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doi:10.2204/iodp.sp.331.2010
Scientific objectives
Previous ODP and IODP expeditions provide lessons in exploring the subvent biosphere: a strong interdisciplinary collaboration of microbiology with related science fields based on clearly defined scientific objectives with a feasible drilling strategy is essential to successfully investigating the subvent biosphere and clarifying the structure and community dynamics of a subseafloor microbial ecosystem associated with hydrothermal activity. Here, we propose IODP Expedition 331, Deep Hot Biosphere, with the goals of obtaining direct evidence of the existence of a functionally active, metabolically diverse subvent biosphere in its physical, geochemical, and hydrogeologic context within the Iheya North field in the mid-Okinawa Trough. This proposal directly addresses one of the primary themes of the IODP Initial Science Plan, the Deep Biosphere and the Subseafloor Ocean.
This project will be conducted by an interdisciplinary group of onboard and shore-based scientists including microbiologists, geochemists, hydrogeologists, mineralogists, geophysicists, and geologists. The major scientific objectives of the Expedition 331 drilling program are as follows:
To directly prove the existence of a functionally active, metabolically diverse subvent biosphere associated with subseafloor hydrothermal activity in the Iheya North field by drilling.
To clarify the architecture, function, and impact of subseafloor microbial ecosystems and their relationship to physical, geochemical, and hydrogeologic variations along the overall hydrothermal fluid pathway on the basis of
- Characterizing variability in the biomass, diversity, structure, and function of microbial communities in various habitats occurring in vertical and horizontal extensions of subseafloor hydrothermal flow;
- Characterizing compositional and isotopic variability in fluid chemistry associated with hydrothermal abiotic and biotic processes in vertical and horizontal extensions of subseafloor hydrothermal flow;
- Characterizing compositional and isotopic variability in mineral formation and alteration of sediments throughout hydrothermal abiotic and biotic processes; and
- Reconstructing subseafloor hydrothermal fluid pathway and hydrogeologic structure by means of distribution and transportation of naturally existing microbial and chemical tracers relevant to hydrothermal activity together with geophysical site survey data.
To recover core samples of hydrothermal sulfides for mineralogical and geochemical studies. Mineral assemblage and chemical composition would constrain physical and chemical conditions of mineralization. Studies of fluid inclusions in sulfide minerals would constrain fluid temperature and provide evidence for subseafloor phase separation. Isotopic study of sulfur, lead, and possibly iron would provide information on magmatic and crustal contributions to the hydrothermal system. Dating of hydrothermal deposits would constrain duration and timing of hydrothermal activity. Application of recently developed microanalytical techniques such as laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) would provide new insight into the sources of hydrothermal metals. Drilling can help to constrain the size and geometry of the sulfide deposits and hydrothermal alteration zones, which can modify sediment permeability and influence hydrothermal flow paths. Because microbial habitat relies on hydrological structure as well as temperature structure, these studies would provide important insights into the settings of the subseafloor biosphere.
To recover and analyze interstitial waters from interbedded pumiceous volcanic material and terrestrial sediment, which are expected to reveal further cases of lateral fluid intrusion and possibly the existence of a hydrothermal reservoir within the sediment layer. Geochemical studies of pore fluid composition would document the contribution of any hydrothermal component. The isotopic composition of oxygen and hydrogen would constrain the fluid migration history, including phase separation and possible formation of CO2 hydrate. Organic geochemistry together with carbon and sulfur isotopic compositions would provide evidence for in situ diagenesis enhanced by fluid intrusion and may demonstrate effects of microbial metabolism supported by the hydrothermal fluid.
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