Monterey Bay Aquarium Research Institute

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

July 25, 2002: Day #6

Log Entry:

Today was an exceptionally long dive, and the most successful thus far.
Best Wishes,
Jim McClain

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.

--Rob Zierenberg

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This is the recruitment experiment, where blocks of wood have been placed on the seafloor during Dive 452 on the Tiburon. The blocks (encased in mesh bags) are made of fir and oak, and are expected to provide nutrients and living space for new communities of benthic animals. Markers are also placed in the area so that the wood will be recovered after at least one year. The experiment is being conducted by Dr. Janet Voight of the Field Museum in Chicago.
Measuring the temperature of the clear, shimmering fluid coming out of an orifice in the NESCA hydrothermal vent field. A high-tech green broom-stick is helping to keep rigid the thin, metal thermistor probe. The highest temperature we've found so far is 217 degrees C.

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Enormous clams, up to 27 cm long, are the largest vesicomyid clams we believe have been found yet in hydrothermal systems other than at the East Pacific Rise. We have also found Ridgia (vestimentiferan tubeworms), some snail species typical of the Juan de Fuca Ridge and one that is endemic (known only at Gorda Ridge vents), Paralvinella (an orange plumed polychaete worm of a family known only at hydrothermal vents), polynoids (predatory scaleworms), and Solenogaster (mollusks that are worm-shaped and have spicules rather than shells).
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A tumulus (lava tube that inflated and cracked the overlying, older rock) discovered SE of the NESCA site. It was found at a higher elevation than the majority of the lava flow and near ponded sheet flows and collapse features, which indicates a high effusion rate near the vent.


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