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Scientific objectives and hypotheses

An operational goal of Expedition 337 was to extend the riser drilling/coring depth at Site C0020 (i.e., JAMSTEC Site C9001) to at least 2200 mbsf (and possibly deeper if time and safety concerns permitted) in order to obtain samples from the terrigenous to shallow-marine coalbeds that were believed to be situated below marine sediments. This expedition was thus anticipated to provide the unique opportunity to examine geobiological and diagenetic processes associated with coal formation in deeply buried marine sediment. Moreover, microbial life or its activities had never been documented at the 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 coalbed-derived organic substances influences microbial and diagenetic processes in the overlying, shallower strata.

  3. Subsurface coalbeds have the potential to serve as a seal for CCS within the porous sand layers between the coal formation 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 guided 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 function in the deep subseafloor biosphere; and

  • Examine whether distinct and active microbial communities inhabit the deeply buried coalbed, the overlying sediments of terrigenous origin, and the even shallower marine sediments and determine microbial response to high CO2 concentrations.

We also addressed 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 deep subseafloor coal-sand formation?

In order to address these scientific objectives, we performed (1) spot coring of marine sediments and coalbed layers, (2) in situ wireline logging of various geophysical and geochemical properties, and (3) in situ sampling of formation fluids associated with coalbeds using a wireline fluid sampling tool. These materials and data were used for extensive microbiological, biogeochemical, geological, and geophysical analyses on board the ship and will be used 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. The examination of the effects of high CO2 concentrations and the associated decrease in pH under conditions of hypothetical CO2 sequestration into the deep coal/sand layers is an important objective. Therefore, 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.