Detailed Observations and Sampling of the Sea Cliff Hydrothermal Field (GR-14) on the Northern Gorda Ridge

During the summer of 2000, on a research cruise of the MBARI research ship, Western Flyer, we deployed the ROV Tiburon for a series of dives at the Sea Cliff Hydrothermal site on the northern Gorda Ridge. The Sea Cliff site is near the top of a terrace on the west facing rift valley wall (fault), about 300 meters above, and 3 km east of the ridge axis. The 1996 Gorda Ridge eruption occurred on axis in the region west and south of the vent site. The vents were first predicted on the basis of water column anomalies and seafloor structure, and the field was discovered in 1988 during dives of the Sea Cliff submersible. In 2000, we made 4 dives at the site and collected a suite of rock and vent fluid samples. The high temperature water vents from as many as 10 individual chimneys. Measured vent temperatures at several of the chimneys fall in a narrow range of around 304\deg C. The chimneys are arrayed along two low ridges that are oriented roughly perpendicular to the strike of the rift valley. Venting fluids have low salinity indicating subsurface phase separation. The waters are isotopically enriched (\delta ¹⁸O = 1.9%), suggesting extensive water-rock interaction. The chimneys themselves are primarily anhydrite and a pale green Mg-rich clay with minor amounts of amorphous silica, pyrrhotite, wurtzite, and isocubanite. The chimneys are delicate and are surrounded by aprons (5 -10 m) of collapsed chimney material. As a result, no macro fauna were observed colonizing the high temperature vents. Silica-rich hydrothermal crust and talus cover the fault slope. A broad region of diffuse venting surrounds the active chimneys and locally supports a rich biological community that includes blue ciliate mats near the vents, that give way to tube worm fields and low tube worm mounds formed on massive barite. The Sea Cliff Hydrothermal site is unusual in that it lies off axis and above the rift valley floor. Faulting must play a role in its location and perhaps geometry, and the orientation of the chimney ridges is consistent with east-west lineaments noted in previous work. It is unclear if the faults link the site to the axis, where recent volcanism has been observed. Temporal variations in the Sea Cliff site are difficult to assess based on the limited information from previous dives in the area. In general, the geometry of the field appears unchanged since first visited in 1988, and we observed many of the same types of hydrothermal features. We found slightly higher vent temperatures and many more chimneys than observed previously, but that may reflect the improved navigation and the increased bottom time permitted by the use of the ROV. Future studies will better constrain the answers to the questions about the stability of the field.