The samples included in this report are a subset of those examined by the second author for the presence of calcareous nannofossils. Diatoms are the main focus of this report, as they are the most abundant fossil group in these samples; the presence of silicoflagellate species is also recorded. Tables T1 and T2 provide abundance or presence/absence data of all species reported in this study, and Plates P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, and P11 illustrate selected diatoms and silicoflagellates.
The two zonations referred to and applied herein are those employed by Barron (1985a) and Yanagisawa and Akiba (1998) for the equatorial Pacific and northwest Pacific, respectively. Table T3 illustrates the zones and species that define the boundaries of each, along with the samples and depths at which these were observed in this study. Figure F2 illustrates the cores of the two holes with their approximate depths, the position of the zonal boundaries for both biostratigraphies, and some species that are present in higher abundances and/or are significant in the two zonations.
Overall, diatom abundance varies throughout the samples from Hole 1179C. In general, the Miocene portion of the section (Cores 191-1179C-19H to 13H) has a lower abundance, with more samples falling into the few and common categories than are observed in the overlying Pliocene and Pleistocene. Even so, there are two intervals of lower abundance in the younger sediments. These are from Sections 191-1179C-7H-7 to 7H-4 (105.83–101.55 meters below seafloor [mbsf]) and 191-1179B-4H-7 to 3H-7 (36.0–26.85 mbsf), from the middle and upper Pliocene, respectively. Samples from the second of these intervals fall into the rare and scarce categories, and the lowest abundances are reported in these two holes. Diatom preservation is most often moderate, with the lower-abundance samples often having poor preservation. Broken and fragmented valves rather than dissolution distinguish poor preservation.
The diatom assemblage from the deepest core in this study, 191-1179C-19H, places the core in the Rouxia californica (Yanagisawa and Akiba, 1998) and Nitzschia miocenica (Barron, 1985a) Zones. This assemblage is characterized by a consistent presence of Actinocyclus tenellus, Actinoptychus spp., Azpeitia nodulifer, Hemidiscus cuneiformis, Nitzschia fossilis, Nitzschia reinholdii, Nitzschia rolandii, Rhizosolenia spp., Thalassiosira leptopus, and Thalassionema/Thalassiothrix spp. Azpeitia tabularis is present in moderate numbers in the middle of Core 191-1179C-19H and then is present only rarely in scarce abundance further uphole.
The transition from the N. miocenica Zone to the Thalassiosira convexa Zone (Barron, 1985a) is distinguished by the first occurrence (FO) of T. convexa, which is present in Sample 191-1179C-18H-2, 75–77 cm (203.05 mbsf). The last occurrence (LO) datum of R. californica marks the top of the R. californica Zone, between Samples 191-1179C-16H-7, 72–74 cm, and 16H-6, 75–77 cm (191.52–190.05 mbsf). Concurrent with this LO is the FO of Thalassiosira oestrupii. N. miocenica was observed only within the R. californica Zone. The base of the range of this species was not observed, but the LO is between Samples 191-1179C-11H-5, 75–77 cm, and 11H-4, 75–77 cm (141.05–139.55 mbsf). This quite distinctive species establishes a good temporal reference for the lower part of the R. californica Zone.
Within the Neodenticula kamtschatica (Yanagisawa and Akiba, 1998) and T. convexa (Barron, 1985a) Zones there are two LOs of Asteromphalaceae. Asterolampra acutiloba is absent from the assemblage after Sample 191-1179C-16H-4, 75–77 cm (187.05 mbsf), and the relatively consistent presence of Asteromphalus symmetricus ceases at Sample 14H-4, 73–75 cm (168.03 mbsf). The stratigraphic top of A. acutiloba is not well confined, however, as there is a 14.48-m gap between its LO and the next available adjacent sample. The FO of N. kamtschatica is present at 153.55 mbsf (Sample 191-1179C-12H-7, 75–77 cm), well within the N. kamtschatica Zone and very near the top of the T. convexa Zone. This species has a later appearance at this Leg 191 site relative to those from Legs 86 and 87 (Koizumi and Tanimura, 1985; Akiba, 1986). Yanagisawa and Akiba (1998) report observing the FO of this species in the R. californica Zone of Holes 438A and 584; they also comment on the difficulties in discerning the differences between N. rolandii and N. kamtschatica as the first gradually evolves into the second.
The top of the T. convexa Zone and bottom of the Nitzschia jouseae Zone (Barron, 1985a) are delineated by the FO of N. jouseae, which is observed in Sample 191-1179C-12H-5, 75–77 cm (150.55 mbsf). The consistent presence of N. rolandii ends upsection within the lower part of the N. jouseae Zone. This species is randomly present above Sample 191-1179C-11H-7, 35–37 cm (143.65 mbsf), and is absent above 116.05 mbsf.
The dominant members of the diatom assemblage in the lower part of the Pliocene in Hole 1179C are Actinocyclus curvatulus, A. nodulifer, Coscinodiscus marginatus, H. cuneiformis, N. kamtschatica, N. reinholdii, N. fossilis, Thalassiosira eccentrica, T. oestrupii, T. leptopus, and Thalassionema/Thalassiothrix spp. T. leptopus is only frequent in the lower Pliocene, and Stephanopyxis spp. conversely becomes a more frequent member of the assemblage in the middle and upper Pliocene.
The FO of Rhizosolenia praebergonii at the top of the N. jouseae Zone and base of the R. praebergonii Subzone A (Barron, 1985a) is not observed in Hole 1179C. Another datum reported to have a similar age (Yanagisawa and Akiba, 1998) is the FO of Neodenticula koizumii, which was placed at Sample 191-1179C-8H-3, 75–77 cm (109.55 mbsf). This datum is the lower boundary for the N. kamtschatica–N. koizumii Zone (Yanagisawa and Akiba, 1998). This zone is relatively short in Hole 1179C, as the upper boundary was placed 8 m upcore based on the LO of N. kamtschatica (Sample 191-1179C-7H-4, 75–77 cm); it is succeeded upsection by the N. koizumii Zone (Yanagisawa and Akiba, 1998). Another datum observed within the same part of the hole is the LO of N. jouseae at 112.55 mbsf (Sample 191-1179C-8H-5, 75–77 cm). This datum is the boundary for Subzone B of the R. praebergonii Zone (Barron, 1985a).
The FO of Azpeitia neocrenulata is observed in Sample 191-1179C-6H-7, 60–62 cm (96.4 mbsf). This species became a rare yet consistent part of the assemblage in the upper Pliocene and Pleistocene. The assemblage as a whole is very similar to that of the lower Pliocene with a few exceptions. N. kamtschatica is no longer present in the upper Pliocene, with N. koizumii and Neodenticula seminae replacing it in the assemblage. Proboscia barboi and Stephanopyxis turris have higher abundances and are more frequently present in the upper Pliocene.
The upper part of Hole 1179C is Pleistocene in age and contains several important datums. The LO of N. koizumii in Sample 191-1179C-2H-4, 75–77 cm (54.05 mbsf), marks the top of the N. koizumii Zone and the base of the Actinocyclus oculatus Zone (Yanagisawa and Akiba, 1998). In Sample 191-1179C-2H-7, 71–73 cm (58.51 mbsf), the FO of Pseudoeunotia doliolus denotes the boundary between the R. praebergonii Subzone C and the N. reinholdii Zone (Barron, 1985a). Slightly upcore from both of these datums, at 52.55 mbsf, is the FO of Proboscia curvirostris (Sample 191-1179C-2H-3, 75–77 cm).
Hole 1179B is latest Pliocene and Pleistocene in age. Diatom abundance and preservation are good, with the exception of the previously mentioned interval from 36.0 to 26.85 mbsf that has low abundance. Four northwest Pacific and two equatorial zones are represented in sediments from Hole 1179B. There is a 5.61-m overlap between the two holes, which reveals no noticeable change in the diatom assemblage. In fact, the top of the N. koizumii Zone (Yanagisawa and Akiba, 1998) is present within this overlap, occurring between 52.35 and 49.35 mbsf (Samples 191-1179B-6H-5, 75–77 cm, and 6H-3, 75–77 cm) in Hole 1179B and 54.05 and 52.55 mbsf (Samples 191-1179C-2H-4, 75–77 cm, and 2H-3, 75–77 cm) in Hole 1179C. The upper boundary for this zone is based on the LO datum of N. koizumii. This datum point is defined in Leg 191 sediments as the consistent decrease in abundance (from few to rare) of N. koizumii, as it does not disappear entirely from the assemblage. Several authors have noted the difficulties in separating N. koizumii and N. seminae (Maruyama, 2000; Yanagisawa and Akiba, 1998), so this change in abundance was chosen as the best basis for this datum.
The top of the A. oculatus Zone and base of the P. curvirostris Zone (Yanagisawa and Akiba, 1998) is defined by the LO of A. oculatus, which is present in Sample 191-1179B-5H-7, 70–72 cm (45.8 mbsf). Curiously, the age at which this datum occurs in these sediments (1.21 Ma) is closer to the age presented by Maruyama (2000) from the northeastern Pacific (1.01–1.46 Ma) than the 1 Ma age for this datum presented by Yanagisawa and Akiba (1988). The upper boundary of the N. reinholdii Zone (Barron, 1985a) is defined by the LO of N. reinholdii (Sample 191-1179B-3H-6, 75–77 cm). The diatom assemblage from this depth upward in Hole 1179B belongs to the youngest equatorial Pacific zone, P. doliolus (Barron, 1985a).
The last and highest significant datum in this hole is the LO of P. curvirostris, which is present in Sample 191-1179B-2H-5, 75–77 cm (14.35 mbsf). This marks the change from the P. curvirostris Zone to the N. seminae Zone of the northwest Pacific biostratigraphy (Yanagisawa and Akiba, 1998). Throughout the Pleistocene, the diatom assemblage experiences little change; the most marked distinction is the loss of N. koizumii, N. fossilis, and N. reinholdii as constant members of the assemblage. Dominant taxa are A. curvatulus, Alveus marina, Coccolithus radiatus, N. seminae, T. eccentrica, T. oestrupii, and Thalassionema/Thalassiothrix spp. Several species have periods of higher abundance (A. neocrenulata, C. marginatus, Fragilariopsis doliolus, and S. turris) but are not present continuously throughout the interval.
Because the diatom biostratigraphic record documented by Koizumi and Tanimura (1985) from DSDP Leg 86 was compiled from cores collected in a geographic area slightly southeast of the Leg 191 drill sites, it is of interest for comparison to this study. As would be expected, there are many datums in common between the two studies, some ages are comparable and some are not. Table T4 lists these datums. The ages for the Leg 191 diatom datums are estimated by the depths at which they occur in the cores and the ages assigned to these depths by the magnetostratigraphic sedimentation rate curve (Shipboard Scientific Party, 2001); the ages given for the Leg 86 datums are age ranges assigned to each datum from the four holes of this leg (Koizumi and Tanimura, 1985). Most datums are present in sediments of similar ages from the cores of the two cruises; they tend to be older in the Leg 191 cores with a few exceptions. The youngest datum listed is the LO of P. curvirostris, which is very close in age in both data sets. The LO of N. reinholdii is older than that observed in Leg 86, as is the following LO of N. fossilis. The second of these is downhole from an interval with low abundances of diatoms, making this LO datum rather tenuous. The next three datums (LO of A. oculatus, FO of P. doliolus, and LO of Thalassiosira antiqua) occur within the range of ages presented for them from Leg 86 sediments. From this point downhole, only the FO of N. jouseae and the FO of T. convexa are present in sediments younger than their equivalent observances in Leg 86. The rest of the Pliocene/Miocene datums in the table have ages older than those in Leg 86.
The silicoflagellate data presented in this report are meant only to record the taxa observed, without creating an accompanying zonation based on this fossil group. Tables T1 and T2 present the distribution of silicoflagellate species observed in each sample. A plus sign indicates the presence of a particular species in that sample. Age constraints provided by these taxa agree with those of the diatoms, but there is also obvious reworking of older species. One example is Distephanus crux ssp. loeblichii in Sample 191-1179B-2H-4, 74–77 cm (12.85 mbsf), to which Perch-Nielson (1985) assigned a confined age of upper Oligocene. The overall abundance of silicoflagellates in the samples was generally rare, with some reaching the few category of abundance. Preservation in general is moderate, as it also is for the diatoms, although some breakage did inevitably occur. Time constraints prevented a more detailed identification of species in the middle Pliocene, which are mostly grouped by genus in the upper part of the time column. It is hoped that the inclusion of these data will provide some assistance to others who work with this group.