March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 13, 2001: Leg 2; Day 13
Kyra Schlining writes: We are still busy sampling rocks out here off the west coast of Hawaii. Tonight breaks our record for number of samples collected in a single dive, but I will let Dave Clague tell you about that in his next report. I thought it might be interesting to walk our web viewers through the rock sampling procedure we have out here on the Western Flyer.
The chief scientist identifies an interesting piece of pillow basalt from which they would like to collect a sample.
The ROV pilots very carefully pick out the wedge that the chief scientist points out using the manipulator arm on the ROV. A frame grab is taken from the video for each sample and stored in MBARI's extensive annotation and samples database for later access by the scientists.
The sample is then given a number (this pillow wedge is #T290-R13 for Tiburon Dive 290 Rock 13) and placed into one of the numbered compartments in a drawer located under the ROV for safe transport back to the ship. The time of collection, latitude and longitude, and collection depth is also recorded for each sample.
Once our sample, Rock 13, is brought back on board it is labeled, then put through a rigorous assembly line of processing in the wet lab. First the sample is scrubbed to get any mud or biological growth off of it. Tonight is Kristen's turn to wash.
Then the sample is sketched and described. Upon closer observation, Rendy notices some large glass fragments still present in the rind of Rock 13. This is good news because the glass can be used to identify the parent magma chemical composition for this rock. Each rock gets an official sample photograph with its sample number and a scale bar for archival purposes.
The next day Rock 13 will have a 1 cm thick slab sawed out of it from which thin sections will be made for the geologists to observe the microscopic features of the rock. Finally, all the carefully labeled samples and slabs are boxed up to be shipped back to MBARI after the cruise gets back to port.
Dave Clague writes: Our dive operations continue to be limited by strong trade winds and surface currents. We tried again at several of our highest priority sites on the Mahukona terraces before completing yet another dive on the 400 m reef this morning. The dive was mainly designed to examine the reef lithologies of the carbonates that make up the reef face, the reef flat, the lagoon, and the elongate ridges in the lagoons. Dive 292, across a segment of the 400m reef near its southern end, began at about 19 degrees 53 minutes N and 156 degrees 6 minutes west, from close to the summit of a knoll just seaward of the main reef escarpment. The dive descended the east side of the knoll and went up the main escarpment to the break in slope at its summit. Then it went southward along the summit plateau, traversing and sampling outcrops of limestone exposed above the modern sediment cover, before turning eastward to explore low outcrops farther shoreward on the summit plateau. The dominant type of rock cropping out along the dive traverse (in fact virtually the only type of rock) is the now-familiar massive limestone with "swiss-cheese" weathering that creates small caverns with arched roofs and upright pillars in some places inhabited by colorful fish. Corals in good condition are not evident at the outcrop but several were revealed in sawed specimens. The outcrops at the summit level are low knobs and whale-back ridges, some aligned parallel to the main escarpment, some normal to the main escarpment, suggesting construction or shaping by currents.
By the end of the shallow dive, the winds, seas, and currents had died off enough that we returned to a site on the 1150 meter deep terrace and finally dove on this important feature. This terrace is thought to have drowned about 430 thousand years ago. The terrace is different from the others in the region in having two distinct levels, an outer terrace bench at about 1195 meters and an inner one at about 1150 meters. We designed a dive to sample both. We landed in a fairyland of coral debris and upright corals in growth position, apparently exposed as the surrounding cement preferentially dissolved over time. Not knowing when the strong winds or currents might return, we quickly collected an array of these corals. We drove along the top edge of the reef, dropped back down the reef face, and returned again to the top edge of the reef. The entire area was a blanket of coral fragments and in-place corals. Occassional slabs of black rock littered the surface. These slabs turned out to be composed of phosphorite. The dive ended on the shallower terrace where we collected more corals, although these were smaller and cemented in place to a far greater degree than those on the deeper terrace step. The phosphorite slabs formed a near continuous pavement on parts of the upper terrace. The dive set a new record for samples collected on a single dive, returning to the surface with 57 samples of coral and phosphorite! The initial labelling, cleaning, and photography of these samples took until after midnight and the sawing and descriptions will keep our science party quite busy in the lab for a good part of Saturday.
With a forecast for improving conditions, and the success of the dive on the 1150-meter terrace, we decided to stay in the Mahukona area and try to complete two of our highest priority dives. One of these, on the 1275 m terrace, several volcanic cones and a spatter rampart formed along an eruptive fissure in the western part of the terrace region. The second site is an elongate collapsed feature (a graben) with a pit crater near the 1335 m terrace. This graben and pit collapsed over a shallow volcanic dike. The pit is about the same depth as the terrace and might provide a cross section of the terrace. We had not yet attempted the first because of its location in the Alenuihaha channel (between Maui and Hawaii), but had been to the pit crater site three times previously, but had been unable to dive there due to the sea and wind conditions. Morning arrived with stronger winds and a new gale warning for the channel. For two hours, we searched for a place where we could work. At about 8 am we finally conceded and set sail for some dives on an unsampled rift zone southwest of Kahoolawe Island. Hopefully this site will be protected from the trade winds by Maui.