doi:10.2204/iodp.proc.331.103.2011
Site C00131
Expedition 331 Scientists2
Background and objectives
Integrated Ocean Drilling Program (IODP) Site C0013 is located ~100 m east of the main hydrothermal mound chains of the Iheya North hydrothermal field (see Fig. F3 in Expedition 331 Scientists, 2011a). This area is characterized by patchy occurrences of numerous diffuse flow sites and associated chemosynthetic animal colonies, chiefly mussels and galetheid crabs. Most of the seafloor in the area is covered with pumiceous breccia and barite-anhydrite-carbonate crusts, whereas some of the bathymetric depressions host thin hemipelagic sediments.
A heat flow survey of the entire hydrothermal field (Fig. F1) shows that Site C0013 is located in a zone of relatively high heat flow, with a surficial temperature gradient estimated before drilling to be ~3°C/m, and is highly impacted by high-temperature fluids. A multichannel seismic survey predicts that nonstratified pumiceous pyroclastic deposits underlie the seafloor to a pronounced seismic reflector at 50–60 meters below seafloor (mbsf) (see Fig. F5 in Expedition 331 Scientists, 2011a). This reflector may be the boundary between volcanic basement and the overlying pyroclastic deposits, or it may simply be a hard layer within these deposits that would likely impede hydrothermal flow. Discovering the nature of this seismic reflector at 50–60 mbsf was an important drilling objective at Site C0013 that we failed to achieve because we were unable to drill deeper than 54.5 mbsf.
The scientific objectives for Site C0013 are similar to those for Expedition 331 as a whole, namely, to test for direct evidence of a “subvent biosphere”; to clarify the architecture, function, and impact of subseafloor microbial ecosystems; and to establish their relationship to the physical, geochemical, and hydrogeologic variations within a high-input hydrothermal mixing zone. Pore water chemistry and microbial community characterization of cores that penetrated <1 mbsf (J. Ishibashi et al., unpubl. data) suggest that microbial community structure changes rapidly with depth, from a psychrophilic-mesophilic methane-consuming community at shallow depth to a thermophilic-hyperthermophilic methane-producing community deeper. The shift in microbial community is likely associated with physical and chemical variations in pore water, in which the hydrothermal input increases with depth and where it will be heavily affected by spatial and temporal fluctuations in mixing between discharging hot hydrothermal fluid and recharging cold seawater.