March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 2, 2001: Leg 2; Day 2
Dave Clague writes: We arrived at our first dive target offshore the north Kona coast following an all night transit from Oahu to Hawaii. Two striking characteristics of the area are a series of drowned coral reefs and several submarine fissures consisting of volcanic cones aligned in lines. Extensive lava flows erupted from these fissures can be discerned on the high-resolution bathymetry we collected in 1998
The series of coral reefs each formed close to sea level and have now subsided with the rest of the island of Hawaii. The subsidence is caused by flexure of the lithosphere as the Hawaiian Islands grow on top of the Pacific Plate. Previous studies, based on dredged samples and samples collected by manned submersibles, have demonstrated that the deeper reefs are progressively older than the shallower reefs. The reefs form as series of stairsteps because the reefs grow when sea level is falling during glacial climate periods and they drown when sea level rises during interglacial periods. The reefs are inferred to have drowned at about 15, 130, 245, 335, 430, and 590 thousand years ago. The dive was designed to collect samples from an eruptive fissure that crosscuts several of the drowned reefs off the Kona coast and to examine the reef face on the -950 m reef (335 thousand years old). We began the dive in the summit crater of the largest volcanic cone along the line of vents. The crater is about 50 m deep and the inside walls are mainly comprised of angular blocks of basalt. On leaving the cone, we finally encountered some in-place pillow lava at the crater rim, but the outer slopes were also comprised of angular basalt blocks. Few of the blocks have the characteristic wedge-shape of pillow lava fragments, suggesting that the surface consists are blocky 'a'a-like flows. We progressed to the next cone in the line of vents and found similar outcrops, although this cone has no summit crater. On our way to the third cone, we found a flat plain of reef carbonate. This carbonate is the top of the -1150 m reef (430 thousand years old). We were able to collect several samples of staghorn coral (genus Acropora) that may be suitable for age dating using the uranium-thorium system. These corals were on the top of the reef and presumably represent the last corals to grow before the reef drowned.
We then collected samples from the third volcanic cone, which had abundant gravel-sized debris on its lower slopes. At this point, it became evident that our slow progress, due to strong surface currents that affected the ship's ability to closely follow the remotely operated vehicle, would prevent us from reaching our last objectives. We decided to return to the -1150 m reef, since on that heading the current would help the ship keep up with the vehicle. We crossed a series of lava flows and sediment covered bottom as we headed to the northwest. When we were about halfway to the target, the wind suddenly picked up and within a short time was blowing 35 kts. We terminated the dive and began to recover in deteriorating conditions. The recovery was long and difficult, but at last the vehicle was back on board-safe and sound.
We are currently collecting gravity cores in the lagoons behind the -400 m reef (130 thousand years). The first two cores consisted of about 50 cm of foraminifera-pteropod ooze. The sediment is quite sandy, which inhibits the core penetration, so we have not reached the lagoonal deposits that formed behind the old reef. We will try a few more cores and then call it a day.
Jenny Paduan writes: 4/2/2001 23:50 local Hawaii time (no daylight savings needed for us! We are now 3 hours behind Pacific Standard Time)
Aloha from off the Kona Coast of the Big Island! We're nearly finished gravity coring for the night and the seas have settled significantly. We will dive tomorrow starting at 06:30.