MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 10, 2005
Tiburon dive 876, South Cleft, Juan de Fuca Ridge.
The sky has been overcast since we left Newport, but the wind and chop have diminished so the ride is quite pleasant. Today's dive was a National Undersea Research Program (NURP) funded dive for Bill Chadwick to recover instruments deployed five years ago.
Bill Chadwick writes: The main task for today’s dive was to recover an array of monitoring instruments. The instruments are called acoustic extensometers and they are designed for measuring extension across the ridge axis. The Juan de Fuca Ridge is the boundary between two plates that are spreading apart at an average rate of 6 cm/yr. Basically, one side of the ridge is moving west toward Japan, and the other side is moving eastward toward north America. However, the spreading probably takes place sporadically instead of continuously. In other words, no spreading may occur for many years, and then there is a sudden spreading event. We don’t yet know how often these kinds of spreading events occur, because we have just developed the technology to measure them. That is the goal of this experiment—to continuously monitor for several years at a time in order to detect and quantify spreading events.
The array spans a distance of 1.2 km and instruments measure the distance across the array with a resolution of about 1 cm. They measure the distance by precisely timing the round trip travel time of acoustic pulses. The instruments record data internally and have to be recovered to download the data and replace their batteries. To get the instruments back we used an “elevator mooring” with giant tubes that hold the instruments.Tiburon's manipulator arm picked up each instrument in turn and placed it in the elevator (see image on right). It will take two loads with the elevator to get all the instruments back. Has the Cleft segment been spreading during the last 5 years? We don’t know yet, because we won’t be able to take a look at the data until we get the instruments back to the lab, but we are looking forward to finding out. In the meantime it is extremely gratifying just to get the instruments back. In oceanography, when you put something over the side of a ship, you never know if you will ever seen it again!
The elevator holding six extensometers is hoisted onto the fantail of the Western Flyer.
Jenny Paduan writes: First we collected six extensometers. One by one, Tiburon navigated to an extensometer, lifted it carefully off its base, carried it to an elevator that had been dropped off the stern before the ROV was launched, and gently loaded it into the elevator. Each traverse took us past lobate and sheet flows and numerous shallow, drained lava ponds. After the elevator was full, we shifted our focus to the next instruments. We transited down into the Cleft, a narrow graben along the ridge axis, and retrieved two high-temperature probes from the vent orifices of tall, slim, vigorous black smokers and replaced them with fresh ones. After the ROV was on deck, the anchor release on the elevator was tripped, and about 45 minutes later it bobbed to the surface right next to the ship and was brought aboard. It was an intense day for Bill, Nick and Bob, but the rest of us had a chance to catch up on sample processing from the first two dives, in between our regular watches in the control room: operating the ROV's science camera, annotating video, taking notes, and running the real-time mapping system.
Linda Kuhnz writes: As a biologist, it would be hard to imagine a better day. On the last two dives we have been looking at relatively new lava flows. Because it takes such a long time for animals to re-colonize after a devastating eruption, we saw few animals there. Today we saw older lavas and observed organisms that are hardy enough to manage in this relatively food-poor environment. Small brittle stars seem to be the most abundant and there are crabs here too. We saw several species of fish, numerous small sponges, a few seastars, plus some soft corals. As we traversed a dramatic wonderland of layered arches and spires, we even found an octopus on a tall lava pillar (see image on right). As we crossed over South Cleft we met up with a deep-sea skate swimming in the water column.We topped our day off at a hydrothermal vent where there were amazing sights; shimmering hot water, tall black smokers, tube worms, scale worms, snails, and crabs. Leaves me wondering what adventures await us tomorrow!