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| IODP Expedition 331 "DEEP HOT BIOSPHERE" |
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Overview |
| Expedition Number |
331 |
| Science Plan |
The Deep Biosphere and the Subseafloor Ocean |
| Location |
Okinawa Trough, Japan |
| Period |
From 1 September to 3 October, 2010 |
| Drilling Vessel |
CHIKYU |
| The goal of Expedition 331 is to directly sample hydrothermally active mounds in the Okinawa Trough to obtain evidence for microbial communities. This includes variations in biomass and species diversity, as well as the function and impact of such active subseafloor microbial ecosystems underneath the hydrothermal vents. The results are expected, not only to provide clues to the role of a subsurface biosphere left untouched deep under the sea, but also to help understand the mechanism of microbial methane generation and gas transport in seafloor sediments. The expedition also expects to reveal the relationship between the extents of subseafloor microbial communities and methane commonly found at high concentration in the hydrothermal fluids, and the formation of hydrothermal mineral deposits. |
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Drilling Plan |
1. Sub-seafloor hydrothermal fluid structure and hydrothermal alternation zone
The temperature readings from the bottom of the borehole were higher than those expected at C0013 (about 100 m east to the center of a high-temperature hydrothermal fluid plume), and C0014 (350 m further east of C0013). There, the Expedition recovered cores of volcanic sediment containing hydrothermally-altered sulfate minerals. Horizontal flows of hot fluids were also found at several depths. This indicate that in the eastern Iheya North hydrothermal field, there are several layers of caprock, where high-temperature vent fluids are trapped but can migrate laterally. Such interaction of hydrothermal waters and seawater passing through the layers causes hydrothermal alteration of rock (hydrothermal alteration zone).
2. Sub-seafloor hydrothermal fluid reservoir
The chemical composition of pore water contained in the cores revealed the presence of lighter vapor-rich water in the upper portion of the hydrothermal fluid reservoir; heavier high-salinity water accumulated at the bottom.
It is theorized that a hot fluid with high salinity sinks to the bottom of the hydrothermal fluid reservoir. The drilling into the hydrothermal mounds in Expedition 331 brought back the first direct evidence for this theory. The huge and deep expanse of the hydrothermal fluid reservoir in Iheya has also overturned the concept that the extent of fluid circulation and its flux volume are relatively small at plate convergence boundaries, such as the Okinawa Trough.
3. Distribution and mineralogy of hydrothermal sulfide minerals: clues to uncover the hydrothermal ore genesis.
The recovered cores contained a wide range of minerals composed of hydrothermally synthesized metal sulfides, providing clues to the chemical and depositional environment. It is commonly known that hydrothermal mounds (e.g. Site C0016) consist mainly of sulfide minerals. The Iheya hydrothermal system is not an exception; especially at C0013 and C0014, veins of sulfide minerogenic layers are extending through the bottom portion of the thick hydrothermal alteration zone. The finding has scientific significance as it may reveal the processes of the generation of hydrothermal deposits.
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Ken TAKAI
Institute of Biogeosciences, JAMSTEC (JAPAN) |
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Michael Mottl
Department of Oceanography, University of Hawaii at Manoa (USA) |
| Expedition Project Manager |
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Simon Harder Holm Nielsen
IODP Department
Center for Deep Earth Exploration, JAMSTEC
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Expedition Logo |
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