SUMMARY

Two sites were drilled during ODP Leg 200: the H2O site (Site 1224) and the Nuuanu Landslide site (Site 1223). Results are summarized as follows:

  1. In Hole 1224D we drilled basaltic basement to 59 mbsf and installed a reentry cone with cemented casing for future installation of a borehole broadband seismometer.
  2. During the H2O operations, broadband seismometer records obtained from the shallowly buried H2O seismometer were analyzed. In the midocean, the double-frequency microseism peak splits into two peaks associated with two different kinds of noise sources. Microseism levels in the band ~0.2–0.5 Hz correlated with sea state and wind speed observed from the drillship, indicating that this noise is generated locally at the sea surface. Microseism levels in the band 0.1–0.2 Hz correlate with high sea states impacting distant shorelines. Large quasi-periodic noises generated by whales and drilling-related noise were also identified.
  3. Hole 1224F, 20 m away from Hole 1224D, was drilled to 173.5 mbsf. Drilling in Hole 1224F recovered >100 m of ~46-Ma basaltic basement that is composed of three units. Although these three units are geochemically distinct, they are more highly evolved than most MORB (i.e., they have experienced a protracted history of cooling and crystal fractionation within the crust).
  4. Analysis of the alteration at this site showed that most of basalts are <5% altered to secondary minerals assemblages of Fe oxyhydroxides, celeadonite, saponite, Ca carbonate, trace pyrite, and rare phillipsite and quartz. Ca carbonate formed within 20 m.y. of crustal formation and occurred at low temperatures (4°–11°C).
  5. Logging and physical property measurements show good correlation to lithology, geochemisty, and alteration of minerals in basalt. The fractures and low-temperature alteration greatly reduce VP, suggesting a cause of VP ~ 4.5 km/s in oceanic Layer 2B.
  6. The discovery of fungi in calcite veins in abyssal basalts was made by microbiological analyses. This is evidence that eukaryotic life existed in the extreme environment below the seafloor.
  7. The Nuuanu Landslide site, ~260 km northeast of the island of Oahu near the crest of the 500-m-high Hawaiian Arch, was also drilled. We drilled to 41 mbsf and identified eight sandy layers. The layers contain fresh and angular glass fragments typical of Hawaiian shield volcanoes. Degassed glasses are estimated to be subaerially erupted. Seven of the sandy layers are thought to be associated with the Koolau Landslide prior to 1.77 Ma. Of the four thicker sandy layers, it is unclear which are related to the Nuuanu Landslide. Analysis of the Site 1223 cores suggests repeated occurrences of collapsing Hawaiian volcanoes and debris flows to ~260 km distance.

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