The first Leg of the HYDROTRACE cruise (S0109-1) ended in Victoria/Vancouver Island on June 4, 1996. On June 5, the R/V SONNE was boarded by the shipboard scientific party of Leg S0109-2. The Canadian remotely operated vehicle ROPOS, the hydraulic unit, and a ROPOS service container were also taken on board. During S0109-2, the Hepburn winch for ROPOS was under repair in Victoria in preparation for diving operations during Leg S0109- 3. After the scientific equipment was loaded and the installation of all systems completed, R/V SONNE left port on June 8 at 10:06 local time heading for Axial Volcano (46° N, 130°
W).
On arrival at Axial on June 9 at 13:38, an array of five transponders was deployed and calibrated in the North Rift Zone. Sediment sampling by box corer in bathymetric basins NW and SE of Axial Seamount, as well as in the Helium Basin to the NE, was carried out in order to determine the extent of hydrothermal signatures recorded in distal sediments. The box corer in the Helium Basin recovered two distinct layers of volcanic ash (about 2-3 cm and
> 90 cm thick), separated by a thin layer of pelagic sediment and topped by 5 cm of pelagic sediment. Radiometric dating of those layers will help to unravel the depositional history at this site. A CTD with 12 Niskin water samplers (10 I) was used to map the hydrothermal plume above Axial Volcano in three dimensions. A total of 18 CTD stations was carried out at locations inside and outside the caldera. Two CTD stations close to the ASHES Vent Field were transponder-navigated and sampled the near-field fluids. Shipboard methane analyses have shown values up to 680 nl/1 CH4, potential temperature anomalies up to 0.4°C, as well as distinct anomalies in light transmission, all about 10 to 20 m above bottom. Thermophile (> 55°C) and hyperthermophile (> 90°C) microorganisms were cultivated onboard from vent
plume samples collected with the CTD/rosette system. This is the first time that the presence of thermophile and hyperthermophile bacteria has been demonstrated in a hydrothermal plume. Similar bacteria were also cultured from samples of sphalerite and barite and in samples of gut from Paralvinellid worms collected with the TV-grab from active chimneys in the caldera. DNA analyses and epifluoreszence microscopy will further characterize these bacteria.
A mapping program using SONNE’s Hydrosweep system and Differential GPS (DGPS) was carried out to survey the area of Axial Volcano including the North and South Rift Zones. A total of 53 Hydrosweep profiles was compiled to generate a bathymetric map with 10 m contour intervals as well as a separate map for the caldera, encompassing the CASM and ASHES Fields, the South Rift Zone (5 m contour interval), and the North Rift Zone (5 m contour intervals). The bathymetry provides an important baseline for ongoing monitoring of volcanic and tecontic activity at Axial and complements existing Sea Beam bathymetric maps of the neovolcanic zone completed by NOAA in 1991. The Hydrosweep maps will be published jointly by the principal investigators as an Open File Report of the Geological Survey of Canada.
The North Rift Zone was investigated with the EXPLOS TV/camera sled system which is run on a 8,000 m fiber optic cable and rated to a water depth of 5,000 m. EXPLOS was navigated using the transponder net. A zig-zag tow along the North Rift Zone indicated abundant fissure- and fracture-controlled diffuse hydrothermal activity, marked by the presence of bacterial mats and hydrothermal fauna. A second camera tow was performed to investigated hydrothermally active areas in greater detail, and a deployment of the TV- controlled grab sampled a young lava flow observed during the camera runs. The grab recovered a large, 350 kg pillow, partly covered with fresh glass. In an attempt to measure horizontal extensions of the seafloor related to dike intrusions, five acoustic extensometer instruments were deployed in a line across the North Rift Zone at 100-200 m spacings.
These instruments are battery powered and calibrated to measure baseline distances up to about 1km with an accuracy of about 1 mm. The instruments can make one measurement per day for a year. The data are stored in a data unit which will be recovered together with the instruments in summer 1997. After operations at the North Rift Zone, three of the five transponders were released.
A deployment of the EXPLOS system in the South Rift Zone mapped additional areas of hydrothermal activity. NOAA's chemical scanner SUAVE (Submersible System Used to Assess Vented Emissions) was fitted at sea for use on the EXPLOS frame and has measured temporally coherent anomalies in temperature, light scattering, dissolved Mn, and H2S. For each parameter measured, isolated anomalies were found. The scanner was run in an unattended monitoring mode at low sensitivity with detection limits of 20nmol/l Mn,
3 5nmol/l Fe, and 40nmol/l H2S. The thermochemical anomalies (Mn/heat(=Q), Fe/Q, and H2S/Q) at the South Rift Zone are characteristic of diffuse fluid venting.
The CASM Hydrothermal Field in the northern part of the Axial Seamount caldera was sampled extensively with the TV-grab. The first deployment recovered 300 kg of basaltic material with some Fe-staining and pyrite as well as pyrrhotite occurring in vesicles. The second grab recovered 150 kg of basaltic sheet flows with distinct sulfide mineralization, anhydrite in cavities, and abundant vent fauna. The occurrence of sphalerite and chalcopyrite on the underside of broken sheet flows and the fact that tube worms observed during sampling were growing from the surface down into lava cavities and collapse pits indicates hydrothermal fluid flow beneath the lava surface. The presence of chalcopyrite in the mineral assemblage points to fluid temperatures of more than 300°C. Fluid inclusion analyses of coexisting anhydrite will constrain the fluid temperatures. Compared to earlier observations by PISCES IV and ALVIN, the main eruptive fissure at CASM appears to have been hydrothermally reactivated after a period of relative quiescence. Further deployments of the TV-grab at CASM recovered about 100 kg of massive barite with some associated sulfides, abundant vent fauna, and 500 kg of weakly altered basalt. The barite is part of the CASM chimneys which were located with the TV-grab during several crossings of the main fissure zone.
Reactivation of hydrothermal activity was also observed in the ASHES Vent Field. In 1986, ALVIN temperature measurements indicated that the Inferno chimneys were venting at about 330°C. Repeat measurements in 1995 showed that the fluid temperatures had increased by about 20° to 348°C. At a water depth of about 1,500 m (equal 150 bars or 15mPa), these fluids cross the two-phase boundary of seawater and start to boil, as observed at Inferno during ALVIN diving. A major task of the forthcoming ROPOS dives will be to sample boiling fluids and associated precipitates from the same vent to document the impact of boiling on the fractionation of metals and sulfur isotopes. A transponder net consisting of three transponders was installed at ASHES in preparation for EXPLOS and ROPOS diving. Sampling with the TV-grab in the vicinity of Virgin Mound recovered about 100 kg of massive sulfides consisting of pyrite, marcasite, sphalerite, and chalcopyrite, together with abundant tube worms, sulfide worms, and other vent fauna. Some of the samples represent small sulfide mounds of about 50 cm in diameter and 30 cm in height showing distinct high-temperature fluid channels lined with chalcopyrite. This hydrothermal area was not known previously and likely represents a new vent that has grown during the recent thermal intensification of the field. In order to preserve the active chimneys at ASHES for time-series measurements and sampling by ROPOS during Leg S0109-3, further TV- grabs were avoided. However, a camera tow just east of the chimney field revealed new areas of venting, extensively covered by white bacterial mats with abundant tube worms and
spider crabs indicative of diffuse hydrothermal flow. Several small active vents were photographed.
The CoAxial Segment, which was the site of a major eruption just north of Axial in 1993, was surveyed using EXPLOS and SUAVE. A traverse along the main eruptive fissure up to the HDV (High Diffuse Venting) site indicted several areas of diffuse hydrothermal fluid flow and associated biological activity. Both vent specific fauna and bacterial mats were found, but their extent appears to have diminished dramatically since 1993, owing to the cooling of the lava flows and the collapse of low-temperature circulation. During one survey, the SUAVE system detected 6 -8 distinct thermal and chemical anomalies, although no visible indications of active venting were observed during this tow. The H2S/Q value determined by SUAVE for CoAxial (5.3 nmol/J) is similar to the value measured in 1995 (3.8 nmol/J) but much lower than the pulse of high H2S/Q (32 nmol/J) that immediately followed the dike intrusion associated with the 1993 eruption. A TV-grab at the HDV site successfully sampled vent fauna, but the sample was washed out during heaving of the grab.
On June 22, R/V SONNE started an intense three-day work program at the Cascadia Margin (90nm off-shore Astoria). Four transponders were deployed and calibrated in preparation for ROPOS dives at this site during S0109-3. However, DGPS was not available on the second and third day of operations due to a failure in the ship's INMARSAT system. The aim of the Cascadia Margin program was to observe and sample cold seeps and associated fauna located at a bioherm in this area during Leg S0109-1 and on previous ALVIN dives. Work during S0109-2 included detailed bathymetric mapping with Hydrosweep, CTD/rosette sampler depolyments, EXPLOS runs, and sampling with the TV-grab. Shipboard analyses of water samples have revealed methane concentrations up to 4,700 nl/l and clearly documented extensive degassing at the seafloor. However, tides and currents appear to significantly affect the distribution pattern of plumes in this environment. H2S is an important constituent of gas hydrates (CH4 + H2S), and abundant sulfidic muds were sampled during Leg S0109-2. The SUAVE scanner was mounted on the EXPLOS system in an experiment to map H2S in the water column. However, no anomalous values were observed, suggesting that insufficient heat is available to support a buoyant plume of H2S. Several EXPLOS runs were succsessful in delineating areas with abundant cold seep fauna, often located in bathymetric depressions similar to collapse pits. Sampling of those areas with the TV-grab recovered indurated sediments and limestone as well as abundant cold seep fauna.
Degassing of H2S was obvious during heaving of one of the grabs, and precautions were taken to avoid any hazards resulting from H2S degassing on deck (personnel wearing gas masks). The content of this grab consisted of spectacular samples of indurated sediments, with complex degassing channels similar to small chimneys, and abundant fauna specific to cold seeps.
On June 25, at 05:24 local time, the R/V SONNE departed the Cascadia Margin for Astoria/Oregon. Leg S0109-2 ended at 15:00, when the ship was tied up at the pier of Astoria. After exchange of personal and loading of equipment and supplies, the ship departed for Victoria on June 26. On June 27, the ROPOS winch, with 3,000 m of fiber optic cable, was loaded and testing was carried out in preparation for ROPOS dives at Axial Volcano during Leg S0109-3.
4.3 O perations Background