Samples studied from Core 199-1219A-1H were found to be barren of silicoflagellates; calcareous nannofossils were not present in Cores 199-1219A-1H and 2H (Shipboard Scientific Party, 2002). The first downhole occurrence of silicoflagellates was in Sample 199-1219A-2H-2, 70–71 cm, representing the D. varia Zone of the middle Miocene. This zone is represented by four samples, one of which was barren of silicoflagellates. Silicoflagellates were fairly uncommon and of low diversity, with only four silicoflagellate taxa found in this interval. The lowermost of the samples in this interval, Sample 199-1219A-2H-5, 70–71 cm, shows a modest increase in silicoflagellate abundance that continues through the next 6.5 m.
The C. triacantha Zone occurs in Samples 199-1219A-2H-6, 70–71 cm, through 4H-1, 70–71 cm. Nine samples were studied in this interval, with silicoflagellates having moderate abundance (19–47 specimens per slide) in the upper five samples and showing very low abundance (7 specimens or fewer per slide) in the lower four samples. Diversity is fairly high, with 12 taxa found in the interval. D. fibula fibula is usually the most abundant taxon, except in the bottom two samples, 199-1219A-4H-2, 70–71 cm, and 4H-3, 70–71 cm, where Dictyocha is absent and Distephanus and Naviculopsis are first present in numbers greater than 10 specimens per slide.
The transition from Dictyocha to Distephanus dominance between Samples 199-1219A-4H-1, 70–71 cm, and 4H-2, 70–71 cm, suggests a change in surface water temperature or some environmental variable associated with water temperature (McCartney and Loper, 1989). Several silicoflagellate researchers (Poelchau, 1974; Ciesielski, 1975; Bukry, 1981a; Bukry and Monechi, 1985; Schrader et al., 1986; and others) have suggested that the Distephanus/Dictyocha ratio may be useful as a temperature indicator, although the several proposed scales do not compare closely, showing that there is only regional utility at best (see Perch-Nielsen, 1985). Nevertheless, the general absence of Dictyocha below Sample 199-1219A-4H-1, 80–81 cm, and of Distephanus above Sample 4H-1, 80–81 cm, strongly suggest a fairly significant change in sea-surface temperature.
The N. ponticula Zone is found in only three samples, 199-1219A-4H-2, 70–71 cm, through 4H-4, 70–71 cm. The zone is commonly much thicker, for example, 20 m in Hole 575A (Bukry, 1985) and at least 23 m in Hole 495A (Bukry, 1981a). However, the top and bottom of the zone appear to be preserved in Hole 1219A, as Sample 199-1219A-4H-2, 70–71 cm, includes Naviculopsis ponticula spinosa, which Bukry (1985) noted to be dominant in the upper part of the zone, and Sample 4H-4, 70–71 cm, has especially abundant N. ponticula, which is typical of the bottom part of the zone (Bukry, 1985, 1982). Thus, unless there are missing sediments in the middle part of the zone, sedimentation rates in the early Miocene were apparently lower at Site 1219 than elsewhere in the east equatorial Pacific.
The N. lata Zone is also very thin in this region, applied to three samples, 199-1219A-4H-5, 70–71 cm, through 4H-7, 50–51 cm (31.7–34.5 mbsf). Silicoflagellates are fairly abundant in this interval, with 39–86 specimens per slide representing nine taxa (one reworked). Sample 199-1219A-5H-1, 70–71 cm, immediately below where we place the bottom of the N. lata Zone, is barren, and silicoflagellates are found in significantly lower numbers through the rest of the lower Miocene section.
The abundance and diversity of silicoflagellates are relatively low in the N. biapiculata Zone, with four of the nine samples in this interval being barren. Eight taxa were found (one reworked). The dominant silicoflagellates in this interval are D. crux and D. speculum, with N. biapiculata present only in Samples 199-1219A-6H-1, 70–71 cm, and 6H-2, 70–71 cm.
Silicoflagellates were found in all 11 samples placed in the B. apiculata Zone, with the number of specimens per slide ranging from 2 to 64. Diversity was generally low, with D. speculum speculum being the dominant taxon throughout the interval. B. apiculata was present in all but 2 of the 11 samples. Below this zone is a lengthy unzoned interval where silicoflagellates, when present, do not include diagnostic zonal indicators. N. biapiculata and N. constricta are unusually abundant in Sample 199-1219A-14H-6, 70–71 cm, which is the lowermost sample of the unzoned interval.
The N. constricta Zone represents a lengthy interval of 36 samples covering >50 m. Only nine of these samples were found to contain silicoflagellates. There is a thick barren interval covering 21 sections from Sample 199-1219A-16H-3, 70–71 cm, to 19H-5, 70–71 cm (47.7–79.2 mbsf). There are two features of interest in the interval covered by this zone. The topmost sample, 199-1219A-14H-7, 70–71 cm, contains the most silicoflagellate specimens observed in this study. This sample was dominated by N. constricta and was also the last occurrence of that taxon. The second feature of interest is a four-sample interval dominated by D. byronalis, considered here as a horizon within the primary zonal interval. D. byronalis was described and is very abundant in the Kellogg Shale of northern California (Barron et al., 1984), although that is dated to the middle Eocene.
The lowest zone identified in this study is the D. hexacantha Zone, extending from Sample 199-1219A-14H-7, 70–71 cm, to 23H-5, 70–71 cm (128.2–212.2 mbsf). Most of the samples in this interval were barren, but there was a short interval of five samples that contained sporadic D. spinosa, which was especially abundant in Sample 199-1219A-21H-6, 70–71 cm. The D. spinosa Subzone also contained abundances of C. bimucronata and C. hastata. No silicoflagellates were found in the 12 sections below Sample 199-1219A-14H-7, 70–71 cm (129.7 mbsf).