SUMMARY

The primary results of this study on the depositional patterns of radiolarians during a diatom event in the equatorial Pacific Ocean during the middle Eocene are as follows:

1. Most radiolarian groups were not affected by the mass production of diatoms and the deposition in the area of the ITCZ, indicating major radiolarian taxa distribution in deeper marine tropical environments and a deep thermocline with temperatures similar to surface temperatures.
2. Changing environmental conditions are responsible for rapidly evolving distinct zones of higher productivity in the equatorial area. The Eocene radiolarian sedimentation, which is characterized by an extended marine zonation lacking steep gradients, is controlled by slight cooling of the circulation system and rich input of nutrients and silica after an extremely warm period (early Eocene).
3. Radiolarian assemblage compositions indicate that lateral transport, together with the increasing South Equatorial Current, induced an overprint from above, adding diatoms and shallow marine radiolarians (actinommids) to the faunal spectrum during and immediately after maximum diatom production.
4. Some specific forms of radiolarians ("robust" artostrobids; Casey, 1993) seem to react at the beginning and end of the diatom occurrence. Nassellarians, mostly occurring in intermediate zones between the warm- and cold-water levels at greater depths, could be specialized to collect food and silica from the sinking diatom tests, which are mostly dissolved at these bathymetric levels.