Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser drilling technology. The site examined during this expedition (Site C0020) is located off the Shimokita Peninsula, Japan, at a water depth of 1180 m in a forearc basin formed by the subduction of the Pacific plate under the Okhotsk plate. Previously conducted seismic profiles suggested the presence of deep, coal-bearing horizons at ~2 km below the seafloor. Our primary scientific objectives during Expedition 337 were thus to study the relationship between these deep coalbeds and carbon cycling, as well as to explore the distribution of subseafloor life at the greatest depths that have ever been sampled by scientific ocean drilling. A key question that guided our research strategy was “Do deeply buried hydrocarbon reservoirs, such as coalbeds, act as geobiological reactors that sustain subseafloor life by releasing nutrients and carbon substrates?” To address this question and other objectives, we drilled through a 2466 m deep sedimentary sequence with a series of coal layers at ~2 km below the seafloor. Hole C0020A is thus the deepest borehole in the history of scientific ocean drilling, surpassing the previous maximum penetration depth by 355 m and providing the opportunity to extend the maximum depth of subseafloor life detection by >800 m. Site C0020 also provides the first geological record of a dynamically changing depositional environment 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 continual use of drilling mud during riser drilling operations required implementation of a rigorous program dedicated to sample quality assurance and quality control. We successfully added chemical tracers to drilling mud to monitor levels of drilling mud contamination of samples and quantified levels of mud-derived solutes in interstitial fluid. Therefore, our data provide a framework for differentiating signals of indigenous microbes from those of contaminants. For the first time ever in scientific ocean drilling, we conducted downhole in situ fluid analysis and sampling as part of the logging operations. Logging operations yielded data of unprecedented quality that provide a comprehensive view of sediment properties at Site C0020. With an estimated temperature gradient of 24.0°C/km, temperatures in coal-bearing horizons are near 50°C and thus well within the known temperature range of life. We conducted gas analyses using a newly installed mud-gas monitoring laboratory. Gas chemistry and isotopic compositions provide the first indication of biological activity in deep horizons associated with the coalbed. Last but not least, this expedition 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 community are enormous, but its implementation will require the adaptation and further fine tuning of riser drilling technology to the needs of basic research.
|ScienceOpen disciplines:||Earth & Environmental sciences, Oceanography & Hydrology, Geophysics, Chemistry, Geosciences|