The semiquantitative abundance of radiolarians, diatoms, sponge spicules, silicoflagellates, rock fragments, and amorphous material (clumps of sediment that failed to break down) are shown in Figure F4 and listed in "Appendix A". Abundances and variation in abundance of biosiliceous components are both rather high throughout much of the studied interval. Diatoms are the most abundant component, followed by sponge spicules, radiolarians, and silicoflagellates. Radiolarians frequently vary from abundant to rare between adjacent samples with a similar variability, albeit at lower absolute values, for silicoflagellates as well. This degree of variability is unusual in coeval pelagic sediment sections from the Southern Ocean and suggests either rapidly altering local upper-water environmental conditions or frequent episodes of nonbiogenic sediment dilution and lowered preservation during deposition at the seafloor. Diatoms are less variable and usually common to abundant in all samples. This may well be an artifact (recording range limit error) of the pipette method of sample preparation because residue is added to the slide until an optimum slide density is obtained. Diatoms, as the most abundant component in the residue, would, thus, normally be at least common in the large majority of slides made and equally are not allowed to ever be more than the maximum value of "abundant." There is a weakly developed pattern of variation in mean abundances, particularly in the silicoflagellates, at a scale of ~100 m (~1 m.y.), but the pattern and possible correlations between components is not very obvious. Biosiliceous material is essentially absent below Core 178-1095B-46X, to a degree that suggests either substantially different primary upper water environmental conditions or a major change in preservation (pre- or postdepositional).
The radiolarian faunas recovered from Hole 1095B (Table T1) are fairly typical pelagic assemblages for this time interval (the full taxonomic list given in "Appendix B"), with some significant differences. Basal Pliocene and latest Miocene faunas are dominated by Antarctissa denticulata and Antarctissa strelkovi, which give way in older late Miocene deposits to forms transitional to Antarctissa deflandrei. Accessory taxa such as Cycladophora pliocenica, Prunopyle titan, Lychnocanium grande (group) in the basal Pliocene to latest Miocene and Dendrospyris rhodospyroides/haysi, Prunopyle hayesi, and Siphonosphaera vesuvius in older late Miocene samples are also typical. Despite the lack of a full objective treatment of the entire fauna, the general (subjective) impression given by the faunas is of somewhat lower-than-average diversity, even in assemblages with common to abundant individuals and moderate to good preservation. Certain groups, in particular the actinommids and the prunoid/lithelids, seem to be poorly represented in both numbers of taxa and individuals. On the other hand, in at least some of the better preserved samples, there is a wealth of plagoniid taxa, although this latter observation may equally reflect the fact that most prior work on Southern Ocean faunas has been done using a 63-µm sieve, which does not retain many of the small plagoniid species.
The less-than-full diversity and somewhat skewed composition at higher taxonomic levels suggest that ecologic restrictions on the distribution of taxa were important in this region in this time interval. The causes are unknown; but in any case, this observation has a significant bearing on the interpretation of the biostratigraphic data.
The studied interval corresponds to the following zones: basal part of the Tau, Amphymenium challengerae, Amphymenium labrata, S. vesuvius, and upper Amphymenium australis Zones of Lazarus (1992). All zonal markers are seen, and the order of events conforms to expectations (Table T2). However, the relative duration between events, as judged from relative sediment thicknesses, is often rather different than reported by Lazarus (1992) and others. Local ecologic and/or preservational control of event positions may be responsible for this, a suspicion reinforced by the general assemblage characteristics noted above and by the ranges of individual taxa. A. labrata, for example, ranges downcore in more open ocean sections to approximately the top of A. australis and S. vesuvius (e.g., Site 751) (Lazarus, 1992). In Hole 1095B, there is a >100-m gap between these events. It is also possible that the last appearance datum of A. challengerae and the last abundant appearance datum of L. grande are further upsection than reported here, as the observed tops may be due to the intermittent presence of the taxa within their ranges. Thus, although the section can be assigned standard zonal ages, the accuracy of these age assignments is somewhat questionable.
In addition to the standard marker taxa, several as-yet unnamed taxa were seen within the late Miocene with what appear to be restricted stratigraphic ranges and that are also known to occur in approximately the same time intervals in other Southern Ocean sites (e.g., Sites 745 and 746) (D.B. Lazarus, unpubl. data). Two of these are recorded in Table T1 in this report, are illustrated in Plate P1, and are briefly described in open nomenclature in "Appendix B" (specimens from Kerguelen Site 746 were used for illustration because of better preservation of specimens). New taxa such as these may prove to be useful additions to the radiolarian stratigraphy of late Neogene sediments in the Antarctic region.