it is proposed that IBM peridotites during Stage 1 underwent decompression partial melting and contributed to arc volcanism as actual mantle source. Successively, they were emplaced at relatively shallow levels (Stage 2), constituting the top of a strongly refractory mantle column and being percolated by melts produced by plumbing sources of the arc volcanism.

Serpentine and brucite of ultramafic clasts from the South Chamorro Seamount (Ocean Drilling Program Leg 195, Site 1200): inferences for the serpentinization of the Mariana forearc mantle

AbstractSerpentine minerals and brucite in ultramafic rocks from the South Chamorro Seamount were characterized chemically to investigate the serpentinization of the Mariana forearc mantle. Relict primary minerals of the serpentinites are olivine, enstatite and minor Cr-spinel and diopside. The secondary minerals are mostly serpentine and brucite with minor magnetite. The serpentine minerals, mostly lizardite and chrysotile, display large compositional variations. Al2O3 and Cr2O3 contents depend generally upon the nature of the primary mineral from which the serpentine was derived. Both serpentine minerals and brucite exhibit wide Mg, Fe and Mn substitution: the Mg# ranges are 95.1–77.2 and 88.9–60.8, respectively. These mineralogical and chemical features allowed us to estimate an upper temperature limit for serpentinization of ∼200–300°C, in agreement with recent thermal models which suggest that the serpentinized mantle wedge of the Izu-Bonin-Mariana subduction zone is cold. The high degree of serpentinization (40–100%, average >75%), and the serpentine + brucite paragenesis of these ultramafics imply that the Mariana forearc mantle has a significantly reduced density and strength down to ∼30 km, which provides a driving mechanism for serpentinite diapirism. Pervasive serpentinization of the forearc by fluids released from the décollement zone also explains the low seismicity of the Izu-Bonin-Mariana subduction zone.

Deep‐slab fluids fuel extremophilic Archaea on a Mariana forearc serpentinite mud volcano: Ocean Drilling Program Leg 195

As the Pacific plate subducts beneath the Mariana forearc it releases water that hydrates the overlying mantle wedge, converting it to serpentinite that protrudes to form mud volcanoes at the seafloor. Excess H2O ascends through these mud volcanoes and exits as cold springs at their summits. The composition of this deep‐slab derived water has been determined by drilling on two of these seamounts. It has a pH of 12.5 and, relative to seawater, is enriched in sulfate, alkalinity, Na/Cl, K, Rb, B, light hydrocarbons, ammonia, 18O, and deuterium, and depleted in chloride, Mg, Ca, Sr, Li, Si, phosphate, and 87Sr. Within the upper 20 m below seafloor at South Chamorro Seamount a microbial community operating at pH 12.5, made up overwhelmingly of Archaea, is oxidizing methane from the ascending fluid to carbonate ion and organic carbon, while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia.