Ko-ichi Nakamura

Hydrothermal vent fields and chemosynthetic biota on the world's deepest seafloor spreading centre.

The Mid-Cayman spreading centre is an ultraslow-spreading ridge in the Caribbean Sea. Its extreme depth and geographic isolation from other mid-ocean ridges offer insights into the effects of pressure on hydrothermal venting, and the biogeography of vent fauna. Here we report the discovery of two hydrothermal vent fields on the Mid-Cayman spreading centre. The Von Damm Vent Field is located on the upper slopes of an oceanic core complex at a depth of 2,300 m. High-temperature venting in this off-axis setting suggests that the global incidence of vent fields may be underestimated. At a depth of 4,960 m on the Mid-Cayman spreading centre axis, the Beebe Vent Field emits copper-enriched fluids and a buoyant plume that rises 1,100 m, consistent with >400 °C venting from the worlds deepest known hydrothermal system. At both sites, a new morphospecies of alvinocaridid shrimp dominates faunal assemblages, which exhibit similarities to those of Mid-Atlantic vents.

Submarine venting of liquid carbon dioxide on a Mariana Arc volcano

Although CO2 is generally the most abundant dissolved gas found in submarine hydrothermal fluids, it is rarely found in the form of CO2 liquid. Here we report the discovery of an unusual CO2-rich hydrothermal system at 1600-m depth near the summit of NW Eifuku, a small submarine volcano in the northern Mariana Arc. The site, named Champagne, was found to be discharging two distinct fluids from the same vent field: a 103°C gas-rich hydrothermal fluid and cold (<4°C) droplets composed mainly of liquid CO2. The hot vent fluid contained up to 2.7 moles/kg CO2, the highest ever reported for submarine hydrothermal fluids. The liquid droplets were composed of ∼98% CO2, ∼1% H2S, with only trace amounts of CH4 and H2. Surveys of the overlying water column plumes indicated that the vent fluid and buoyant CO2 droplets ascended <200 m before dispersing into the ocean. Submarine venting of liquid CO2 has been previously observed at only one other locality, in the Okinawa Trough back-arc basin (Sakai et al., 1990a), a geologic setting much different from NW Eifuku, which is a young arc volcano. The discovery of such a high CO2 flux at the Champagne site, estimated to be about 0.1% of the global MOR carbon flux, suggests that submarine arc volcanoes may play a larger role in oceanic carbon cycling than previously realized. The Champagne field may also prove to be a valuable natural laboratory for studying the effects of high CO2 concentrations on marine ecosystems.

Microbial community in a sediment-hosted CO2 lake of the southern Okinawa Trough hydrothermal system.

Increasing levels of CO2 in the atmosphere are expected to cause climatic change with negative effects on the earths ecosystems and human society. Consequently, a variety of CO2 disposal options are discussed, including injection into the deep ocean. Because the dissolution of CO2 in seawater will decrease ambient pH considerably, negative consequences for deep-water ecosystems have been predicted. Hence, ecosystems associated with natural CO2 reservoirs in the deep sea, and the dynamics of gaseous, liquid, and solid CO2 in such environments, are of great interest to science and society. We report here a biogeochemical and microbiological characterization of a microbial community inhabiting deep-sea sediments overlying a natural CO2 lake at the Yonaguni Knoll IV hydrothermal field, southern Okinawa Trough. We found high abundances (>109 cm−3) of microbial cells in sediment pavements above the CO2 lake, decreasing to strikingly low cell numbers (107 cm−3) at the liquid CO2/CO2-hydrate interface. The key groups in these sediments were as follows: (i) the anaerobic methanotrophic archaea ANME-2c and the Eel-2 group of Deltaproteobacteria and (ii) sulfur-metabolizing chemolithotrophs within the Gamma- and Epsilonproteobacteria. The detection of functional genes related to one-carbon assimilation and the presence of highly 13C-depleted archaeal and bacterial lipid biomarkers suggest that microorganisms assimilating CO2 and/or CH4 dominate the liquid CO2 and CO2-hydrate-bearing sediments. Clearly, the Yonaguni Knoll is an exceptional natural laboratory for the study of consequences of CO2 disposal as well as of natural CO2 reservoirs as potential microbial habitats on early Earth and other celestial bodies.

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Roughly 60% of the Earth’s outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85° E, to acquire photographic and video images of ‘zero-age’ volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km2) area. At least 13.5 wt% CO2 is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

Archaeal Diversity and Distribution along Thermal and Geochemical Gradients in Hydrothermal Sediments at the Yonaguni Knoll IV Hydrothermal Field in the Southern Okinawa Trough

ABSTRACT A variety of archaeal lineages have been identified using culture-independent molecular phylogenetic surveys of microbial habitats occurring in deep-sea hydrothermal environments such as chimney structures, sediments, vent emissions, and chemosynthetic macrofauna. With the exception of a few taxa, most of these archaea have not yet been cultivated, and their physiological and metabolic traits remain unclear. In this study, phylogenetic diversity and distribution profiles of the archaeal genes encoding small subunit (SSU) rRNA, methyl coenzyme A (CoA) reductase subunit A, and the ammonia monooxygenase large subunit were characterized in hydrothermally influenced sediments at the Yonaguni Knoll IV hydrothermal field in the Southern Okinawa Trough. Sediment cores were collected at distances of 0.5, 2, or 5 m from a vent emission (90°C). A moderate temperature gradient extends both horizontally and vertically (5 to 69°C), indicating the existence of moderate mixing between the hydrothermal fluid and the ambient sediment pore water. The mixing of reductive hot hydrothermal fluid and cold ambient sediment pore water establishes a wide spectrum of physical and chemical conditions in the microbial habitats that were investigated. Under these different physico-chemical conditions, variability in archaeal phylotype composition was observed. The relationship between the physical and chemical parameters and the archaeal phylotype composition provides important insight into the ecophysiological requirements of uncultivated archaeal lineages in deep-sea hydrothermal vent environments, giving clues for approximating culture conditions to be used in future culturing efforts.

Explorations of Mariana Arc volcanoes reveal new hydrothermal systems

Some 20,000 km of volcanic arcs, roughly one-third the length of the global mid-ocean ridge (MOR) system, rim the western Pacific Ocean. Compared to 25 years of hydrothermal investigations along MORs, exploration of similar activity on the estimated ∼600 submarine arc volcanoes is only beginning [Ishibashi and Urabe, 1995; De Ronde et al, 2003] . To help alleviate this under-sampling, the R/V T G Thompson in early 2003 (9 February to 5 March) conducted the first complete survey of hydrothermal activity along 1200 km of the Mariana intra-oceanic volcanic arc. This region includes both the Territory of Guam and the Commonwealth of the Northern Mariana Islands.

Surface expression of fluid venting at the toe of the Nankai wedge and implications for flow paths

Abstract Cold seeps have been observed with manned submersibles at many sites on the Tokai margin. Most sites are found on the hanging wall of major thrust faults, within 1–2 km of their outcrop. Few sites relate to purely erosional features. This relationship between thrusting and venting may be explained either by fluid flow along fault zones, by fluid flow along permeable layers outcropping at the fault scarp, or by diffuse upward flow of fluids released during footwall compaction. We use deep-towed side-scan sonar and sediment sounder profiles acquired at the toe of the Nankai accretionary wedge to identify the context of seepage at a smaller scale. Modes of fluid expulsion inferred from observations in this zone include flow channelling along strata, along thrusts, along gravity faults, and along joint fractures as well as diffuse flow. We then combine visual observation and monitoring of electrochemical potential (Eh) to assess the relative importance of seepage sites. We show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites, cemented by authigenic carbonates. These were sampled and have permeability high enough to channel fluid flow (1–4×10−14 m2). We suggest they are the main conduits near the seafloor. However, seepage sites are also observed directly on the outcrop of major thrusts, indicating that they do play a role as fluid conduits. We observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance.

High-resolution surveys along the hot spot–affected Galápagos Spreading Center: 1. Distribution of hydrothermal activity

The spatial density of hydrothermal activity along most mid-ocean ridges is a robust linear function of spreading rate (or magmatic budget), but extreme crustal properties may alter this relationship. In 2005–2006 we tested the effect of thickened crust on hydrothermal activity using high-resolution mapping of plumes overlying the hot spot–affected Galapagos Spreading Center from 95° to 89°42′W (∼560 km of ridge crest). Plume mapping discovered only two active, high-temperature vent fields, subsequently confirmed by camera tows, though strong plume evidence indicated minor venting from at least six other locations. Total plume incidence (ph), the fraction of ridge crest overlain by significant plumes, was 0.11 ± 0.014, about half that expected for a non–hot spot mid-ocean ridge with a similar magmatic budget. Plume distributions on the Galapagos Spreading Center were uncorrelated with abrupt variations in the depth of the along-axis melt lens, so these variations are apparently not controlled by hydrothermal cooling differences. We also found no statistical difference (for a significance level of 0.05) in plume incidence between where the seismically imaged melt lens is shallow (2 ± 0.56 km, ph = 0.108 ± 0.045) and where it is deep (3.4 ± 0.7 km, ph = 0.121 ± 0.015). The Galapagos Spreading Center thus joins mid-ocean ridges near the Iceland (Reykjanes Ridge), St. Paul-Amsterdam (South East Indian Ridge), and Ascension (Mid-Atlantic Ridge) hot spots as locations of anomalously scarce high-temperature venting. This scarcity implies that convective cooling along hot spot–affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid-ocean ridges.

Deep sea underwater robotic exploration in the ice-covered Arctic ocean with AUVs

The Arctic seafloor remains one of the last unexplored areas on Earth. Exploration of this unique environment using standard remotely operated oceanographic tools has been obstructed by the dense Arctic ice cover. In the summer of 2007 the Arctic Gakkel Vents Expedition (AGAVE) was conducted with the express intention of understanding aspects of the marine biology, chemistry and geology associated with hydrothermal venting on the section of the mid-ocean ridge known as the Gakkel Ridge. Unlike previous research expeditions to the Arctic the focus was on high resolution imaging and sampling of the deep seafloor. To accomplish our goals we designed two new Autonomous Underwater Vehicles (AUVs) named Jaguar and Puma, which performed a total of nine dives at depths of up to 4062m. These AUVs were used in combination with a towed vehicle and a conventional CTD (conductivity, temperature and depth) program to characterize the seafloor. This paper describes the design decisions and operational changes required to ensure useful service, and facilitate deployment, operation, and recovery in the unique Arctic environment.

High SO2 flux, sulfur accumulation, and gas fractionation at an erupting submarine volcano

Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite >100 mmol/kg, total dissolved sulfide 1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO 2 , CO 2 , H 2 , and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (S e ) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO 2 injected into seawater. S e accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated S e can be heated, melted, and pushed upward by rising magma to produce molten S e flows and lavas saturated with S e . Molten S e near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of S e accumulation and refluxing may underlie the relatively widespread occurrence of S e deposits of many sizes found on submarine arc volcanoes.