MST data, collected at 2-cm intervals from Holes 1088A through 1088C, and color reflectance data (650-750 nm), collected at 4-cm intervals from Holes 1088A through 1088C, were used to determine depth offsets in the composite section. Gamma-ray attenuation (GRA) bulk density, color reflectance, and magnetic susceptibility measurements were the primary parameters used for the limited hole-to-hole correlation at Site 1088. The three types of data are presented on a composite depth scale in Figures F10, F11,and F12, respectively. The depth offsets are given in Table T2 (also in ASCII format in the TABLES directory).
Only two short depth intervals (0-5.5 and 122-129 meters composite depth [mcd]) were cored multiple times. Thus, continuity of the sedimentary section could not be documented at Site 1088. The composite data show that Holes 1088A and 1088B can be spliced together at intervals 177-1088B-14H-6, 12 cm, and 177-1088C-2H-4, 18 cm (127.12 mcd), to form a single (albeit discontinuous) section down to 230 mcd.
Although the upper 5.5 mbsf was cored in all three holes, no attempt was made to construct a continuous sampling splice over this interval for two reasons. First, Cores 177-1088B-1H and 177-1088C-1H are almost the same length (5.5 and 5.8 m, respectively), and the recovered material from these two cores is nearly identical. Second, Core 177-1088A-1H was highly disturbed and its depth relationship to the uppermost cores in the other holes could not be confirmed. Thus, the gap between Cores 177-1088B-1H and 2H could not be spliced with material from either Holes 1088A or 1088C.
Sediments recovered from Site 1088 provide a nearly continuous record for the Pleistocene through middle Miocene. Calcareous nannofossils are abundant or very abundant, with good to medium preservation of assemblages. Overgrowth is a common feature observed in Discoaster spp. but, in general, does not prevent identification. One to two samples per section were examined to obtain an accurate age assignment of the cores. The age model used for nannofossil event calibration is mainly derived from tropical and subtropical areas (see "Biostratigraphy" in the "Explanatory Notes" chapter), and diachronism can occur. For the same reason, characteristic events included in the Martini (1971) and Okada and Bukry (1980) standard zonations have not been identified (Tables T3, T4, both also in ASCII format in the TABLES directory). The adopted calcareous nannofossil biochronology for the late Neogene, compared to the geomagnetic polarity time scale (GPTS) of Berggren et al. (1995), is included in "Biostratigraphy" in the "Explanatory Notes" chapter. Table T5 (also in ASCII format in the TABLES directory) summarizes the calcareous nannofossil biostratigraphic results.
The Pleistocene interval in Hole 1088B is represented from the top to 20.2 mcd, where the first occurrence (FO) of medium-sized Gephyrocapsa marks the Pliocene/Pleistocene boundary (Table T5).
The acme and the FO of Emiliania huxleyi define the base of Subzones NN21b and NN21a at 1.10 and 2.65 mcd, respectively. The top of the Gephyrocapsa caribbeanica acme is contemporaneous with the FO of E. huxleyi and represents a good alternative event for identifying the base of Subzone NN21a, especially when dissolution occurs. The last occurrence (LO) of Pseudoemiliania lacunosa is observed around 4.40 mcd and defines the base of Zone NN20. The top and the base of the small Gephyrocapsa acme (see "Biostratigraphy" in the "Explanatory Notes" chapter) are not clearly recognized; closer sampling and quantitative analyses will be necessary to define these events. However, an obvious dominance of the "very small Gephyrocapsa" complex is observed between 8.0 and 15.0 mcd. The LO and FO of Reticulofenestra asanoi are recognized at 11.80 and 15.07 mcd, respectively. The re-entrance of medium Gephyrocapsa is found at 11.8 mcd. The LO of large Gephyrocapsa (>5.5 µm) is present at 15.07 mcd, and its FO is present at 16.80 mcd (Tables T3, T5). The LO of Calcidiscus macintyrei is observed at 18.30 mcd.
Typical Pliocene biostratigraphic markers, such as taxa of the genera Discoaster and Amaurolithus, were found only at low abundance. Nevertheless, a semiquantitative analysis conducted on the upper Pliocene assemblages allowed the recognition of the LOs of Discoaster brouweri (Zone NN19), Discoaster pentaradiatus (Zone NN18), Discoaster surculus (Zone NN17), and Discoaster tamalis (top of Zone NN16) at around 21.30, 22.8, 23.89, and 25.55 mcd, respectively. A hiatus of about 0.5 m.y. may occur between 20.95 and 22.80 mcd (Table T5; Fig. F13). Assemblages characterized by Sphenolithus spp. (S. abies/neoabies) and Reticulofenestra pseudoumbilicus were assigned an early Pliocene age. The LO of Sphenolithus spp. is present between 31.90 and 32.70 mcd, whereas the LO of R. pseudoumbilicus falls between 33.40 and 33.90 mcd. The top of the small Gephyrocapsa acme is recognized close to the LO of Sphenolithus spp. (32.30 mcd); the base of this acme is less clear than the top, but it seems to begin close to 40.0 mcd. This acme was defined by Marino (1994) from the Mediterranean area. Events defined by ceratoliths and related forms, such as Triquetrorhabdulus rugosus, have not been identified because of the extremely low abundance or absence of these taxa (Tables T3, T5).
The Miocene interval is represented in both Holes 1088B (late Miocene) and 1088C (middle-late Miocene). The upper Miocene assemblages are characterized by few to rare specimens of Discoaster spp. and Amaurolithus spp. Nevertheless, some events are tentatively recognized. In Hole 1088B, the LO of Discoaster quinqueramus, the FO of Amaurolithus s.l., the FO of D. quinqueramus, and the LO of Discoaster hamatus are placed at around 46.05, 71.29, 97.30, and 115.2 mcd, respectively (Table T5). Rare specimens of Amaurolithus amplificus are observed in Sample 177-1088B-7H-3, 140 cm, but it is not possible to establish the range of distribution. The above cited events allow us to identify Zones NN10 through NN12 for the latest Miocene. Rio et al. (1990) referred to a late Miocene interval of absence of R. pseudoumbilicus <7 µm (paracme). This characteristic feature is not observed at Site 1088 for the corresponding interval (Zones NN10 and NN11).
In Hole 1088C (Table T6, also in ASCII format in the TABLES directory), the FO of D. hamatus between 142.14 and 142.84 mcd defines the base of Zone NN9. The LO of Coccolithus miopelagicus is clearly identified between 169.71 and 169.97 mcd, approximately defining Zone NN8. This last event is the most useful for identifying the upper/middle Miocene boundary (Raffi and Flores, 1995). Unfortunately, it is not possible to recognize the zones of the middle Miocene because standard markers such as Sphenolithus heteromorphus, Discoaster kugleri, and Catinaster coalitus are absent. The LO of Coronocyclus nitescens, FO of Calcidiscus macintyrei, LO of Calcidiscus premacintyrei, and last common occurrence (LCO) of Cyclicargolithus floridanus are observed between 205.80 and 215.37 mcd, and can be considered indicative of the base of Zone NN6 (Tables T3, T5). The identification of these events suggests the possibility of a hiatus at ~207.0 mcd. (Figs. F13, F14). No characteristic markers from Zone NN5 have been observed at the bottom of Site 1088, which is tentatively assigned to this interval of time (<13.2 Ma) (Tables T3, T5).
Planktic foraminifers are abundant in the >63-µm fraction throughout the cores examined from Holes 1088B and 1088C (Table T6). Samples from Sections 177-1088B-1H-CC through 14H-CC and 177-1088C-1H-CC through 13X-CC were studied. The ages of the sections range from Quaternary to middle Miocene. The Pliocene/Pleistocene boundary can be defined in Hole 1088B between Samples 177-1088B-2H-CC, 12-19 cm (14.44 mcd), and 3H-CC, 14-19 cm (23.97 mcd), on the basis of the FO of Globorotalia truncatulinoides in Sample 2H-CC, 12-19 cm, which indicates an age of less than ~2 Ma (Berggren et al., 1995). In Hole 1088C, only Sample 177-1088C-1H-CC, 12-17 cm, contains G. truncatunlinoides, suggesting a Quaternary age (Table T6). The absence or low abundance of several important marker species makes it difficult to subdivide the studied sections into the subantarctic zonal scheme of Jenkins and Srinivasan (1986) (see "Biostratigraphy" in the "Explanatory Notes" chapter). In addition, absolute ages are currently only available for a few subantarctic and transitional foraminifer datums (Berggren et al., 1995). However, the presence of ventroconical forms of Globorotalia miotumida (=Globorotalia conomiozea according to Kennett and Srinivasan, 1983) in Sample 177-1088B-6H-CC, 13-18 cm indicates an age <6.9 Ma (Berggren et al., 1995) (Table T6). The FO of this form was difficult to determine because of the gradation between different forms within the G. miotumida complex.
The Pliocene-Pleistocene assemblages of Site 1088 are dominated by Globigerina bulloides, Neogloboquadrina pachyderma (sinistral), Globorotalia crassula, G. crassaformis, G. inflata, and G. puncticuloides. The Miocene assemblages of Site 1088 are dominated by G. miotumida, G. panda, and G. mayeri. Globorotalia quinqueloba and Globigerinita glutinata are present throughout the studied cores. In addition, abundant to rare occurrences of Globigerinoides sacculifer, G. obliquus, Globigerina falconensis, Globigerinita uvula, Globigerinella aequilateralis, Neogloboquadrina humerosa, and Globoquadrina dehiscens were recorded at Site 1088.
Planktic foraminifers are generally well preserved in Sections 177-1088B-1H-CC through 5H-CC. Pliocene and Miocene assemblages are moderately preserved in both Holes 1088B and 1088C. Preservation changes were easily detected in the studied sections as an increase in the ratio of Globorotalia vs. Globigerina in the less well-preserved samples.
Benthic foraminifers were present in all the CC samples from this site, generally constituting less than 5% of the total foraminifer fauna from the >63-µm fraction studied. Quantitative estimates of relative species abundance were made with counts of as many as 190 specimens per sample. Diversity is variable, with a maximum of 39 taxa recorded in Sample 177-1088B-8H-CC, 10-15 cm, and a minimum of less than 20 taxa in Samples 177-1088B-1H-CC, 7-12 cm, and 177-1088C-1H-CC, 12-17 cm. Preservation is generally good throughout, showing some deterioration downhole, particularly below ~120 mcd. Absolute foraminifer abundances, while variable, exhibit a clear trend toward higher values uphole, reaching a maximum of 246 specimens/cm3 in Sample 177-1088B-5H-CC, 12-18 cm. These changes in foraminifer abundance appear to reflect the general pattern of sedimentation at Site 1088, from nannofossil ooze (low abundance) during the middle and late Miocene to nannofossil foraminifer ooze (higher abundance) during the Pliocene-Pleistocene.
Biostratigraphic differentiation of Site 1088 is limited to the LO of Stilostomella lepidula in Section 177-1088B-2H-CC, which supports a late Pleistocene age above 14.44 mcd (e.g., Thomas, 1987). Interesting assemblage changes are evident in the transition from middle to upper Miocene, characterized by increased relative abundances of the genus Uvigerina, notably U. hispidicostata. In addition, Cibicidoides mundulus, C. wuellerstorfi, Epistominella exigua, Gyroidinoides soldanii, and Bolivina spp. all tend to be more common downhole (Table T7, also in ASCII format in the TABLES directory).
Diatom identification was conducted on smear slides. Because of the high carbonate content, ranging between 85 and 95 wt%, of the total sediment throughout the entire recovered section at Site 1088 (Fig. F6), selected samples were acid cleaned for the study of the noncarbonate biosiliceous residue (Table T8, also in ASCII format in the TABLES directory).
In general, uncleaned smear slides contain only low concentrations of diatoms, silicoflagellates, ebridians, and Actiniscus, and some samples have been identified as barren of these microfossils. However, sponge spicules are recorded in all investigated samples at varying abundances. Only one interval (177-1088B-3H-2, 60 cm) contains abundant and well-preserved diatoms, with an assemblage dominated by taxa of the Thalassiothrix antarctica-longissima group.
In the noncarbonate residue slides, moderately preserved diatom assemblages were recovered in five other sediment intervals representing the time between ~14 and 3 Ma. The uppermost interval (177-1088B-3H-CC, 14-19 cm) represents the late Pliocene Thalassiosira insigna/T. vulnifica Zone and, thus, places the Thalassiothrix-rich interval encountered in the same core in the same time interval (Fig. F14; Table T4). Interval 1088B-10H-CC, 10-15 cm, has a diatom assemblage marked by the common occurrence of Actinocyclus ingens var. ovalis, a zonal marker that ranges in age between 8.7 and 6.3 Ma. The co-occurrence of Nitzschia cylindrica places this interval in the early F. reinholdii Zone on the basis of the FO of N. cylindrica at 7.6 Ma (Barron, 1992). However, the calcareous nannofossil age assignment of this interval (see "Calcareous Nannofossils") indicates an age ranging in the lower portion of the F. reinholdii Zone (~8 Ma). Interval 1088B-13H-CC, 11-16 cm, contains an assemblage dominated by Coscinodisus marginatus and the Thalassiothrix antarctica-longissima group. Age-indicative taxa were not encountered. The co-occurrence of Actinocyclus ingens and Denticulopsis dimorpha places interval 177-1088C-8X-5, 10-12 cm, in the middle-late Miocene D. dimorpha Zone. The oldest interval containing significant diatoms in the acid-cleaned residue is 177-1088C-11X-2, 48-50 cm, and it is assigned to the middle Miocene A. ingens var. nodus Zone.
Evidence for eolian transport of diatoms is present in intervals 177-1088B-5H-CC, 12-18 cm, and 10H-CC, 10-15 cm, where the freshwater diatom Aulacoseira granulata and opaline phytoliths were identified, respectively.
In general, diatoms do not contribute significantly to the establishment of an age-depth model at Site 1088 (Table T4; Fig. F14). However, their occurrences indicate the potential of sediment intervals that are rich in biosiliceous components at the northernmost site of the Leg 177 transect. These biosiliceous intervals might be related to short-term paleoceanographic changes leading to increased paleoproductivity at Site 1088.
Radiolarians are present in all the CC samples from Holes 1088A, 1088B, and 1088C. In general, the radiolarian preservation is excellent to moderate, and abundance varies from abundant to moderate, except for Sample 177-1088C-13X-CC in which only a few specimens were present in a strewn slide (Table T9, also in ASCII format in the TABLES directory).
The radiolarian fauna of Site 1088 indicates a Quaternary to middle Miocene age and is characterized by the dominant occurrence of low- to mid-latitude species. Typical Antarctic species, such as Antarctissa denticulata, occur only in the upper part of Hole 1088B (Samples 177-1088B-1H-CC through 3H-CC).
The Quaternary/Pliocene boundary is placed between Samples 177-1088B-2H-CC (14.44 mcd) and 8H-CC (71.37 mcd). This interval can be assigned to the Pleistocene Eucyrtidium matsuyamai to Pliocene Sphaeropyle langii Zones on the basis of the common occurrence of Lamprocyrtis heteroporos in Samples 1088B-3H-CC through 7H-CC (Fig. F14; Table T4).
A distinctive late Miocene assemblage containing Diartus hughesi is first recognized in Sample 177-1088B-10H-CC, which is correlative with the Didymocyrtis antepenultima Zone. Therefore, the interval from Sample 1088B-7H-CC through 10H-CC, which contains Stichocorys peregrina, spans the Pliocene to upper Miocene.
Samples 177-1088B-11H-CC through 14H-CC and 177-1088C-2H-CC, 99.16 to 130.47 mcd, are assigned to the middle Miocene Diartus petterssoni Zone on the basis of the sporadic presence of the nominal species and persistent occurrence of Didymocyrtis laticonus.
Didymocyrtis mammifera, ranging in age from early to middle Miocene, was identified in Samples 177-1088C-3H-CC, 9X-CC, 10X-CC, and 13X-CC. On the basis of the previously known range of D. mammifera, an interval from Sample 177-1088C-3H-CC to 13X-CC, 141.49 to 233.27 mcd, may be correlative with the Dorcadospyris alata Zone, although the nominal species is absent. Further investigation is necessary to make precise correlation. Below Sample 177-1088C-4H-CC, 150.54 mcd, radiolarian assemblages are particularly monotonous. Cyrtocapsella japonica predominates in Samples 1088C-4H-CC through 8X-CC, whereas the assemblages below Sample 1088C-9X-CC are characterized by the presence of C. tetrapera. This type of radiolarian assemblage is very similar to that in the mid-latitude northwestern Pacific region and is considered to be middle Miocene in age. The LO of Cyrtocapsella japonica, which is placed between Samples 177-1088C-3H-CC and 4H-CC, is dated at 8.9 Ma in the northwest Pacific (Motoyama, 1996).
Archive halves of all APC cores recovered at Site 1088 were measured using the shipboard pass-through magnetometer. Measurements were made at 5-cm intervals. For Cores 177-1088A-1H through 10H, six measurement steps were conducted after alternating-field demagnetization at peak fields of 0, 5, 10, 15, 20, and 25 mT. For Cores 177-1088A-11H through 14H, and all APC cores from Hole 1088C, three measurement steps were conducted after 0-, 10-, and 25-mT peak-field demagnetization.
Natural remanent magnetic moments were about 10-4 Am2 throughout the core, approximately two orders of magnitude above the noise level of the magnetometer. Inclinations show a tendency to steepen during progressive demagnetization. Throughout the demagnetization sequence, however, inclinations remain less than those expected (60°) for the site location (Fig. F15) and declinations are highly scattered. This suggests a low-inclination (possibly radial) remagnetization of the core. The perceived drill-string remagnetization may have been exacerbated by physical disturbance of the poorly consolidated nannofossil/foraminifer oozes.
A 233.4-m-thick sedimentary section spanning the interval from the Pleistocene through the middle Miocene was recovered at Site 1088. The basal age was estimated to be ~13-14 Ma. Holes 1088A, 1088B, and 1088C were cored with the APC to 9.5, 129.0, and 162.3 mbsf, respectively. Drilling in Hole 1088C continued with the XCB to a total depth of 233.4 mbsf. A continuous sedimentary section could not be documented at Site 1088. Furthermore, a magnetostratigraphy was not obtained at Site 1088 because of core-barrel-induced magnetization problems.
Age assignment and calculation of sedimentation rates for Site 1088 are based primarily on calcareous nannofossil biostratigraphy. Calcareous nannofossil Zones NN6 through NN21 and CN5 through CN15, respectively, were identified on the basis of examination of CC and additional samples, indicating rather continuous sedimentation. Planktic foraminifers and diatoms provided only a few datum levels. Radiolarian assemblages, which were encountered at excellent to moderate preservation in all CC samples, contain only few age-indicative taxa that allow the identification of long-range zones that are not well tied to the GPTS (Fig. F14; Table T4). On the basis of combined biostratigraphic results, 25 age-depth control points were selected. The control points represent mean depths (mbsf and mcd) of samples below and above the biostratigraphic datums (Table T3). The depth uncertainty associated with the calcareous nannofossil datums is about ±0.7 m, corresponding to a resolution of one-half core section.
The resulting age-depth interpretation shows a rather continuous sedimentation. Low sedimentation rates, which might be indicative of a hiatus or condensed sections, are found only between 1.95 and 2.45 Ma (20.95-22.8 mcd) and from 12.3 to 12.7 Ma (206.1-207.0 mcd; Fig. F13). These discontinuities, however, may be an artifact of age uncertainties associated with the calcareous nannofossil datums. On the basis of the age model derived from the biostratigraphic control points, the Pliocene/Pleistocene boundary can be placed at 19.61 mbsf (19.61 mcd), the Miocene/Pliocene boundary at 43.90 mbsf (43.90 mcd), and the middle/late Miocene boundary at 183.31 mbsf (181.45 mcd).
Calculated sedimentation rates average 12 m/m.y. in the Pleistocene and 7 m/m.y. in the Pliocene (Fig. F13; Table T3). Late Miocene sedimentation rates average 28 m/m.y., twice those of the younger Pliocene-Pleistocene section. Before 12.3 Ma, sedimentation rates drop, reaching values of <1 m/m.y.