March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 28, 2001: Leg 3; Day 8
Judith Connor writes: Saturday morning looks encouraging; it's clear, sunny and relatively calm on the south side of Molokai. Overnight, the ROV pilots repaired the umbilical and are now making the final preparations for our next dive to 650 m. In the galley, the early morning discussions range from planning vehicle and ship maintenance to human impacts on coral reefs and the date humans arrived on the Hawaiian Islands. From technicians and scientists to captain and crew, the staff all show great dedication to their work at MBARI.
Gary Greene writes: Dive 308 was located at the head of a canyon offshore of the Amikopala area of the southern leeward side of Molokai. The canyon is the only one that exists along the southern side of the island of Molokai. This is a second priority site, picked as an alternate site to study because the weather would not allow for operations in the high priority areas. The objective was to compare canyons that formed on the windward sides of the islands with ones that formed on leeward sides.
Six rock samples and one push core sample were collected and continuous water sampling was done through about half of the dive. Bathymetric data for the canyon is not high quality (no multibeam data exists for the area) and the sonar on the vehicle did not operate during the dive. Therefore, the Tiburon’s position with respect to the topography of the canyon was poorly constrained.
A NE directed dive transect was initiated at a depth of 689 m near the axis of the canyon. The first half of the transect was entirely over a muddy bottom. Here the surface was slightly bioturbated, supporting a few holothurians and an occasional flytrap anemone, probably attached to a rock buried beneath the mud. Shallow grooves similar to trawl marks were also observed.
At a depth of 608 m the base of an extensive rock outcrop was encountered. This outcrop represents the eastern wall of the canyon and is composed of cemented mudstones and sandstones. The base of the outcrop is a sharp clean contact with mud sediment. At 606 m a carbonate grainstone overlain by white cement and a black (basaltic?) pebble/cobble conglomerate, a possible shoreline deposit, was encountered. From 605 m to 383 m water depth the transect ran up a steep (45° to near vertical) 222 m high slope comprised of extensive carbonates that stair-stepped upwards and alternated between carbonate exposures and mud-covered slopes that locally were rippled. Ripple marks were oriented N-S indicating E-W flowing bottom currents. The carbonate exposures were generally gray to brown in color, locally coated with black manganese deposits, were vuggy (full of irregular cavities) and highly grooved and fluted. In many places caves and tunnels could be seen. However, these units were very difficult to sample. The few pieces that were recovered are firmly cemented grainstones, lacking corals. While these rocks appear to have a shallow water origin, they differ from the reefal carbonates that we have now sampled at numerous other locations. The dive terminated at 383 m on a gentle slope.
Initial assessment of Dive 308 suggests that the canyon is inactive. No evidence for significant scouring of walls was observed, as was common in the canyons on the windward side of the island. Little volcanic rocks were found and the canyon appears to have been eroded into carbonate bank deposits that since have subsided and been covered with mud.
Jenny Paduan writes: We just completed the second dive of the day (dive 309), and 35 knot winds and a 1.5 knot current may preclude us from doing the third. We've just run 34 samples through the assembly line: scrubbing, describing, drawing, and photographing. Next they will get sawed and dried.