June 17 - June 24, 2007
Kristina, Laura R, and Lonny write: After transiting south from Pioneer Seamount through the night, we arrived at Davidson Seamount. We deployed ROV Tiburon at approximately 6:30 A.M. and hit the seafloor after an hour-long descent. The dive started in a lava pond which we discovered last fall, using our high-resolution mapping AUV. During the course of the day, the Tiburon traversed four volcanic cones and covered a depth range of 1765 m to 1380 m. These cones were selected based on previous bamboo coral observations and we were able to collect samples over the entire depth range despite some navigational challenges due to strong currents. The biology was sparse inside the lava pond, but as we ascended the steep slopes of the pond and several other cones, coral density increased. We found several species that we had not yet seen during this expedition, including a variety of primnoids. Dense stands of the bamboo coral Keratoisis sp. were found on the slopes and saddles and on one of the shallower summits we found a rich coral garden of Paragorgia arborea. These large, bright pink "bubblegum corals," were often overhanging sheer cliffs, seeming to defy gravity. Davidson Seamount also provides a unique opportunity to observe the effects of mixing between the two main currents off the coast of California, the cold California current and the warmer Davidson current.
We collected two sediment cores in the interior of a collapse pit in a young lava flow we explored today, and just beyond the edge of the flow. We are hoping the collected sediment contains many foraminifera for environmental analysis. We also collected a number of water samples.Dave writes: Today’s dive started on an unusual volcanic feature we had discovered during a high-resolution bathymetric survey conducted last fall using the new MBARI Mapping AUV D. Allan B. The map showed a fluid lava pond between several of the steep-sided cones that are the normal expression of volcanic activity on Davidson. Our dive today showed that this flow is indeed a fluid flow formed by rapid eruption of fluid lava and that the lava drained out of the pond, perhaps back down the eruptive vents.
The mapping data hinted that the pond was younger than all the cones around it, but the samples and dive observations provide evidence that the flow may be much younger than other Davidson eruptions, which range from 9.8-14.8 million years old. We observed only a thin cover of sediment on the pond, thin Mn-oxide crusts on the rocks (about 2 mm from visual observation), and the presence of volcanic glass beneath the Mn-crusts. We collected 6 samples on the flow for chemical analysis and age dating so hope to be able to rigorously test our ideas about the youthfulness of this eruption. Another seamount located offshore Pt. Conception named San Juan Seamount, on which we will also be diving this voyage, had a very prolonged eruptive history with lavas dated from 18.7 to 2.7 million years. We think Davidson may turn out to have had a similarly long history.
Much of the rest of the dive returned to volcanic cones we have collected volcanic rocks from previously and we did not add to those collections. At the end of the dive, we located the cone from which an unusual lava type named trachyte erupted 14.8 million years ago. The dated sample and one other both came from dives that had experienced some navigation problems and the location of these important samples was uncertain.After today's dive was completed we returned to a sedimented area observed at the start of the dive and attempted to collect a 3-meter gravity core, but the core apparently bounced on the gravel surface andno core was recovered. Determining the thickness of and recovering the foraminifera ooze beneath the gravel will have to await a return trip, perhaps with different sampling gear. We wanted to recover thissediment because the foraminifera, which live in the surface water, record the climatic conditions during their lifetimes in their shells and so might record the history of climate offshore California for the last several hundreds of thousands to perhaps millions of years.
Craig writes: Today, Day 3, we moved on from Pioneer Seamount, approximately parallel with San Francisco, to Davidson Seamount further south, approximately parallel with the Big Sur coast. Davidson Seamount will likely soon be added to the Monterey Bay Marine Sanctuary and has received an increased amount of attention both by scientists and the public. This is my second time to visit Davidson and I was no less in awe this time around. Previously, my work focused on the southern-most region of Davidson. Today we dropped down on the northern end, a potentially younger volcanic region. Lava pillows, tubes, and domes dominated the flanks. We eventually transversed the flank and explored around an extinct lava pond. Most of the pond and flanks were either void of life or possessed very low densities of organisms. Although speculative, its is likely that these regions experience little current and thus settling larvae rarely make it to this area. Alternatively, low current speeds may prevent many organisms that survive on filtering the water for food to flourish here.
The highlights of today’s dive were capturing lengthy video of both a chimera and a blob sculpin (Psychrolutes sp., photo on right) swimming above the seafloor. From the pond we ascended onto shallower cones and explored the coral gardens and sponge meadows that Davidson is known for. I lucked out again today capturing another of the, apparently not so rare, nudibranchs. Tomorrow, we steam south to the Patton Escarpment, our deepest dives during the expedition.