Diatoms are very sparse in the vast majority of the slides that were prepared from Site 1016. Sponge spicules and fragments of reworked Miocene taxa are commonly observed. Where present, the in situ diatom assemblages contain fragments of Thalassiothrix, Thalassionema, and Proboscia; whole valves of resistant genera like Coscinodiscus and Stephanopyxis; and resting spores of Chaetoceros. Because of the poor and sporadic nature of the assemblages, no diatom assemblage counts were completed.
Diatoms are very sparse in virtually all of the samples that were prepared for Site 1021. This is presumably because Site 1021 was seaward (west) of the coastal zone of diatom productivity during the mid-Pliocene. Based on modern oceanographic conditions, this may imply that spring SSTs exceeded ~17°C (Dowsett et al., 1996).
Diatoms are relatively common in nearly all the samples that were examined, implying that Site 1022 lay within the coastal zone of higher diatom productivity during the mid-Pliocene.
Diatom assemblages are dominated by Chaetoceros spores (a coastal upwelling indicator whose numbers may be augmented by lateral advection off the continental shelf), the cold-water (subarctic) taxa Neodenticula kamtschatica and its descendant Neodenticula koizumii, and Thalassionema nitzschioides, a temperate taxon that appears to represent spring-season production within a broader region extending seaward from the coastal zone (Sancetta, 1992). Other significant components include Coscinodiscus marginatus (up to 15% of the assemblage) and undifferentiated forms of Thalassiosira (e.g., T. convexa, T. eccentrica, T. leptopus, T. jacksonii, and T. pacifica) (up to 18% of the assemblage; Table 1).
Significant diatom biostratigraphic events include the last common occurrence of N. kamtschatica (2.68 Ma) between 82.5 and 81.0 meters below seafloor (mbsf) and the last occurrence of Nitzschia jouseae (2.79 Ma) between 90.5 and 88.5 mbsf. These diatom events suggest a sedimentation rate approximating 70 m/m.y.
Color reflectance data for the mid-Pliocene of Hole 1022A displays cycles about every 2.8 m (10 cycles between 117 and 89 mbsf; Shipboard Scientific Party, 1997). If these cycles reflect 41-k.y. obliquity cycles, a sedimentation rate of ~68 m/m.y. results, which is in good agreement with diatom biostratigraphy. A sedimentation rate of 68 m/m.y and diatom biostratigraphic datum levels indicate that the sequence studied from Hole 1022A ranges in age from ~2.50 (71.5 mbsf) to ~3.22 Ma (118.5 mbsf). Comparison of the last occurrence of the calcareous nannofossil Reticulofenestra pseudoumbilicus (3.82 Ma; Shipboard Scientific Party, 1997) between 174.80 and 165.20 mbsf in Hole 1022C with the diatom datum levels suggests a slightly higher sedimentation rate of ~78 m/m.y. For the purposes of this report, an age model based on diatom biostratigraphy and a 68-m/m.y sedimentation rate has been used to assign ages to the samples (Table 1).