Sediments recovered from Site 1086 represent a continuous pelagic section spanning the upper Miocene to lower Pleistocene. The biostratigraphic framework developed for Site 1086 was based on calcareous microfossils. Siliceous microfossils are rare or absent in much of the core. There is generally good agreement between stratigraphic position of the calcareous nannofossil and planktonic foraminiferal datums and between the biostratigraphic and magnetostratigraphic age model (see "Paleomagnetism" section, this chapter). The superior quality of the paleomagnetic data for Site 1086 allowed us to examine in detail the differences in time scales in the early Pliocene.
A comparison of the calcareous nannofossil-based age model for this interval with an age model generated from paleomagnetic data indicates a significant offset in the timing of changes in sedimentation rates. The biostratigraphic age model based on the Lourens et al. (1996) time scale produces an increase in sedimentation rate that occurs 0.7 m.y. earlier than the one produced by the Berggren et al. (1995) paleomagnetic age model. There are known problems associated with the radiometric dating in Chron C3n (4.2–5.2 Ma; Berggren et al., 1995). The Berggren et al. (1995) time scale lacks high precision 40Ar/39Ar dating in Chron C3n. The Lourens et al. (1996) time scale represents an improvement to the Berggren et al. (1995) time scale for this interval because it integrates radiometric dating and astronomical tuning.
This deviation in the early Pliocene was not detected at other sites during Leg 175 because of lower sedimentation rates, lower quality paleomagnetic data, and/or carbonate dissolution.
Calcareous nannofossils were studied in core-catcher samples from Hole 1086A. Because additional samples from within the cores were not examined, the depth range of the datum events identified at Site 1086 is approximately ±5 m. Nannofossils are abundant and well preserved throughout the entire section.
Based on the oldest identified datum (last occurrence [LO] of Amaurolithus primus; 7.2 Ma) as well as paleomagnetic data (see "Paleomagnetism" section, this chapter), Site 1086 terminated within Zone NN11 (late Miocene). The top core of Hole 1086A probably does not contain material younger than 1.25 Ma, as shown by the presence within Sample 175-1086A-1H-CC of Helicosphaera sellii, a species whose LO event is dated at 1.25 Ma. Of the 10 biohorizons identified within Hole 1086A, six are zonal boundary markers (Table 2). Within the sampling resolution, the sedimentation appears continuous throughout the entire section (Fig. 10). The nannofossil-derived biostratigraphy agrees with the magnetostratigraphy derived from this site (see "Paleomagnetism" section, this chapter), except between 3.8 and 5.5 Ma (early Pliocene), as discussed above.
The top 21 mbsf of Hole 1086 are placed within Zone NN19. Consequently, drilling at Site 1086 did not recover any late Pleistocene material. Based on sedimentation rate estimates (Fig. 10), the Pleistocene/Pliocene boundary can be placed at ~15 mbsf.
The top of Zone NN18 is defined by the LO of Discoaster brouweri, a datum event recognized between Samples 175-1086A-2H-CC and 3H-CC. Zones NN18 and NN17 were lumped together because the LOs of both D. pentaradiatus (NN18/NN17 zonal boundary event) and D. surculus (NN17/NN16 zonal boundary) were identified within the same sampling interval. The base of Zone NN17 was identified between Samples 3H-CC and 4H-CC at the mean depth of 30.9 mbsf.
This interval is constrained between 30.9 and 48.9 mbsf. Discoaster brouweri, D. pentaradiatus, and D. surculus are the only star-shaped coccoliths identified within this sequence at Hole 1086A. The diagnostic species for the lower part of Zone NN16 (Subzone CN12a), D. tamalis, D. variabilis, and D. challengeri, are missing from the investigated samples, pointing either to a condensed CN12a interval, or an overall low diversity of Discoaster specimen because of cool surface-water conditions.
The top of this interval is defined by the LO of Reticulofenestra pseudoumbilica (3.82 Ma), a datum event identified between Samples 175-1086A-5H-CC and 6H-CC. This interval is marked by the first downhole occurrence of Sphenolithus sp. The Zone NN15/NN14 boundary is defined by the LO of Amaurolithus tricorniculatus (4.5 Ma), a datum identified at the mean depth of 59.4 mbsf (Samples 6H-CC through 25H-CC).
The base of this 0.52-m.y. interval was identified between Samples 175-1085A-26H-CC and 27H-CC (first occurrence [FO] of Discoaster asymmetricus; 5.02 Ma).
This interval is constrained between the mean depths of 78.3 and 97.3 mbsf. Zone NN13 is lumped together with Zone NN12 because of the sparse occurrence of Ceratolithus spp. throughout this interval. The LO of Discoaster quinqueramus (5.54 Ma), identified between Samples 175-1086A-10H-CC and 11H-CC, defines the NN12/NN11 zonal boundary.
This 3.06-m.y. interval is defined as the range of D. quinqueramus. The range of Amaurolithus amplificus (LO between Samples 175-1086A-15H-CC and 16H-CC; FO between Samples 175-1086A-17H-CC and 18H-CC) and the LO of Amaurolithus primus (7.2 Ma) were used to refine the age model.
The top two samples (175-1086A-1H-CC and 2H-CC) are assigned to Pleistocene Zone Pt1 (1.77–0 Ma). Zone Pt1 was not differentiated, although Sample 1H-CC (6.72 mbsf) is older than 0.65 Ma, based on the presence of G. tosaensis. The presence of G. crassaformis viola (latest Pliocene to early Pleistocene) corroborates the age. Sample 1H-1, 19–21 cm was examined, but no upper Pleistocene foraminifers were found in the core top. The base of Zone Pt1 is placed at 12 mbsf, based on the LO of G. truncatulinoides in Sample 2H-CC (Table 3).
Zonation was difficult in the Pliocene because many of the marker species are present only sporadically or are dissolution susceptible. For example, Globorotalia margaritae (last-appearance datum [LAD] at 3.58 Ma) is a diagnostic index fossil for the early Pliocene and is present from 97.4 to 135 mbsf (Samples 175-1086A-11H-CC through 14H-CC). The paleomagnetic and calcareous nannofossil data, however, indicate that the top of the lower Pliocene sequence occurs higher up in the section at ~ 60 mbsf. Although G. margaritae is present in temperate regions, it is a dissolution-susceptible species (Bolli and Saunders, 1985), and so its anomalous occurrence at Site 1086 is attributed to dissolution. Paleoceanographic changes as well as dissolution affect the distribution of the species downcore. For example, S. seminulina (LAD at 3.12 Ma) and D. altispira (LAD at 3.09 Ma) have premature LOs at this site, some ~50 m lower in the section than the nannofossil and paleomagnetic age models predict (Fig. 10). Both of these species are tropical to subtropical species (Kennett and Srinivasan, 1983), and this may indicate a cooling in the surface waters at that time (late Miocene to early Pliocene).
The late Pliocene Zones P6–P3 (97–12 mbsf) could not be differentiated (Samples 175-1086A-3H-CC through 7H-CC). The early Pliocene (135–97.4 mbsf) is identified by the presence of G. margaritae (Samples 175-1086A-11H-CC through 14H-CC). Zones Pl2 and Pl1 are not differentiated because the boundary is the LAD of G. nepenthes, a species that is not present in this interval. The top of earliest Pliocene Zone PL1a is defined on the LO of G. cibaoensis (Sample 12H-CC) at 116 mbsf, although G. cibaoensis is rare in these samples. G. juanai ranges through the lowermost part of Pl1a (Zone N18 of Blow, 1969), and the LO of G. juanai in Sample 11H-CC (106.8 mbsf) therefore restricts that interval to the lowermost Pliocene. This is in agreement with calcareous nannofossil datums and suggests that the top of Zone P11a should be moved up stratigraphically. The base of Zone Pl1 is placed at 135 mbsf and is coeval with the FO of G. sphericomiozea (5.6 Ma; Sample 14H-CC).
The Miocene transitional Zone Mt10 ranges from Samples 175-1086A-15H-CC through 22H-CC (the base of the core), based on the presence of G. conomiozea (FAD at 6.9 Ma) in Sample 22H-CC and the absence of G. sphericomiozea. (FO at 5.6 Ma) in Samples 15H-CC and 16H-CC. The absence of temperate to warm subtropical (Kennett and Srinivasan, 1983) species G. sphericomiozea in Samples 15H-CC (140 mbsf) and 16H-CC (149.73 mbsf) may be caused by ecological forcing.
Benthic foraminifers are abundant throughout Hole 1086A, but as at Site 1085, there is a very high planktonic to benthic ratio. The preservation of the benthic foraminiferal assemblages is good throughout Hole 1086A (Table 4).
The uppermost core catcher (Sample 175-1086A-1H-CC; 6.72 mbsf) is dominated by Cassidulina laevigata and Trifarina bradyi. The presence of Uvigerina spinulosa in this sample confirms an early Pleistocene age because this species only persists into the earliest Pleistocene (Boersma, 1984).
The core catchers in the Pliocene part of Hole 1086 (Samples 175-1086A-3H-CC through 13H-CC; 26.15–120.86 mbsf) are dominated by Cibicidoides pachyderma, Stilostomella spp., and Uvigerina auberiana and, in the lower part of this interval, T. bradyi. Samples from the early Pliocene and the uppermost late Miocene contain high abundances of Uvigerina hispida.
Studied samples from the late Miocene (Samples 175-1086A-15H-CC through 21H-CC; 140.04–197.04 mbsf) contain mostly similar species as the subsequent Pliocene interval, but with the addition of high percentages of Cibicidoides bradyi, which at Hole 1086A is restricted to the late Miocene.
Radiolarians are absent or extremely rare (one specimen per sample) in all core-catcher samples from Hole 1086A, except for Sample 175-1086A3H-CC, which yielded rare and poorly preserved radiolarians. The radiolarian assemblage from Sample 3H-CC, characterized by spherical spumellarians, Axoprunum stauraxonium, and the Ellipsoxiphus attractus group, suggests a warm and low productivity oceanographic condition. No age indicator species nor reworked specimens were identified.
Smear slides and the 63-µm-sieved fraction of each core-catcher sample from Hole 1086A were analyzed for their diatom content. Diatoms are absent or extremely rare (one Coscinodiscus specimen in Sample 175-1086A-5H-CC, and another one in Sample 11H-CC in the sieved fraction) at Hole 1086A. Silicoflagellates are absent. Sponge spicules are present in Samples 175-1086A-2H-CC through 4H-CC only; they are particularly abundant in Sample 3H-CC (26.15 mbsf; ~1.95 Ma).
As was the case at Site 1085, dinoflagellate cysts are also present at Site 1086; abundances range from frequent to common in Samples 175-1086A-15H-CC (140.04 mbsf) through the end of the hole (206.56 mbsf). It is interesting to note that the age of the LO of dinoflagellate cysts at Site 1086 approximates that of Site 1085 (~5.8 Ma), suggesting that both sites were affected by the same oceanographic process simultaneously.