BIOSTRATIGRAPHY AND SEDIMENTATION RATES

Holes 1165B and 1165C represent a composite section of ~1000 m, which is divided into three primary lithostratigraphic units. Unit I (0 to ~64 mbsf) consists of a brown diatom-bearing silty clay spanning the uppermost Pleistocene to lowermost Pliocene. A disconformity (~1 m.y.) between uppermost Pleistocene and lower Pleistocene sediments is identified within Unit I at ~6 mbsf, based on integrated diatom and magnetostratigraphic data. Unit II (~64 to ~305 mbsf) is a greenish gray diatom-bearing clay spanning the uppermost Miocene to middle Miocene. Unit III (~305 to ~999 mbsf) is composed of dark gray, thinly bedded fissile claystones assigned to the lower middle Miocene to lower Miocene. Two possible hiatuses are identified in Unit III from biostratigraphic data. A middle Miocene hiatus (~2 m.y. in duration) at ~328 mbsf is identified from radiolarian data, and a possible hiatus of ~1 m.y. is also identified in the lower Miocene, at ~485-495 mbsf, from diatom data.

The approximate positions of the upper Pliocene to lower Miocene subepoch boundaries in Hole 1165B are recognized from diatom and radiolarian biostratigraphy. The lower Pliocene/upper Pliocene boundary is placed at ~30 mbsf (within Core 188-1165B-4H). The Miocene/Pliocene boundary is identified at ~50 to ~70 mbsf (between Cores 188-1165B-6H and 8H). The middle Miocene/upper Miocene boundary is placed at ~190-215 mbsf (within Cores 188-1165B-24X to 26X). The lower Miocene/middle Miocene boundary is recognized at ~450-485 mbsf (within Cores 188-1165B-51X to 55X).

Biostratigraphic zonal assignments and paleoenvironmental interpretations for Site 1165 are based on shipboard analysis of diatoms, radiolarians, benthic foraminifers, planktonic foraminifers, and calcareous nannofossils. The results of these initial investigations are summarized in Figures F28A, F28B, and F29 and are described below.

Introduction

Planktonic foraminifers are present in few samples at Site 1165. All records presented here are based on examination of 10-cm³ samples, predominantly from the core catchers. Preparation was designed to produce the residue on a 125-µm sieve, but the 63- to 125-µm fraction was also retained and examined.

Planktonic foraminifers were very rare in samples from Site 1165, which is not surprising in light of the water depth at the site and estimates of the depth of the CCD at ~1500 m (Quilty, 1985). Planktonic foraminifers contribute little to the chronostratigraphy at the site except, potentially, in the Pliocene-Pleistocene section.

Hole 1165A

The record from Hole 1165A consists of one mudline core, Core 1H. A sample from the core catcher contains a few Neogloboquadrina pachyderma (Ehrenberg), which show signs of dissolution.

Hole 1165B

N. pachyderma is very abundant in the >125-µm fraction in Sample 188-1165B-1H-CC and constitutes ~80% of the residue. Specimens in this sample show signs of dissolution and commonly are partly infilled with black manganese oxides. They are accompanied by a minor benthic fauna. N. pachyderma is present through Cores 1H and 2H but is rare below that depth because of dissolution effects. Sample 188-1165B-6H-CC also contains very rare specimens of N. pachyderma.

Sample 188-1165B-2H-1, 70-75 cm, contains the most diverse late Neogene foraminifer fauna recovered in Hole 1165B. It is affected by CCD influence, but planktonic foraminifers are the dominant contributors to the residue. The sample contains N. pachyderma accompanied by Globorotalia puncticulata Deshayes and Globorotalia scitula Brady, an association that is not present elsewhere in the hole. This fauna seems anomalous, as it was at Site 747 (Berggren, 1992; ODP Leg 120). G. puncticulata is now routinely taken as a Pliocene species (some even regard it as more restricted to the early and mid-Pliocene) and G. scitula ranges through to the Holocene, although not at Site 747 nor here. Berggren (1992) referred to the Site 747 range as anomalous for G. puncticulata. At Site 747, the three species are present together at the bottom of Core 120-747A-2H, and Sample 188-1165B-2H-1, 70-75 cm, seems to be the equivalent. In Hole 747A, this level appears to be at ~2 Ma, applying the time/depth curve in Harwood et al. (1992)—an age that puts the expected foraminifer-based age in Hole 1165B in conflict with the paleomagnetic/diatom data. It is clear that the Pliocene-Pleistocene section at Site 747 needs to be reexamined. Another consequence of this sample is that it implies a warm-water influence to explain the presence of two species of Globorotalia.

An interesting "fauna" is present in Sample 188-1165B-6H-5, 70-75 cm. This sample contains thousands of particles in the 100- to 300-µm fraction that appear to be composed of dolomite. Compositional documentation will require postcruise investigation. Many are in the form of modified rhombohedra with an elongate A-axis and very curved faces. Others are in the form of pseudomorphs after three or four species of planktonic foraminifers, in which case coiling pattern and chamber arrangement are discernible but fine details such as apertural characteristics and wall structure are not. It is possible that the rhombohedra also represent pseudomorphs. With the limited information available, the pseudomorphs are consistent with species of late Eocene-early Oligocene age. They are present in concert with an enhanced glauconite content and a "dolomite"-glauconite intergrowth. They are thus from the same source, probably a Paleogene sequence in Prydz Bay or on the Mac. Robertson Shelf (Harris et al., 1997a; Quilty et al., 1999).

Below Sample 188-1165B-6H-5, 70-75 cm, no further planktonic species are documented until Sample 188-1165B-24X-CC, where planktonic forms constitute 65% of the assemblage, including Globoturborotalita woodi (Jenkins), Globorotaloides variabilis, Globigerina bulloides d'Orbigny, and Globigerina praebulloides Blow. This sample is tentatively placed in Zone AN5, even though the nominate zone marker (Neogloboquadrina nympha) is absent. The reason for the absence is not clear, but this location is several degrees farther south than Berggren's (1992) study area; the reference taxon may not have reached this far south.

Well-preserved Catapsydrax unicavus Bolli, Loeblich, and Tappan is present in Sample 188-1165B-74X-1, 26 cm, and indicates that this sample is no younger than 17.3 Ma and is in Zone AN3 or older in the scheme of Berggren (1992).

A single poorly preserved specimen of Catapsydrax dissimilis (Cushman and Bermudez) was recovered from Sample 188-1165B-74X-CC and is consistent with the early Miocene age assigned by other fossil groups.

Hole 1165C

The richest pre-Pliocene planktonic fauna in Hole 1165C was recovered from Sample 188-1165C-2R-CC. It contains two genera (Globorotalita of the woodi group and Catapsydrax sp.), but no specimens are well enough preserved for identification to the species level. This level marks a dramatic change in the preservation of planktonic foraminifers. C. unicavus in Sample 188-1165B-74X-1, 26 cm, was well preserved, and C. dissimilis in the same sample was uncrushed and identifiable. Below this depth, planktonic species are severely crushed and unidentifiable. In contrast, benthic species are often very well preserved, even if the test seems delicate (e.g., those identified as Eponides sp. 1 and sp. 2).

Despite crushing, a few specimens identifiable to generic level were recovered from Sample 188-1165C-2R-CC, and C. dissimilis is present in Sample 188-1165C-11R-CC.

Benthic foraminifers are present in many samples at Site 1165, but there is no obvious pattern to their stratigraphic location. Dissolution is expected at these water depths, and the sporadic presence of Cyclammina incisa throughout the sequence is consistent with this generalization. Even where faunas are present, they have little in common in structure or species composition. Thus, it is difficult to comment about their significance. Below Core 188-1165B-20X, evidence of dissolution is not prominent, and faunas from below that depth seem complete.

The few faunas that yielded calcareous benthic faunas at Site 1165 contain very little evidence of infaunal species (e.g., buliminid species are virtually absent); thus, the waters where these faunas accumulated were fully oxygenated. Cyclammina as an epifaunal genus (e.g., Gooday, 1990) supports this interpretation.

Epistominella vitrea is present in two samples at Site 1165 and appears to be roughly equally divided into dextrally and sinistrally coiled types.

Samples 188-1165A-1H-CC, 188-1165B-1H-CC, and 188-1165B-2H-CC contain a few calcareous benthic species that are outnumbered by the planktonic content. They are well preserved and probably in situ.

Sample 188-1165B-14H-CC yielded an abundant and diverse benthic foraminiferal fauna, containing both simple agglutinated and common calcareous forms and showing only minor evidence of dissolution. Planktonic species are absent. The dominant benthic form is Oridorsalis umbonatus (Reuss) (~50%), which van Morkhoven et al. (1986) regard as indicative of a mid-lower neritic depth. This suggests that the fauna, which shows no evidence of dissolution effects, probably originated on the nearby continental shelf and made its way to the present site by mass movement.

Between Samples 188-1165C-12R-CC and 22R-CC, there is an association of agglutinated species that have many characteristics in common. Samples almost routinely yield fragments of more than one species, but the remains are poorly preserved because the tests are poorly cemented and do not survive processing well. They are clear in hand-specimen examination and consist of flattened white tubes or fragments parallel to bedding. Some may not be foraminifers, but they are included here. If foraminifers, they are astrorhizid/allogromiid or simple ammodiscid species lacking chamber partitions and thus referable to Bathysiphon in most instances. Two species can be recognized. One (Bathysiphon sp. 1, present throughout the section) is large and robust, up to 7-8 mm long, and straight or slightly meandrine. The other (Bathysiphon sp. 2) is soft, small (to ~1 mm), thin walled, and usually in fragments.

Another fauna, with both benthic and planktonic species, is present in Sample 188-1165B-24H-CC, and again there is no evidence of dissolution. The benthic component of this fauna is dominated by Epistominella exigua (Brady), accompanied by Cibicidoides mundulus (Brady, Parker, and Jones), Cibicidoides subhaidingeri (Parr), and Hanzawaia mantaensis (Galloway and Wissler), which suggests that the fauna is from a bathyal environment and is thus in situ. Also present are several species each of Lagena, Fissurina, and miliolid foraminifers.

A small, well-preserved fauna of two species, Pullenia cf. subcarinata and E. vitrea, is present in Sample 188-1165B-26H-CC.

Sample 188-1165B-58X-1, 134-136 cm, contains a diverse agglutinated/calcareous benthic-only fauna, dominated by C. subhaidingeri (Parr). This fauna has a few specimens (Stilostomella and Virgulina) that may represent some infauna and is accompanied by echinoid spines.

The labyrinthine agglutinated taxon C. incisa (Stache) is found in many samples in Holes 1165B and 1165C and is the most commonly present benthic species at this site. Deeper in the section, C. incisa, in addition to Bathysiphon sp. 1, is interpreted to constitute an assemblage found sporadically higher in the section, for example, in Samples 188-1165C-2R-CC and 3R-CC. In Samples 188-1165C-2R-1, 37 cm; 3R-4, 67-101 cm; 15R-2, 41 cm; and many levels in Cores 188-1165C-29R and 30R, there are zones in which these two species are clearly visible with a hand lens and sometimes are quite abundant (up to eight specimens of Cyclammina on one side of a "biscuit" of core). They are more likely to be found through hand-specimen examination than after vigorous processing. In some of these co-occurrences, they are accompanied by coarse sand detritus, in marked contrast to the dark gray siltstone above and below. C. incisa is also present deeper in Hole 1165C in association with a variety of other foraminifers.

Below Sample 188-1165C-58X-1, 134-136 cm, no other benthic species are recorded until Sample 188-1165B-74X-CC, which contains a crushed and unidentifiable specimen of Trochammina. At this depth (~660 mbsf), preservation changes and the tests assume a yellowish color that contrasts with the gray of the enclosing sediment, thus making separation easier. A few benthic species (normally one to three specimens per sample) are identified in Samples 188-1165C-3R-CC; 4R-CC; 17R-CC; 18R-2, 56-58 cm; and 18R-CC.

Sample 188-1165C-30R-3, 142-144 cm, contains Repmanina (formerly Glomospira charoides), known commonly from bathyal and abyssal assemblages, but in contrast to other assemblages, indicates the existence of an infaunal mode of life (Gooday, 1990).

Because of the erratic nature of faunas downhole, it is difficult to identify meaningful faunal assemblages from the foraminifers. Two agglutinated assemblages, however, seem to be recognizable: a Cyclammina/Bathysiphon sp. 1-dominated assemblage and a less well-constrained Bathysiphon assemblage. Both assemblages seem to imply that the sediment containing them has not moved more than a few tens of meters.

The Cyclammina/Bathysiphon sp. 1 assemblage is well defined and recurrent. It is present sporadically throughout the section (see preceding paragraph) but is best established in the lower cores of Hole 1165C, where it is apparent with the use of a hand lens. The large (to ~3-mm diameter) tests of C. incisa stand out because they are lenticular in vertical section and because the labyrinthine wall is clear. Bathysiphon sp. 1 (perhaps other nomenclature will evolve with more work) is normally present with C. incisa. It is a robust, thick-walled form ~1 mm in diameter, as long as 7-8 mm, and normally is present as flattened ovoids with a dark center. Usually there is no calcareous component of this assemblage, although Cyclammina does occur sporadically in association with other faunas. This assemblage is best developed in a distinctive lithofacies consisting of units as thick as 20 cm that are coarser grained than the black siltstone/shale prevalent in the sequence.

The Bathysiphon assemblage is present particularly in Cores 188-1165C-12R through 22R. It is not as well characterized as the Cyclammina/Bathysiphon sp. 1 assemblage. Two species of Bathysiphon are present in this assemblage and may be accompanied by others (e.g., Psammosphaera sp. 1 and rarely by a calcareous species). These are common in the sandstone beds in the lower part of the dark gray shale prevalent in the hole. The larger, more robust species has roughly the same size characteristics as Bathysiphon sp. 1, but it is much less robust but visible in the unprocessed rock. The second species (very tentatively placed in Bathysiphon) is much smaller, up to ~1-2 mm long and 0.3-0.5 mm in diameter. It is present in processed residues as fragments, and complete specimens are found only in the unprocessed rock. Because these specimens are so delicate, they probably can be found only where the sequence is in situ. They are accompanied by horizontal burrows (some of which, on further examination, may be placed in foraminiferal genera), and all are classic examples of the question of what constitutes a foraminifer. This assemblage is found where the prevailing black shale/siltstone contains abundant coarser, thin, white siltstone lenses and interbeds.

It is clear that some of the faunas have arrived at their resting place by transport and are not in situ. That is, they are thanatocoenoses (death assemblages) instead of biocoenoses (life assemblages). If so, where did they originate, and how did they reach their current site? For most faunas, no answer can be given because of the small, unrepresentative faunal content, but an attempt is possible for those from Samples 188-1165B-14H-CC, 24X-CC, and 58X-1, 134-136 cm. In two of these cases, however, there is an inconsistency between planktonic percentage and benthic content.

Sample 188-1165B-14H-CC

The fauna present in this sample probably slumped to its present site or was carried in some form of turbidity current. It contains O. umbonatus (dominant: >40%), Eggerella bradyi, Karreriella bradyi, and Laticarinina pauperata. Knowledge of the global distribution of these species (van Morkhoven et al., 1986) suggests an outer neritic to mid-bathyal source, well above the CCD. As noted above (see "Benthic Foraminifers"), the absence of planktonic species is a puzzle and could be used to indicate a much shallower source, which would be in conflict with the species composition.

Sample 188-1165B-24X-CC

The fauna present in Sample 188-1165B-24X-CC has a planktonic component of ~65%, which in other environments would indicate an outer shelf/upper slope depth. Because it is a single sample and not part of a sequence with the same characteristics, use of planktonic percentage as an indicator of depth of deposition must be used with caution. Other features include the presence of E. bradyi, C. subhaidingeri, and dominant (30%) E. vitrea. These suggest an upper depth limit of ~600 m, although the upper depth limit of E. vitrea is considerably shallower than this estimate. A mid- to lower bathyal depth is therefore interpreted.

Sample 188-1165B-58X-1, 134-136 cm

C. subhaidingeri is dominant (45%), L. pauperata is present, and there are no planktonic foraminifers in this sample. Although in conflict with the absence of planktonic species, the benthic fauna indicates a mid-lower bathyal source.

Introduction

Calcareous nannofossils are present sporadically throughout Holes 1165A, 1165B, and 1165C, with moderate preservation and high abundance in only a few samples. Dissolution at this site is prevalent, which is consistent with a corrosive deeper water environment. Where nannofossils are present, the assemblages are characterized by low diversity with one or two dominant taxa and lack the usual age-diagnostic marker species. Assemblages are comparable to those previously described from Prydz Bay and other Southern Ocean drill sites (Wise, 1983; Wei and Wise, 1990; Wei and Thierstein, 1991; Wei and Wise, 1992a, 1992b). A detailed nannofossil biostratigraphy was not achieved for these drillcores. Nannofossil-bearing samples, however, could be assigned to rough zonal combinations and broad age designations (Figs. F28A, F28B, F29).

Hole 1165A

One core was taken in Hole 1165A in order to recover Pleistocene-Holocene sediments for high-resolution study. The core is tentatively assigned to the late Pliocene-Quaternary nannofossil Zones CN12-CN15 of Okada and Bukry (1980). No nannofossils were noted in a sample from the core catcher, but few to common specimens were noted from discrete samples within the core. Sample 188-1165A-1H-1, 20-21 cm, contains few to common, poor to moderately preserved nannofossil specimens possibly assignable to the Pleistocene to Holocene species Emiliania huxleyi, the first occurrence (FO) of which marks the base of Zone CN15. Because of the small size of this taxon and less than ideal preservation, this assignment is only tentative and must be confirmed with scanning electron microscopic analysis. Additional species present show affinities to Gephyrocapsa; however, no specimens were noted with a bar spanning the central area, as is characteristic of this genus and as was noted in Hole 1165B (see "Hole 1165B"). Sample 188-1165A-1H-8, 17-18 cm, contains moderately preserved but rare Coccolithus pelagicus, Calcidiscus leptoporus, and possible Gephyrocapsa sp. (no central bar). E. huxleyi may also be present in this sample but could not be confirmed aboard ship.

Hole 1165B

Early Miocene- to Pleistocene-age sediments were recovered in Hole 1165B with sporadic calcareous nannofossil recovery. Only general zonal age assignments were possible. Overall, nannofossils are poorly to moderately preserved with etching and dissolution of specimens reflecting deposition in deep corrosive waters.

Sample 188-1165B-1H-CC contains etched nannofossils dominated by common C. pelagicus with few specimens of C. leptoporus and Gephyrocapsa sp. (no central bar preserved). Specimens of Gephyrocapsa caribbeanica with a central bar intact are present in the upper two sections of Core 2H, which is indicative of an age of late Pliocene-Pleistocene. Questionable specimens of a smaller oval morphotype of Pseudoemiliania lacunosa were also noted in this core. According to Perch-Nielsen (1985), the smaller morphotype is more prevalent in the higher latitudes. If these specimens are the cold-water variety of P. lacunosa, then the bottom of this first core can be assigned to Zones CN12-CN14a of late Pliocene to mid-Pleistocene age.

Core-catcher samples from Cores 2H through 13H are barren of calcareous nannofossils. Sample 188-1165B-14H-CC, however, contains abundant moderately preserved specimens. The assemblage is dominated by R. perplexa (syn. Dictyococcites antarcticus) and Reticulofenestra producta. C. pelagicus and several varieties of small- and medium-sized Reticulofenestra spp. are few to common. The latter have been variably assigned by previous authors to a number of species including Reticulofenestra haqii, Reticulofenestra minuta, medium and small Reticulofenestra gelida, and Reticulofenestra minutula. Very rare Reticulofenestra pseudoumbilica and large R. gelida are also present. A surprise in this sample was the presence of rare specimens of Minylitha convallis, which is noted from Zones CN8 to CN9 of the late Miocene. Single specimens of the warmer-water taxa Sphenolithus abies and Discoaster sp. cf. D. variabilis were also noted. To our knowledge, these species, including M. convallis, have not been observed in upper Miocene sediments from the high austral latitudes, and in particular, the Antarctic margin.

Core-catcher samples from Cores 15H through 23X are also barren of calcareous nannofossils, but very abundant specimens were noted from a thin chalky layer within the core catcher of Core 24X. This sample contains an assemblage completely dominated by R. perplexa with fewer R. producta, Reticulofenestra spp., and rare C. pelagicus. The assemblage is characteristic of middle to late Miocene of the higher latitudes and can be roughly assigned to Zones CN5 to CN11. A single specimen of a Reticulofenestra hesslandii was noted, which, if not reworked, suggests an age of middle Miocene for the assemblage. Samples 188-1165B-26X-CC and 27X-CC contained similar but less abundant assemblages. Unfortunately, Core 25X was not recovered from within this relatively nannofossil-rich interval of an otherwise barren section. Core catchers from Cores 28X through 57X are barren of nannofossils.

Cores 58X and 59X contain several intervals of common to abundant nannofossils. The presence of Cyclicargolithus floridanus, whose last occurrence (LO) roughly corresponds to the top of Zone CN4, along with abundant R. hesslandii and Reticulofenestra spp., is indicative of the lower to middle Miocene of the austral high latitudes (e.g., Wei and Wise, 1992a). Rare Cyclicargolithus abisectus suggests an age of early Miocene. In light of the diatom data, in addition to the absence of C. leptoporus, an age of late early Miocene is probable (nannofossil Zones CN2-CN3). Rare reworked Oligocene specimens of Chiasmolithus sp., Dictyococcites bisectus, Reticulofenestra daviesii, and Reticulofenestra samodurovii were noted in Sample 188-1165B-58X-2, 34-35 cm.

Below Core 59X down to Core 76X (674.96 mbsf), nannofossils are very rare with sporadic occurrences of R. hesslandii, C. floridanus, and small species of Reticulofenestra. Assemblages are generally in agreement with diatom stratigraphy, which places the section in the lower Miocene.

Hole 1165C

Calcareous nannofossils are moderately preserved but few and sporadic in the predominantly dark brown to gray-green mudstones of Hole 1165C. A good biostratigraphy could not be achieved, but assemblages recovered are indicative of an early Miocene age for the hole, most likely encompassing nannofossil Zones CN2 to CN1 (Fig. F29).

Nannofossils, where present in Hole 1165C, are dominated by small and medium-sized nondescript species of Reticulofenestra (R. haqii, R. minuta, and R. producta) and C. pelagicus. Few R. hesslandii are present down through Core 14R and questionably below that. Very rare and sporadic C. floridanus are found throughout, and rare to few C. abisectus are present in Core 29R and below. Very rare non-age diagnostic Sphenolithus spp. are present in Samples 188-1165C-12R-4, 24-26 cm, and 18R-CC. A single poorly preserved specimen of Discoaster sp. aff. D. deflandrei was also noted in Sample 188-1165C-12R-4, 24-26 cm, and very rare specimens of the early- to mid-Miocene age taxa Coccolithus miopelagicus are present in Samples 188-1165C-18R-CC and 22R-CC.

The bottom core, Sample 188-1165C-35R-1, 19-20 cm, contains few to common specimens of a relatively rich nannoflora (>6 species) that includes common Reticulofenestra spp., few C. pelagicus, rare C. abisectus, and single specimens of Helicosphaera sp. cf. H. paleocarteri, Helicosphaera sp., and Umbilicosphaera jafarii. The latter is only known from Miocene and younger strata, whereas the remaining assemblage is generally assignable to the lower Miocene (nannofossil Zones CP19b-CN3).

When compared to previously drilled sections at nearby Kerguelen Plateau and elsewhere in the Southern Ocean, the overall assemblages noted in discrete samples of Hole 1165C are consistent with an early Miocene age (Wei and Wise, 1990; Wei and Thierstein, 1991; Wei and Wise, 1992a). The presence of C. abisectus in Core 188-1165C-29R and below broadly limits the section to the late Oligocene-early Miocene age interval. However, in Southern Ocean sections, the uppermost Oligocene is characterized by few to common D. bisectus, whose LO marks the top of the Oligocene, and abundant R. daviesii, which actually ranges just into the lower Miocene. D. bisectus specimens were not observed in bottom assemblages, and only questionable (overgrown?) specimens of R. daviesii were noted. These species were reported in Antarctic margin sections such as nearby Prydz Bay, Site 739 (Wei and Thierstein, 1991), and in Cape Roberts cores (Watkins and Villa, in press). Thus, it would be reasonable to assume their presence at this site if Oligocene sediments were reached. Poor preservation is ruled out as a potential cause of the absence of D. bisectus because it is more robust (8-10 µm for smaller morphotypes) than most of the species observed in these samples. In addition, reworked specimens were noted upsection, which further indicates their presence in the region and supports the interpretation that the Oligocene was not cored at Site 1165.

Because of the sporadic presence of nannofossils, little paleoenvironmental interpretation is possible beyond speculation. Nannofossil abundance at this site may be a factor of several processes or a combination of processes. High nannofloral productivity in surface waters around Prydz Bay (associated with warmer intervals?) perhaps periodically depressed the CCD and permitted the preservation of nannofossils, whereas under conditions of normal production, dissolution was prevalent. Occurrences can also be attributed to rapid downslope transport, burial, and preservation of nannofossils. Two examples of this latter process are evident at Site 1165. In Sample 188-1165B-14H-CC, nannofossils are abundant and fairly well preserved, along with a well-developed fauna of neritic benthic foraminifers, which were obviously brought downslope. Similarly, in Sample 188-1165C-12R-4, 24-26 cm, nannofossils are abundant in a carbonate-rich interval that sedimentologists have interpreted as a distal mud-flow deposit. Further corroborating this interpretation is the presence of rare reworked Oligocene-Eocene nannofossils in this sample.

Interpreting the lack of nannofossils throughout most of the section is difficult. As cited above, dissolution below the CCD is likely and/or nannoplankton surface productivity was low due to cool surface-water temperatures—or it may simply be a result of dilution of an already low number of nannofossils by a copious supply of terrigenous sediment. Intervals of higher nannofossil abundance may be merely the result of a diminishing terrigenous supply, possibly associated with interglacial periods.

As previously mentioned, assemblages recovered at Site 1165 are comparable to those of equivalent age found elsewhere in the high austral latitudes, particularly in Prydz Bay and on the Kerguelen Plateau (Wei and Thierstein, 1991; Wei and Wise, 1992a). These assemblages are composed of a typical cold-water nannofossil flora. An example is R. perplexa, which achieves very high abundances and comprises almost 100% of the assemblage at various intervals in upper Miocene to lower Pliocene austral sections (Wei and Wise, 1992b). A nearly monospecific assemblage of very abundant R. perplexa is present at Site 1165 in Sample 188-1165B-24X-CC and may represent a period of extremely high nannofossil productivity (depressing the paleo-CCD and allowing carbonate deposition?; see Wei and Wise, 1992b). It is also possible to infer warmer intervals from abundance increases of C. pelagicus (Wei and Wise, 1992b). At Site 1165, a few intervals of C. pelagicus relative abundance increase are noted in Core 188-1165B-2H (7.1 mbsf); Samples 188-1165B-14H-CC (124.51 mbsf); 58X-2, 34.5 cm (512.85 mbsf); 188-1165C-18R-CC (829.57 mbsf); and to a lesser extent, 35R, 19-20 cm (989.69 mbsf). These intervals are generally characterized by overall nannofossil abundance increase as well, so it remains speculative at this time as to whether increases in C. pelagicus is a significant warm-water proxy at Site 1165.

Introduction

Neogene sediments recovered at Site 1165 contain rich diatom assemblages, although in variable states of abundance and preservation. Poorly preserved diatoms are present in one sample at the base of Hole 1165A (5.33 mbsf) and indicate a Quaternary age for this core. In Hole 1165B, an expanded upper Pliocene to lower Miocene section is present from ~9 to ~650 mbsf and is disconformably overlain by a thin upper Pleistocene section. An excellent diatom record is represented in the upper Pliocene to lower Miocene cores of Hole 1165B, which provides for detailed biostratigraphic analysis. Cores recovered in Hole 1165C were barren of diatoms, except for one sample recovered from a burrow at ~754 mbsf. This sample is assigned an age of early Miocene.

The diatom zonation of Harwood and Maruyama (1992) was applied to Neogene cores of Holes 1165B and 1165C (see "Biostratigraphy and Sedimentation Rates" in the "Explanatory Notes" chapter). Diatom zonal datums recognized in Holes 1165B and 1165C are listed in Table T5. Lower Miocene to upper Pliocene zones are well represented in these cores, although some zones were combined because of the absence of marker taxa or apparent juxtaposition of datum events. Initial biostratigraphic "reconnaissance" of core-catcher samples also suggests that a few zones may be absent in the section. Further work with narrow sample spacing is required to identify whether these zones are present or are missing as a result of hiatuses. Contamination of core-catcher samples from the drilling slurry may also be a problem in some samples, and examination of discrete core samples will provide clarification on the stratigraphic ranges of specific taxa.

Hole 1165A

One piston core was taken in Hole 1165A, and a diatom sample from the base of the core at 5.33 mbsf (Sample 188-1165A-1H-CC) was examined. The presence of Thalassiosira elliptipora (FO = 2.2 Ma) indicates the section is Quaternary in age, and the presence of Neogene and Paleogene taxa, including Denticulopsis praedimorpha (LO = 11.5 Ma) and Pyxilla reticulata (LO = 30.7 Ma), indicates Oligocene and Miocene reworking. Apparent reworking in this sample prohibits the use of Quaternary LO datums (e.g., the LO of Actinocyclus ingens at 0.66 Ma); as a result, this section has been left unzoned.

Hole 1165B

Poorly preserved diatom assemblages are present above ~17 mbsf in Hole 1165B, with the interval above 1.70 mbsf representing a thin upper Pleistocene section. Moderately to well-preserved diatom assemblages were recovered from ~17 to ~606 mbsf through a succession of lower Pleistocene to lower Miocene strata. Initial zonal assignment and age interpretation were carried out using core-catcher samples. In some sections, additional core samples have been examined to further constrain the depth of zonal boundaries. A summary of diatom zonal assignments for Hole 1165B is shown in Figure F28A and F28B, and a summary of datums applied to designate zonal boundaries is presented in Table T5.

Diatom abundance varied from absent to very abundant throughout Hole 1165B. In general, diatom preservation and abundance were best in lower Pliocene sediments (~30 to ~50 mbsf). Preservation and abundance varied with lithology in the Miocene section, where preservation appears to be best and abundance highest in light green rather than in gray-green to gray-black claystones. Barren samples were predominantly recovered below ~606 mbsf (Sample 188-1165B-67X-CC), except for a few carbonate-cemented horizons and burrows where diatoms were preserved. Samples at 646.10 mbsf (Sample 188-1165B-73X-1, 30-32 cm) and 753.77 mbsf (Sample 188-1165B-10H-3, 127-129 cm), for example, contain poorly preserved siliceous microfossil assemblages. Barren and poorly preserved samples below ~606 mbsf indicate that the hole reached the level of complete opal-A dissolution at this depth.

Neritic diatoms associated with shallow-euphotic water depths (<100 m) are noted throughout diatom-rich intervals of Hole 1165B. Many intervals contain low abundances of Cocconeis spp., Paralia sulcata, Rhabdonema spp., Grammatophora spp., and Entopyla spp. These taxa maintained a benthic or tychopelagic habitat within the photic zone and are interpreted to indicate reworking from a continental shelf source during deposition of the sequence.

Numerous samples were examined from Cores 1H and 2H. Most samples in this interval contain very poorly preserved diatom assemblages with many reworked taxa. Moderately preserved assemblages, however, were noted in Samples 188-1165B-1H-1, 20-21 cm; 1H-5, 20-21 cm; and 2H-2, 95-96 cm. The highest occurrence of A. ingens is noted between Samples 188-1165B-1H-2, 20-21 cm (1.70 mbsf), and 1H-5, 20-21 cm (6.20 mbsf), placing the base of the Thalassiosira lentiginosa Zone between these samples. An interval of poor preservation between these samples inhibits the accurate placement of this zonal boundary, as this taxon can be easily reworked into overlying sediments. An age of <0.66 Ma is therefore interpreted for sediments above 1.70 mbsf.

The interval between 1.70 and 9.25 mbsf is presently left unzoned because of poor diatom preservation and the presence of Fragilariopsis sp. cf. barronii, as opposed to the zonal datum Fragilariopsis barronii. Taxonomic problems associated with the identification of the LO of F. barronii have been noted by several workers (e.g., Gersonde and Bárcena, 1998). Lower Pleistocene assemblages documented in several Southern Ocean drill cores contain transitional forms of F. barronii that are difficult to distinguish from early forms of Fragilariopsis ritscherii or Fragilariopsis kerguelensis, thereby limiting the use of the LO datum of F. barronii as a zonal marker.

The highest occurrence of Thalassiosira kolbei is noted between Samples 188-1165B-2H-2, 95-96 cm (9.25 mbsf), and 3H-1, 95-96 cm (17.25 mbsf), which serves as a rough estimation of the Pliocene/Pleistocene boundary. Again, this datum cannot be constrained to a narrow depth interval because of poor preservation between these samples. The presence of F. barronii sensu stricto at 9.25 mbsf (Sample 188-1165B-2H-2, 95-96 cm), however, places the sample at this level within the F. kerguelensis Zone (1.35 to 1.8-2.0 Ma).

The absence of A. ingens (LO = 0.66 Ma) at 1.70 mbsf and the presence of F. barronii (LO = 1.35 Ma) at 9.25 mbsf indicates an ~1-m.y. hiatus within this interval. Numerous short hiatuses may exist between 1.70 and 9.25 mbsf but presently cannot be resolved from the diatom data.

Sample 188-1165B-3H-1, 95-96 cm (17.25 mbsf), is placed within the T. kolbei Zone (1.8-2.0 to 2.2-2.3 Ma). T. kolbei is present within this sample, and the highest occurrence of Thalassiosira vulnifica is recognized below this level between Samples 188-1165B-3H-1, 95-96 cm (17.25 mbsf), and 3H-2, 20-21 cm (18.00 mbsf).

The highest occurrence of Thalassiosira insigna is between Samples 188-1165B-3H-3, 20-21 cm (19.50 mbsf), and 3H-2, 20-21 cm (18.00 mbsf). The overlying interval, which includes Sample 188-1165B-3H-2, 20-21 cm (18.00 mbsf), is therefore placed in the T. vulnifica Zone (2.2-2.3 to 2.5-2.6 Ma), based on the presence of T. vulnifica and the absence of T. insigna.

Samples 188-1165B-3H-3, 20-21 cm (19.50 mbsf), through 3H-CC (25.01 mbsf) are assigned to the T. insigna-T. vulnifica Zone (2.5-2.6 to 2.8-3.2 Ma). This zone is constrained by the combined presence of T. vulnifica and T. insigna through the section. The lowest occurrence of T. vulnifica is identified between Samples 188-1165B-3H-CC (25.01 mbsf) and 4H-1, 20-21 cm (26.00 mbsf).

The highest occurrence of Fragilariopsis weaveri in Sample 188-1165B-3H-5, 95-96 cm (23.25 mbsf), may allow the T. insigna-T. vulnifica Zone to be further subdivided into Subzones "a" and "b." Several rare specimens of F. weaveri, however, were noted above this level. These occurrences may represent reworking, but further work is required to validate its use as a subzonal marker.

The lowest occurrence of Fragilariopsis interfrigidaria is noted between Samples 188-1165B-5H-1, 127-128 cm (36.57 mbsf), and 5H-2, 127-128 cm (38.07 mbsf). The F. interfrigidaria Zone (2.8-3.2 to 3.7-3.8 Ma) is therefore identified between Samples 188-1165B-4H-1, 20-21 cm (26.00 mbsf), and 5H-1, 127-128 cm (36.57 mbsf), based on the presence of F. interfrigidaria and the absence of T. vulnifica throughout this interval. The boundary between the upper and lower Pliocene lies within the F. interfrigidaria Zone and is roughly placed at ~30 mbsf.

The lowest occurrence of F. barronii is identified between Samples 188-1165B-5H-4, 95-96 cm (40.75 mbsf), and 5H-5, 60-61 (41.90 mbsf). Accordingly, the interval between 38.07 and 40.75 mbsf is placed within the F. barronii Zone (3.7-3.8 to 4.2-4.4 Ma), which is constrained by the presence of F. barronii and the absence of F. interfrigidaria. The highest occurrence of Rhizosolenia costata between Samples 188-1165B-5H-3, 95-96 cm (39.25 mbsf), and 5H-4, 95-96 cm (40.75 mbsf), may allow this zone to be further subdivided into Subzones "a" and "b," as defined by Harwood and Maruyama (1992).

The lowest occurrence of Thalassiosira inura is documented between Samples 188-1165B-6H-3, 95-96 cm (48.75 mbsf), and 6H-4, 59-60 cm (49.89 mbsf). The interval between 41.90 and 48.75 mbsf is placed within the T. inura Zone (4.2-4.4 to 4.8-4.9 Ma), as indicated by the presence of T. inura and the absence of F. barronii. It should be noted that morphologies taxonomically similar to T. inura are found below the lowest occurrence of T. inura presently identified here. These forms have a reduced central hyaline area and are tentatively identified as Thalassiosira jacksonii. In the present study, T. inura is limited to specimens with a central hyaline patch that spans at least one-fourth of the valve diameter.

The lowest occurrence of Fragilariopsis praeinterfrigidaria in Sample 188-1165B-8H-CC (73.61 mbsf) lies well below the lowest occurrence of T. inura at ~49.89 mbsf. Previously, the FOs of both taxa were considered to be contemporaneous (Harwood and Maruyama, 1992). Recovery of an expanded upper Miocene section at Site 1165 will allow refinement of a number of secondary datums, such as the FO of F. praeinterfrigidaria.

Within some intervals of the T. inura Zone, the occurrence of silicoflagellates and diatoms with a subantarctic affinity is noted. Sample 188-1165B-5H-CC (44.08 mbsf), for example, contains a high abundance of Dictyocha spp. silicoflagellates (relative to Distephanus spp.). Future work on this interval will possibly allow early Pliocene warming and cooling phases to be identified from both diatom and silicoflagellate assemblage data.

Below the lowest occurrence of T. inura (~49.89 mbsf), the Thalassiosira oestrupii to Nitzschia reinholdii Zones (4.8-4.9 to ~6.4 Ma) are represented, as indicated by the presence of Thalassiosira oliverana (FO = 6.4 Ma). The position of the base of the N. reinholdii Zone is approximated in Hole 1165B by the lowest occurrence of Hemidiscus triangularus (FO = 6.2 Ma) between Samples 188-1165B-9H-CC (81.39 mbsf) and 10H-1, 20-21 cm (83.00 mbsf), which places the T. oestrupii to N. reinholdii Zones in the interval from 48.75 to 81.39 mbsf. It is noteworthy that H. triangularus (FO = 6.2 Ma, calibrated from Leg 120 data) is used here as a substitute datum for the FO of Thalassiosira miocenica. The usefulness of the FOs of T. oestrupii and T. miocenica as zonal datums requires investigation, as these datums could not be identified at Site 1165. T. oestrupii was only observed in the T. inura Zone, which may indicate the presence of a disconformity below this zone or that the FO of T. oestrupii is unreliable as a high southern latitude marker datum.

In the Southern Ocean diatom zonation of Harwood and Maruyama (1992), the Pliocene and Miocene boundary is placed within the T. oestrupii Zone; no datums, however, recognized in Hole 1165B distinctly identify this boundary. The Miocene/Pliocene boundary is therefore tentatively placed between the lower part of Cores 6H and 9H, within the T. oestrupii to N. reinholdii Zones.

The section between Samples 188-1165B-10H-1, 20-21 cm (83.00 mbsf), and 11H-CC (100.32 mbsf) is assigned to the Hemidiscus ovalis Zone (~6.4 to ~8.7 Ma). The base of this zone is identified by the lowest occurrence of H. ovalis, which lies between Samples 188-1165B-11H-CC (100.32 mbsf) and 12H-CC (101.80 mbsf). As noted above, the top of the H. ovalis Zone is identified by the lowest occurrence of H. triangularus between 81.39 and 83.00 mbsf.

Sample 188-1165B-12H-CC (101.80 mbsf) is assigned to the Thalassiosira torokina Zone (~8.7 to ~8.5-9.0 Ma), as indicated by the presence of T. torokina and the absence of H. ovalis. The use of the lowest occurrence of H. ovalis as the top of this zone is tentative, as morphologies similar to H. ovalis were observed below this level. These forms may only represent slightly oval-shaped specimens of A. ingens and will be further investigated in postcruise work.

The section between Samples 188-1165B-13H-CC (116.02 mbsf) and 23H-CC (194.70 mbsf) is placed within the Asteromphalus kennettii to Denticulopsis simonsenii Zones (8.5-10.0 to 11.0-11.1 Ma). These zones are identified by the occasional presence of A. kennettii, with an acme in Samples 188-1165B-13H-CC (116.02 mbsf) and 22X-CC (185.08 mbsf); the absence of T. torokina; and the infrequent occurrence of Denticulopsis dimorpha. The inconsistent occurrence of A. kennettii prevented the separation of the A. kennettii and D. simonsenii Zones, which are divided by the FO of A. kennettii. The presence of A. kennettii in Sample 188-1165B-22X-CC may represent downhole contamination because this taxon was not observed below Sample 188-1165B-13H-CC.

The D. dimorpha to D. praedimorpha-Nitzschia denticuloides Zones (11.0-11.1 to 12.2-12.8 Ma) are recognized from Samples 188-1165B-23X-CC (194.70 mbsf) to 27X-CC (231.69 mbsf) between the highest common occurrence and lowest occurrence of D. dimorpha. The stratigraphic position of the highest occurrence of N. denticuloides could not be identified during shipboard investigations and prevents the separation of the D. dimorpha and D. praedimorpha-N. denticuloides Zones.

The upper to middle Miocene boundary is within the D. dimorpha Zone in the Southern Ocean diatom zonation. In Leg 120 cores, the full range of Denticulopsis meridionalis is recorded only in the upper D. dimorpha Zone; as a result, the lowest occurrence of D. meridionalis is used here to approximate the position of this boundary. This datum occurs between Samples 188-1165B-24X-CC (199.15 mbsf) and 26X-CC (213.90 mbsf).

Sample 188-1165B-28X-CC (242.68 mbsf) is assigned to the D. praedimorpha Zone (12.2-12.8 to 12.8-13.1 Ma), as indicated by the presence of D. praedimorpha and the absence of D. dimorpha.

The section from Samples 188-1165B-30X-CC (253.71 mbsf) through 33X-CC (281.33 mbsf) is placed within the N. denticuloides Zone (12.8-13.1 to 13.5-13.8 Ma), as marked by the presence of N. denticuloides and the absence of D. praedimorpha. The lowest occurrence of N. denticuloides is recorded between Samples 188-1165B-33X-CC (281.33) and 34X-CC (290.89 mbsf). Rare occurrences of N. cf. denticuloides, however, were noted below this level, which may represent transitional forms between Denticulopsis maccollumii and N. denticuloides. The lower range of N. denticuloides, therefore, remains uncertain.

The interval from Samples 188-1165B-34X-CC (290.89 mbsf) through 35X-CC (298.38 mbsf) is assigned to the D. simonsenii-Nitzschia grossepunctata Zone (13.5-13.8 to 14.1-14.6 Ma), as indicated by the absence of N. denticuloides and the lowest common occurrence of D. simonsenii in Sample 35X-CC.

The A. ingens var. nodus Zone (14.1-14.6 to 14.5-14.7 Ma) is identified in the interval between Samples 188-1165B-36X-CC (309.52 mbsf) and 38X-CC (327.78 mbsf), based on the absence of D. simonsenii and the presence of A. ingens var. nodus. The lowest occurrence of A. ingens var. nodus is noted in Sample 188-1165B-38X-CC (327.78 mbsf) but was rare in all samples examined, except for Sample 188-1165B-34X-6, 20-21 cm (288.90 mbsf).

The interval between Samples 188-1165B-39X-CC (336.89 mbsf) and 50X-CC (443.99 mbsf) is placed within the N. grossepunctata to A. ingens-D. maccollumii Zones (14.5-14.7 to 15.9-16.4 Ma). This assignment is based on the absence of A. ingens var. nodus and the common occurrence of A. ingens. The boundary between the N. grossepunctata and A. ingens-D. maccollumii Zones was not determined because of the similarities between N. grossepunctata and Nitzschia sp. 17 (of Schrader, 1976), as the latter species extends into the underlying D. maccollumii Zone (e.g., in Holes 747A and 751A on the Kerguelen Plateau [Harwood and Maruyama, 1992]).

The D. maccollumii Zone (15.9-16.4 to 16.7-17.3 Ma) is recognized between Samples 188-1165B-51X-CC (449.80 mbsf) and 55X-CC (487.52 mbsf). The top of this interval is marked by the lowest common occurrence of A. ingens, and the base is identified by the lowest occurrence of D. maccollumii.

The lowest occurrence of Crucidenticula kanayae is noted in Sample 188-1165B-55X-CC (487.52 mbsf), in addition to the lowest occurrence of D. maccollumii. In the Southern Ocean zonal scheme, the FO of C. kanayae (17.5-17.7 Ma) and the FO of D. maccollumii (16.7-17.0 Ma) define the C. kanayae Zone. The lowest occurrence of these two taxa in the same sample possibly indicates the presence of a hiatus between Samples 188-1165B-55X-CC and 56X-CC. Calibrated ages from Hole 744B for the FO of C. kanayae and D. maccollumii and the first common occurrence (FCO) of A. ingens suggest that this hiatus is ~0.7-1.4 m.y. in duration.

The middle to lower Miocene boundary is placed within the D. maccollumii Zone in the Southern Ocean zonal scheme of Harwood and Maruyama (1992). As noted above, Samples 188-1165B-51X-CC (449.80 mbsf) through 55X-CC (487.52 mbsf) are assigned to combined zones (D. maccollumii to C. kanayae) because of uncertainty in the level of the lowest occurrence of D. maccollumii. The middle/lower Miocene boundary is therefore placed between ~450 and ~487 mbsf.

The Thalassiosira praefraga "c" Subzone (17.5-17.7 to 18.3-19.1 Ma) is identified between Samples 188-1165B-56X-CC (495.81 mbsf) and 59X-CC (527.92 mbsf). This section represents an interval zone between the lowest occurrence of D. maccollumii and the highest occurrence of T. praefraga.

The section from Sample 188-1165B-60X-CC (534.60 mbsf) to the bottom of Hole 1165B (Sample 188-1165B-76X-CC) is assigned to the T. praefraga "a" to "b" Subzones (18.3-19.1 to 19.9-20.8 Ma). Subzones "a" and "b" are constrained by the full biostratigraphic range of T. praefraga. T. praefraga is frequent to common in Samples 188-1165B-60X-CC through 67X-CC. Below this level (~606 mbsf), all samples are poorly preserved to barren of diatoms, which indicates complete opal-A dissolution below this level. T. praefraga, however, is present in one sample from Hole 1165C, stratigraphically below the bottom of Hole 1165B (see "Hole 1165C" in "Diatoms"), allowing the entire lower section of Hole 1165B to be placed in the T. praefraga "a" and "b" Subzones. The absence of Rossiella symmetrica (LO = 19.4 Ma) may place the lowermost section of Hole 1165B in the T. praefraga Subzone "b." Its absence, however, may merely represent biogeographical exclusion from the high southern latitudes.

Hole 1165C

Core 1R was recovered to complete the missing section that resulted from the failed retrieval of Core 188-1165B-7H. A distinct color change is noted in Core 1R, from dark tan at the top to gray green at the bottom. Samples were taken across this contact, but no distinct differences in the diatom assemblages were observed. With further work, the presence of a disconformity may be discerned from detailed analysis of multiple samples above and below this level, but a hiatus is not presently identified. Samples from Core 1R are assigned to a combined T. oestrupii to N. reinholdii Zone (4.8-4.9 to ~6.4 Ma), as was the bottom of Core 188-1165B-6H-CC (54.39 mbsf) to the top of Core 8H-1, 95-96 cm (64.75 mbsf). This zonal assignment is based on the absence of T. inura and the presence of H. triangularus lower down in the section (see "Hole 1165B" in "Diatoms").

Rotary drill core retrieval from Hole 1165C recommenced at a depth of ~673 mbsf, stratigraphically below Hole 1165B. Diatom recovery was marginal and patchy, with most samples barren of diatoms or containing only traces of recrystallized diatoms. Moderately preserved diatom assemblages were recovered from only one carbonate-cemented burrow in Sample 188-1165C-10R-3, 127-129 cm (753.77 mbsf). The presence of T. praefraga in this sample indicates an age <20.8 Ma and places the interval above this level in the T. praefraga "a" and "b" Subzones. This age assignment is corroborated by the presence of Dactyliosolen antarcticus, which has a FO at ~21 Ma in the CRP-2A drill core in the southern Ross Sea (Scherer et al., in press).

Introduction

Radiolarian faunas were recovered from both Holes l165B and 1165C. Well-preserved and age-diagnostic radiolarian samples were limited to 487.5 mbsf and above in Hole 1165B.

The radiolarian zones proposed by Lazarus (1990, 1992) and Abelmann (1990, 1992) for high-latitude Pliocene and Miocene radiolarians are summarized in Table T2 in the "Explanatory Notes" chapter. Several of these Neogene zones were recognized in Holes 1165B and 1165C (Figs. F28A, F28B, F29).

Discussion of unconformities or hiatuses in the stratigraphy of Holes 1165B and 1165C, based on missing radiolarian zones, is premature because this initial work on the radiolarian fauna has been based entirely on core-catcher samples. In addition, there are numerous sections barren of radiolarians (Figs. F28A, F28B, F29). Final conclusions regarding zonal determinations and hiatuses in Holes 1165B and 1165C will be postponed until all the individual core samples are processed and analyzed. The following zones were recognized on initial examination of samples from Holes 1165B and 1165C.

Upsilon Zone

Samples 188-1165B-3H-CC, 4H-CC, and 5H-CC are assigned to the Upsilon Zone (Lazarus, 1992). The base of the this zone is marked by the first appearance of Helotholus vema Hays (Table T6), which is present in all these samples. Samples 188-1165B-4H-CC and 5H-CC may represent the lower part of the Upsilon Zone, as Prunopyle titan Campbell and Clark is present in those samples. The last appearance of P. titan is noted in Sample 188-1165B-4H-CC (Table T6) and represents the top of the middle Upsilon Zone (Lazarus, 1992). Additional samples from the core will be processed postcruise to determine more precise boundaries between the lower and upper Upsilon Zone.

Tau Zone

Samples 188-1165B-6H-CC and 188-1165C-1R-CC are tentatively assigned to the Tau Zone (Lazarus, 1992). The base of this zone is marked by the last appearance of the species Amphymenium challengerae Weaver. This taxa, however, was not seen in any of the samples from Holes 1165B or 1165C. The top of the Tau Zone is marked by the first appearance of H. vema, which is not present in Samples 188-1165B-6H-CC or 188-1165C-1R-CC but does appear in the overlying Sample 188-1165B-5H-CC (Table T6). Sample 188-1165C-1R-CC may represent the lower part of the Tau Zone because Lynocanoma grande Campbell and Clark (Table T6) is very abundant in this sample. Lazarus (1992) defined the lower part of the Tau Zone by the last common occurrence of L. grande.

The A. challengerae Zone was not recognized in the core-catcher samples, but the processing of additional samples from Cores 188-1165C-1R and 188-1165B-8H may result in the identification of this very short zone (6.1 to 6.6 Ma).

Acrosphaera? labrata Zone

Samples 188-1165B-8H-CC, 9H-CC, 10H-CC, and 12H-CC were assigned to the Acrosphaera? labrata Zone of Lazarus (1992). The base of this zone is marked by the FO of A.? labrata in Sample 188-1165B-11H-CC (Table T6), and this species is present in all the above samples.

The Siphonosphaera vesuvius Zone (Lazarus, 1992) has not been identified in Hole 1165B during shipboard investigations but may be recognized with more detailed sampling of Cores 188-1165B-13H, 14H, and 15H.

Acrosphaera australis Zone

Samples 188-1165B-13H-CC, 14H-CC, 15H-CC, 16H-CC, 17H-CC, 18H-CC, and 19X-CC were assigned to the Acrosphaera australis Zone of Lazarus (1992), based on the presence of A. australis Lazarus. The base of the A. australis Zone is marked by the evolutionary transition from Acrosphaera murrayana (Haeckel) Strelkov and Reshetnyak to A. australis (Table T6). Also present in these samples is Cycladophora spongothorax (Chen), which has its LO at the top of the A. australis Zone (Table T6). Ceratocyrtis stigi Nigrini and Lombari is also present in Sample 188-1165B-16H-CC, which supports assignment of this sample to the A. australis Zone.

Cycladophora spongothorax Zone

Samples 188-1165B-22X, 24X-CC, 34X-CC, 38X-CC, and 39X-CC are assigned to the C. spongothorax Zone. The base of this zone is marked by the first appearance of C. spongothorax in Sample 188-1165B-38X (Table T6). This species is present in all samples assigned to this zone. The top of this zone is represented by the evolutionary transition from A. murrayana to A. australis. A. australis is present in Sample 188-1165B-19X-CC, which has been assigned to the A. australis Zone. Sample 188-1165B-22X-CC may represent the top of the C. spongothorax Zone and the bottom of the A. australis Zone, as there are rare specimens of A. australis in the sample. Actinomma golownini is present in Sample 188-1165B-24X-CC (Table T6) and is reported to have its last appearance near the top of the C. spongothorax Zone.

The A. golownini, Cycladophora humerus, and Eucyrtidium punctatum Zones of Abelmann (1992) have not been recognized in samples from Holes 1165B or 1165C. It is not likely that these zones will be identified in these holes even with postcruise work because Samples 188-1165B-40X-CC (347.04 mbsf) through 54X-CC (479.50 mbsf) are barren of radiolarians (Figs. F28A, F28B, F29).

Cycladophora golli regipileus Zone

Sample 188-1165B-55X-CC is assigned to the Cycladophora golli regipileus Zone of Abelmann (1992). The base of this zone is defined by the FO of C. golli regipileus (Table T6), which was identified in Sample 188-1165B-55X-CC.

Drilling at Site 1165 yielded a composite sedimentary section of ~999 m through Pleistocene to lower Miocene strata with only a few minor disconformities (i.e., 2 m.y. in duration). An excellent record of siliceous microfossils is present in Hole 1165B, allowing the application of the Neogene high-latitude zonal schemes for both diatoms and radiolarians (Fig. F28A, F28B). Twenty diatom biostratigraphic datums and 12 radiolarian datums were identified in Miocene to Pleistocene cores of Holes 1165B and 1165C from initial analysis of siliceous microfossil assemblages (Table T6). These datums are used to construct an initial age-depth model for Holes 1165B and 1165C (see "Sedimentation Rates").

Diatoms are well preserved and abundant down to ~500 mbsf and absent below ~606 mbsf in Hole 1165B, except for two deeper horizons cemented by carbonate. Diatom assemblages are predominantly oceanic planktonic in character. The low abundance of neritic benthic diatoms recorded throughout the section indicates a fraction of "background" recycled biosiliceous sedimentation derived from areas within the photic zone on the continental shelf.

Well-preserved radiolarian assemblages are identified down to ~488 mbsf in Hole 1165B. Six radiolarian zones are currently identified, providing a well-defined upper Miocene to lower Pliocene biostratigraphy.

Nannofossils are generally rare and sporadic at Site 1165, with only a few discrete intervals of higher abundance and moderate to good preservation. Assemblages are characterized by low diversity and are comparable to those previously described from Prydz Bay and other Southern Ocean drill sites. Because of the inconsistent occurrences, a detailed nannofossil biostratigraphy could not be achieved. However, broad age assignments are possible and indicate that Pleistocene to lowermost Miocene sediments were recovered.

Fossil recovery in Hole 1165C was very poor, and diatoms, radiolarians, and planktonic foraminifers are very rare to absent. A burrow with poor to moderately preserved diatoms at ~754 mbsf indicates an age of <21 Ma. Biostratigraphic age control otherwise is provided primarily by calcareous nannofossils in this section. A lowermost Miocene position for the base of Hole 1165C is interpreted from nannofossil data (CN1 to CN2 Zones).

Planktonic foraminifers provide little assistance in chronostratigraphy at Site 1165 but place upper limits on the age of a few samples. Postcruise Sr dating of Pliocene-Pleistocene planktonics may clarify some apparent anomalies. Benthic foraminifers are noted more commonly than planktonics throughout the section recovered at Site 1165. Benthic foraminifer assemblages and accessory components noted in foraminiferal residues provide useful paleoenvironmental information on the source of some sediment units and help identify times at which major environmental changes occurred. Several faunas in the upper levels of Hole 1165B show evidence of being derived from shallower waters.

Figure F27 is a plot of age vs. depth for the composite section of Holes 1165B and 1165C. Approximately 20 diatom and 12 radiolarian FO and LO datums were used (Table T6) along with magnetostratigraphic data to construct this age model and to estimate sedimentation rates for the section. Good age control is available down through middle Miocene sediments to ~600 mbsf, at which point siliceous-microfossil dissolution is prevalent. In general, calcareous nannofossil and planktonic foraminifers are too sporadic to provide reliable datum events. Below 600 mbsf, magnetostratigraphic chron designations are based mainly on extrapolation of sedimentation rates from limited diatom and nannofossil biostratigraphic data. These data suggest that the lower part of the hole is of early Miocene age.

Biostratigraphic data were used to obtain a fit for the observed paleomagnetic reversal stratigraphy. This fit yields a set of dates with several constraints at certain depths (see "Paleomagnetism"). In Figure F30, paleomagnetic and biostratigraphic data are combined to constrain an age-depth plot. For this purpose, we assume the errors on depth and age of the paleomagnetic datums are negligible and take the most conservative estimate within the range of the biostratigraphic datums. This defines an age-depth "envelope;" for a given depth, the age is constrained to lie within the gray box.

Three lithologic units represented in Figure F27 have been identified (see "Lithostratigraphy"), which very roughly correspond to major shifts in sedimentation rates. An overall trend of decreasing sedimentation rates throughout the Neogene is noted. Sedimentation estimated for the lower Miocene is fairly high at ~110-130 m/m.y. (lithologic Unit III) and drops to ~50 m/m.y. through the middle and upper Miocene (lithologic Unit II). Uppermost Miocene to Pleistocene rates during the deposition of Unit I are even less and average ~15 m/m.y. Accumulation of sediment at Site 1165 appears to have been fairly continuous, as few disconformities were noted. Preliminary radiolarian data suggest a possible hiatus of ~2 m.y. within the middle Miocene lithologic Unit III at ~328 mbsf. An additional disconformity (~1 m.y.) is tentatively noted from diatom data at ~485-495 mbsf in the same unit.