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doi:10.2204/iodp.proc.331.105.2011 Site C00151Expedition 331 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site C0015 is located near the crest of the main hill that lies to the west, ~600 m northwest of the main hydrothermal mound chains of the Iheya North field (see Fig. F3 in Expedition 331 Scientists, 2011a). The preexpedition survey of the area surrounding Site C0015, using a remotely operated vehicle (ROV), found many colonies of deep-sea vent mussels, all dead, as well as widespread pavements of carbonate and/or sulfate crusts. We hypothesize that the crest of the hill west of the Iheya North vent sites was an enormous methane seepage field in the past. A heat flow survey of the entire hydrothermal field (see Fig. F1 in Expedition 331 Scientists, 2011c) shows that Site C0015 has relatively low heat flow and a surficial temperature gradient of ~1°C/m, much lower than at Sites C0013 and C0014. A significant feature of Site C0015 revealed in the seismic reflection survey is the presence of a large-scale sequence of negatively polarized reflections, the eastern end of which appears to converge into the seafloor near the vent sites (see Fig. F6 in Expedition 331 Scientists, 2011a). The three-dimensional extension of the negative polarity sequences seems to extend beneath the thick basin-filling sediments that are widespread in the northern and western parts of Iheya North Knoll (Fig. F1). These sequences are thought to consist of highly porous layers of pumiceous volcanic sediment that could serve as a reservoir and/or migration paths for subseafloor hydrothermal fluids. Seismic reflection data suggest further that the negative polarity sequences reach a maximum thickness of ~100 m just beneath the hill west of the hydrothermal vent sites (see Fig. F6 in Expedition 331 Scientists, 2011a) and could represent the final storage volume for hydrothermal fluid before it ascends to the east along the westward-dipping structure and exits the seafloor at the high-temperature vent sites (see Fig. F6 in Expedition 331 Scientists, 2011a). The scientific objective for Site C0015 is to test whether the negative polarity reflectors imaged seismically represent a hydrothermal reservoir and/or fluid migration pathway that might deliver fluids to the vents and associated microbial communities. As much as 400 m of penetration would be required to penetrate the deepest of these reflectors at Site C0015, but we had time to take only two hydraulic piston coring system (HPCS) cores, to 9.4 meters below seafloor (mbsf), before we had to depart for Okinawa, Japan, for a crew change midway through the expedition. |
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