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doi:10.2204/iodp.proc.331.107.2011 Site C00171Expedition 331 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site C0017 is located 1.6 km east of the hydrothermal vent sites in the central valley of Iheya North Knoll (see Fig. F3 in Expedition 331 Scientists, 2011a). Based on the multichannel seismic (MCS) survey (see Figs. F5 and F6 in Expedition 331 Scientists, 2011a), the uppermost 100 meters below seafloor (mbsf) probably consists of hemipelagic sediment. At ~100 mbsf, a strong seismic reflection is continuous from the hydrothermal vent sites at Site C0016 eastward through Sites C0013 and C0014. This reflection represents a potential interface between hemipelagic sediments and pyroclastic flow deposits. Below this reflection the subseafloor structure becomes obscure. Potential volcanic basement, however, may be identifiable at ~450 mbsf at Site C0017. Heat flow measured in the sediments near Site C0017 is quite low (see Fig. F2 in Expedition 331 Scientists, 2011c). The surficial thermal gradient is one order of magnitude lower than the average thermal gradient of the nonhydrothermal, trough-filling sediments in the middle Okinawa Trough. This low heat flow anomaly strongly suggests that Site C0017 is located within a large-scale local recharge zone. Heat flow data also indicate that there are areas with relatively high thermal gradients several hundreds of meters from Site C0017, suggesting that the local recharge/discharge boundary of the Iheya North hydrothermal system may be quite narrow in the east–west direction in the vicinity of Site C0017. The scientific objectives for Site C0017 are to understand the potential discharge and recharge dynamics around the central valley of Iheya North Knoll and to clarify the interaction between subseafloor physical-chemical variations and microbial community structures and functions. |
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