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doi:10.2204/iodp.sp.331.2010

Background

Active subseafloor hydrothermal systems at mid-ocean ridges (MOR), volcanic arcs (VA), backarc basins (BAB), and hotspots are environments with extraordinarily high fluxes of energy and matter. The "subvent biosphere" is the subseafloor biosphere that is predicted to exist just beneath active hydrothermal vents and fluid discharge zones. Subseafloor environments within active hydrothermal systems are the most promising locations for functionally active, metabolically diverse subseafloor microbial ecosystems. The existence of a subvent biosphere has been inferred from many microbiological and geochemical investigations of vent chimney structures and diffuse hydrothermal fluids (generally, those diluted with seawater in the shallow subseafloor). High-temperature hydrothermal fluids with focused discharge and little or no dilution by seawater also provide evidence of indigenous subvent microbes active along flow paths of hydrothermal upwelling, in which abundant H₂, CO₂, CH₄, H₂S, and CO provided as magmatic volatiles and by hydrothermal water-rock reactions are metabolized. Indeed, the possible occurrence of a subvent biosphere has been clearly demonstrated by microbiological and geochemical characterizations of high-temperature hydrothermal fluids in several hydrothermal fields such as the Kairei field on the Central Indian Ridge (CIR) and the Iheya North field in the mid-Okinawa Trough (MOT). In the Kairei field, Takai et al. (2004) proposed the occurrence of a hydrogen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME) in which hydrogenotrophic hyperthermophilic methanogens within the Methanococcales genus dominate. Subseafloor microbial communities are energized by the extraordinarily high concentration of H₂ in the hydrothermal fluids (as high as 8.5 mM), probably derived by serpentinization in the subseafloor. In the Iheya North field, it has been suggested that a variety of microbial communities sustained by different chemolithoautotrophic primary producers are present in subseafloor habitats (Nakagawa et al., 2005). Variability in potential subseafloor microbial communities is likely associated with physical and chemical variation of hydrothermal fluids, controlled by phase separation and phase partition of hydrothermal fluid beneath the seafloor. In addition, hydrothermal environments hosted by organic-rich sediments provide unusual amounts of C₁ compounds (CO₂ and CH₄) in hydrothermal fluids and unique microbial habitats affected by liquid CO₂ and gas hydrates, which have been reported in another deep-sea hydrothermal system (Yonaguni Knoll IV field) in the Okinawa Trough (Inagaki et al., 2004; Nunoura and Takai, 2009; Nunoura et al., 2010). Thus, the abundant supply of energy and carbon sources and the richness of the habitats supported by physical and chemical variations in the MOT Iheya North field provide an ideal setting for the formation of functionally and metabolically diverse subseafloor microbial communities associated with hydrothermal activity.

Geological setting

The Okinawa Trough is a backarc basin extending for ~1200 km, located between the Ryukyu Arc-Trench system and the Asian continent (Lee et al., 1980; Letouzey and Kimura, 1986) (Fig. F1). It is presently undergoing rifting, which began at ~2 Ma and was preceded by an earlier rifting episode during the Miocene. Lee et al. (1980) have proposed that the Okinawa Trough is presently in a "drifting phase" (i.e., oceanic crust spreading characterized by short spreading centers and concomitant transform faults), as typically occurs along mid-ocean ridges and in other actively spreading backarc basins such as the Lau Basin and Mariana Trough. Seismic reflection data also suggest a typical geologic structure for the Okinawa Trough (Letouzey and Kimura, 1986): a high-velocity mantle at ~6000 mbsf overlain by potentially young basalt having an average velocity of 5.8 km/s between ~3000 and 6000 mbsf, an igneous rock layer (4.9 km/s) between ~1000 and 3000 mbsf, and ~1000 m of sediments at the seafloor. Geochemical features of hydrothermal fluids in the Okinawa Trough hydrothermal systems clearly demonstrate a significant contribution from felsic rocks and magma, as described below (Glasby and Notsu, 2003). Thus, it seems likely that the Okinawa Trough is actively rifting a transitional region between continental and oceanic crust.

The drilling proposed here would thus provide the first opportunity to drill into an active hydrothermal system and associated deposits within a backarc basin in continental margin setting. This tectonic difference is reflected in the chemical composition of the deposits. Sulfide samples collected from the Iheya North field show a Zn- and Pb-rich signature that is distinctive from the Fe-rich signature of mid-oceanic ridge sulfides (Halbach et al., 1993) (Fig. F1). This chemical signature is similar to that of Kuroko-type hydrothermal deposits that formed during the Tertiary in northeast Japan, related to opening of the Japan Sea. It is well known, moreover, that many volcanic massive sulfide ore deposits formed throughout geologic time are related to felsic and/or intermediate magmatism, rather than to basaltic volcanism at typical mid-ocean ridges. This suggests that a combination of a sediment-rich setting and arc-backarc magmatism favors formation of hydrothermal ore deposits.

Seismic studies/site survey data

Since the first discovery of submarine hydrothermal activity in the Iheya Knoll and the Izena Hole of the MOT (Momma et al., 1996) (Fig. F2), six highly active, representative hydrothermal fields (Minani-Ensei Knoll, Iheya North, Iheya Ridge, Izena Hole, Hatoma Knoll, and Yonaguni Knoll IV) have been discovered so far (Glasby and Notsu, 2003). The Iheya North field has been the subject of the most intensive interdisciplinary investigations, specifically geochemistry of hydrothermal fluids and sulfide/sulfate deposits and microbial ecology at the seafloor, and has also been monitored for >10 y. The outstanding features of the Iheya North field, some of which are also commonly observed in the other hydrothermal systems of the Okinawa Trough, are as follows:

• It is hosted in thick terrigenous sediments from the Yangtze and Yellow rivers.

• It has extremely high concentrations of gaseous C₁ compounds (CO₂ and CH₄) and ammonia in hydrothermal fluids.

• It contains mostly biogenic CH₄, as evidenced by a highly ¹³C-depleted carbon isotopic composition and high C₁/(C₂ + C₃) ratio, and CO₂ of mixed (magmatic input and organics-derived) source, with a moderately ¹³C-depleted value.

• It contains novel liquid CO₂ and CO₂ hydrates in sediments near hydrothermal fields.

• It exhibits phase separation of hydrothermal fluids.

It is believed that these features greatly impact the structure, activity, and functions of the subseafloor microbial ecosystem. Hydrothermal activity in the Iheya North Knoll (27°47′50N, 126°53′80 E; 150 km north-northwest of the island of Okinawa) was first discovered by a camera survey in 1995. Since then, >40 deep submergence vehicle (DSV) and remotely operated vehicle (ROV) dives have revealed details regarding the location of hydrothermal activity and seafloor events (Fig. F2). Prior to this proposal, however, there had been few geologic and geophysical surveys of the Iheya North Knoll, and recent surveys have provided important insights into the geological setting and the associated heat flow anomaly, suggesting a hydrothermal discharge-recharge model for the Iheya North hydrothermal system (Table T2).

A grid of multichannel seismic (MCS) profiles is available for the Iheya Knoll region (Fig. F3). An MCS profile across the Iheya North Knoll (Fig. F4) (B–B′ lines in Fig. F1A) clearly demonstrates its subseafloor stratigraphic structure and that of the surrounding terrain in the mid-Okinawa Trough. The igneous intrusion penetrates trough sediments that are >1000 m thick (Fig. F4). In the middle of the Iheya North Knoll, a central valley exhibits relatively strong subbottom seismic reflectors (as deep as ~350 mbsf), suggesting the existence of sediments or some materials other than massive igneous rock (Fig. F4). An interval velocity profile from the detailed velocity analysis of the same MCS section reveals more detailed subbottom structures (Fig. F5). Several low-velocity zones are identified within Unit C of the trough sediments (150–200 mbsf) and even within the central valley (150–200 mbsf) (Fig. F5). These low-velocity zones may indicate the existence of fluids in permeable zones, representing potential hydrothermal fluid pathways. In particular, the low-velocity zone in the central valley might be a good candidate for a hydrothermal fluid reservoir in the Iheya North field.

Based on seafloor DSV and ROV observations, the Iheya North hydrothermal field and the surrounding central valley are covered with sediments and are characterized by several faults with a north–south trend, with major hydrothermal vent chimneys (mounds) standing along one of the north–south faults. A gridded single-channel seismic reflection survey of the Iheya North Knoll with shorter intervals of generator-injector gun shots does not clearly show faults but indicates instead an unstratified subbottom structure, sometimes extending to just beneath the seafloor, in the central valley of the Iheya North Knoll. This unstratified subbottom structure likely consists of strongly permeable materials but not layered sediments, as strongly suggested by recent coring expeditions in the central valley, JAMSTEC KY05-14 and KY08-01. In all the cores, thick, pumicious flow deposits with coarse to fine grain sizes were found just below the seafloor (Fig. F6). These pumice layers often contained abundant gas-filled voids and were polluted by elemental sulfur and sulfide deposits (Fig. F7), suggesting the input of gas-rich hydrothermal fluids (Kinoshita et al., 2006). Geochemical analyses of pore water samples also supported the input of typical gas phase–enriched hydrothermal fluid in Iheya North field. These site survey data consistently suggest that multiple and thick volcanic pyroclastic deposits fill the central valley of the Iheya North Knoll, providing both potential hydrothermal fluid recharge paths and reservoir and discharge paths.

Geochemical studies of pore fluid extracted from the pumiceous sediment revealed unusual chemical profiles just below the Calyptogena clam colony, including low Cl and high CH₄ concentrations. The Na/Cl ratio and δ¹³C(CH₄) value are both close to those of the high-temperature venting fluid; these observations can be explained by intrusion of hydrothermal fluid into the surface pumice layer. Together with the occurrence of elemental sulfur in the sediment, these results strongly suggest lateral migration of vapor-rich hydrothermal fluid into the layer of pumiceous sediment. The isotopic compositions of carbon and sulfur can be attributed to microbial sulfate reduction within the sediment, which produces the H₂S utilized by Calyptogena. Microbial population analysis confirmed delta-proteobacteria (including sulfate-reducing bacteria) and ANME 2c archaea (including methane-oxidizing archaea) as predominant groups.

In addition to seafloor surveys of the subvent biosphere, several ODP and IODP drilling explorations have been conducted. For example, microbiologists and biogeochemists participated in drilling expeditions to the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge (ODP Leg 158) (Reysenbach et al., 1998), the hydrothermal fields of Middle Valley on the Juan de Fuca Ridge (ODP Leg 169) (Cragg et al., 2000), the PACMANUS field in the Manus Basin (ODP Leg 193) (Kimura et al., 2003), and on the flank of the Juan de Fuca Ridge (IODP Expedition 301) (Expedition 301 Scientists, 2005). These studies have found that, while there is evidence for microbiological processes occurring in hydrothermal sediments, it is very difficult to sample indigenous microbial communities and document their activity and biological signatures from rocky materials without contamination. Heretofore, potentially indigenous subvent microbial communities have been detected only at ODP Sites 1188 and 1189 in the Manus Basin. Unfortunately, their physiological characteristics as well as their in situ populations remain poorly documented.

The supporting site survey data for Expedition 331 are archived at the IODP Site Survey Data Bank.

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