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doi:10.2204/iodp.proc.331.104.2011 Site C00141Expedition 331 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site C0014 is located 450 m east of the main hydrothermal mound chains of the Iheya North field and 350 m east of Site C0013 (see Fig. F3 in Expedition 331 Scientists, 2011a). Site C0014 has a distinct colony of deep-sea clams and a generally rocky seafloor. Based on a heat flow survey of the entire hydrothermal field (see Fig. F1 in Expedition 331 Scientists, 2011c), Site C0014 is located in a region of relatively low surficial heat flow (1°–1.5°C/m). Compared with Site C0013, Site C0014 is expected to have a deeper and more moderate hydrothermal mixing zone in the subseafloor. The multichannel seismic (MCS) survey (see Figs. F5 and F6 in Expedition 331 Scientists, 2011a) predicts that the upper part of the sediment column, to 70–80 meters below seafloor (mbsf), consists of well-stratified sequences that are probably pyroclastic deposits interbedded with hemipelagic and hydrothermal sediments. The underlying interval to ~120 mbsf seems to be nonstratified pumiceous pyroclastic deposits, and the deepest zone may be volcanic basement. The scientific objectives for Site C0014 are similar to those for Expedition 331 as a whole, namely, to test for direct evidence of a “subvent biosphere” and to clarify the architecture, function, and impact of subseafloor microbial ecosystems and their relationship to physical, geochemical, and hydrogeologic variations within a mixing zone, in this case one with relatively low hydrothermal input. Preliminary pore water chemistry and microbial community characterization from piston cores that penetrated several meters (J. Ishibashi et al., unpubl. data) suggest a gradual change in microbial activity with increasing depth. Microbial communities appear to be dominated by anoxic methane-oxidizing Archaea and Bacteria, together with typical anaerobic archaeal and bacterial phylotypes often identified in deep-sea sediments. With increasing depth below the seafloor, sulfate in pore water gradually decreases whereas methane increases, possibly supplied by a deeper hydrothermal source. |
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