The second phase of the SROF project consisted of a follow-up expedition aboard the R/V T.G. Thompson in March-April 2004 employing the remotely-operated vehicle (ROV) ROPOS to directly explore and sample a selected group of the Mariana Arc submarine volcanoes. Three ROV dives were devoted to exploring NW Eifuku. Approximately 8 hours into the first dive, at a depth of 1604 m, ROPOS discovered a remarkable hydrothermal field (later named Champagne) with small white chimneys discharging buoyant milky fluid. Subsequent surveys with the ROV located several additional sites of hydrothermal discharge on NW Eifuku, although the most intense venting was found at the Champagne site ~80 m WNW of the volcano summit. The summit of NW Eifuku was mapped with an Imagenex scanning sonar on ROPOS (Figure 2c), following the methods described by Chadwick et al. . The highresolution bathymetry shows that the Champagne vent field lies in the steep headwall of a gravitational slope failure that cuts across the top and SW side of the volcano [Chadwick et al., 2004]. Although there were few vent animals right at the Champagne site, an extensive biological community was found within the surrounding few hundred meters, including mussels, shrimps, crabs and limpets.
In addition to the vent fluid discharge at Champagne vent, droplets coated with a milky skin were rising slowly from the seafloor around the chimneys (Figure 3). The droplets were later determined to consist mainly of liquid CO, with HS as a secondary component. The seafloor area of active CO droplet flux was characterized by pumice and whitish/yellowish sulfur-rich material. The droplets were sticky and adhered to the ROV like clumps of grapes, although they did not tend to coalesce into larger droplets (Figure 3e). The film coating the droplets was assumed to be CO hydrate (or clathrate) which is known to form whenever liquid CO contacts water under these P, T conditions [Sloan, 1990]. Liquid CO should be buoyant at the depth of the Champagne site, since it has a density less than seawater at depths shallower than about ~2600 m [Brewer et al., 1999]. At NW Eifuku, droplets percolated out of crevices in the seafloor, and we did not observe the formation of small hydrate pipes as noted at the JADE site in the Okinawa Trough [Sakai et al., 1990a]. The flux of liquid CO droplets increased dramatically whenever the seafloor was disturbed by the ROV. This observation is consistent with the presence of a layer of liquid CO beneath the surface capped by an impeding layer of CO hydrate (see Figure 4). Thus any penetration of the hydrate cap releases the buoyant liquid CO beneath. These observations are similar to those reported by Sakai et al. [1990a], who discovered venting of liquid CO in the Okinawa Trough back-arc basin. A comparison of the video from both sites indicates a higher flux of CO-rich droplets at NW Eifuku compared to the JADE site (see Figure 3 caption).
Figure 3. Photographs of the Champagne hydrothermal site taken with the ROPOS ROV. (a, b, and c) Small chimneys venting 103°C vent fluid. Liquid CO droplets are also visible. (d) Close-up of liquid CO droplets rising in a stream from the seafloor. (e) Liquid CO droplets collecting on the underside of the ROV bumper-bar and camera. (f) Mussel bed only tens of meters from the Champagne vent site. See also Movies 1 and 2. Additional photographs and video clips from the 2004 Submarine Ring of Fire expedition and from NW Eifuku in particular are available at the Ocean Exploration Web site: http://oceanexplorer.noaa.gov/explorations/04fire/logs/april10/april10.html and http://oceanexplorer.noaa.gov/explorations/04fire/logs/photolog/photolog.html.
Figure 4. Diagram of near surface conditions at the Champagne vent field. The "?" indicates possible regions where liquid CO and/or CO hydrate are being entrained into the vent fluid flow.
In October-November of 2005 we had a second opportunity to collect samples at NW Eifuku during cruise NT05-18 aboard the R/V Natsushima. During the Natsushima cruise, the ROV Hyper-Dolphin completed 6 dives on NW Eifuku, 2 on the volcano flanks and 4 on the summit area. During this expedition, the Hyper-Dolphin collected additional samples of both the vent fluids and liquid droplets at the Champagne site.
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