Post-drilling hydrothermal vent and associated biological activities seen through artificial hydrothermal vents in the Iheya North field, Okinawa Trough

In 2010, IODP Expedition 331 was conducted in the Iheya North Field, the Okinawa Trough and drilled several sites in hydrothermally active subseafloor: e.g., the active hydrothermal vent site and sulfide-sulfate mound at North Big Chimney (NBC) and three sites east of NBC at different distances from the active vents [1]. In addition, during the IODP Expedition 331, four new hydrothermal vents were created. These post-drilling artificial hydrothermal vents provide excellent opportunities to investigate the physical, chemical and microbiological characteristics of the previously unexplored subseafloor hydrothermal fluid reservoirs, and to monitor and estimate how the anthropogenic drilling behaviors affect the deep-sea hydrothermal vent ecosystem. The IODP porewater chemistry of the cores pointed to the density-driven stratification of the phase-separated hydrothermal fluids and the natural vent fluids were likely derived only from the shallower vapor-enriched phases. However, the artificial hydrothermal vents had deeper fluid sources in the subseafloor hydrothermal fluid reservoirs composed of brine phases. The fluids from the artificial hydrothermal vents were sampled by ROV at 5, 12, 18 and 25 months after the IODP expedition. The artificial hydrothermal vent fluids were slightly enriched with Cl as compared to the natural hydrothermal vent fluids. Thus, the artificial hydrothermal vents successfully entrained the previously unexplored subseafloor hydrothermal fluids. The newly created hydrothermal vents also hosted the very quickly grown, enormous chimney structures, of which mineral compositions were highly variable among the vents. In addition, the IODP drilling operation not only created new hydrothermal vents but also induced the newly generated diffusing flows by many short drillings in the seafloor where no apparent hydrothermal fluid discharge was observed. The new widespread diffusing flows altered the habitat condition, and provided postdrilling propagation and colonization of indigenous hydrothermal chemosynthetic animals.