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In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock.

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H 2 -and CH 4 -rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the "Shinkai Seep Field (SSF)." The SSF appears to be a serpentinitehosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.Challenger Deep | convergent margin | hydrothermal vent | Shinkai Seep Field | vesicomyid clam H ydrothermal activity plays an important role in Earth evolution by modifying the composition of oceanic crust, affecting ocean chemistry, forming metal-rich deposits, and providing energy and nutrient sources for chemosynthetic biological communities. Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures occur in different tectonic settings. Along divergent plate margins, three basic vent types have been identified. The first type is a basalt-hosted, hightemperature hydrothermal system with fluid temperatures up to approximately 400°C and low H 2 and CH 4 concentrations, but high metal concentrations (e.g., TAG hydrothermal field, 26°10′

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We report the first study of sperm whale-fall ecosystems, based on mass sinking of whale carcasses at shelf depths in the northwest Pacific. We conducted three observations over a 2-year period on replicate sperm-whale carcasses implanted at depths of 219-254 m off the southern part of Japan from July 2003 to August 2005. The study was made possible by a mass stranding of sperm whales in January 2002, and the subsequent sinking of 12 carcasses in the waters off Cape Nomamisaki. Dense aggregations of unique chemosynthesis-based fauna had formed around the whale carcasses after 18 months (July 2003). The mytilid mussel Adipicola pacifica was the most abundant macrofaunal species and covered most of the exposed bone surfaces. The general composition of the fauna was similar to that of deep-water reducing habitats, but none of the species appearing in this study has been found at hydrothermal vents, cold seeps or deep-water whale falls. A new species of lancelet, which was the first record of the subphylum Cephalochordata from reducing environments, a new species of Osedax; a rarely encountered benthic ctenophore, and a rare gastropod species were discovered at this sperm whale-fall site. Benthic communities were similar across all the carcasses studied, although the body sizes of the whales were very different. The succession of epifaunal communities was relatively rapid and the sulphophilic stage was considerably shorter than that of other known whale falls.