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doi:10.2204/iodp.pr.331.2010

Abstract

The Iheya North hydrothermal field is located in the middle Okinawa Trough, an actively spreading backarc basin that extends for 1200 km between the Ryukyu arc-trench system and the Asian continent, in a transitional region between continental and oceanic crust. Because the Okinawa Trough contains both hemipelagic and volcanic sediment, in some places >1000 m thick, its hydrothermal systems provide abundant H₂, CO₂, CH₄, NH₄, H₂S, and CO derived from sedimentary organic matter and from magmatic gases that could feed a variety of microbial communities, sustained by different chemolithoautotrophic primary producers within a range of subseafloor habitats (Nakagawa et al., 2005). Integrated Ocean Drilling Program (IODP) Expedition 331, the Deep Hot Biosphere project, drilled into the Iheya North hydrothermal system in order to investigate metabolically diverse subseafloor microbial ecosystems and their physical and chemical settings.

We drilled five sites during Expedition 331: the active hydrothermal vent site and sulfide-sulfate mound at North Big Chimney (NBC) (Site C0016); three sites east of NBC at distances of ~100, 450, and 1550 m from the active vents (Sites C0013, C0014, and C0017, respectively); and one site on a hill ~600 m northwest of the active vents that represents a potential migration path for hydrothermal fluid (Site C0015). Our maximum penetration was 151 meters below seafloor (mbsf) at recharge Site C0017. We used heavy, triangular, gimbaled guide bases at three holes, one each at Sites C0013, C0014, and C0016, for reentry, casing, and capping, including installation of a steel mesh platform with valve controls for postcruise sampling of fluids.

At Site C0016, drilling at the summit of the active hydrothermal mound failed to recover core, and drilling at the base of the mound yielded only 2.1 m of core from 45 m of penetration, but the core included the first Kuroko-type, sphalerite-rich black ore ever recovered from the modern seafloor. The other four sites yielded interbedded hemipelagic and volcaniclastic sediment and volcanogenic breccias and pumice that are variably hydrothermally altered and mineralized, in the zeolite to greenschist facies. Temperature gradients decrease greatly with distance from the active vents at Site C0016, from >7°C/m at Site C0013, to 3°C/m at Site C0014, to 0.6°C/m at Site C0017. Detailed temperature profiles at Sites C0014 and C0017 display irregularities suggestive of lateral flow. The profile at Site C0017 is concave-upward, consistent with recharge of cold seawater into the hydrothermal system at this site.

Analyses of interstitial water and headspace gas yielded complex patterns with depth and laterally at most sites over distances of only a few meters. Documented processes include formation of brines and vapor-rich fluids by phase separation and segregation, uptake of Mg and Na by alteration minerals in exchange for Ca, leaching of K at high temperature and uptake at low temperature, anhydrite precipitation, microbial oxidation of organic matter and anaerobic oxidation of methane utilizing sulfate, microbial methanogenesis, abrupt changes in composition with depth that result from sealing by relatively impermeable cap rock, and generation of hydrogen at depth, apparently by hydrothermal rather than microbial processes.

Shipboard analyses have not confirmed presence of an active deep hot biosphere. Cell abundances are much lower than those found in previous Ocean Drilling Program/IODP sites on continental margins, and attempts at culturing were generally unsuccessful. We did find ample evidence for microbial activity supported by sedimentary organic matter, but only in sediments within the upper 10–30 mbsf where temperatures were relatively low. At the recharge Site C0014 we found a community of Fe-oxidizers that was successfully cultured.

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