West Coast Expedition
July 20 - August 30, 2002
West Coast of North America

July 30, 2002: Day #11

This frame grab depicts temperature measurements among a tubeworm "bush" in an area of diffuse venting. In addition to tubeworms, limpets and palm worms are in the view. The charred wood is a strength member attached to protect the metal temperature probe.

Log Entry: On Tuesday, July 30, 2002, we undertook the second of three dives at GR-14, the Sea Cliff Hydrothermal Field on the northern Gorda Ridge. Thanks to the superb efforts of the Tiburon pilots, as well as the officers and crew of the Western Flyer, we were able to complete a wide variety of tasks. Each was related to the overall goal of understanding the processes related to this enigmatic hydrothermal site.

During Tuesday's dive we made a detailed survey of the vent field. The purpose of this was to better characterize the field's geometry, and to document any changes that have occurred since our earlier dive series in 2000. Our preliminary observations indicate that the field's overall size has not changed. However, it appears that, within in the vent field, there are more active vents and chimney structures.

As a part of the day's efforts, we sampled water from three vents. These included vents at the southern terminus of the field that have not been sampled before. We recovered 9 rock samples, including 5 samples of the hydrothermal crust that dominates the field. We recovered a temperature probe that sampled temperatures every 12 seconds over 48 hours. It revealed that vent waters do not change on a short time scale, and do not appear to have changed from two years ago.

Finally we recovered a significant sample of tube worms from the diffusely venting periphery of the hydrothermal field. A preliminary examination of the worms indicates that they are malnourished, which suggests that the diffuse flow is diminishing in sulfide concentration.

Overall, there is contradictory evidence about changes in the vent field. We would expect such contradictions if short term changes are driven by earthquakes on the fault zones that provide conduits for fluid flow. That is, the fluids don't change, but the vents do.

-- Jim McClain

Figure 1frame.jpg
A chain of anhydrite chimneys located in the Sea Cliff Field at a depth of 2700 meters below the ocean surface. The consistent orientation for these chains reveals an underlying structural fabric that provides conduits for hot fluids to reach the seafloor. The water emitted from these vents reaches temperatures of over 300 degrees Celsius. The fragile chimneys can only grow one to two meters high before they will collapse, only to be replaced by a new structure.
Sampling an "elephant trunk" bud from a pillow basalt on the seafloor. The Tiburon's manipulator arm is being readied to break off a small piece for later laboratory analysis.


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