The biostratigraphy at Site 1095 was derived from an examination of diatoms, radiolarians, and benthic and planktonic foraminifers. Pliocene to Pleistocene deposits consisted primarily of glacial and interglacial hemipelagic sediments containing foraminifers but only rare intervals of siliceous microfossils. The upper Miocene through lower Pliocene sediments consist mainly of turbidites, together with intervals of hemipelagic sediments containing radiolarians and diatoms and very rare foraminifers. The biostratigraphy of Site 1095 was accomplished using zonations developed for the Southern Ocean (see "Biostratigraphy" in the "Explanatory Notes" chapter).
The diatom biostratigraphy of Holes 1095A, 1095B, and 1095C is complex and contains an incomplete record of Pleistocene through upper Miocene datums and events. Many core intervals are barren or have a low abundance and diversity of diatoms. In samples that contain diatoms, reworking of older species is often noted. In spite of the obvious reworking, there is a strong biostratigraphic signal for the early Pliocene and late Miocene (Fig. F14). Material used in the biostratigraphic analysis of this site was gathered from core catchers as well as from within the split cores (usually one sample per section).
All datums and their hole/core depths at Site 1095 are listed in Table T4. The identification of diatom species at this site follows the taxonomic determinations of Schrader (1976), Akiba (1986), Barron (1985), Akiba and Yanagisawa (1986), Gersonde (1990, 1991), Gersonde and Burckle (1990), Yanagisawa and Akiba (1990), Baldauf and Barron (1991), Harwood and Maruyama (1992), and Gersonde and Bárcena (1998).
The first datum in the zonal scheme is the last occurrence (LO) of Actinocyclus ingens. This was noted early in the drilling, in Sample 178-1095A-2H-3, 90 cm (9.8 mbsf). The LO datums of Thalassiosira elliptipora and Fragilariopsis barronii (178-1095A-3H-6, 94 cm [17.7 mbsf]) were also observed within the A. ingens Zone. Zonal boundary datums for the Thalassiosira kolbei Zone were not noted, but the LO of Thalassiosira inura (which occurs within this zone) was observed at 178-1095A-9H-3, 130 cm (72.6 mbsf). Within the overlap between Holes 1095A and 1095B is the LO of Fragilariopsis interfrigidaria, which was last noted in Sample 178-1095B-1H-1, 95 cm (83.9 mbsf) and was absent from 178-1095A-9H-CC (77.8 mbsf). The Thalassiosira vulnifica and T. insigna/vulnifica Zones were not observed in Hole 1095A. In Hole 1095D, however, we observed some well-preserved specimens of T. vulnifica and T. insigna in Samples 178-1095D-7H-CC (65.6 mbsf) and 8H-CC (75.1 mbsf). These two core-catcher samples would fall within the T. insigna/T. vulnifica Zone because both species are present, but the top and bottom datums for this zone were not noted in Hole 1095D.
The base of the F. interfrigidaria Zone was identified in Sample 178-1095B-3H-CC (111.5 mbsf) with the first occurrence (FO) of F. interfrigidaria. This zone, and the earlier F. barronii Zone, is well represented in the sediments at this site with abundant and diverse diatom assemblages. The base of the F. barronii Zone is marked by the FO of F. barronii in Sample 178-1095B-5H-3, 33 cm (124.3 mbsf); co-occurrent with this datum is the FO of Thalassiosira striata. The FO of Thalassiosira complicata was noted in Sample 178-1095B-6H-CC (140.0 mbsf), and the next biostratigraphically useful datum (FO of T. inura) occurs six cores deeper in Sample 178-1095B-12H-4, 41 cm (192.4 mbsf).
The lower two-thirds of Hole 1095B is late Miocene in age. Diatoms are preserved in Cores 178-1095B-15H through 43X. The FO datum of Thalassiosira oestrupii, defining the base of the T. oestrupii Zone, is found between Samples 178-1095B-14X-4, 133 cm, and 15X-2, 126 cm. The FO of Fragilariopsis praeinterfrigidaria, which has been observed later than that of T. oestrupii (Harwood and Maruyama, 1992), is in Sample 178-1095B-14X-4, 133 cm (210.8 mbsf). The next sample with a good assemblage of diatoms is from 178-1095B-15X-2, 126 cm (217.5 mbsf). T. oestrupii is not found in this sample or in any of the others below, thus placing the FO datum for T. oestrupii in this interval (210.8-217.5 mbsf). It is possible that the order in which these two datums occur in this region is reversed compared to other areas of the Southern Ocean. Another datum in this part of the hole is the LO of Nitzschia donahuensis in Sample 178-1095B-15X-2, 126 cm (217.5 mbsf). Because the stratigraphic age of this datum is older than that of T. oestrupii, the argument for placing this previous zonal boundary between Samples 178-1095B-14X-4, 133 cm, and 15X-2, 126 cm, is reinforced (Table T4).
The LO datum for Actinocyclus ingens var. ovalis is placed in Sample 178-1095B-21X-3, 128 cm (276.7 mbsf). The upper boundary species datum for the A. ingens var. ovalis Zone (FO of Thalassiosira oliverana) was not observed at this site. It would occur just below the A. ingens var. ovalis LO datum, according to the biostratigraphy of other regions of the Southern Ocean. We noted FO datums of two species (A. ingens var. ovalis and T. oliverana var. sparsa) in Sample 178-1095B-28X-5, 104 cm (346.8 mbsf). The FO of Thalassiosira torokina is higher in the sedimentary column than its biostratigraphic range would place it, occurring in Sample 178-1095B-23X-CC (301.2 mbsf). After examining additional samples, we may be able to determine that the late FO of T. torokina resulted from poor preservation within samples from this period and that intervals of good preservation were not sampled in this preliminary investigation.
The samples in Cores 178-1096B-29X through 45X contained a typical late Miocene assemblage, with a large proportion of Denticulopsis species dominating. The species of Denticulopsis used as zonal boundary markers in older sediments, D. dimorpha and D. praedimorpha, were absent from this assemblage. One sieved sample, from Core 178-1095B-37X, contained Asteromphalus kennetii, which has an FO age of 10.29 Ma. This datum also helps constrain the age of the lower part of Hole 1095B to the late Miocene.
The neritic diatom Paralia sulcata has been observed in samples from Cores 178-1095B-21X through 27X and is especially common in Cores 178-1095B-23X and 24X (290-310 mbsf) (see "Sedimentation Rates" [Fig. F52]). Paralia sulcata is a benthic neritic species that lives on coarse sediment in a relatively low-salinity, shallow-water environment and can easily be transported to deep water. The movement from a shallow-water environment can take place through sediment transport or by an increased influx of relatively low-salinity water from the shelf. The sporadic presence of delicate benthic diatoms, Navicula spp. and Pinnularia spp., and of the reworked middle Miocene diatom, Denticulopsis dimorpha var. areolata, in the same interval suggests shallow-water sediment transport. To account for the observed proportion of P. sulcata, it is possible that more of the shelf was in the photic zone and was therefore shallower than today. However, this is a preliminary interpretation based on the shipboard smear-slide observations. Additional future analyses are needed.
Radiolarians from core-catcher samples at Site 1095 range in age from late Pliocene (Phi Zone) to late Miocene (Acrosphaera australis Zone). Occurrences of good to moderately well-preserved radiolarians are sporadic, with long intervals that are barren or have only rare occurrences of radiolarians (Fig. F15). Consequently, first and last occurrences of biostratigraphic indicator species can only be estimated. All assemblages are composed of high-latitude species, and no warm-water species were observed. The youngest sediment with stratigraphically useful species is in Sample 178-1095D-3H-CC (27.6 mbsf) indicated by Triceraspyris antarctica in the Phi Zone. A relatively diverse assemblage is present in this sample, and Helotholus vema does not occur. The shallowest occurrence of H. vema is in Sample 178-1095D-4H-CC (37.1 mbsf), indicating a late Pliocene age (Upsilon Zone).
The youngest core-catcher sample in Hole 1095A with biostratigraphically useful radiolarians is 178-1095A-8H-CC (68.3 mbsf), containing Antarctissa cylindrica, H. vema, Desmospyris spongiosa, and Stylatractus universus. The sediment above this core was barren of radiolarians, so Core 178-1095A-8H-CC is placed within the Upsilon Zone instead of at its upper boundary. However, because Prunopyle titan was not present in Sample 178-1095A-8H-CC (but does appear deeper in Hole 1095B), we conclude that the sample is younger than 3.5 Ma. Prunopyle titan does not occur in Hole 1095A but does in 1095B, suggesting that Hole 1095A does not penetrate sediments older than 3.5 Ma.
In Sample 178-1095B-3H-CC (111.5 mbsf), Lampromitra coronata is present, suggesting an age of at least 3.7 Ma. The common occurrence of D. spongiosa implies a Pliocene age within the Upsilon Zone and not reworking. The last occurrence of H. vema is in Sample 178-1095B-8H-CC (159.0 mbsf). Because the moderate preservation in Sample 178-1095B-9H-CC seems sufficient for the presence of H. vema, the lower boundary of the Upsilon Zone is placed between Samples 178-1095B-8H-CC and 9H-CC (168.5 mbsf).
The top occurrence of Lychnocanium grande is in Sample 178-1095B-10H-CC (178.0 mbsf). This species is common in Sample 178-1095B-16H-CC (233.9 mbsf), where its top common occurrence is placed (5.0 Ma, in the Tau Zone; see "Sedimentation Rates"). In core-catcher samples below Core 178-1095B-18H (243.5-235.1 mbsf), few to no radiolarians were encountered until Sample 178-1095B-29H-CC (359.2 mbsf). Here, fragments of Siphonosphaera vesuvius are present, indicating a middle late Miocene age within the S. vesuvius Zone. No marker species for the Amphymenium challengerae Zone were encountered. The Acrosphaera? labrata Zone is indicated in Sample 178-1095B-19X-CC (262.7 mbsf) by the presence of A.? labrata fragments.
The oldest marker species observed is a fragment of Cycladophora spongothorax in Sample 178-1095B-43X-CC (493.4 mbsf). A fragment of A. australis is also present. The presence of sponge spicules may indicate increased influence of shelf deposits, which would dilute the oceanic component containing radiolarians. On the other hand, this could be an indication of reworking. These sediments are placed in the A. australis Zone, however, because diatom and foraminifer data are consistent with this age; further, none of the several biostratigraphically significant species just below this datum is present. No other biostratigraphically useful specimens were encountered between Sample 178-1095B-44X-CC and the bottom of the hole at 52X-CC.
The sporadic nature of the observed radiolarian biostratigraphic record makes it impossible to estimate a single sedimentation rate for any given time. Nevertheless, a range of sedimentation rates can be generated from the available biostratigraphic data (Fig. F15).
Shipboard analyses of foraminifers at Site 1095 were done using core-catcher and a few additional samples. Rare to abundant planktonic and rare benthic foraminifers were recovered in Cores 178-1095A-1H through 6H (49 mbsf). Preservation of the foraminifers was good to excellent. The planktonic foraminiferal assemblage was dominated by Neogloboquadrina pachyderma sinistral, which is characteristic of Pleistocene to upper Miocene sediments in the Southern Ocean (Berggren, 1992). Rare specimens of N. pachyderma dextral and Globigerina bulloides were also observed in Holes 1095A and 1095D.
Hole 1095B was barren of foraminifers except for two samples. Sample 178-1095B-23X-5, 34-36 cm (298 mbsf), contained a small assemblage consisting of N. pachyderma sinistral and several benthic foraminifer species. The preservation was so poor, however, that identification could only be made to genus level. Sample 178-1095B-44X-CC (494 mbsf) contains a better preserved but small planktonic foraminifer assemblage consisting of Neogloboquadrina continuosa, Globorotalia scitula, and Globoturborotalita woodi. This suggests a middle to late Miocene age and would fall in the G. scitula Zone of Berggren (1992).