Leg 6: April 11, 2012
Day 4: More cores and curious clams
Before sunrise, the winds picked up to 48 kilometers per hour (30 mph) delaying the launch of our first dive of the day for a short while. Up until today, the seas have been flat and the weather balmy, quite different from our spring cruises in Monterey Bay, California. This morning resembled Monterey Bay, with high winds and churned up, gray seas. It was amazing how quickly the weather changed and the pilots prepared the vehicle for launch at Site 4, an area that was surveyed with the mapping autonomous underwater vehicle (AUV) during the R/V Zephyr‘s operation in March. We dove to 1,000 meters (3,280 feet) depth to explore a large depression about 80 meters (260 feet) in diameter that we could see on our bathymetric maps. This depression in the seafloor is most likely a slide scarp from a past seafloor failure event.
Today we observed and sampled an unusual clam belonging to the genus Acesta. It looks somewhat like a scallop, but it is unrelated. The clam appeared to sit in a flat field of soft sediments at about 1,000 meters (3,280 feet) depth. When we collected it with the remotely operated vehicle (ROV) Doc Ricketts’s robotic arm, we found that the clam was attached with byssal threads—strong, fine, silky fibers made from proteins—to a carbonate rock buried beneath a thin veneer of sediment. Acesta clams in the Pacific always seem to be attached to rocky outcroppings.
We hope to find more of these unusual clams tomorrow as we explore seeps near Bahia Concepcion on the west side of the Gulf of California. David Clague also plans to look for more of these clams during his leg of the Gulf of California Expedition later this month. With the added samples, we should be able to determine if the Gulf of California clams can be assigned to Acesta sphoni, a species similarly found in the low-oxygen zones of some seamounts and along the margins of central and southern California. Genetic studies back at MBARI will help to settle that question.
For our second dive, we transited about 1.6 kilometers (one mile) east of the depression site to dive at 850 meters (2,800 feet) on a fault scarp. A number of cruise entries from the previous leg of this expedition have discussed types of faults here in the Gulf of California. Deep-sea faults can provide us with a wealth of information about modern faults. Terrestrial faults, like the San Andreas Fault that runs through California, have been altered by extensive human activity, erosion, floods, and silting, making them difficult to study and interpret. Most deep-sea faults are in pristine condition for analysis with geological tools and their patterns are directly applicable to other faults around the world.
Today we collected several vibracores and push cores from both sides of this fault scarp, the Pacific Plate and North American Plate, to determine the offset between the two and the age of some of the smaller features. We explored the area for some time, looking for evidence of recent activity and surveying the biological organisms that inhabit the outcrops and fields of broken rock along the scarp.