West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
July 25, 2002: Day #6
Today was an exceptionally long dive, and the most successful thus far.
Dive 452 Update:
We completed a second dive at the NESCA hydrothermal vent site. Hot water samples were collected at the same vents that we sampled in 2000; comparison of these will let us see how much the fluid composition has changed. We collected tube worms that had colonized the area since our visit to the area two years ago for genetic studies. We also used a new suction sampler to sample everything from filamentous bacterial mats to samples of basaltic glass. The sampler was particularly helpful in collecting small animals like snails, limpets, pycnogonids, and a poorly known but very pretty blue colonial ciliate that covered a rocky area about 15 meters from low temperature vents that support the tube worms. We made a short traverse to a different massive sulfide mound that has some patches of very long, up to 1.5 m, tube worms that are colonized by anemones. Our biological collections from this site will be compared with the animals found at the mound venting 220 degree C fluids. At present, only low temperature fluids vent from this sulfide mound, but the metallic sulfide minerals that form the mound tell us that this mound previously vented fluids as hot as 330 degrees C, but we can't tell when the hot fluids were released.
Open shells of large clams are occasionally found in this area, but we have never found live clams at this site. In 1988, we found an area at the base of a large massive sulfide deposit about 800 meters away from this site that had live clams in the sediment. The biologists are very interested in these clams, so after we finished our sampling in the main vent area, we set out to see if we could find the clam area, and, with luck, live clams. We were able to find an area with lots of dead clam shells and followed these to the massive sulfide area where we found and sampled clams living in sulfidic sediment. No hydrothermal fluids can be seen to vent at this site, but energy-rich gases must flow diffusely through the sediment here to fuel bacteria that in turn support the large clams. The largest clam was 27 cm long! The clams live with their foot and most of their body buried in the sulfidic sediment but extend their siphons upward in oxygen-bearing bottom water. We found white anemones with red tentacles living on the top of each clam shell where it sticks above the sediment. The sulfide deposit near the clams is very large, but is also very old and weathered. Most sulfide minerals exposed near the seafloor had weathered to iron oxide.
The clams occur in a very complex area of the seafloor characterized by long, deep channels that we think formed when the sediment hill was uplifted and of the slope slumped at its edges. The hill was uplifted by intrusion of basaltic magma into the sediment at a depth of about 400 meters. Basalt lava flows also are present just to the east of the sediment hill near the clam site. The source of the lava flows has been a mystery. Our previous mapping suggested that a high point on the flow just southeast of the clam site was a likely site. Near the end of the dive, we drove to this site and found very young, glassy sheet flows and a lava tube, consistent with this area being near the vent area for the basalt flow. Samples of the basaltic glass were collected for comparison with samples that had been previously collected from this lava flow.