Submarine hydrothermal fluids from JADE and CLAM sites in the mid‐Okinawa Trough Backarc Basin are highly enriched in CO2, K, Li, NH4, CH4 and titration alkalinity compared to MOR fluids so far studied, while their H2S and 3He contents are similar. The He and C isotopic ratios as well as these chemical features indicate that the hydrothermal systems are controlled by reaction between seawater and CO2‐rich intermediate to acid volcanic rocks of island‐arc type, with strong influence of organic matter and are consistent with its initial stage of rifting on a continental plate margin. While the endmember JADE fluid is nearly devoid of Mg and SO4, the CLAM fluid has definite concentrations of Mg (upper limit 22 ± 5 nM) and, most interestingly, of 34S‐enriched SO4 (upper limit 10 ± 2 mM, δ34S= 27 to 39‰).

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[1] We measure expansion rate and bending in a 23-hour time series of acoustic images of the lower 25 m section of a buoyant hydrothermal plume rising from Grotto vent in the Main Endeavour Field, Juan de Fuca Ridge. We then calculate entrainment coefficient, the constant of proportionality relating mean inflow velocity at the plume edge to maximum mean upward velocity within the plume. The plume section alternately bends southwest at relatively high inclinations (37°) and northeast at lower inclinations at irregular intervals twice during this time period, apparently driven by current reversals in the mixed semi-diurnal tidal cycle. The measured expansion rates (0.11 -0.25 m/m) and calculated entrainment coefficients (0.07 -0.18) are directly proportional to the degree of bending (R 2 = 0.75). The loss of buoyancy flux related to enhanced mixing in a stratified environment during bending may contribute to reduction of potential rise height consistent with predicted ($400 m) and measured (300 -350 m) plume tops. Citation: Rona, P. A.,

Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data.