Previous investigations on siliceous organisms in the central equatorial Pacific Ocean have concentrated on radiolarian- and diatom-based biostratigraphic zonation, using as their basis sediments cored during DSDP and ODP legs and others obtained from box cores and piston cores (Moore, 1971; Dinkelman, 1973; Schrader, 1976; Fenner, 1984; Nigrini, 1985). Oceanographic research related to the occurrence of radiolarians and diatoms has focused on their lateral distribution relative to the present current system, sea-surface temperatures, temperature gradients, thermocline influences, salinity, and nutrient supply (comp. Blueford et al., 1990; Goll, 1976). Radiolarian depth zonations (Kling, 1979) have been obtained from different water masses and for low and high latitudes, and radiolarian associations have been recognized that represent specific temperature and bathymetric regimes (Petrushevskaya, 1971; Casey, 1993). Recent paleoceanographic research has investigated depositional patterns of siliceous and calcareous microorganisms and nannoorganisms with the purpose of delineating the spatial distribution of productivity zones and (in combination with ocean-climate models) of investigating the recycling of silica (Leinen, 1985; Ragueneau et al., 2000; Moore et al. 2001; Huber, 2002).
The primary scientific goal of the present study is to describe and interpret the changes in radiolarian associations from the Zone RP-15 diatom-rich unit at Site 1219. The "radiolarites" underlying this unit are composed either of hard radiolarian chert that could not be penetrated, drilled, or recovered without problems or of soft brownish to pink radiolarian accumulations that are identified as radiolarites. These underlying radiolarian deposits range from middle to late Eocene in age and represent the overall signal of unique oceanic conditions described and modeled by Huber (2002). From an ecological standpoint, the radiolarian mass accumulations are a result of the evolutionary potential of radiolarians, the supply of silica into the ocean system, and the availability of nutrients. After the high temperatures of the early Eocene, radiolarians increased both in their diversity and in their individual abundance (hence in their biomass); the result was that everywhere at lower latitudes there was deposited a radiolarian-rich sediment interval (Zones RP-14 through RP-16). The mass accumulation rate of radiolarian silica is constant throughout this interval, suggesting that stable oceanic conditions existed for most of the middle Eocene. Within this radiolarian-rich interval is the diatom-rich unit that forms the subject of this present work; it represents an intercalation of particles produced by photoautotrophic phytoplankton and marks an important change in the eastern equatorial Pacific Ocean current system. This diatom-rich unit is related to a high-productivity belt deposited in the ITCZ during the equator crossing of Hole 1219A and nearby Site 1220. Backtracking and paleomagnetic investigations indicate that the southern sites of the transect crossed the equator at ~29 Ma. This timing favors the hypothesis that the diatom accumulation at Site 1219 is part of the southern productivity lobe of the ITCZ, which was moving southward and transporting the productivity signal in younger sediments of the southern sites. Above the diatom-rich unit are normal radiolarites that are almost lithologically identical to those from the middle Eocene; these occur up to the Eocene/Oligocene boundary.
This pilot study concentrates on the changes in biological associations from the onset to the end of the deposition of the diatom-rich unit. Because other siliceous organisms (e.g., sponges) are almost undetectable in the radiolarite, the analysis is limited to radiolarians. The main point of interest concerns the sedimentological or, possibly, biological relationship between spherical radiolarians (spumellarians) and monopylid radiolarians (nassellarians); this relationship has been used to interpret changes in water depth, temperature, and productivity at other times in earth history (Steiger, 1998). A secondary point concerns the question of whether the faunal development was induced by productivity or whether it was simply a long-term reaction of the radiolarians to gradual changes in oceanic conditions and the geographic location of the depositional area; this question is investigated by analyzing counts of subgroups on the family level or counts of groups of comparable morphotypes from base to top of the diatom-rich unit. Finally, the study tries to determine what were the major radiolarian assemblages, to identify the predominant species in these, and to estimate the degree to which compositional changes in the associations reflect alterations within the water column.