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Geological setting

IODP Site C0020 is located in a forearc basin formed by the subduction of the Pacific plate (~8 cm/y, west-northwest plate motion vector; Seno et al., 1996) beneath northeastern Honshu, Japan (Fig. F1). The Hidaka Trough, a sedimentary basin formed by subsidence in the drilling area, originates just offshore, southwest of Hokkaido, and extends to the Japan Trench. Along the coastal area of the Shimokita Peninsula, both sedimentary and volcanic rocks younger than Late Cretaceous lie scattered on Triassic to Early Cretaceous sedimentary rocks or Cretaceous granites.

Several scientific drilling expeditions have been carried out off Shimokita Peninsula: Deep Sea Drilling Project–International Phase of Ocean Drilling (DSDP-IPOD) Legs 56 and 57 in 1977, DSDP-IPOD Leg 87 in 1982, and ODP Leg 186 in 1999. In addition, well data are available from hydrocarbon drilling exploration carried out between 1977 and 1999 (Japan Natural Gas Association and Japan Offshore Petroleum Development Association, 1992; Osawa et al., 2002). Seismic profiles around Site C0020 show pull-up blanking reflections below bottom-simulating reflectors at ~360 mbsf, suggesting the occurrence of methane hydrates in shallow sedimentary realms and a strong upward flux of free hydrocarbon gases from deep hydrocarbon reservoirs (Fig. F2). A thick and prominent Quaternary sedimentary unit onlaps a Pliocene unit and is thought to be composed mainly of alternating beds of mud and sand with intercalations of thin volcanic tephras and locally developed gravel/sand layers. The Pliocene unit consists primarily of alternating beds of mudstone and sandstone. Below these relatively recent formations, sedimentary deposits are believed to range from Cretaceous to Miocene in age and are cut by many landward-dipping normal faults. The presence of coal formations has been confirmed by natural gas drilling exploration at Site MITI Sanriku-Oki, ~50 km southward of Site C0020 (Fig. F1) (Osawa et al., 2002). Sonic logging data in the MITI Sanriku-Oki well showed that three major tuff layers involving coal layers with 30, 45, and 80 m thickness (40%–60% total organic carbon [TOC] in lignite coal layer and 0.5%–2% TOC in tuffs) are present in Eocene and Pliocene–Upper Cretaceous horizons, in which vitrinite reflection values (Ro) range between 0.5 and 0.7, indicating burial of relatively immature coals in the ocean (Osawa et al., 2002). In situ temperatures are expected to lie well within the range of the habitable zone of microbes, based on the previously reported thermal gradient of 22.5°C/km in that area (Osawa et al., 2002).

In 2002 and 2003, 2-D seismic surveys off Shimokita Peninsula were carried out by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in a 15 km (north–south) × 30 km (east–west) area using the R/V Polar Duke and Polar Princess. During the NT04-01 cruise using R/V Natsushima in 2003, detailed bathymetry mapping was performed using a SeaBat 8160 multibeam echosounder with a frequency of 50 kHz (Taira and Curewitz, 2005) (Fig. F2). Site C0020, which is alternatively designated as Site C9001 by JAMSTEC, is located on the cross point of seismic Lines ODSR03–BS and ODSRW03–H81. During the Chikyu shakedown cruise (Expedition CK06-06) in 2006, 365 m of sediment core were recovered from the upper sedimentary section at Site C9001 (41°10.5983′N, 142°12.0328′E, 1180 m water depth), ~80 km off the coast of Shimokita Peninsula, Japan (Fig. F1) (Aoike, 2007). During the same cruise, riser drilling was tested to 647 mbsf without coring, 20 inch casing was installed to 511 mbsf, and the riser hole was suspended for a future riser drilling opportunity (i.e., Expedition 337).

Pilot studies of shallow subsurface sediment at Site C0020

The sediment cored from JAMSTEC Site C9001 during the Chikyu shakedown cruise was composed primarily of diatom-rich hemipelagic silty clay intercalated with volcanic tephra and sand layers. Preliminary biostratigraphic age models indicated very high sedimentation rates, ranging from 54 to 95 cm/k.y., and an approximate core-bottom age of 640 ka (Aoike, 2007; Aoike et al., 2010; Domitsu et al., 2010). During this shakedown cruise, core temperature anomalies were monitored immediately after recovery by Thermo-View infrared camera in order to identify and locate methane hydrates. Formation of methane hydrates, as well as metabolically active microbial aggregates that could be visualized using the FISH technique (F. Inagaki et al., unpubl. data), were observed in porous ash and sandy layers. Geochemical analyses of interstitial water consistently showed that chloride and other sea salts are depleted within the porous layers as a result of hydrate dissociation (Tomaru et al., 2009). Iodine concentrations and radioisotopic compositions (129I/I) of deep pore waters suggest that the iodine and oldest hydrocarbon sources could be as old as 40 Ma (Tomaru et al., 2009). Acetate concentrations in pore waters were >100 µmol/L throughout the sediment column (maximum = 313 µmol/L; H. Yoshioka et al., unpubl. data), which was tentatively interpreted to be possibly linked to coal diagenesis in the deeper subsurface.

Microbial cell numbers in sediment from Site C9001 were evaluated by the fluorescent image-based automated cell count system, which showed that the sediments contain abundant microbial cells with counts >107 cells/cm3 to 365 mbsf (Morono et al., 2009). The abundance of Bacteria and Archaea was studied by quantitative polymerase chain reaction and slot-blot hybridization techniques, suggesting a significant contribution of Archaea to subseafloor microbial biomass (average is ~40% at DNA level; Lipp et al., 2008). Phylogenetically diverse, reductive dehalogenase-homolog genes (rdhA) were detected, suggesting the occurrence of anaerobic microbial respiration using organohalides as electron acceptors (Futagami et al., 2009).

The metabolic activity of organoclastic sulfate reduction (sulfate reduction coupled to AOM, aceticlastic methanogenesis, and autotrophic [CO2 reducing] methanogenesis rates) investigated using 35S and 14C radiotracers, showed high AOM activity within and below the SMT zone and relatively low methanogenic activity throughout the core column (F. Inagaki et al., unpubl. data). Using a sediment sample from Site C9001, the carbon and nitrogen incorporation rate of deep subseafloor microbes was studied at the single cell level using nanoscale secondary-ion mass spectrometry (NanoSIMS) (Morono et al., 2011). A large fraction of subseafloor microbes was found to incorporate 13C- and 15N-labeled substrates into biomass, indicating that deeply buried microbial cells retain the potential to be physiologically alive.

Cultivation of aerobic and anaerobic microorganisms has been conducted, and a variety of microbes and their enzymatic activities were observed in the core sediments (Kobayashi et al., 2008). Using a continuous downflow bioreactor system, phylogenetically diverse anaerobic microbes were successfully activated, including methanogens, such as the genera Methanobacterium, Methanoccoides, and Methanosarcina, and uncultured archaeal and bacterial lineages (Imachi et al., 2011). Several attempts at traditional batch-type cultivations have led to the isolation of novel anaerobic subseafloor microbes, including Geofilum rubicundum (Miyazaki et al., 2012) and Spinavirga faexivivus (Takai et al., 2013), which are the first representatives of previously uncultivated genera. These successful cultivations and isolations confirmed the presence of metabolically and/or physiologically active microbial populations in deep subseafloor sediments off the Shimokita Peninsula.

During the first operation of riser drilling during the Chikyu shakedown cruise, the shift in microbial communities in the riser drilling mud tank and circulated mud fluid was examined through cultivation and cultivation-independent molecular ecological studies (Masui et al., 2008). Despite the high alkalinity of the mud circulation fluid (~pH 10), the predominance of Xanthomonas DNA and the potential growth of facultatively anaerobic and halophilic bacteria Halomonas suggest the potential utility of using molecular and microbiological signatures of these organisms as tracers of drilling mud contamination during Expedition 337.