Scientific ocean drilling in the central equatorial Pacific Ocean has concentrated on obtaining fundamental information about sedimentary successions and developing an overall biostratigraphic framework. As drilling techniques have improved, the research goals have changed somewhat—moving away from basic investigations and toward specialized objectives such as the reconstruction of changing climatic conditions and ocean-current patterns since the onset of sediment deposition. In addition to its general-interest objectives (those presented in the Ocean Drilling Program [ODP] Leg 199 Scientific Prospectus), Leg 199 focused on the sequence of calcareous and siliceous sediments in the area of the paleoequator during the early Tertiary (between 56 and 40 Ma) and on the evolution of biota there at that time. Of particular interest were (1) the effects of temperature on the position of the carbonate compensation depth and the stability of marine siliceous components and (2) the effects of the Late Paleocene Thermal Maximum on productivity. Previous drilling in this area was carried out during Deep Sea Drilling Project (DSDP) Leg 8 (Tracey, Sutton, et al. 1971), DSDP Leg 16 (van Andel, Heath, et al., 1973), and DSDP Leg 85 (Mayer, Theyer, Thomas, et al., 1985). Improved drilling techniques allowed the collection of high-recovery cores and almost undisturbed sediment sequences for most of Leg 199. In addition, multiple coring of several holes allowed continuous sedimentological analysis by letting gaps in the sediment record that were the result of drilling disturbances be filled with corresponding sediments from other holes at the same site.
Leg 199 Sites 1215–1222 (a north–south oriented transect) were drilled on 56-Ma basaltic crust; an exception was Site 1218, which was drilled on 40-Ma crust. The sedimentary record obtained from Leg 199 sites showed predictable variations from north to south. Significant thicknesses of sediment were intersected at the southern sites for those sites that were at the equator between 40 and 25 Ma. Three sites (Sites 1219, 1220, and 1221) contained signals of both southern and northern hemisphere positions (Shipboard Scientific Party, 2002).
The sediments studied from Leg 199 sites range in age from the late middle Eocene to the Eocene/Oligocene boundary. During that time, the ecological and productivity conditions changed from those characteristic of an oceanic, deepwater milieu to those that marked the change from siliceous to carbonaceous lithologies at the Eocene/Oligocene boundary. The drilled sediments spanned a 20-m.y. time period that covers the passage of the equator over the Inter-Tropical Convergence Zone (ITCZ). The sediments that were deposited contain accumulations of radiolarians, diatoms, and nannofossils that are characteristic of specific productivity conditions within the Eocene ITCZ. This paper is the result of a pilot study of 30 samples from Hole 1219A, from a diatom-rich unit that is both underlain and overlain by radiolarian-rich sediments.