Siliceous microfossils are present in all samples from Cores 183-1138A-29R through 1R (269.41-0.0 mbsf) (Table T1). Diatoms, however, are poorly preserved and in generally low abundance in the interval between Cores 183-1138A-29R and 20R (269.41-181.30 mbsf); siliceous microfossils in this interval are clearly affected by dissolution. In contrast, moderate to good preservation and high diatom abundance characterizes Cores 183-1138A-19R through 1R (174.75-0.0 mbsf); the moderately preserved samples in this interval are predominantly affected by physical breakage. Nannofossils are abundant in Cores 183-1138A-36R through 13R (340.60-113.00 mbsf) and are present in variable abundance above this level (Cores 12R through 1R; 107.03-0.0 mbsf). A graphical representation of relative diatom and nannofossil abundance is shown in Figure F2. The position of the horizontal bars in Figure F2 represents the sample depths examined in the present study.
The stratigraphic distribution of individual diatom, ebridian, and endoskeletal dinoflagellate taxa is recorded in Table T1. Abundant diatoms and moderate-to-good preservation in most samples allowed identification of continuous biostratigraphic ranges for most taxa. Some rare occurrences are attributed to limited reworking and downhole contamination (Table T1). Taxonomic and occurrence notes for diatom taxa identified in Hole 1138A are given in "Systematic Paleontology," and several specimens are illustrated in Plates P1, P2, and P3. Two new diatom species, Fragilariopsis heardensis and Azpeitia harwoodii, are described from lower and upper Pliocene strata of Hole 1138A.