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doi:10.2204/iodp.pr.337.2012

Introduction

Marine subsurface hydrocarbon reservoirs and the associated microbial life in continental margin sediment are among the least characterized systems on Earth that can be accessed by scientific ocean drilling. Our scientific knowledge of the biological and abiotic processes associated with hydrocarbon production is limited because of the highly limited opportunities to conduct scientific ocean drilling initiatives using deep-riser drilling in natural gas and oil fields. A number of fundamentally important questions regarding deep subseafloor hydrocarbon systems have remained unanswered. For example

• What role does subsurface microbial activity play in the formation of hydrocarbon reservoirs?

• Do the deeply buried hydrocarbon reservoirs such as natural gas and coalbeds act as geobiological reactors that sustain subsurface life by releasing nutrients and carbon substrates?

• Do the conversion and transport of hydrocarbons and other reduced compounds influence biomass, diversity, activity, and functionality of deep subseafloor microbial populations?

• What are the fluxes of both thermogenically and biologically produced organic compounds, and how important are these for the carbon budgets in the shallower subsurface and the ocean?

To address these important scientific questions, Integrated Ocean Drilling Program (IODP) Expedition 337 aimed to drill and study a hydrocarbon system associated with deeply buried coalbeds off the Shimokita Peninsula, Japan, in the northwestern Pacific using the riser drilling system of the D/V Chikyu.

Deep subseafloor biosphere

Subseafloor sediment harbors a remarkably sized microbial biosphere on our planet (Parkes et al., 1994, 2000; Whitman et al., 1998; Lipp et al., 2008; Kallmeyer et al., 2012; Hinrichs and Inagaki, 2012). To date, microbial cells have been observed in sediment ranging in age to the Cretaceous and in subsurface depth to 1626 meters below seafloor (mbsf) (Newfoundland margin, Ocean Drilling Program [ODP] Leg 210; Roussel et al., 2008). Diagenetic models of pore water chemical constituents as well as radioactive and stable isotope tracer incubation experiments showed that metabolic activities of deep subseafloor microbes are extremely low because of the low supply of energy-rich substrates (D’Hondt et al., 2002, 2004), whereas most deeply buried microbial cells are physiologically active (Morono et al., 2009) or quiescent as the dormant phase or spore (Lomstein et al., 2012); however, the activity of microbial communities is often stimulated at geochemical and/or lithologic interfaces, such as porous ash layers and sulfate–methane transition (SMT) zones (Inagaki et al., 2003; Parkes et al., 2005; Biddle et al., 2006; Sørensen and Teske, 2006). The metabolic activities of subseafloor microbial communities are controlled by the flux of bioavailable electron donors and/or acceptors, some of which are derived either from the overlying seawater by photosynthetic primary production (D’Hondt et al., 2004, 2009; Lipp et al., 2008) or from crustal fluids underlying sedimentary habitat (Cowen et al., 2003; Nakagawa et al., 2006; Engelen et al., 2008). Fluid flow regimes in the subseafloor environment control availability of energy to microbial life. Hence, the geologic and sedimentological characteristics represent crucial factors controlling habitability of the deep marine subsurface environment. Culture-independent molecular ecological surveys of 16S rRNA gene fragments reveal that the microbial communities in continental margin sediment are predominantly composed of species lacking cultivated relatives, such as the bacterial members within the candidate division JS1, Chloroflexi, and Planctomycetes, as well as the archaeal members within the Deep-Sea Archaeal Group, the Miscellaneous Crenarchaeotic Group, and the South African Gold Mine Euryarchaeotic Group (e.g., Inagaki et al., 2003, 2006; Inagaki and Nakagawa, 2008). The carbon isotopic analysis of intact polar lipids (IPLs) and fluorescence in situ hybridization (FISH)-stained cells suggest that sizeable populations of heterotrophic archaea significantly contribute to microbial biomass in organic-rich sediment (Lipp et al., 2008), even at the SMT zone where the occurrence of anaerobic oxidation of methane (AOM) mediated by methanotrophic archaea and sulfate-reducing bacteria takes place (Biddle et al., 2006). Despite the significance of an organic-fueled microbial ecosystem in biogeochemical cycling within continental margin sediment, the metabolic characteristics of organic matter degradation and fluxes of secondary metabolites remain largely unknown (e.g., Hinrichs et al., 2006).

Coal diagenesis: microbiological significance for biogeochemical cycles

Within the generally energy-starved deep subseafloor biosphere, the deeply buried immature coal (e.g., lignite) is a potential source of nutrients and energy for microbial communities. Previous studies of terrestrial coal deposits suggest that microorganisms play important ecological roles in coal diagenesis, resulting in substantial quantities of coalbed methane and secondary products of microbial activities (e.g., Brown et al., 1999; Detmers et al., 2001; Fry et al., 2009; Krüger et al., 2008; Shimizu et al., 2007; Strapoc et al., 2008; Jones et al., 2008; Glombitza et al., 2009; Orem et al., 2010; Ünal et al., 2012). The microbial communities in terrestrial coaly habitats are phylogenetically highly diverse with relatively low cell density of <10⁶ cells/cm³ For example, methane-producing archaea (i.e., methanogens) such as the genera Methanoculleus, Methanobacterium, Methanolobus, and Methanosarcina, as well as some potential acetate-producing bacteria (i.e., acetogens), such as Acetobacterium, were predominant in a deep borehole aquifer directly connected to the coal deposits in Hokkaido Island, Japan (Shimizu et al., 2007). Using incubation tracer experiments and the FISH technique, active aceticlastic methanogenesis was found to occur even in a highly altered graphite deposit (Krüger et al., 2008). Fry et al. (2009) reported sizable cultivable populations of potential sulfate-reducing bacteria, methanogens, acetogens, and lignite-utilizing heterotrophs in the uplifted coaly sediment of northern New Zealand based on results from the most probable number (MPN) cultivation method. Metabolic activities were stimulated at the geologic interfaces between coal and sand/silt layers as reported from other terrestrial deep subsurface black shales (e.g., Krumholz et al., 1997), and the concentrations of organic acids in the coal layers were higher than in normal deposits, consistent with the co-occurrence of coal diagenesis and microbial processes.

Despite the microbiological and (bio)geochemical significance of coaly deposits for the global carbon cycle, there have been no studies of coal layers that are deeply buried in the subseafloor, mainly because of the safety regulations applied to hydrocarbon gas–related hazards during riserless drilling. In continental margin sediment, large quantities of gaseous hydrocarbons and its derivatives (e.g., H₂, organic acids) are potentially generated by thermogenic and/or biogenic degradation processes of deeply buried organic matter like lignite coal. All of these diagenetic compounds are potential nutrient and energy sources that support energy-retrieving redox chemical reactions mediated by the deep subseafloor microbial communities. Hence, coalbeds and microbial life may influence characteristics of dissolved gases and organic matter along with depths, as well as influencing the accumulation of gas hydrates in the shallow sedimentary sequence. In this regard, the connectivity between deep subseafloor microbial ecosystem and dynamics of deep subseafloor hydrocarbon system is a frontier research theme in geobiology and geochemistry that can only be studied by a dedicated initiative such as Expedition 337.

Exploring the feasibility of CO₂ sequestration in deep offshore geological repositories

To date, CO₂ capture and sequestration (CCS) into deep subsurface environments such as oil, gas, and porous aquifers is considered as a solution for reducing the emission of substantial amounts of anthropogenic CO₂ and preventing dangerous consequences of the anticipated future climate change. CCS offshore deep subseafloor environments has a number of advantages, including a positive risk assessment compared to shallow-water CCS (House et al., 2006; Schrag, 2009). It has been predicted that CCS can potentially reduce future world emissions from fuel energy by 20% (Dooley et al., 2006). In a terrestrial saline aquifer located in Germany, CO₂ injection has stimulated deep microbial population and activities, indicating that deep microbial communities may adapt to the drastic environmental change and play some roles in biogeochemical cycles (Morozova et al., 2010; Krüger et al., 2011). However, the behavior and stability of CO₂ as well as its geochemical and biological reactions in deep marine subsurface repositories are still almost completely uncertain (Onstott, 2005).

Using cored samples from Expedition 337, multiple scientific issues regarding the geological CO₂ sequestration will be addressed through shore-based ex situ experimentations. For example

• How does liquid or supercritical CO₂ spatially penetrate into various lithostratigraphic settings?

• How does CO₂ react with minerals, organic matter, and life in the deep subsurface?

• What are the impacts of long-term CO₂ storage on biogeochemical carbon cycling and the subsurface biosphere on different timescales?

Conducting various multidisciplinary ex situ experimental studies using cored materials as well as in situ logging characterizations of the deep-riser hole, Expedition 337 will significantly expand our knowledge of the coalbed subseafloor hydrocarbon system, including the physiochemical and biological factors that determine the potential for CO₂ sequestration.

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