Final Report: 15 February 1998
Despite constant winds in the 25-40 knot range and confused seas of 10-18 ft., the response team managed to stage nearly continuous CTD-rosette operations from Thursday until the time of departure from Axial Volcano on the Saturday afternoon (days 4-6) . A total of 16 vertical CTD casts were made during the cruise. Whereas no hydrothermal signals were detected at the southern portion of the rift areas where seismicity swarms had also been noted, very strong signals were found in the southern portion of the Axial Volcano caldera as well as at stations to the southwest of the caldera. Temperature anomalies of over 0.15oC and light attenuation coefficients (relative particle loading) of nearly 0.2 (1/m) were measured. Depending on the specific station the vertical profiles were dominated by a shallow (1200 to 1400 m) and/or a deep (1400-1550 m) maxima in hydrothermal signals. The cast taken over the "Ashes" site revealed a very strong plume that had considerable vertical structure and extended from nearly 1200 m to the seafloor at ~1565 m. Both light attenuation and temperature (>.1 deg. C) anomalies were present with double maxima at 1375 and 1425 m. Shipboard analysis of methane and hydrogen showed concentrations as high as 600 and 200 nM, respectively, in the plumes at stations near the intersection of the south rift zone with the caldera. The German Hydrotrace (RV Sonne) cruise measured order of magnitude lower methane concentrations the during a May-July, 1996 cruise. The intensity of the present hydrothermal plumes indicates greatly elevated discharge rates at the seafloor relative to previous surveys of this same region in 1996 as well as last summer.
The prevailing currents over the past several weeks were clearly southwesterly based on the very strong hydrothermal signals in this direction at least as far away as 18 km from the caldera, and absence of signal at stations due west, north, or south of the caldera. However, a small but distinctly hydrothermal signal was found between 1200 and 1300 m at a station within the central seismic cluster, about 11 miles south of the caldera. Water samples from this plume remnant had 5-20 nM CH4, significanly greater than background values (<1 nM) for these depths. There was no elevated H2 signal detected which could indicate very low H2 content of the source hydrothermal fluids or that the small plumes here represented remnants of an older plume in which the H2 has been lost to microbial oxidation.
We departed Axial Volcano at 1400 hr, Saturday, arriving at Thompson Seamount, the Sound Source mooring deployment site, 5 hours later. The mooring was successfully deployed over a seafloor depth of ~1500 m. The anchor drop position was 46o 01.86' N; 128o 37.73' W. We then transited south overnight to the Tsunami mooring site to recover 2 moorings before heading back to Newport. As of 1515 hr, Sunday afternoon, we have fully recovered the bottom mooring and have the surface mooring buoy on deck. The 2600 m of mooring line is being reeled in as this message is sent off.
We should be waiting at the Newport harbor mouth tomorrow morning.
Aloha,
Jim Cowen and the Axial Response Team (ART I)
February 15, 1998