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doi:10.2204/iodp.proc.331.101.2011

Background

Active subseafloor hydrothermal systems at mid-ocean ridges, volcanic arcs, backarc basins, 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 (Deming and Baross, 1993). Subseafloor environments within active hydrothermal systems are 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 (Nunoura and Takai, 2009; Nunoura et al., 2010; Takai et al., 2008, 2009; and references in Takai et al., 2006, and Huber and Holden, 2008). High-temperature hydrothermal fluids with focused discharge and little or no dilution by seawater also provide evidence for 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 (Takai et al., 2004; Huber and Holden, 2008). Indeed, the possible occurrence of a subvent biosphere has been clearly demonstrated by microbiological and geochemical characterization of high-temperature hydrothermal fluids in several hydrothermal fields (Nakagawa et al., 2005; Nunoura and Takai, 2009; Takai et al., 2004, 2008, 2009). In the Iheya North field, it has been suggested that a variety of microbial communities sustained by different chemolithoautotrophic primary producers is 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 as carbon sources, as well as unique microbial habitats affected by liquid CO₂ and gas hydrates (Nakagawa et al., 2005; Kawagucci et al., 2011). Thus, the abundant supply of energy and carbon and the richness of the habitats supported by physical and chemical variations in the Iheya North field provide an ideal setting for the formation of functionally and metabolically diverse subseafloor microbial communities associated with hydrothermal activity.

Geological setting and hydrothermal activity in the Okinawa Trough

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, the current phase of which began at ~2 Ma and was preceded by an earlier rifting episode during the Miocene. Lee et al. (1980) 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), consisting of a high-velocity mantle at ~6000 meters below seafloor (mbsf) overlain by potentially young basalt with 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 sediment immediately beneath the seafloor. Geochemical features of hydrothermal fluids in the Okinawa Trough hydrothermal systems clearly demonstrate a significant contribution from felsic rocks and magma (Glasby and Notsu, 2003), as described below. Thus, it seems likely that the Okinawa Trough is an actively rifting transitional region between continental and oceanic crust.

Integrated Ocean Drilling Program (IODP) Expedition 331 provided an opportunity to drill into an active hydrothermal system and associated deposits within a backarc basin in a continental margin setting. Characteristics of this tectonic setting are reflected in the chemical composition of hydrothermal sulfide deposits there. Sulfide samples collected from the Iheya North field are distinctly more rich in Pb than mid-oceanic-ridge sulfides (Halbach et al., 1993) (Fig. F2). The polymetallic Zn-Pb-Cu chemical signature of Iheya North sulfides is similar to that of Kuroko-type hydrothermal deposits formed during the Tertiary in northeast Japan and related to the opening of the Japan Sea. Moreover, 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 (Urabe and Marumo, 1991).

The chemistry of hydrothermal fluids collected from active sulfide chimneys in the Okinawa Trough is characterized by higher concentrations of CO₂, CH₄, NH₄, I, and K and higher alkalinity than those in typical sediment-free mid-ocean-ridge hydrothermal fluids (Sakai et al., 1990a, 1990b; Gamo et al., 1991; Konno et al., 2006; Takai and Nakamura, 2010; Kawagucci et al., 2011). The distinctive hydrothermal fluid chemistry is strongly linked with the geologic setting and the thick terrigenous sediments of the Okinawa Trough. Philippine plate subduction along the Ryukyu arc-trench system supplies dacitic-rhyolitic magma to the Okinawa Trough that is rich in K and volatile components (Sakai et al., 1990b; Gamo et al., 2006). Organic-rich terrigenous sediment filling the Okinawa Trough (Narita et al., 1990) supplies not only the sedimentary chemical inputs (NH₄, I, etc.) (Gamo et al., 1991; You et al., 1994) but also promotes the widespread occurrence of functionally active microbial communities that impact hydrothermal fluid chemistry and circulation (Nakagawa et al., 2005; Inagaki et al., 2006; Nunoura and Takai, 2009; Nunoura et al., 2010; Takai and Nakamura, 2010). In addition to the chemical aspects, the relatively shallow water depth of many Okinawa Trough hydrothermal systems serves to induce subcritical phase separation (Suzuki et al., 2008) and subsequent phase segregation, as the boiling temperature of seawater decreases steeply with decreasing pressure at ~100 bar. Phase separation and segregation sometimes produce hydrothermal fluids of quite different chemical composition at different vent sites in the same hydrothermal field, even though they are derived from the same source fluid (Kawagucci et al., 2011).

Since the discovery of submarine hydrothermal activity at Iheya Ridge and Izena Hole (Halbach et al., 1989; Sakai et al., 1990b) in the middle Okinawa Trough in 1988, six active hydrothermal fields (Minani-Ensei Knoll, Iheya North, Iheya Ridge, Izena Hole, Hatoma Knoll, and Yonaguni Knoll IV) have been discovered. The Iheya North field has been investigated by interdisciplinary methods, specifically geochemistry of hydrothermal fluids and sulfide/sulfate deposits (Glasby and Notsu, 2003; Kawagucci et al., 2011) and microbial ecology at the seafloor (Takai and Horikoshi, 1999; Takai et al., 2003; Nakagawa et al., 2005; Takai et al., 2006; Takai and Nakamura, 2010). It has also been monitored for ~15 y.

Seismic studies and site survey data

Hydrothermal activity in the Iheya North Knoll (27°47.50′N, 126°53.80′E; 150 km north-northwest of the island of Okinawa, Japan) was first discovered by a camera survey in 1995. Since then, deep submergence vehicle (DSV) and remotely operated vehicle (ROV) dives have revealed details regarding the location of hydrothermal activity and seafloor events (Fig. F3). Recent seismic and geophysical surveys (Table T1) have provided insights into the subseafloor geologic structure and the pattern of heat flow through the seafloor, suggesting a possible hydrothermal fluid flow model for the Iheya North hydrothermal system.

A grid of multichannel seismic (MCS) profiles has been completed for the Iheya North Knoll region (Fig. F1). MCS profiles across the Iheya North Knoll demonstrate an igneous intrusion penetrating the trough-filling sediments, which are >1000 m thick (Fig. F4). In the middle of Iheya North Knoll, a central valley exhibits relatively disordered seismic reflectors as deep as 400–500 mbsf, suggesting the presence of pumiceous volcaniclastic deposits beneath the surficial hemipelagic sediments rather than massive igneous rocks (Fig. F5).

Pumiceous volcaniclastic deposits have been recovered in numerous shallow (<2 m) gravity cores obtained from the central valley of Iheya North Knoll (Oiwane et al., 2008). In all cores, thick pumice layers with coarse to fine grain sizes were found just below the seafloor. These layers often contain abundant gas-filled voids accompanied by elemental sulfur and sulfide minerals, probably deposited by gas-rich hydrothermal fluids (Masaki et al., 2011). These site survey data consistently suggest that multiple and thick volcaniclastic deposits fill the central valley of Iheya North Knoll, providing potential hydrothermal fluid recharge paths, fluid reservoirs, and discharge paths (Kumagai et al., in press).

A significant discovery of the seismic reflection survey is the existence of large-scale negatively polarized reflection sequences deep beneath the hill west of the Iheya North hydrothermal vent sites. The eastern end of these westward-dipping sequences appears to converge at the seafloor where the vent sites are located (Fig. F6). These negative polarity sequences are interpreted to consist of layers of highly porous pumiceous volcanic deposits, which could serve as potential migration paths for subseafloor hydrothermal fluids. The sequences appear to be as thick as 100 m just beneath the western hill near the vent sites. The negative polarity sequences could also represent a potential fluid reservoir that feeds ascending hydrothermal solutions eastward up to the discharge sites along the westward dipping structure. To determine the possible role of these structures in the hydrothermal system, it would be desirable to penetrate and sample these negative polarity sequences, a process which was not attempted during Expedition 331.

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

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