West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook
July 23, 2002: Day #4
Rob Zierenberg writes: The second dive of our cruise was on a field of hydrothermal vents at the southern end of Gorda Ridge. Gorda Ridge is a seafloor spreading center, where two oceanic plates are slowly spreading apart, causing magma to erupt from the fissure and form new oceanic crust. This site is somewhat unusual, in that the spreading center is covered by sediment, and the magma flows into the sediment rather than erupting to the surface.
These vents were discovered and sampled in 1988 using the Alvin submersible. When scientists returned in 2000, several of the hydrothermal vents had stopped releasing fluids, and many of the communities of animals that live around the vents had vanished. This year we returned to what had been the highest temperature hydrothermal vent, where fluids of about 215° C had been found in 1988. When this mound was sampled in 2000, we found that the temperature had not changed, and the chemical composition of the fluids was very close to what was measured in 1988. 
Yesterday’s dive showed that the temperature has remained steady, but we will not know until we get back to the lab what the fluid composition is. The sulfide mound surrounding the vent still has a dense covering of vent fauna, including tube worms, palm worms, snails, limpets, anemones, scale worms, and pycnogonids, or sea spiders. A small new cluster of tube worms has grown near the site we sampled in 2000. We collected a few specimens of these worms for the biologist on our cruise to study.
We also returned to two drill holes that were made in 1996 by the Ocean Drilling Program. The program drilled a series of holes in this area, up to 450 meters below the seafloor, to investigate how the sulfide deposits form beneath the hydrothermal mounds, and to try to understand what controls the flow of hydrothermal fluid beneath the sea floor. One of our drill holes (1038A) is on the hydrothermal mound we sampled today. Hydrothermal fluid is now leaking from the drill hole and the walls of the hole have been colonized by vent animals. We also visited drill hole 1038H, located a few meters from a tubeworm colony on a different hydrothermal mound. The tube worms next to the drill hole were healthy and happy in the low temperature vent. This drill hole had no hot water leaking from it and therefore was not colonized by animals. Our overall impression is that the drilling did very little to change the vent field and the areas that are still active look very similar to how they were in 2000 and 1998. However, it is still clear that this vent field is in its declining years and that the vents on the edges of the field are shutting down. We still don’t know how long the vents we sampled will remain active, serving as an oasis for the bizarre and beautiful animals that live here.
