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doi:10.2204/iodp.pr.331.2010

Scientific objectives

The goal of Expedition 331, Deep Hot Biosphere, is to determine directly 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 work addresses one of the primary themes of the IODP Initial Science Plan, the Deep Biosphere and the Subseafloor Ocean.

This project was conducted by an interdisciplinary group of onboard and shore-based scientists including microbiologists, geochemists, sedimentologists, petrologists, geologists, and geophysicists. The major scientific objectives of Expedition 331 drilling are

1. To prove the existence of a functionally active, metabolically diverse subvent biosphere associated with subseafloor hydrothermal activity in the Iheya North field;

2. To clarify the architecture, function, and impact of subseafloor microbial ecosystems and their relationship to physical, geochemical, and hydrogeologic variations within the hydrothermal mixing zones around the discharge area; and

3. To establish artificial hydrothermal vents in cased holes, using casing with perforated screen pipes at those depths that exhibit hydrothermal flow, and to prepare a research platform at each cased hole for later study of fluids tapped from various parts of the hydrothermal system and their associated microbial and macrofaunal communities.

In our shipboard studies, and continuing postcruise, we have addressed these objectives by a combination of compositional and isotopic biogeochemistry and cellular, molecular, and functional microbiology. We have used downhole temperature measurements, alteration mineral assemblages, and the chemical composition of hydrothermal fluids to constrain the physical and chemical conditions of mineralization and of the microbial habitats. Studies of fluid inclusions in alteration minerals will constrain present and past fluid temperatures and provide evidence for subseafloor phase separation. Isotopic studies of sulfur, lead, and possibly iron will provide information on magmatic and crustal contributions to the hydrothermal system. Dating of hydrothermal deposits will constrain the duration and timing of hydrothermal activity. Application of recently developed microanalytical techniques such as laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) will provide new insight into the sources of hydrothermal metals. Finally, drilling will provide constraints on the hydrology of the active hydrothermal system and the size and geometry of the associated sulfide deposits and alteration zones. Because microbial habitat relies on hydrologic as well as physical-chemical structure, these studies will provide important insights into the range of conditions within the subseafloor biosphere.

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