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doi:10.2204/iodp.pr.337.2012

Abstract

Integrated ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser drilling technology. IODP drill Site C0020 is located in a forearc basin formed by the subduction of the Pacific plate off the Shimokita Peninsula at a water depth of 1180 m. Seismic profiles strongly suggested the presence of deep, coal-bearing horizons at ~2 km subseafloor depth. Our primary objectives during Expedition 337 were to study the relationship between the deep microbial biosphere and the subseafloor coalbed and to explore the limits of life in horizons deeper than ever probed before by scientific ocean drilling. Among the questions that guided our research strategy was: Do deeply buried hydrocarbon reservoirs such as coalbeds act as geobiological reactors that sustain subsurface life by releasing nutrients and carbon substrates? To address this question and other objectives, we penetrated a 2466 m deep sedimentary sequence with a series of coal layers at ~2 km below the seafloor. Hole C0020A is currently the deepest hole in the history of scientific ocean drilling. Drilling at this site extended the previous maximum penetration depth in scientific ocean drilling by 355 m and provided the chance that our postcruise research will extend the current evidence of deepest subseafloor life by more than 800 m. Riser drilling at Site C0020 provided an unprecedented record of dynamically changing depositional environments in the former forearc basin off the Shimokita Peninsula during the late Oligocene and Miocene. This record comprises a rich diversity of lithologic facies reflecting environments ranging from warm-temperate coastal backswamps to cool-water continental shelf. The use of riser drilling technology in very deep sediment created both unique opportunities and new challenges in the study of subseafloor life. The use of drilling mud during riser drilling required implementation of a rigorous program dedicated to quality assessment and quality control of the sampled materials and data. We successfully added chemical tracers to monitor the levels of drilling mud contamination of samples and quantified levels of mud-derived solutes in interstitial fluid. These data provide the framework for differentiating signals of indigenous microbes from those of contaminants. For the first time in scientific ocean drilling, we conducted downhole fluid analysis and sampling, and logging operations yielded data of unprecedented quality that provide a comprehensive view of sediment properties at Site C0020. The estimated temperature gradient was 24.0°C/km or slightly lower; estimated temperatures in coal-bearing horizons are ~50°C and thus provide comfortable conditions, temperature-wise, for many microbes. We conducted gas analysis using a newly installed mud-gas monitoring laboratory. Gas chemistry and isotopic compositions provide the first indication of the existence of a subseafloor biosphere in deep horizons associated with the coalbed. Last but not least, this expedition also provided a testing ground for the use of riser drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling. Potential benefits of deep riser drilling for the scientific communities are enormous. Its implementation will require the adaptation of this technology to the needs of basic sciences.

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