Gorda Ridge Cruise
August 5 - 21, 2002
August 6, 2000: Day #2
Dave Clague writes:
August 6 Report from Gorda Ridge
Dive 185, in the axial valley of the northern Gorda Ridge, was short due to a thruster failure. However, we collected two lava samples from old sediment-covered flows that were cut by many faults and fissures. We also tested the glass sampler, recovering one sample, but discovered that the glass rinds on flows were easy to shatter, but difficult to suction into the sample carousel. After a rapid repair of the thruster and some work on the glass sampler, we dove again and went to the "SeaCliff" off-axis hydrothermal field that was to have been the end of the planned first dive. Moving upslope on the eastern wall of the axial valley from 2850 m, we first encountered abundant talus composed of hydrothermal deposits. We followed this talus upslope to a large field of active chimneys and areas of more diffuse flow characterized by abundant masses of vestimentiferan tubeworms and large patches of limpets and palm worms. Some blind sea spiders (pycnogonids) were recovered with the worms and limpets.
An unusual blue bacterial mat coats much of the hydrothermal crust in this region. We attempted to measure temperatures in the vents, but the temperature probe did not work properly. On recovery, we found that the high and low temperature sensors were incorrectly connected to the pressure housing with the control electronics. We sampled vent fluids from one vent, melting the hydraulic line to the trigger for the bottles (but verifying that these vents are high temperature!), so the gas tight and titanium major bottles were triggered using two manipulators-one to hold the bottle and one to trigger it.
Preliminary analysis of the recovered fluids shows that the vent water has lower salinity than seawater and higher pH than vent water from most other vents. The fluids have precipitated 1-2 m chimneys that are dominated by coarse anhydrite with an inner lining of pale green clay and trace amounts of fine-grained sulfides. These chimneys look like those from Middle Valley. Basalt talus in the vent field is extensively altered. Pillow lava flows above and below the region of hydrothermal activity appear old, but are not excessively altered. We plan to return to this vent field in two days for more complete water and rock sampling, temperature measurements, and biologic collections, in addition to mapping the entire deposit. This site might be located near an off-axis eruption that produced hyaloclastite and pillow flows that drape the scarp, so we will also map and collect these volcanic rocks.
On Monday, we will be mapping and collecting lavas in the axial valley in an attempt to locate the vents and flows from a fissure eruption that occurred in 1996. At night, we have been using the "rock crusher" to sample basalt glass from specific volcanic cones and ridges identified using MBARI's Simrad EM300 bathymetry.