July 24 - August 6, 2006
July 31 update
Tiburon dive T1009, Axial Seamount
Dave Clague writes: Today we began the second major objective of the cruise and went to Axial Volcano to collect vibracores of clastic deposits around the summit caldera. Axial Volcano is an active large central volcano with rift zones extending north and south from a summit caldera. The last eruption took place in 1998 on the south edge of the summit and down the south rift zone. The volcano is astride the Juan de Fuca Ridge and rises about 1 kilometer above the adjacent ridge axes to a summit depth a little shallower than 1400 m. During a dive last year to service some seafloor instruments monitoring this active volcano, we discovered a surprisingly thick deposit of fine-grained clastic volcanic and hydrothermal debris that we thought might be related to the collapse that formed the summit caldera. To test this idea, we decided to collect the full thickness of this deposit and map the variation in thickness, fragment types, and grain size away from the caldera to see if the patterns observed matched what we would expect from a source in the caldera.
Since the 30 cm pushcores we collected last year had barely reached the underlying lava flows, we decided to employ a vibracorer deployed from Tiburon. The vibracore is a core barrel that penetrates into the sediment with the aid of a large vibrator that literally shakes the barrel, which helps it penetrate into sandy sediments as weight is applied to the top. We are able to collect 9, 2-meter cores during a single dive by placing most of the cores in an elevator that is dropped to the bottom separately from the ROV. The ROV then locates the elevator on the seafloor and switches filled core barrels with unused ones, carrying the elevator along as we move from site to site. When all the cores are filled, the elevator is released and rises to the surface, where it is recovered by the ship’s crew as the dive continues on the seafloor. The planned dive on the southwest side of the caldera was high risk since we really had no way to know, in advance, how thick the deposits were, nor how quickly they might thin away from the caldera. Because of this high uncertainty, we had planned entire alternate programs in case the deposits were too thin to sample with the vibracorer.
When the first vibracore penetrated a little more than a meter, we were greatly relieved, knowing that our general plan was likely to accomplish our goals. We collected cores in a sequence, spaced about 200-300 meters apart, but only the first three cores penetrated more than a meter, and the cores quickly got shorter and shorter, until the deposits were too thin to sample with the vibracorer. We continued further away from the caldera using pushcores, until we had filled all of them. Although the cores were not as long as we had thought they might be, the deposit clearly thinned quickly as we moved away from the caldera. We also have already determined that the deposits are still quite coarse-grained far from the caldera. Most of our work on this project will have to wait until we are back in our labs and can split the cores to sample them in detail.
During the dive we also collected lava samples (15 in all) that will define the composition of the lava that erupted just prior to the collapse of the caldera. We located a low-temperature hydrothermal vent site and collected a few animals along the way. The nearly-sediment covered region has sparse and non-diverse megafauna communities consisting almost entirely of two sea star species, a small white urchin, two types of sea cucumber (pannychia was one), spider crabs, and rarer anemones and barnacles.
The dive met our hopes and we are going to try to repeat the same experiment on the northeast side of the caldera tomorrow. Hoping again to be able to collect lava samples when possible, and push cores when the deposit thins beyond when vibracores are practical. No doubt, further surprises await us!
Kristen writes: Definately the best day on the ship so far. Not only did we find some pretty amazing creatures, but I got to enjoy an afternoon of sun on the deck and there was chocolate cake for dessert!! On the dives today, our main objective was to take some vibracores on the Axial seamount, which meant not too much biology for us biologists. But when the ROV came up at the end of the day, we were pleasantly surprised. We has some animals we haven't seen yet such as tunicates, and my personal favourite, barnacles! The barnacles were huge, not the same ones you see on rocks or docks on the shoreline. They had to be about the size of my thumb. So naturally, I wanted to dissect one! They were pretty hard to open, I almost needed a nutcracker. Inside, they look like any other barnacle with their legs sticking out on top, and their heads glued to the bottom. I'm glad that I am finally getting to see some new and interesting things on this cruise.