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doi:10.2204/iodp.sp.337.2010

Background

Drilling site off Shimokita Peninsula

In 2002 and 2003, two-dimensional (2-D) seismic surveys off Shimokita Peninsula were carried out in a 15 km (north–south) × 30 km (east–west) area by the R/V Polar Duke and Polar Princess. During the NT04-01 cruise using the R/V Natsushima in 2003, the detailed bathymetry mapping was performed using SeaBat 8160 Multibeam Echosounder with a frequency of 50 kHz (Taira and Curewitz, 2005) (Fig. F4). Site C9001 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 cores 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 (Figs. F1, F4) (Aoike, 2007). Riser drilling was also tested to 647 mbsf without coring at Site C9001, 20 inch casings were installed to 511 mbsf, and the riser hole was suspended for the future riser drilling opportunity. Given those pilot surveys, the geologic resistance and potential safety hazards for the riser drilling operations at Site C9001 have already been evaluated as feasible.

Geological setting

Site C9001 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: International Phase of Ocean Drilling (IPOD) Legs 56 and 57 in 1977, IPOD Leg 87 in 1982, and ODP Leg 186 in 1999. In addition, well data are available from hydrocarbon drilling explorations 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 C9001 show pull-up blanking reflections below bottom-simulating reflectors (BSRs) at ~360 mbsf, suggesting the occurrence of methane hydrates and a strong upward flux of free hydrocarbon gases (Fig. F5). 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 range from Cretaceous to Miocene in age and are cut by many landward-dipping normal faults. The presence of thick coal layers was confirmed by natural gas drilling exploration at Site MITI Sanriku-Oki, ~50 km southward of Site C9001 (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% 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 (R_o) ranged between 0.5 and 0.7, indicating relatively immature coal (Osawa et al., 2002). In situ temperatures are well within the range of the habitable zone of microbes, based on the reported thermal gradient of 22.5°C/km (Osawa et al., 2002).

Preliminary scientific results from shallow sedimentary column at Site C9001

The cored sediments taken from Site C9001 during the Chikyu shakedown cruises were composed primarily of diatom-rich hemipelagic silty clay intercalated with volcanic tephras and sand layers (Fig. F6). Preliminary biostratigraphic age models indicate 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 the Expedition CK06-06 Chikyu shakedown cruise, core temperature anomalies were monitored immediately after recovery by ThermoView infrared camera in order to identify and locate methane hydrates. We observed methane hydrate formations (Fig. F7) as well as microbial aggregates (Fig. F8) in porous ash and sandy layers. Geochemical analyses of interstitial waters consistently showed that the chloride concentrations (and other sea salts) were notably depleted within the porous layers as a result of hydrate dissociation (Tomaru et al., 2009). Iodine concentrations and radio-isotopic compositions (¹²⁹I/I) of deep pore waters suggest that the iodine and oldest hydrocarbon sources could be as old as 40 Ma. Acetate concentrations in pore waters were >100 µmol/L throughout the sediment column (maximum 313 µmol/L), which is presumably related to coal diagenesis in the deeper zone (H. Yoshioka et al., unpubl. data).

A newly developed cell counting technique using a computer image showed that the cored sediments contain abundant microbial cells with counts >10⁷ cells/cm³ down to 365 mbsf (Morono et al., 2009); these counts were approximately two orders of magnitude higher than those in sediments from the Nankai Trough seismogenic zone (i.e., microbiological samples from IODP Expeditions 315 and 316; see Fig. F2). The abundance of Bacteria and Archaea was studied by quantitative real-time PCR and slot-blot hybridization techniques, suggesting a significant contribution of Archaea to the subseafloor microbial biomass (average = 40% at DNA level) (Lipp et al., 2008).

The metabolic activity of organiclastic sulfate reduction, sulfate reduction coupled with AOM, aceticlastic methanogenesis, and autotrophic (CO₂ reducing) methanogenesis rates were investigated using ³⁵S and ¹⁴C radiotracers, showing high AOM activity below the SMT zone and relatively low methanogenic activity throughout the core column (F. Inagaki and H. Yoshioka, unpubl. data). Using a sediment sample from Site C9001, the carbon and nitrogen incorporation rate of deep subseafloor microbes was studied at single-cell level using nano-scaled secondary-ion mass spectrometry (NanoSIMS) (Y. Morono, unpubl. data). A large fraction of subseafloor microbes was found to incorporate ¹³C- and ¹⁵N-labeled substrates into the biomass, and their metabolic rates provide in vitro evidence for energy starvation.

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, the cored sediments were anaerobically incubated at 10°C. Synthetic seawater containing glucose, yeast extract, acetate, and propionate as energy sources was supplemented into the bioreactor. After 289 days of operation, significant methane production was observed (Imachi et al., submitted). The δ¹³C_CH4 value was approximately –80‰, strongly suggesting the occurrence of microbial methanogenesis. 16S rRNA and its gene-based clone analyses of the bioreactor enrichment culture revealed that phylogenetically diverse microbes were cultivated in the bioreactor system and the dominating phylotypes were closely related to the typical environmental phylotypes that have been frequently observed in various subseafloor zones. Predominant archaeal members enriched in the reactor were affiliated with methanogens, such as the genera Methanobacterium, Methanoccoides, and Methanosarcina and the uncultured archaeal lineages (Fig. F9). Several attempts at transferring into traditional batch-type cultivations successfully led to the isolation of several methanogens and anaerobic microbes. These provide direct evidence for the presence of metabolically active and cultivatable microbial populations in subseafloor habitats off Shimokita Peninsula.

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