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doi:10.2204/iodp.pr.337.2012

Scientific objectives and hypotheses

During Expedition 337, extending the riser drilling/coring depth at Site C0020 (JAMSTEC Site C9001) was originally planned to 2200 mbsf (maximum penetration depth at 2500 mbsf), where the terrigenous to shallow-marine coalbed is situated beneath the overlying marine sedimentary realm. The riser drilling exploration of the deep hydrocarbon reservoir off Shimokita provided the unique opportunity to examine geobiological and diagenetic processes associated with coal formation in deeply buried strata. No microbial life or its activities have been documented to date at the maximum targeted burial depths in any marine environment. Expedition 337 was driven by three overarching testable hypotheses:

1. The deeply buried coalbeds act as geobiological reactors that release dissolved organic compounds such as methane, acetate, and other substances.

2. The conversion and transport of the coalbed-derived organic substances influence microbial and diagenetic processes in the overlying, shallower strata.

3. The subsurface coalbed has the potential to serve as a cap rock for potential future activities of CO₂ sequestration and can support biological conversion of CO₂ into biomass and organic compounds even at high CO₂ concentrations.

The following operational objectives addressed during Expedition 337 are tied to the above hypotheses and guide our research strategy:

• Constrain the impact of a thermally immature coalbed on the diagenetic and microbial processes at great burial depths;

• Quantify the upward fluxes of dissolved organic compounds, such as gaseous hydrocarbons and volatile fatty acids, out of the coalbed and evaluate their impact on microbial processes in shallower strata;

• Assess the limits of life and potential geochemical and geophysical constraints to microbial population, diversity, metabolic activity, and functioning in the deep biosphere; and

• Test whether distinct active microbial communities inhabit the deeply buried coalbed, the overlying sediments of terrigenous origin, and the even shallower marine sediments and how they respond to high CO₂ concentrations.

We also address the following set of specific research questions:

• What is the ecological and biogeochemical relevance of deeply buried lignite in the natural hydrocarbon system offshore the Shimokita Peninsula?

• What are the fluxes of both thermogenically and biologically produced methane and other diagenetic products, such as organic acids, into shallower strata and how important are these for the carbon budget?

• How does coal diagenesis affect subseafloor microbial biomass, diversity, and metabolic activities?

• Does the presence of the low-maturity coalbed stimulate heterotrophic and autotrophic microbial communities?

• How do minerals and organic matter react in the coalbed formation, how will this change the physical and chemical characteristics, and how will it affect the microbial communities?

• What paleoenvironmental information and sedimentary regimes are recorded at Site C0020?

• What is the natural flux of CO₂ and CH₄ from the coalbed hydrocarbon system, and what is the potential for CO₂ sequestration in the Shimokita system?

In order to address these scientific objectives, we performed (1) spot coring of marine sediments and coalbed layers, (2) wireline logging of various geophysical and geochemical properties in situ, and (3) sampling of in situ formation fluids associated with coalbeds using a wireline fluid sampling tool. These materials and data are used for extensive microbiological, biogeochemical, geological, and geophysical analyses on board the ship and in shore-based laboratories. Achieving these scientific objectives will expand our knowledge of geobiological and biogeochemical properties in the coalbed hydrocarbon system. Similar coaly environments are widely distributed along the western coast of the Pacific Ocean, and hence our results will be of great societal relevance. Because the effect of high CO₂ concentrations and the associated decrease in pH under conditions of CO₂ sequestration into the deep coal/sand layers is one of the primary objectives to be addressed, the shore-based laboratory experiments will include quantitative evaluation and modeling of fluid flow and biological systems in the subseafloor environment, including their response to high CO₂ concentrations. These applied scientific aspects will add an important new component to IODP.

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