March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 9, 2001: Leg 2; Day 9
Kyra Schlining writes: We just launched on our third dive for the day. We are close to the spot where we dove yesterday. We are still struggling with less than ideal weather, but managing to find some pretty interesting geological features on every dive. Jerry Winterer and Josh Plant are busily sawing rocks outside, in search for carbonate containing ancient corals.
Oh, down to 300-meter, better run!
Dave Clague writes: Monday began with the same strong winds and surface currents that have hampered our operations all week. This morning we steamed to 5 sites before we found a place with low enough winds and currents. The dive was in the lagoon behind the 400-meter reef, where we wanted to explore some linear features in the bathymetric map and to ground truth the seafloor characteristics of the areas with high and low amplitude sonar backscatter. The dive turned out to be quite brief, as we had our first mechanical problem when a thruster controller failed. We recovered the vehicle, fixed the problem in a few minutes and resumed the dive where we left off. The outcrops of limestone are very eroded and many appear to be displaced blocks. The areas of low amplitude sidescan are sandy and the high amplitude areas consist of limestone. We did not observe anything that clarified the origin of the linear ridges and troughs. A highlight of the dive was recovery of one coral, suitable for U-Th dating, from the lagoon region. Corals in this location would be the last ones growing prior to drowning of the reef as sea level rose rapidly at the end of the last glacial period.
The next dive returned to the 585-meter reef because only one of the 25 limestone samples that we collected yesterday turned out to be coral. We elected to dive a few miles north of our previous dive, hoping to avoid the thick basalt flow that created the vertical cliff at the top of the terrace on yesterday's dive. We landed near the base of the reef in a mix of limestone and basalt fragments, worked our way up the reef face and then drove along the top edge of the terrace. Within a short time we located an area with abundant coral fragments, and we filled several of Tiburon's sample bins. The dive was visually very dramatic because the edge of the cliff varied from basalt flows, to rippled sand, to coral patches. We traversed down the reef face, which is a steep, slope-parallel pavement of cemented reefal debris. At the base, we found many caves where a resistant horizontal limestone layer had been left in place after limestone had eroded or dissolved out from underneath it. Similar cantilevered sheets of limestone were seen earlier near the rim of the terrace. We were hoping to find more corals in place near the base of the terrace and finally located an area with many fragments and larger corals, including a staghorn variety we have not seen previously. Having accomplished all our goals and then some, we ended the dive and recovered the vehicle. Most of the carbonate samples recovered are coral, so we will be able to determine the age of both the top and bottom of this reef.
In case I forgot to explain why coral is so important... Reef corals make their skeletons from aragonite, a metastable form of calcium carbonate, whereas most other marine organisms make their skeletons from calcite. Many limestones recrystallize to calcite over time and this resets their isotopic "clocks". If they begin as calcite and end as calcite, you cannot determine whether they have recrystallized. Corals, however, begin as aragonite and if they are still aragonite when we analyze them then we know that they have not been recrystallized.
On all of our dives we have observed a great variety of benthic animals, including gorgonians, sponges, fishes, stars, sea cucumbers, anemones, hydroids, crabs, and shrimp. We are all learning the names of the more common ones, but we have been unable to identify several of them. Some animals have been inadvertently recovered on rocks and have been preserved for study by biologists not on the cruise.
Tomorrow we will make our third attempt to dive on the deepest terrace on Mahukona, but alternate sites have already been selected!