Mendocino Fracture Zone Cruise
August 24 - 31, 2000
Over 300 km off the California-Oregon Coast
August 29, 2000: Day #6
Log Entry: The Tiburon was finally diving again on Tuesday when the high winds and seas abated. Thus began a series of four dives in 38 hours. Our general plan for Dive 205 is to explore the internal lithology of the Gorda Escarpment by following a steep gully where basement rocks might peek from beneath the thick sediment cover. Once we reach the ridge summit, we plan to hop over to the adjacent peak by flying through mid-water. There we would finally, finally locate the contact between the ocean crust and overlying sediments. The rock coring sled was left on the vehicle because we might need this if these sediments were old and lithified.
Our strategy of tracking up the tight steep contours of a major gully paid off. We collected a continuous suite of basaltic rocks from 2400 to 2000 meters depth. These include both the crystalline rocks of the lower ocean crust as well as the overlying basaltic volcanic rocks. The outcrops would occasionally show shapes that were reminiscent of the young pillowed basalts of the Juan de Fuca Ridge. However, most bore evidence of their transport along the Mendocino Fracture Zone and uplift along the Gorda Escarpment. The glassy outer layers were now hackly piles of talus partially covered with sediment. Their interiors were rounded boulders and everything was covered with manganese crust. There were fewer benthic fauna clinging to the steep slopes here. The octopus is still the ever present witness to our exploration. Tiny red jellies sit on the seafloor or fly through the air. We trapped one in a pushcore to bring home for more examination.
The rock coring sled has been working well during the cruise, even though we have had to visually estimate such parameters as the weight on bit, rpm and depth of penetration. The sensors for these measurements were installed just before the cruise. They work on deck and, on today's dive, MBARI engineer Paul McGill double checked that they were indeed working during the descent. We also had a brief scare on one dive when the fitting between the rock coring motor and the rock coring string became partially pulled apart. A quick-thinking pilot shoved the connection back together and we continued with no incident.
Toward the top of the slope, the rock coring sled struggled against strong currents to line up on a target. The vehicle position was precarious, and this was made worse by a brief loss of hydraulic power. Nevertheless we were able to core 10 centimeters into fractured and veined basalt. During the withdrawal of the rock coring string, we were plagued with the problem of the insufficient connector between the rock coring motor and the rock coring string. Only this time the connection was pulled completely free. We could not rotate the rock coring manifold, and so could not take another rock core. We had also already filled up the one small compartment for grab samples that was attached to the rock coring sled. So once again, just before we go to find the sediment contact, we have to terminate the dive and head for the surface.
After only 30 minutes on deck, we are ready to redeploy the Tiburon for Dive 206. The second deployment of the day rewarded us with a visit by a ~1.3 meter long fish that looked like a silver ribbon and which no one onboard had ever seen before. The purpose of this short dive will be to recover samples across the sediment-basement contact imaged in the seismic line through this area. The seismic data display a homogeneous basement rock (which we now know is uplifted oceanic crust) overlain by deformed sediments of unknown age and lithology. Given how many times this contact has eluded us, we decide to dive straight to just below the depth predicted by the seismic record. Our plan is then to drive uphill and sample both lithologies. We first find outcrops of a dark manganese coated rock which we interpret as basalt. This gives way to patches of sand and a light-colored siltstone. These dark rocks show abundant evidence of erosion into low outcrops that we cannot core. The sediments are too soft to use the rock core or grab and we end up scooping them up in a pushcore. Finally we have the contact. Upon our return however, we discover that all the dark-colored rocks are not basalts but sediments covered with manganese. So the outcrop must be near where we landed, either covered by the talus slope or at a greater depth than we explored on this brief second dive of the day.
After Dive 206, we conferenced with the Chief Pilot and Captain to decide how to conduct the remaining dives. We had lost 2.5 dives to weather and had to leave for Eureka by the wee hours of Thursday morning. We decided that our most important science goals could be met by two dives of moderate length. One dive up the central Gorda Escarpment to follow the deepest gully and recover samples from the basement interior and surface sediments. The second dives would be much more risky. We would explore a slump deposit adjacent to a buried layer of gas hydrate to look for linkages between the formation of the slump and loss of methane from the sediments. The most dramatic linkage of course would be a cold seep community. The pilots want to begin the first dive almost immediately and follow with the second dive after only a few hours of sleep. We all agree and catnap in preparation.