Gorda Ridge Cruise
August 5 - 21, 2002
August 14, 2000: Day #10
Dave Clague writes: Our dive today went to the hydrothermal vent field at the northern Escanaba Trough (NESCA). This site was drilled by the Ocean Drilling Program (ODP) and one of our objectives was to see if the drill-holes had become hydrothermal vents. We started well, landing almost within sight of a marker left during a previous ALVIN dive and only a short distance from the first of the drill-holes we wanted to visit. This drill hole is located near some high-temperature vents, which we relocated and sampled for fluid chemistry after measuring their temperatures. These vents were still about the same 215 degree centigrade temperature they were in 1988 when they were last sampled. The samples collected for gas analyses are the first ever collected here.
The vents support large clumps of Riftia tubeworms and associated vent fauna and form chimney structures several meters tall. After completing the fluid collection (4 bottles), we searched for the first drill hole and found it colonized by vent fauna and emitting a small amount of shimmering water. We then drove down slope to a site where 108 degree C water had been sampled in 1988, but the mound was inactive and no longer supported vent fauna. We located another drill hole, discovering a previously unknown field of inactive barite chimneys on the way. This hole was not venting any fluids. We returned to the first site, where we had left a homer beacon, and then traversed across the summit of a large ridge of massive sulfide to two more drill holes, neither of which was venting fluids. We then visited the top of the hill where the hole was drilled to basaltic basement, but no fluid was venting here either. We continued to the southeast towards a region where active vents with chemosynthetic clams have been seen in the past.
On our way to this site we encountered a drill-string that was dropped during the ODP operations here. The ship was located more than half a kilometer from the site where we encountered the drill pipe, indicating that the pipe is more-or-less laid out across the seafloor rather than in a pretzel-shaped pile. The pipe was not in a straight line but had buckled and pipe was crossing pipe and we decided to avoid the area for fear of entangling the tether. At this point, we triggered the last major water sampler to obtain a background seawater sample. We then went to the northern flank of the hill and searched for new vent sites and clams. Numerous large inactive sulfide mounds were seen, but no active vent sites and no live clams. We returned to the area where we had started and collected samples of sulfide chimneys and mounds, vent fauna, and pushcores of bacterial mats. One of the cores also contained hydrocarbons along with the sulfide materials. We triggered the final gas-tight water sampler in the same vent we had collected a sample in earlier this morning. We recovered the homer beacon and returned to the surface having accomplished nearly all of our objectives.
On Tuesday we will be diving on a cluster of small volcanic cones at the northern end of the Escanaba Trough, collecting lava samples and examining the structure of these volcanic cones.