Silicoflagellates are planktonic, microscopic algae with siliceous skeletons characterized by their simple geometries and remarkable variability (see McCartney and Wise, 1990; McCartney et al., 1995). They can be considered the least important of the major fossil-forming plankton groups; they are also the least studied. Biostratigraphic zonations have been developed for silicoflagellates in all oceans throughout the Cenozoic, though the zones typically range over longer intervals of time than foraminiferal or diatom zones. Silicoflagellate skeletons may have considerable potential as ecophenotypic indicators (see, e.g., Cornell, 1974; McCartney and Wise, 1990, 1993). Mathematical studies (McCartney, 1988; McCartney and Loper, 1989, 1992) suggest that skeletal shape is influenced by environmental factors, with more complex skeletal morphologies resulting from a need to optimize surface area.
Eight sites were drilled in the eastern equatorial Pacific Ocean during Ocean Drilling Program (ODP) Leg 199 from October through December 2001. The sites are divided into two complementary north–south transects (Fig. F1) across an area that was located at the equator during the Eocene. Plate motions during the past 40 m.y. have moved this region northward, away from the equatorial upwelling region, into an area of lower productivity. The goal for Leg 199 was to drill through the relatively thin overlying Neogene sequence to recover Paleogene sediments that were not buried deep enough to be lithified. Because the region was situated at the equator during the Eocene, the expectations prior to drilling were that the Paleogene sequence would show high productivity with abundant and diverse plankton.
The drilling region for Leg 199 was also selected to complement the Neogene sequences that were recovered during ODP Leg 138, which was drilled in the same region but slightly southward, straddling the equator. Study of these cores by McCartney et al. (1995) showed silicoflagellates to be abundant and diverse at the equator and rapidly diminishing away from the equator. Silicoflagellates demonstrating unusual skeletal variability, such as in the Bachmannocena/Neonaviculopsis plexus, were very abundant in the late Miocene within 1°–2° of the equator. Sites located farther from the equator, such as in ODP Holes 845B (9°N) and 854B (11°N), were generally found to have few silicoflagellates and low diversity.
Preliminary silicoflagellate analysis of Leg 199 sites showed a continued trend of low biogenic sedimentation away from the equator, with most of the samples studied having few or no silicoflagellates. The southernmost of the eight sites, Site 1219, was selected for detailed study; a data report for Sites 1215–1218 and 1220 is published elsewhere (McCartney et al., this volume). Site 1219 is situated ~3° north of the Clipperton Fracture Zone. Hole 1219A (7°48.019´N, 142°00.940´W; water depth = 5063 m) consists of a 250.8-m section, of which an ~120-m early Miocene and Oligocene section is primarily composed of nannofossil ooze with variable radiolarian and clay content. The Eocene section comprises 95 m of radiolarian and zeolithic clays, radiolarian and diatom oozes, and nannofossil oozes and chalks. Diversity of silicoflagellates is moderate, and the preservation is good. A middle Miocene to early Eocene section was recovered in Hole 1219A, with sporadic occurrences of silicoflagellates throughout (Fig. F2). Radiolarian and nannofossil oozes throughout the core dominate the sediments with carbonate and clay units interrupting the section.
In the present Pacific Ocean, wind systems in the two hemispheres come together at high altitudes and are pushed to a lower elevation near sea level, where they then move back toward the pole. These winds push surface waters away from the equator, where deep nutrient-rich waters then come to the surface to produce an extraordinarily high productivity of phytoplankton and zooplankton. Investigation of the Leg 199 cores (see other papers in this volume) show that the equatorial circulation system was quite broad during the Eocene, with very low plankton productivity. The fossil assemblages are generally dominated by radiolarians. This reflects a very warm Eocene climate that transitioned to a cool Oligocene climate at ~33.7 Ma (Shipboard Scientific Party, 2002). This transition is associated with the first significant buildup of ice on Antarctica and initiated the change toward more modern oceanic circulation patterns and plankton ecologies.