Leg 138

Eastern Equatorial Pacific

Leg 138 is a key element of a global experiment designed to study the role and response of the tropical oceans in global climate change on time scales of thousands of years. Building on the experience gained on earlier legs and taking advantage of advances in coring technology and automated laboratory analyses, the Leg 138 scientific program was designed to maximize the chances of recovering complete, high-resolution paleoclimatic records of the eastern equatorial Pacific. Leg 138 drilled 11 sites, collecting 5537 m of sediment along two transects across the eastern equatorial circulation system, centered at about 95 deg W (Site 844 to Site 847) , where the Pacific equatorial circulation is influenced by the eastern boundary of the basin, and at 110 deg W (Site 848 to Site 854), where the equatorial currents are more zonal and not influenced by the eastern boundary.

The sediment is dominated by the remains of upper-water-column golden-brown algae (nannofossil ooze) with varying amounts of phytoplankton and zooplankton, particularly diatom ooze. As is the case today, proximity to the equator amplified sediment accumulation in the past; however, variations in the flux to the seafloor and sediment burial were dominated by temporal changes which appear to be basin-wide "events", typically reflected by changes in calcium carbonate content (probably related to dissolution) or as intervals rich in the diatom Thalassiothrix longissima. Some of these intervals contain millimeter-scale laminated beds. In the modern ocean, T. longissima is generally associated with enhanced upwelling and high productivity, and the presence of these intervals here indicate major productivity events. These events may be synchronous across the equatorial Pacific and are imaged with seismic-profiling techniques. The broad pattern of temporal variations in sedimentation and accumulation rates is consistent with that found in previous drilling in the central equatorial Pacific, the western equatorial Pacific, and the equatorial Indian Ocean.

Superimposed on the long-term temporal changes are higher frequency fluctuations in the ratio of sedimentary components (typically carbonate to silica) which are reflected in the continuous core logs of density, color, and susceptibility. The density fluctuations, which for the most part represent changes in carbonate content, show a periodicity consistent with orbitally-induced Milankovitch forcing. The near-continuous density, susceptibility, and color-reflectance core logs demonstrate that these detailed records, extending well into the late Neogene, can be correlated over thousands of kilometers.