A total of 120 radiolarian species were recorded in the samples from Hole 1138A, as summarized in Table T2. A full documentation of the taxonomy of recovered radiolarians is given in "Appendix," and in Plates P1, P2, P3, P4, P5, P6, P7, P8, P9, P10.
Radiolarians are abundant and usually well preserved in Sections 183-1138A-34R-1 through 34R-3. Sporadically, an abundant diatom flora is present. Diatoms dominate the siliceous microfossil assemblage in Sample 183-1138A-34R-3, 20-22 cm. Stratigraphic markers are present (e.g., Axoprunum irregularis, Lychnocanoma conica, Amphistylus? sp. sensu Takemura, 1992) but are only moderately preserved and rare. Even the relatively well preserved samples of Sections 183-1138A-34R-1 through 34R-3 are dominated by fragmented radiolarians and diatoms rather than by complete specimens.
The high abundance of Litheliidae in all samples of Sections 183-1138A-34R-3 through 34R-1 is remarkable. Lithelius sp. A gr. (Pl. P3, figs. 1-4), Lithelius sp. C (Pl. P3, fig. 6), Lithelius sp. D (Pl. P3, fig. 7), and Pylonid sp. 1 (Pl. P2, fig. 8) are common throughout. Spumellaria generally dominate the radiolarian assemblage. Among the nassellarians, Artostrobiidae are the most abundant family, and within the Artostrobiidae, Siphocampe and Artostrobus dominate. In all samples of Sections 183-1138A-34R-3 through 34R-1, Siphocampe and Artostrobus are the dominant nassellarian genus, and Artostrobus pusillum gr. is the most frequent species (Pl. P9, figs. 6-10).
Radiolarians are generally rare to common and poorly to moderately preserved in Core 183-1138A-35R. Stratigraphic markers are present (e.g., A. irregularis, L. conica, Amphistylus? sp. sensu Takemura, 1992, and Eucyrtidium spinosum). Spumellarians in general and Litheliidae in particular dominate the assemblages in this interval. Lithelius sp. A (Pl. P3, figs. 1-4), Lithelius sp. C (Pl. P3, fig. 6), Lithelius sp. D (Pl. P3, fig. 7), and Pylonid sp. 1 (Pl. P2, fig. 8) are especially conspicuous. Additional common spumellarians are Actinommidae, especially Cenosphaera sp. A gr. (Pl. P2, figs. 6-7), Amphistylus? sp. A (Pl. P1, fig. 2), Actinomma henningsmoeni (Pl. P2, fig. 1), and Stylosphaera radiosa gr. (Pl. P1, figs. 6-8). Among the nassellarians, Siphocampe and Artostrobus are most common, particularly A. pusillum (Pl. P9, figs. 6-10), Siphocampe nodosaria (Pl. P9, fig. 11), Siphocampe acephala gr. (Pl. P9, figs. 14-17), and Siphocampe arachnea gr. (Pl. P9, figs. 12, 13). Peridium? sp. A and Peridium? sp. B (Pl. P5, figs. 18, 19) are generally abundant in these samples except for the two levels at which diversity declines (see below).
The overall radiolarian diversity is generally lower than that found in Sections 183-1138A-34R-1 through 34R-3. The highest diversity in Core 183-1138A-35R is found in Samples 183-1138A-35R-2, 105-107 cm, and 35R-2, 24-26 cm. There is a conspicuous diversity decline in Sample 183-1138A-35R-1, 140-142 cm, where the assemblage is dominated by Litheliidae. Another diversity drop is observed in Sample 183-1138A-35R-3, 101-103 cm, where Artostrobiidae and Litheliidae dominate the poorly to moderately preserved fauna. Diatoms are common to abundant in this core (Table T3), but diatom diversity is remarkably lower than in Sections 183-1138A-34R-1 through 34R-3.
Stratigraphic markers are present but rare. A. irregularis was observed in Samples 183-1138A-35R-5, 51-53 cm; 35R-4 (all); 35R-3, 101-103 cm; 35R-2 (all) to 34R-3 (all); 34R-2, 23-25 cm; and Section 34R-1 (Table T2). E. spinosum is present only in Sample 183-1138A-35R-1, 103-105 cm (Table T2). L. conica is present in all samples of Section 183-1138A-35R-4; in Samples 183-1138A-35R-2, 105-107 cm, and 35R-1, 140-142 cm; all samples of Section 183-1138A-34R-3; and in Sample 183-1138A-34R-1, 103-105 cm (Table T2).
Radiolarians are rare and poorly preserved in Samples 183-1138A-36R-1, 0-2 cm, through 36R-1, 41-43 cm. Almost exclusively, radiolarian fragments rather than complete specimens were observed. Artostrobiidae are dominant, and some additional Actinommidae are present, which contribute most to the observed diversity. Although there is a continuous trend toward declining diversities downcore in Section 183-1138A-36R-1, the very top of the section is characterized by a profound diversity decline. Below interval 183-1138A-36R-1, 40-42 cm, samples are barren of radiolarians. Diatoms are generally rare in all samples of Core 183-1138A-36R.