Mendocino Fracture Zone Cruise
August 24 - 31, 2000
Over 300 km off the California-Oregon Coast
August 30, 2000: Day #7
Log Entry: Tiburon Dive 207 was conducted on the evening of August 29, 2000 after a remarkably brief turn-around period between dives. Dive 206 left the seafloor at about 5 pm and was back on board at about 6:30 pm. Samples were quickly off-loaded and then the pilots exchanged the rock coring sled for the standard benthic toolsled. This latter toolsled includes many more compartments for grab samples, more quivers for push cores and a biobox to collect animals. We anticipated that our last two dives would focus on a collecting a large number of diverse samples. The toolsled change-out was completed in less than an hour, despite the relatively rough seas. By 8:30 pm the vehicle was once again en route to the bottom to examine the interior of the Gorda Escarpment exposed in the largest gully on the north side. The seafloor was in site at 10 pm at a depth of over 2500 meters.
Our landing spot was a ramp of extremely fresh fault talus-the angular pieces weren't covered with manganese or sediment. We took this as evidence of very recent tectonic activity to the south, which sent the piles of broken rock pieces tumbling down to the foot of the north-facing wall. Samples from the foot of the talus ramp included conglomerates (mixtures of eroded basalt and sediment pieces glued together like cement) and well as crystalline basaltic rocks cut by veins of carbonate and sulfides. The columnar jointed outcrops, found higher up on the wall, yielded the same crystalline basalt, while the hackly vestiges of pillows yielded finer-grained basalt. Our gully proved to be a good choice with almost continuous outcrop of basement rocks, with rare benthic animals. The vehicle continued to struggle against bottom currents, however, making sampling difficult. We ultimately collected 16 grab samples of basaltic rock and sediment, along with five sediment push-cores and were back on deck by 4:30 am. The successful completion of this short, but critical, dive has provided us with adequate samples to show that the basement reflectors of the Gorda Escarpment are in fact a continuation of the Mendocino Ridge. What must await radiogenic age-dating is whether these slivers of uplifted oceanic crust derived from the Pacific plate, where they are now, or were somehow displaced from the Gorda plate to the north.
This is the last dive of our Mendocino Fracture Zone series and the final dive for the Western Flyer's expedition to the northeastern Pacific. For us, it is a risky dive that will lead us either to monotonous sediment and old faults or to a discovery of cold seeps in a new environment. The original dive plan called for three dives to look at diverse features that could be related to the presence of gas hydrates and methane venting. Because of our limited dive time, we consolidated all of these goals into a single short dive. The target for this dive is a small section of seafloor that is highly reflective to acoustic energy. It is also adjacent to a slump juxtaposed against a bright gas hydrate layer in the adjacent sediment. Methane deriving from such hydrate layers has been observed along the Oregon continental margin in a site called Hydrate Ridge. We call our site the Hydrate Slump and wonder whether the dissociation of the hydrate could have caused the slump feature to form. When present in high enough quantities, the methane derived from hydrate, or any organic-rich sediment, supports chemosynthetic communities via the coupled microbial reactions of methane oxidation and sulfate reduction. The reduced sulfide is the primary nutrient for the chemosynthetic fauna diagnostic of such benthic communities.
We have tentatively placed our divetrack to begin at the base of the slump and climb up and through the areas that are bright in our EM300 acoustic reflectivity maps. In the Monterey Bay, such areas of high reflectivity have been connected to the deposition of authigenic carbonate. The isotopic compositions of the carbonate show that it is primarily composed of carbon from the methane oxidation inherent in the cold seep environment. The third piece of the puzzle was a very subtle characteristic of the bottom morphology. The slump was in fact not a straight line, but a series of cuspate or horse-shoe shaped scarps. Once again using Monterey Bay as our model, we assumed that the headwalls of such scarps might channel the gas and fluids evolving from the adjacent sediments.
Based upon this house of cards list of assumptions and analogies, we set forth to look for evidence of seeps. After a few hours of sleep the vehicle is once again launched just before noon on August 30, 2000. We land on a sedimented bottom covered with large sea stars and brittle stars. Proceeding up the step of the slump we encounter an increasing number of normal benthic inhabitants, included many colors of sea stars, brittle stars, worm burrows, silver snails, pink hermit crabs with pink anemones riding on their back, as well as the now familiar venus flytrap anemone and octopus. Our initial discouragement over the apparent absence of carbonate (the bright reflectors were mostly basaltic rocks) was replaced by awe over the sheer numbers and varieties of benthic species. Tunicates, which had been occasionally observed on the Davidson Seamount off Central California were everywhere. Then on a patch of sediment we saw some open clamshells, which might be artifacts of a vent community. While we looked around, I thought I saw a white galatheid crab similar to those which inhabit the cool vents in the Galapagos Spreading Center. A movement caught our eye-it was a small scallop swimming away from the ROV. While we tried to catch it we found a field of clamshells including some live ones still buried in the sulfide-rich mud. As we drove around the horseshoe scarp, we found a series of fields of living chemosynthetic clams and large tubeworms living on the dark mud filling the cracks between the basalt. There are also large fish and boulders covered with what appeared to be pink fish eggs. We found abundant colorful soft corals, white sponges, gorgonians, shrimp and crabs along with the other common benthic animals, all apparently living in harmony with the chemosynthetic animals.
This is the first vent community found on an active transform and related to the presence of gas hydrate. The community is also unusual in that the animals appear to live both on sediment and tucked in and around boulders. We could not find vents with convincing fluid flow, but that can wait until next year. We decided to call it a day, a dive and a season with a basket filled to the brim with rocks and animals from a most spectacular final dive.