V51C-1489
Mission Accomplished: Deep Submergence Science Routinely Supported Using Multiple Vehicles Throughout the Hawaii Undersea Research Laboratory's 2005 South Pacific Expedition
The Hawaii Undersea Research Laboratory (HURL) recently completed an internationally partnered 5-month, 14,500 nautical mile multiple leg expedition to the South Pacific that included 21 study sites in the waters of American Samoa, New Zealand, Tonga, and the U.S. Line Islands to commemorate its 25th anniversary of supporting deep submergence science in the Pacific Ocean. During this voyage, HURL successfully operated its two human occupied vehicles ({\it Pisces IV} and {\it Pisces V}) each capable of diving to 2000 m from their support ship, the R/V {\it Ka'imikai-o-Kanaloa} ({\it KoK}). In addition, a remotely operated vehicle ({\it RCV-150}) with a nearly 1000-m depth limit was utilized alternately with the {\it Pisces} HOV's. The size and organized placement of these vehicles on the compact but efficiently run {\it KoK} (70-m length, 2000-tons displacement, 14 crew) allowed for deployment of a CTD rosette system and recovery of instrument package moorings during the same cruise leg. The {\it Pisces} submersibles are 20-ft long, 13-ton, 3-person vehicles with 7-10 hours duration, up to 350-lb payload capacities, and three forward looking viewports. The small size of the {\it Pisces}' relative to much larger deeper diving HOV's increases their agility, thus allowing maneuvering into more difficult sampling site terrain. The smaller package also facilitates rapid launch (8 min avg, stdev=1) and recovery (12 min avg, stdev=2) in heavier seas (up to sea state 5), as routinely experienced in the South Pacific during the austral winter. In addition to the enhanced safety aspect of having two compatible submersibles aboard, scientific efficiency has benefited by allowing the rotation of vehicles on extended deployments prior to battery servicing, thus maintaining an overall dive time average of 7.1 hr (stdev=1.52) for an average dive depth of 891 m (stdev=431) in 2005. Having the two fully operational submersibles also provides a contingency for equipment malfunction while on site that saved 7 dive days in 2005 alone. The final dive count resulted in 61 out of 56 scheduled {\it Pisces} science dives completed; made possible by careful planning and on-site cruise management and aided by our can-do crew. The {\it RCV-150} was also used on 17 dives when depth and sea conditions allowed, typically in a scouting mode for potential HOV dives to increase the efficiency of the entire operation. The total package described here was made available for less than \$30K per dive day in 2005. Standard equipment on the {\it Pisces} pair and the {\it RCV-150} are listed at the URL below. In addition, science team equipment was interfaced including the NOAA VENTS fluid sampler and gas tight bottles, Imagenex pencil beam bathymetric sonar system, and the WHOI TowCam external digital still camera. Tracking and navigation accuracy to within 20 m at 1500-1800 m depth was provided by a TrackLink 5000HA ultra-short baseline system interfaced with a MAHRS motion sensor to provide pitch and roll correction. Additional vehicle details, operational results, and statistics for the 2005 South Pacific expedition will be presented.
V51C-1490
First Submersible Dives on Brothers Volcano, Kermadec Arc, Offshore New Zealand
Previous NZAPLUME and other cruises between 1999 and 2004 established that at least three major hydrothermal sites occur at Brothers; the NW caldera and cone sites are actively venting while the SE caldera site is extinct. In October 2004, the submersible {\it Shinkai 6500} did 4 dives on Brothers as part of the SWEEP Vents expedition, the first dives anywhere along the 2,500 Kermadec-Tonga arc. Between April and May, 2005, an additional 5 dives were made at Brothers using the HURL submersible {\it Pisces V} as part of a New Zealand and NOAA co-sponsored expedition. In total, 6 dives have been done at the NW caldera site and 3 at the cone site. The NW caldera site is host to a significant sulfide deposit. It mainly crops out along a zone of discrete, narrow, fault bounded ledges between ~1600 and 1680 m, over strike lengths up to ~500 m. Locally, bands of active and extinct black smoker chimneys up to 7 m tall occur in this zone and strike orthogonal to the caldera walls, indicative of cross structures. Abundant sulfide talus and subcropping massive sulfides also occur at this site. The submersible dives recorded extensive alteration of the caldera walls and sampled individual chimneys, Fe-oxide crusts, and altered rocks. The cone site is hosted by ash and talus and is dominated near its summit by extensive deposition of elemental S and Fe-oxide crusts. No sulfides have been recovered from this site. Preliminary vent fluids results are from samples collected by the {\it Shinkai 6500}. They show two very different hydrothermal fluids. Those from the NW caldera site have temperatures up to $302°C, pH down to 2.8, low Mg and SO$_{4}$ values, Cl between 510 and 760 mM, elevated Si and increasing Fe and Mn values with increasing Cl concentrations, consistent with a Cl-enriched endmember. By contrast, vent fluids from the cone site are gas-rich (up to 220 mM total gas), have temperatures <70°C, pH down to 1.9, Mg values near seawater concentrations and higher, Cl <550 mM and low levels of Si, Fe and Mn, consistent with a vapor-dominated phase. Water δD values for both sites are <0 per mil. Together, these results suggest subseafloor phase separation with the NW caldera site a chronic system dominated by evolved seawater and the cone site a nascent magmatic-hydrothermal system.
V51C-1491
Petrology of Newly Discovered Submarine Volcanoes From the Kermadec-Tonga arc: Initial Results
The NZAPLUME III research cruise to the northern Kermadec - southern Tonga arc during September-October 2004 mapped and dredge-sampled eight newly discovered volcanic centres along the arc-front, including Monowai, now known to have a caldera complex immediately to the north of the previously mapped cone. These centres, which typically include one or more calderas with multiple associated volcanic cones, have an average spacing of 47 km, slightly greater than the average spacing of 41 km for the entire Kermadec arc (i.e., from Whakatane in the south to Monowai to the north). All of the newly discovered volcanic centres show evidence of hydrothermal venting and, in the case of Monowai, recent eruptive activity. The dredged lavas have calc-alkaline compositions ranging from high-alumina basalt to rhyolite, confirming the occurrence of evolved lava compositions associated with caldera structures along the entire Kermadec arc. Three of the volcanic centres exhibit the full compositional range from basalt to rhyolite, three are basalt-andesite, and the remaining two entirely rhyolitic. In contrast to published results for the southern Kermadec arc, there is no evidence for compositional bimodality, with the data set containing sub-equal proportions of basalt, basaltic andesite, andesite, dacite, rhyodacite and rhyolite. Despite the variety of volcanic edifices sampled, all of the volcanic centres yield tight geochemical correlations for major and trace elements, which are interpreted to indicate simple mixing or fractionation processes. There is a suggestion, yet to be fully established with trace element or isotopic modelling, that three of the four southern-most centres, spanning some 200 km of arc-front, are genetically related.
V51C-1492
Tin-bearing chalcopyrite and platinum-bearing bismuthinite in the active Tiger sulfide chimney, Yonaguni Knoll IV seafloor hydrothermal system, Okinawa Trough, Japan
The active submarine hydrothermal field at the Yonaguni Knoll IV is located in the southern Okinawa Trough, behind the Ryukyu trench-arc system, Japan. This field consists of seven active hydrothermal venting sites (Mosquito chimney, Carp Chimney, Abyss vent, Shallow Chimney, Tiger Chimney, Lion Chimney, Crystal Chimney) which are hosted by thick sediments and an underlying felsic volcanic rock of rhyolitic composition. The sulfides from the flank of the Tiger chimney consist of chalcopyrite, bismuthinite, pyrite, galena, sphalerite and gangue mineral of anhydrite which is slightly different to the mineral assemblage of sphalerite, pyrite, wurtzite, chalcopyrite, galena, tennanite-tetrahedrite series, stibnite, As-Sb-Tl-Hg-S bearing phase, bornite, covellite, nukundamite, alabandite and gangue minerals of barite, anhydrite, calcite, and rhodocrosite seen in the other chimneys in this field. Electron microprobe analysis of the chalcopyrite and bismuthinite from the flank of the Tiger chimney, indicates that the chalcopyrite and bismuthinite contain significantly high tin (0.51 to 2.40wt.% Sn, n = 16 ) and platinum (1.30 to 1.69 wt.% Pt, n = 9)respectively and are quite different to the sulfide chemistry of the other chimneys in this field The high Sn and Pt content in chalcopyrite and bismuthinite respectively, are significantly high and has never been reported previously for the submarine hydrothermal systems. The high Sn content in chalcopyrite confirms that the Sn enters the chalcopyrite as a solid solution towards stannite by the coupled substitution of Sn4+Fe2+ for Fe3+Fe3+ while the high Pt content in bismuthinite might indicate that Pt probably enters the bismuthinite by interstitial substitution of Pt2+Cu1+ for Bi3+ although very limited published data is available to verify this observation. Fluid inclusion data of anhydrite (297-313°C) and measured end-member temperature of the vent fluid (325°C) does not exceed 400°C. Previous experimental studies suggest that high Sn content in chalcopyrite and Pt content in bismuthinite precipitated metastably in the Tiger chimney. The present mineralogical observation strongly suggests that the Yonaguni Knoll IV site in the southern Okinawa Trough is slightly different to the middle Okinawa trough and other known submarine hydrothermal systems of the back-arc basins in terms of mineralogy and sulfide chemistry.
V51C-1493
Sr, Nd and Pb Isotope Geochemistry of Near-ridge Seamounts in Eastern Pacific: Implications for Upper Mantle Composition and EPR Magmatic Segmentation
Near-ridge seamount lavas tend to reflect the true composition of the upper mantle source of MORB because these are generated by relatively smaller degrees of melting of smaller volumes of the mantle compared to nearby axial lavas; they also by-pass the axial chamber mixing and fractionation processes that are responsible for the relatively more uniform chemical and isotopic composition of normal-MORB. New Sr, Nd and Pb isotope data combined with published data for lavas from near-ridge seamounts on either side of the EPR segment between the 11$^{o}$45' OSC and Orozco Transform at 15$^{o}$00' show latitudinal isotopic variation very similar to that shown by the rise axial lavas (Castillo et al., G3 1, 1999). Seamount and axial lavas at both ends of the rise segment have on average slightly higher and more limited range of $^{143}$Nd/$^{144}$Nd, but slightly lower $^{206}$Pb/$^{204}$Pb and $^{87}$Sr/$^{86}$Sr ratios than lavas at the center of the segment. Some of the seamounts are located on ~8 Ma rise flank crust although most of the seamount lavas are fairly young (e.g., lavas from Seamount 6 on ~3 Ma crust are only 3 to 900 kyr - Graham et al., Nature 326, 1987). Thus near-ridge seamount isotope data provide the first documentation for a large-scale (~350 km long x ~720 km wide), systematic compositional variation of the upper mantle source of EPR MORB. Such a scale of variation is larger and longer than the size and <1 myr life span of the majority of non-transform offsets, which are supposed to be responsible for the along-axis compositional variations of EPR MORB according to the "bottoms up" model of magmatic segmentation.
V51C-1494
Submarine Volcanoes and High-Temperature Hydrothermal Venting on the Tonga Arc, SW Pacific
The first submarine hydrothermal vents and associated seafloor mineralization on the Tonga arc have been found in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents ($245-265 °C) occur at water depths of 385-540 m near the summit of a large volcano at $24°S. They comprise clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys surrounded by extensive deposits of Fe-oxyhydroxides. The chimneys are characterized by vigorous venting of clear fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) discharging from the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges. A second, spatially distinct, vent field occurs at water depths of 850-985 m within a large caldera on the western side of the volcano. Venting is more diffuse and at lower temperature in this field (to $112 °C), which features large clusters of Fe-oxyhydroxide and silica chimneys at its core. Both vent fields are spatially related to basaltic dike swarms. Low-temperature venting (to $152 °C) was also found on a second volcano at $21°S. There, venting occurs at the southern end of a chain of youthful explosion craters, and is most intense in water-depths of 160-210 m. Large fields of mussels and white filamentous bacteria cover the seafloor around these craters, and the vents are characterized by voluminous gas streaming (most likely CO2 bubbles) through sulfur-cemented ash.
V51C-1495
Plume-Vent Fluid Connections along the Tonga-Kermadec arc
The full extent of the 2530 km-long Tonga-Kermadec intra-oceanic arc was systematically surveyed for hydrothermal plume emissions during 5 expeditions between 1999 and 2005: NZAPLUME I, II and III covered the Kermadec arc using the New Zealand {\it RV TANGAROA} and the TELVE and NoToVE cruises surveyed the Tonga arc aboard the Australian {\it RV SOUTHERN SURVEYOR} after preliminary swath mapping by the German {\it RV SONNE}. At least 71 volcanic centers comprise the arc front, with about 75% of these first mapped or newly discovered during the expeditions. Hydrothermal plumes were detected as light scattering and chemical anomalies over more than half (36) of the volcanoes surveyed. Forty submersible dives were made on one-third (12) of the hydrothermally active volcanoes during the past year: 4 inaugural dives by the Japanese submersible {\it SHINKAI 6500} on Brothers volcano, and 36 dives using the NOAA/HURL submersibles {\it PISCES IV} and {\it V} on Brothers and an additional 11 volcanoes spread along the arc front. At least 25 venting sites were observed, and most were sampled for gaseous and liquid hydrothermal fluid emanations. We will summarize the pre-dive plume detection results with emphasis on the variability in the magnitude and character of the chemical signals over the active sites and compare/test predictions of the nature of seafloor sources at dive sites with what was actually observed. While most chemical determinations for the seafloor samples are pending, analyses conducted shipboard confirm that simple extension of plume results to the seafloor can be misleading and in contrast to the MOR experience.
V51C-1496
Observations of Local Seismicity and Harmonic Tremor Using an Ocean Bottom Hydrophone Array at Brothers Volcano, South Kermadec Arc.
The submarine Brothers volcano is an important link in the volcanic chain of the southern Kermadec Arc system in the Southwest Pacific Ocean north of New Zealand. The 3-3.5 km wide caldera has a center depth of 1850m and steep surrounding walls of 300-450m. Active hydrothermal venting distinguished Brothers as a point of focus for the New Zealand American Submarine Ring of Fire (NZASRoF) expeditions in 2004 and 2005. Due to its remote location, moderate to small magnitude seismicity around the Brothers area is largely unknown. In late September 2004, four ocean bottom hydrophones (OBHs) were deployed on the caldera floor. In April 2005, three of the four instruments were recovered intact. These three OBHs continuously recorded, for seven months, the low frequency (0-110Hz) acoustic field around Brothers volcano, in particular seismic P- and S-waves propagating through the crust and acoustic T-waves in the water column . Preliminary analysis reveals seismicity rates on the order of 106 earthquakes per month. In addition to seismic arrivals, low frequency harmonic tremor is frequently and independently observed on each of the OBH instruments, often occurring subsequent to the larger seismic events. Qualitative comparisons of these signals with tremor observed from the Volcano Islands south of Japan (Dziak and Fox, 2002) show them to be nearly equivalent in frequency structure, suggesting the origin of the tremor observed at Brothers may also be attributed to resonance of a magma-gas mixture in a large chamber or conduit near the water/ seafloor boundary.
V51C-1497 INVITED
Tempest in Vailulu'u Crater
The summit crater of the Samoan submarine volcano, Vailulu'u, has been actively erupting since 2001. Based on water chemistry, CTD and temperature logger data from 2000 and 2001, we formulated a model for the hydrothermal system in the crater involving a tidally-modulated "breathing" (Staudigel et al., 2004). During low stands of internal waves (exterior to the crater), the crater exhales warm buoyant hydrothermal water that forms a "halo" around the crater rich in Mn, 3He, and particulates. During "high tides", cold dense external water is inhaled into the crater through the three breaches, and cascades to the crater floor. In April 2005, we used the HURL PISCES V submersible to deploy various temperature and particulate loggers and current meters in and around the crater; these were retrieved by Pisces V in July 2005. In addition, continuous CTD profiling was carried out over 12 hour tidal cycles at one location inside, and one outside, the crater. The accumulated data set fully reinforces our "breathing" model. An ADCP, deployed for 93 days in the NW breach at 752m, showed dominant easterly inflow currents and westerly outflow currents, with maximum velocities of approximately 25 cm/s. The flows were coherent for distances up to 50-60m above the ADCP; the mean inflow velocity and azimuth (20-40 m interval above the ADCP) was 7 cm/s due east; the mean outflow velocity and azimuth was 5 cm/s at 260 degrees. Mean inflows were consistently colder than outflows (5.00 C vs 5.20 C); the maximum observed range in temperature was 1.1 C, correlated with peak flow velocities. The coldest inflows would require very large regional internal wave amplitudes, up to 50-100 meters. A 2-D acoustic current meter was deployed on top of the west crater rim summit (582m) for 90 days, and in the S breach (697m) for 4 days. The summit flows are presumed to represent the regional scale currents; these were largely from the SW quadrant, with typical velocities of 8-15 cm/s, and peaks to 25 cm/s. The S breach flows had a clear semi-diurnal tidal modulation, with strong NE quadrant inflows at high velocity (15-30 cm/s), separated by short outflow spikes of 1-2 hour duration at much lower velocity (greater than10 cm/s). The outflow water was typically warmer by 0.1-0.2 C; the maximum temperature range was 0.6 C, about half of that observed at the NW breach. A 12-hour continuous profiling CTD-LBSS station was serendipitously sited on top of a large diffuse-venting hydrothermal field, in the crater moat just north of the new volcanic cone. The water column here was incredibly dynamic, with a 5-10m bottom boundary layer, 1 C above ambient, forming by diffuse flow from the basalt substrate in a matter of minutes; this layer would destabilize, detach, and rise with velocities of ~ 1 cm/sec. This buoyant water was both warmer and less saline than either the ambient crater water, or the cold outside water which occasionally cascaded onto this site from the nearby NW breach; it was also laden with particulates, with LBSS readings up to 1.7 NTU. Water chemistry and He isotope analyses are in progress.
V51C-1498
Carbon Fluxes from Submarine Arc Volcanoes - examples from the Mariana and Kermadec Arcs
Recent investigations of volcanic arcs have revealed unusually high fluxes of CO2 from several submarine arc volcanoes. In 2004 the ROPOS ROV was used to map and sample ~10 active volcanoes along the Mariana arc, and in 2005 a similar study of volcanoes along the Kermadec arc was conducted using the HURL Pisces submersible. Of particular interest are 3 volcanoes that, in addition to discharging hot vent fluid, were found to be venting a separate CO2-rich phase in the form of gas bubbles or, in one case, droplets of liquid CO2. The Champagne hydrothermal site situated at ~1600-m depth near the summit of NW Eifuku volcano (21.49°N, 144.04°E) in the northern Mariana Arc, was discovered in 2004 during NOAAs Submarine Ring of Fire (SROF) project. This unusual site was discharging two distinct fluids from the same vent field: a 103°C gas-rich hydrothermal fluid, and cold (4°C) droplets of liquid CO2. The hot fluid contained ~2.2 moles/kg CO2, the highest ever reported for submarine hydrothermal fluids and about twice the saturation value at that p,T. The carbon flux from this site was estimated to be ~23 moles CO2/sec, about 0.1% of the global MOR carbon flux. Two similar but much shallower CO2-rich systems were discovered on the Kermadec arc. Pisces dives on Giggenbach volcano (30.04°S, 178.71°W) in the Kermadec arc discovered a mixture of gas bubbles and 203°C fluid discharging at 164-m depth. The fluid contained 250 - 500 mM/kg total gas. At Volcano 1 (21.15°S, 175.75°W), Pisces found streams of gas bubbles rising from the seafloor at ~100 m depth. This vent area had areas of diffuse discharge (30 to 150°C) with gas contents up to 130 mM/kg. Although analyses are still in progress for these two sites, the gas bubbles are assumed to be mainly CO2. It is notable that discharges of pure CO2 have never been reported for MOR hydrothermal systems, and only one other submarine occurrence of liquid CO2 has been reported (in the Okinawa Trough, a back-arc system). This suggests that such CO2-rich systems occur much more frequently in subduction zone systems compared to MOR systems, probably due to the supply of subducted marine carbonates and organic matter. It seems likely that the high CO2 levels arise from direct degassing from a magma chamber and/or de-volatilization of the subducting slab. The apparent high carbon flux from these sites suggests that submarine arc volcanoes may play a larger role in oceanic carbon cycling than previous realized.
V51C-1499
Hydrothermal plumes at the Myojinsho submarine caldera, the Shicito-Iwojima Ridge, Izu-Bonin Arc
For the purpose of locating hydrothermaly active site, water column anomalies of manganese concentrations were searched in the Myojinsho submarine caldera ($31°53'N, $139°57'E), the Shichito-Iwojima Ridge, Izu-Bonin Arc, during KAIREI (JAMSTEC) KR05-11 cruise in August 2005. An in-situ manganese analyzer GAMOS and CTD attached on autonomous underwater vehicle (AUV) r2D4 (IIS, Univ. Tokyo) were used for the plume survey. We could find manganese anomalies inside the caldera by intensive progressive ascent and descent observations (Prog-AD), similar movement with tow-yo by CTD rosette sampler, using AUV r2D4. The manganese anomalies have four maxima at 630m, 680m, 750m and 850m in depth with maximum concentration of 250 nM manganese. These high manganese enrichment is considered to be due to contribution of hydrothermal manganese. The slope of seawater density at 300 - 700m and 700 - 1,100m in depth is 0.033 and 0.001g/cm3 per meter, respectively. It changed rapidly in the point of 700m in depth, so it is difficult for hydrothermal plume from deep site (>700m) to go up to 700m in depth or more. The shallow plumes (630m and 680m in depth), therefore, come from shallower site than deep plumes (750m and 850m in depth).
V51C-1500
Phylogenetic and Physiological Diversity of Subseafloor Microbial Communities at Axial Seamount, Juan de Fuca Ridge: Summary of Results From the New Millenium Observatory (NeMO), 1998-2004
Axial Seamount (45 ° 58' N; 130 ° 00' W) is an active submarine volcano located on the Juan de Fuca Ridge, approximately 300 miles off the coast of Oregon. Lying at the intersection of a seamount chain and a spreading axis, Axial is a unique study site from both the geological and biological perspective. In January of 1998, Axial experienced a week-long series of earthquakes, and subsequent water column and seafloor observations on the southeast portion of the caldera found temperature and chemical anomalies, extensive new seafloor lava flows, large "snow blower" type vents, and other characteristics commonly associated with diking-eruptive events. Due to its high activity and close proximity to shore, Axial was chosen as a site for a multi-year observatory (New Millenium Observatory, NeMO) to document changes and interactions between geology, chemistry, and biology on the mid-ocean ridge system. From 1998 through 2004, we extensively sampled diffuse vents at Axial Seamount to determine the physiological and phylogenetic diversity of subseafloor microbial communities and their relationship to the geochemical environment. Here we present a summary of those studies, including molecular-based phylogenetic surveys of bacteria, archaea, and potential nitrogen-fixing organisms, culturing results of thermophiles and hyperthermophiles from over 20 sites, and the distribution of one particular group of hyperthermophiles at diffuse vents throughout the caldera and how that distribution may be linked to the geochemical habitat. Results indicate that Axial supports a diverse subseafloor microbial community, including hydrogen and sulfur oxidizers, hyperthermophilic methane producers and heterotrophs, and many organisms with the potential to fix nitrogen. In addition, we find that the species composition of the microbial community changes in response to changes in the physical and chemical conditions at each vent site. The extent of seawater mixing with hydrothermal fluids has the most pronounced effect on microbial community diversity. Even at vent sites dominated by seawater, thermophilic and hyperthermophilic anaerobes are still detected and point to a hot, anaerobic subseafloor zone where biological carbon dioxide fixation and biofilm formation are dominant physiological strategies. Axial represents one of the most well-studied seamounts to date, and with a wealth of contextual information and samples, we are continuing our work there with a focus on using metagenomics to determine the genetic content of the subseafloor microbial community and how these genes relate to the biogeochemical environment.
V51C-1501
Role of a unique population of lithotrophic, Fe-oxidizing bacteria in forming microbial Fe-mats at the Loihi Seamount.
The Loihi Seamount, located 30 km SE of the island of Hawai'i, is among the most active volcanos on Earth. The summit, at a depth of 1100m, includes a 250m deep caldera (Pele's Pit) formed by an eruption in 1996. The summit, and especially Pele's Pit, are the site of extensive low to intermediate temperature (10° to 65°C) hydrothermal venting, emanating both from diffuse fissures and orifices that have substantial flow rates. The vent fluid is characterized by a low sulfide content, high CO2 concentrations and Fe(II) amounts in the 10s to 100s of æM. Associated with all vents are extensive deposits of iron oxyhydroxides that typically have 107 to 108 bacterial cells/cc associated with them. The morphology of the Fe-oxides are indicative of biological origins. We have isolated microaerophilic, obligately lithotrophic Fe-oxidizing bacteria from Loihi and describe here 'Mariprofundus ferroxydans' a unique bacterium that forms a filamentous iron oxide mineral. 'M. ferroxydans' is the first cultured representative of a novel division of the Proteobacteria, known previously only from clones from different hydrothermal vent sites. Molecular evidence from Loihi mats based on clone libraries and terminal restriction length polymorphism (T-RFLP) analysis of 16S rRNA genes indicate that this lineage of Fe-oxidizing organisms are common inhabitants at Loihi. We speculate that this organism and its relatives form the basis of an active microbial mat community that owe their existence to the inherent gradients of Fe(II) and O2 that exist at the Loihi vents. In a geological context this is interesting because the Loihi summit and caldera are in an O2-minima zone; O2 concentrations in the bulk seawater are around 0.5 mg/l. In effect, Loihi could serve as a proxy for the late Archaean and early Proterozoic periods when the Earth's atmosphere went from reducing to oxidizing, and it is speculated that abundant Fe(II) in the Earth's oceans served as a major sink for O2 production preventing its accumulation in the atmosphere. Better understanding of extant conditions at Loihi might help us frame questions concerning the role of lithotrophic iron-oxidizing bacteria in the rusty ocean of the late Archaean Earth.
V51C-1502
Elucidating the Mechanisms of Microbial Weathering of Submarine Basalts
In recent years there has been as increasing interest in microbe-mineral interactions, specifically the molecular mechanisms of mineral formation and dissolution. While not a true mineral, submarine basaltic glass represents an important rock surface and one of the most reactive components of the ocean crust. The high solubility of reduced glasses and the large disequilibrium with oxygenated seawater leads to large scale chemical exchange of Ca, Mg, Si, Al, Mn, Sr, as well as the pervasive oxidation of Fe(II). A variety of different mechanisms can be envisioned to contribute to the weathering of basalt, yet our basic understanding of what mechanisms actually occur and which are the most important is exceedingly small. To gain a comprehensive understanding of the mechanisms of basalt weathering it is necessary to be able to measure weathering rates, distinguish between biotic and abiotic components of weathering, and relate these rates to the various microbial processes that may be occurring. This requires an integration of geochemical, microbiological, molecular biological and mineralogical approaches. In addition, comparative studies between laboratory and field experiments and between different environments are necessary to assess the dominant pathways for basalt weathering. Given the chemical abundance and availability of reduced Fe and to a lesser extent, reduced Mn in basalts which may serve as energy sources, our group is focusing on bacteria that carry out redox transformations of these metals or produce compounds that complex these metals. Our approach includes cultivation and characterization of bacteria from natural basalt surfaces of various ages and from different environments, and using these isolates for laboratory studies of basalt colonization and weathering. Natural basaltic glass as well as synthetic basaltic substrates amended with enhanced concentrations of Mn, phosphate and varying Fe oxidation states have been placed back in the environment for exposure and retrieval after months to years for subsequent analysis of microbial populations and rates of weathering. Our primary study sites, the seamounts Loihi in Hawai'i and Vailulu'u in American Samoa, provide access to a wide range of environments characterized by different temperatures and chemistry and will allow us to assess the variety mechanisms of basalt weathering and their universality.
V51C-1503
Endolithic Mn-oxidizing bacteria commonly associated with basalts at active Seamounts
Mn is a trace component of volcanic rocks that is commonly enriched by 1-2 orders of magnitude within the secondary mineral assemblages associated with submarine basalts. Our analysis of relatively young basalts recovered from active seamounts such as Loihi Seamount (Hawaii) and Vailulu'u Seamount (American Samoa) shows that Mn(IV)-oxides readily form during short time-periods (10 years) of low-temperature (~2C) alteration, although the abiotic kinetics of Mn(II)-oxidation are slow at this temperatures and pH. We suggest that the formation of these secondary minerals are likely due to the common presence of heterotrophic bacteria with the functional capability of Mn(II)-oxidation, which accelerate the rates of oxidation several orders of magnitude faster than predicted for water-rock interaction alone. To identify and isolate endolithic Mn(II)-oxidizing microorganisms from naturally-weathered basalt surfaces, samples were recovered from the cold outer-flanks of Loihi and Vailulu'u Seamount via submersible with a sealable biobox. Using a variety of oligotrophic to organic-rich seawater-based media, we have isolated over 40 strains of Mn(II)-oxidizing bacteria. These isolates are primarily alpha- and gamma-Proteobacteria that can grow on low concentrations of simple to complex organics, but not Mn(II) as a sole energy source. None of the isolates, nor their closest relatively, were previously recognized as Mn(II)-oxidizing bacteria. In particular, we have found that there are several strains that are common to the basalts recovered from Loihi & Vailulu'u Seamount, as well as from basalts collected at the East Pacific Rise, particularly Pseudoalteromonas and Sulfitobacter sp. The 16S rRNA gene sequences of the Pseudoalteromonas isolates are also observed in T-RFLP data and 16S clone libraries for microbial mats at Loihi, indicating that these isolates are environmentally-relevant and abundant in-situ. The ubiquitous distribution of these isolates also suggests that they may play an important role in the submarine alteration of volcanic rocks and the formation of ferromanganese crusts, and thereby affect the net chemical fluxes associated with water-rock exchange.
V51C-1504
Heterotrophic Fe-Oxidizing Bacteria Associated With Basalt Surfaces Supporting Life On Vailulu'u Seamount, American Samoa
Fe, the fourth-most abundant element in the Earth's crust, is also one of the most biologically essential ones. The reduced form, Fe(II), is often considered to be biologically limiting as a result of its low solubility and rapid chemical oxidation to Fe(III)(hydr)oxides at circumneutral pH. The alteration of basaltic glass, enriched in Fe(II), however, provides an abundant supply of reduced iron and, thus, has a major influence on local ocean chemistry and Fe bioavailability. Despite the fact that chemical Fe(II) oxidation takes place very rapidly, we demonstrate that alteration processes of freshly formed basaltic glass can be crucially enhanced by microbial activity.Cultivation of bacteria from basalt surfaces collected from two active submarine volcanoes, Loihi (Hawaii) and Vailulu'u (American Samoa) show a large number of heterotrophic bacteria capable of oxidizing Fe(II) and that these bacteria. not only enhance basalt dissolution but also play a major role in precipitating large amounts of thick Fe(hydr)oxides mats on Vailulu'u Seamount, particularly in the vicinity of low temperature hydrothermal vents. These mats contain substantial quantities of organic carbon that may serve as food sources for some of the macrobiological life on Vailulu'u Seamount. This very prominently includes a substantial population of eels that is found in close spatial association with up to 1m thick Fe oxide/microbial mat at Nafanua volcano, a recent volcanic cone that grew from the crater floor of the seamount. Microbial community analysis on different substrates ranging from basalt surfaces to microbial mats were performed on specially designed culturing media for detection and isolation of heterotrophic bacteria capable of Fe(II)-oxidation. Clone libraries from microbial mats originating from an eel dominated area of Vailulu'u crater are being compared to libraries made from eel guts in order to provide information to what extent these mats are being used as a food source in otherwise nutrient poor and extreme habitats like volcanic seamounts and their hydrothermal systems.
V51C-1505
Iron Oxidizing and Reducing Bacteria as Contributors to Basaltic Glass Colonization and Subsequent Weathering in Active Hydrothermal Vent Systems on Loihi and Vailulu'u Seamounts
The extreme oligotrophic nature of the oceanic crust was once believed to be an inhospitable environment to support microbial life. However, numerous studies in the past two decades have revealed diverse chemolithotrophic microbial communities inhabiting the deep biosphere within the oceanic crust. Vailulu'u Seamount in American Samoa and Loihi Seamount in Hawai'i provide access to the deep biosphere environments through the study of the interaction of hydrothermal vent water, basaltic substrates and microbial communities. Both seamounts have been found to exhibit similar iron-encrusted microbial mats surrounding both high and low temperature hydrothermal vent orifices. We are targeting iron as the main electron donor/acceptor in these environments due to the relative abundance and availability in basalts. Through the use of the HURL Pisces submersibles, we exposed amended basaltic glasses of several different compositions to a host of different environments on both seamounts in order to study the colonization and biofilm characteristics of the microbial communities. A large culturing effort reveals multiple iron oxidizing and reducing bacteria as members of the microbial community responsible for the colonization and subsequent dissolution and alteration of basaltic glass. We employ an annular reactor to expose the same suite of chemically altered basaltic glasses to a sample of iron microbial mats taken from Vailulu'u to provide a laboratory complement the environmental exposure experiments. Here cell counts reveal a 90% enhanced colonization and growth on the basalt glass versus the surrounding epoxy and borosilicate glass. The ability of microbes to leach nutrients (such as iron) out of the host substrate has far reaching astrobiological implications for nutrient sources available to sustain life in a Mars or Europa biosphere.
V51C-1506
Initial Investigations into the Microbial Diversity at Brothers Seamount, Kermadec-Tonga Arc, New Zealand
Brothers Seamount, a submarine volcano located along the Kermadec-Tonga Arc in New Zealand, results from intraoceanic plate subduction and as such, exhibits substantially different chemistries to those found on mid-ocean spreading ridges. Furthermore, Brothers Seamount has two major venting sites that exhibit disparate geochemistries. Combined, these factors suggest that microbial diversity studies of Brothers Volcano have potential to identify an array of novel microbial species. Two manned submarine expeditions, the first in November 2004 with the JAMSTEC R/V Yokosuka and Shinkai 6500 and the second in April/May 2005 with the HURL R/V Kiamikai-o-Kanaloa and Picses V dove on the Brothers Seamount a total of 9 times collecting vent, sediment and fluid samples used, in part, for microbial enrichments and molecular diversity studies. Clone libraries of the 16S rRNA gene were generated using DNA extracts from multiple samples at the two venting sites on Brothers Volcano. So far, more than 60 unique bacterial sequence types have been found out of an overall clone count of greater than 200 including, â, a, ä and ?-Proteobacteria, and several Planctomycetes. The ?-Proteobacteria account for most of the diversity. Archaeal 16S rRNA clone banks, including amplifications using primers specific for the phylum Nanoarchaeota, are currently being constructed, with preliminary results showing a diverse range of archaeal species.
V51C-1507
Preliminary Investigations Into the Geomicrobiology of Iron-Rich Seafloor Sediments Obtained From the Tonga Ridge
Two samples (designated P5-636 and P5-637) of seafloor sediment were collected from a depth of 900m at Volcano 19 in the Tonga Arc, an area with diffuse active venting at temperatures ranging from 30 to 112 degrees Celsius. Another sample, P640, was collected from a depth of 399m at Volcano 18, an area with no active venting and a maximum sediment temperature of 13.8 degrees Celsius. Macroscopic observations at Volcano 19 indicated white filamentous microbial mats growing in close association with putative iron oxide and silica-rich sediments. Microbial-sediment assemblages formed vertical chimneys up to 2 metres tall which were easily disrupted and collapsed when disturbed. Scanning electron microscopy of sediments P5-636 and P5-637 revealed abundant microbial filaments, 1 to 2 micrometres wide and up to tens of micrometres long which were often, though not always, encrusted by mineral material. Bacilli and coccoid forms were also observed, but at a much lower frequency. Energy-dispersive X-ray spectroscopy (EDS) of non-encrusted cell surfaces revealed light elements commonly associated with biological material and/or saline evaporites (e.g., Na, K, Mg, Ca). EDS of the encrusting mineral phase resulted in large increases in signals for iron, silicon and oxygen. Morphologically, P5-636 and P5-637 were a mixture of amorphous spherical aggregates and abundant, well-defined, acicular and platy crystals measuring several micrometres in length. X-ray diffraction (XRD) analysis yielded several peaks corresponding to goethite and lepidocrocite, with perhaps some magnetite. This suggests that P5-636 and P5-637 are mixtures of goethite and/or lepidocrocite with a more amorphous phase (e.g., 2-line ferrihydrite). EDS analysis of P640 resulted in large signals for iron, oxygen and silicon. However, the mineral morphology of P640 sediment was largely that of amorphous spherical aggregates. The well-developed goethite/lepidocrocite crystals seen in P5-636 and P5-637 were markedly absent. XRD analyses of P640 sediments yielded only two broad reflections at 2.53 and 1.48 angstroms, suggesting that sediment P640 is composed primarily of 2-line ferrihydrite. Sediment P640 also showed a marked absence of the filamentous organisms seen in the other two samples. The few putative filaments which were observed were heavily encrusted and likely non-viable. Temperature differences between the two fields are an obvious factor in the differences in sediment mineralogy described here, since the higher temperatures recorded for P5-636 and P5-637 would tend to favour a more crystalline iron oxide. It remains to be determined, however, whether the same temperature difference might also account for the difference in microbial abundance seen between the two sediment fields. An intriguing alternative is that the microbes themselves may be exerting an additional (metabolic) control over the nature of the Tonga Arc sediments. Further investigation into the geomicrobiology of this site is clearly warranted. The authors gratefully acknowledge the work of the crew of Support Vessel R/V Ka'imikai-o-Kanaloa in collecting these samples.
V51C-1508
Spatial and Temporal Variability in Microbial Communities from Pre- and Post-Eruption Microbial Mats Collected from Loihi Seamount, Hawaii
Loihi Seamount is an active submarine volcano that marks the southernmost extent of the Hawaiian hotspot. Loihi rises over 3000 meters from the seafloor and summits nearly 1000 meters below sea level. Hydrothermal activity was discovered at Loihi in 1987, yielding diffuse vent effluent (Tmax $37°C) with associated high CO2 and Fe(II) concentrations and luxuriant microbial mats near the summit of the volcano. The Seamount erupted in 1996 forming a new 300 meter deep caldera (Pele's Pit) with hydrothermal venting up to $200°C. Pele's Pit now contains multiple hydrothermal vents with hydrothermal fluids ranging from $8-65°C with concentrations of Fe(II) between 50 and 750 μM. Community fingerprints from over 50 microbial mat samples collected from Loihi Seamount from 1993 to 2004, with temperatures ranging from ambient ($4°C) up to ~$200°C, were analyzed using cluster analysis of terminal restriction fragment length polymorphisms (T-RFLP) coupled with traditional clone library and sequence analysis. These mat samples form two distinct community clusters (Loihi Cluster I and Loihi Cluster II) representing a combined 82% of all samples collected. Loihi Cluster I is the largest group and contains the most mat samples collected over time. Loihi Cluster I is dominated by phylotypes related to the obligate lithotrophic Fe-oxidizing isolate ''{\it Mariprofundus ferroxydans}'' and contains lesser amounts of {\it α-Proteobacteria}, {\it δ-Proteobacteria} and {\it Flavobacteria}. Loihi Cluster II is comprised of only post-eruption communities that generally contain more diversity (in terms of richness) than Loihi Cluster I commuities. Loihi Cluster II communities are dominated by a unique clade of phylotypes belonging to the {\it Nitrospira} division and by {\it ε-Proteobacteria}. Loihi Cluster II also contains phylotypes associated with {\it Thiomicrospira spp.} and from within the ''{\it M. ferroxydans}'' clade. The presence of {\it ε-Proteobacteria} and {\it Thiomicrospira spp.} in this group suggest these microbial mats are potentially more involved with sulfur-cycling than Loihi Cluster I, which are dominated exclusively by putative iron-cycling bacteria and do not contain any known sulfur-oxidizing bacteria.
V51C-1509
Extreme Spatial Variability in Microbial Mat Communities from Submarine Hydrothermal Vents Located at Multiple Volcanoes along the Mariana Island Arc
Volcanic arc systems are the most active tectonic feature in the world, but are among the least studied. The Western Pacific contains ~20,000 km of volcanic arcs, of which only ~2% have been systematically surveyed. The lack of comprehensive knowledge of volcanic arcs is compounded by the incredible variability found in relatively short distances. The complex source history of hydrothermal fluids and the variable depths of seamounts found in island arc systems result in highly variable vent chemistries and therefore unique microbial habitats within relatively short distances. The Mariana Island Arc was surveyed in 2003 and areas with suspected hydrothermal activities were identified for targeted remote operating vehicle (ROV) exploration and sampling in 2004. Sixteen microbial mat samples from five seamounts ranging from 145-1742 mbsl and from ambient to $222°C were collected and analyzed with quantitative PCR (Q-PCR), cluster analysis of terminal restriction length polymorphism (T-RFLP) community fingerprints, and by clone library analysis of small subunit ribosomal rDNA genes. The microbial mat communities from the Mariana Island Arc exhibit greater spatial variability within their community structure than microbial mats sampled from mid-ocean ridge or hotspot hydrothermal vents from a comparable scale. Microbial communities from the summit of NW Eifuku Volcano are dominated by putative iron-oxidizing phylotypes at the Yellow Top and Yellow Cone Vent sites, but are dominated by sulfur-oxidizing {\it ε-Proteobacteria} at the Champagne Vent site. Mats collected at the Mat City Vent site on E Diamante Seamount contained nearly three times as much biomass as any other mat sample collected, and is dominated by a {\it Planctomyces} phylotype. Hydrothermal sediments at the Fish Spa site located on Daikoku Seamount contained the second highest biomass detected and supported a large community of flatfish indicating a direct route for biomass being channeled up the food chain. The microbial community at Fish Spa consists of a highly diverse assemblage of {\it Bacteroidetes}, {\it α-Proteobacteria} and {\it Firmicutes}. While in contrast, the microbial mat at the Iceberg Vent site on NW Rota I is dominated by a single phylotype of {\it ε-Proteobacteria}.
V51C-1510
Mariana Forearc Serpentine Mud Volcanoes Harbor Novel Communities of Extremophilic Archaea
Since the Eocene (45 Ma) the Pacific Plate has been subducting beneath the Philippine Plate in the western Pacific ocean. This process has given rise to the Mariana Islands. As a direct result of this non-accretionary subduction, the Mariana Island Arc contains a broad forearc zone of serpentinite mud volcanoes located between the island chain and the trench. Forearc faulting, due to high pressure and low temperature build-Up, produce slurries of mud and rock that mix with slab derived fluids and rise in conduits. Due to dehydration of the overlying mantle, native rock is converted to serpentinite, which squeezes out at fractures along the sea floor. This results in giant mud volcanoes (~30 km diameter and ~2 km high) that form a chain between 50 and 150 km behind the trench axis. Microbial samples were collected using Jason II from seven mud volcanoes along the length of the forearc and community fingerprinting was applied to genomic DNA using terminal restriction length polymorphism (T-RFLP). The resulting data were compared with traditional clone library and sequence analysis from samples obtained from the southernmost mud volcano, South Chamorro, site 1200, holes D and E, sampled during ODP Leg 195. The dominant archaeal phylotypes found clustered into two groups within the {\it Methanobacteria}, a class of anaerobic methanogens and methylotrophs. These phylotypes were detected at three of the seven mud volcanoes sampled and comprised 61% of the archaeal clone library from 1200 E. The first group was most closely related to the order {\it Methanobacteriales}, however, these novel phylotypes had similarity values of up to 0.90 at best with some resulting at 0.48. The second novel group of phylotypes were most closely related to order {\it Methanosarcinales}, with similarity values in the range of 0.50 to 0.22, indicating a relatively weak association with known phylotypes. At 1200 D, phylotypes associated with non-thermophilic Marine Group I {\it Crenarchaeota} were detected exclusively. These were also detected among all seven mud volcanoes and were found to comprise 39% of the library at 1200 E, potentially indicating near surface seawater mixing within some of the mud volcano sites. Phospholipid fatty acid (PLFA) content analyzed from 1200 E samples at 1.4 mbsf and 13.4 mbsf yield an increase of archaeal to bacterial biomass by 1000 and 500 times, respectively. These data, in conjunction with clone sequencing and fingerprinting information, points towards a novel archaeal subsurface community occurring within at least three of the seven Mariana forearc sites sampled. Additional sampling will be necessary to resolve the spatial variability detected.
V51C-1511
Large Lava Pond Complex on the Juan de Fuca Ridge: an Effusive, Energetic Eruption that Drained Away
We explored an unusually large, deep, drained lava lake complex on the south rift of Axial Seamount on the Juan de Fuca Ridge during three dives with the ROV Tiburon in August 2005. The complex of five large ponds, first identified from EM300 multibeam bathymetry, is 5 km long and more than 1 km wide. The ponds are separated from one another by narrow levees that rise about 90 meters above the pond floors. The levees are all about the same depth, which suggests that the ponds formed at the same time. The volume of the lake, prior to draining, was 0.2-0.4 km3, making it the largest lava lake known along the ridge system. The outer slopes of the pond levees are constructed of elongate pillows that flowed down the steep slopes. The rims are narrow, level plateaus of lobate flows with many collapses. The inner walls are vertical cliffs, overhanging in places, with horizontal shelves from the top of the levees down to the floors of the ponds. Left like bathtub rings, these shelves mark former surfaces of the lava pond as it drained away while the lava was still molten. In many places, this veneer has collapsed to reveal truncated lobate flows and pillows. The floor of one small pond was entirely talus blocks. However, the floors of the other, larger ponds had little talus and, instead, were vast expanses of thin broken crusts, lobate flows, and very fluid, chaotic, folded and jumbled sheet flows. The lavas from each pond have abundant large feldspar and rarer olivine crystals, suggesting that all were from the same eruption. This eruption apparently began with sheet flows whose advance was limited by topography. It then ponded and built up the levees that were left when the lava drained away. On the floor of one pond we found a deposit several meters tall that was delicate and difficult to sample, and turned out to be agglutinated spatter. Limu o Pele (lava bubble wall fragments) was abundant in all the sediment samples in and around the ponds. The spatter and limu demonstrate that the eruptions were magmatic-gas-rich and mildly explosive to the end, with strombolian-like bursts and even fire fountains, though such activity had been presumed to be impossible at 2300m depth. We did not find obvious signs where the lavas went that drained from the ponds. A delta-like fan of partially drained and collapsed lobate flows extended from breaches in two adjacent levees, but since the pond floors are considerably deeper than the breaches and delta surface, the lake must have drained elsewhere after it breached the levees. The ridge axis outside the pond complex is severely tectonized, with numerous faults, gaping fissures, and shattered lavas of similar, unusually feldspar-rich composition, so there is no evidence that the ponds drained down-rift. We propose that the drained lava was recycled back into a crustal magma chamber below the ponds.
V51C-1512
Geochemistry of Basalt Lava and Hyaloclastite From Young (President Jackson) and old (Taney) Near-ridge Seamount Chains
Short linear chains of seamounts are common near Pacific spreading centers. They are most abundant near fast spreading centers but also occur at moderate and slow spreading ridge segments. We explored two such near-ridge chains of vastly different ages. The young President Jackson chain, located at $42°20' N west of the northern Gorda Ridge, resides on ocean crust that is 2.2 to 4.2 million years old. An Ar-Ar age of ~26 Ma (G.B. Dalrymple as cited by Davis et al., 1998) for the Taney seamounts indicates they formed during the Miocene on the later subducted Farallon-East Pacific Rise spreading center. Three dives with MBARI's ROV Tiburon explored the eastern end of the President Jackson seamounts. Two dives explored two of the eastern seamounts of the Taney chain. Volcanoes in both chains are similar steep-sided, flat-topped structures with multiple, nested, overlapping calderas. The Taney seamounts, however, are considerably larger with volumes roughly three times that of the typical seamount in the President Jackson chain. Samples of glassy rimmed pillow lava and hyaloclastite were recovered from both chains, although those from the Taney chain are covered with thick manganese oxide crusts. The hyaloclastite samples from both sites consist of dense angular glass shards compositionally identical to pillow rims, suggesting they formed by quench granulation. One of the President Jackson Seamount samples is loosely consolidated ash, containing fluidal and bubble wall fragments, indicating some mildly explosive eruptions occurred. Glass data for the eastern President Jackson seamounts are all low-K2O N-MORB (0.04-0.13% K2O). Two distinct groups can be identified. One more depleted composition with exceedingly low K2O and MgO >8% and the second, overlapping group includes somewhat more evolved (7.3 -8.3% MgO) and slightly more enriched (>0.1-0.15% K2O) compositions. Both compositions erupted on a single volcano. Glass compositions of dive samples from the Taney Seamounts are also all N-MORB although whole rock samples dredged from the westernmost seamount include E-MORB and an alkalic basalt. The N-MORB glasses from the Taney's also include high MgO (>8%) and low K2O (0.05%) compositions, but many are more evolved (MgO 6.4-7.6%) with slightly higher K2O (0.1-0.15%). They are higher in Na2O at comparable MgO than the President Jackson glasses, possibly suggesting melting at greater depth. Like the President Jackson seamounts, both highly depleted and more enriched compositions exist on the same volcano. Similar to other near-ridge seamount chains, the President Jackson glass compositions are generally high-MgO, relative to basalts from the adjacent Gorda Ridge (Davis and Clague, 1987, 1990). Because of the primitive compositions found on most near ridge seamounts, the prevailing view has been that magma bypasses magma chambers under the spreading center and erupts without storage in shallow crustal reservoirs. The large calderas require magma storage at shallow depths within the volcanoes. As suggested by Clague et al. (2002), these collapse features probably formed by a series of voluminous eruptions of magma that rapidly passed through the crustal reservoirs. The >8% MgO lavas may represent those erupting rapidly whereas the slightly lower MgO lava resided briefly in the crustal reservoir where they cooled and fractionationated to some extent.