Microfossils belonging to seven major groups were recovered from the sedimentary succession at Site 1168 off west Tasmania. The combined results suggest that there are no major time breaks in the sequence from the late Quaternary to late Eocene (Fig. F19). The site may prove to be a standard reference section for the marine sequences of southern Australia. Of the siliceous groups, diatoms are generally rare and sporadic and with moderate preservation, whereas radiolarians are generally rare but occasionally are abundant with poor to good preservation. Calcareous microfossils (foraminifers and nannofossils) are generally abundant with preservation ranging from poor to good; the best preservation is in the Pleistocene to upper Oligocene. Well-preserved organic walled dinoflagellate cysts (dinocysts) are abundant throughout the succession of Hole 1168A, except for the middle Miocene and upper Eocene intervals where they are common. Benthic foraminifers are present throughout the entire sequence, with the upper Eocene showing low abundances and diversity of mainly agglutinating species. Moderately preserved sporomorphs dominate the palynomorph samples from the lower part of Hole 1168A assigned to the late Eocene. Sedimentation rates were relatively low, ranging from 1.65 to 6.9 cm/k.y. (Fig. F19).
The Pliocene/Pleistocene boundary is placed between Samples 189-1168A-2H-CC and 3H-CC on nannofossil evidence; definite Pleistocene foraminifers are present only in Sample 189-1168B-1H-CC. The upper/lower Pliocene boundary is placed between Samples 189-1168A-6H-CC and 7H-CC on nannofossil evidence; planktonic foraminiferal Subzone SN12b, which contains this boundary, is present only in Samples 189-1168A-6H-CC and 189-1168C-6H-CC. The Miocene/Pliocene boundary is placed at the base of planktonic foraminiferal Subzone SN12a, which is present between Samples 189-1168A-7H-CC and 8H-CC; nannofossil evidence, however, suggests that this is present between Samples 189-1168A-9H-CC and 10H-CC. The upper/middle Miocene boundary falls within planktonic foraminiferal Zone SN8 and nannofossil Zone CN6 and, therefore, cannot be located with accuracy on the basis of the present set of samples; however, it must be above Sample 189-1168A-21X-CC. Late Miocene radiolarians and mixed assemblages of diatoms are present in Samples 189-1168A-12H-CC, 13H-CC, and 14H-CC with radiolarians continuing down to Sample 189-1168A-20X-CC. The middle/lower Miocene boundary is placed just above the base of planktonic foraminiferal Zone SN5, just below the top of nannofossil Zone CN3, and again cannot be precisely located. Planktonic foraminiferal Zone SN5 is based on the presence of Praeorbulina suturalis below the first occurrence (FO) of Orbulina suturalis; this zone has not been recognized, but the two species coexist in Sample 189-1168A-28X-CC. This suggests that the boundary may be present within Core 189-1168A-29X.The Miocene/Oligocene boundary falls within the uppermost part of planktonic foraminiferal Subzone SP14b (but is normally placed for convenience at the base of Zone SN1) and at the base of nannofossil Subzone CN1a. Planktonic foraminifers indicate the presence of this boundary between Samples 189-1168A-42X-CC and 43X-CC; nannofossil evidence, however, indicates its occurrence between Samples 189-1168A-46X-CC and 47X-CC.
Bolboformids typical of the late Miocene are recorded only from Samples 189-1168A-16X-CC and 17X-CC. The few available dinocyst and siliceous microfossil datums are in broad agreement with the age assignments presented above. There is evidence that Paleogene dinocysts have been reworked into the Pliocene.
The Neogene dinocyst assemblages are generally represented by relatively warm water, oligotrophic (oceanic) forms. Influence of colder water masses is only evident in the uppermost interval (late Pliocene/Pleistocene). The other microfossil groups suggest temperate to cool-temperate conditions throughout this interval.
The upper/lower Oligocene boundary coincides with the base of planktonic foraminiferal Subzone SP14b. The base of SP14b is defined by the last occurrence (LO) of Chiloguembelina cubensis, which is recorded between Samples 189-1168A-59X-CC and 60X-CC. In terms of planktonic foraminifers, the Eocene/Oligocene boundary in temperate regions is considered to fall within the upper part of Zone SP12. This zone is defined by the total range of Subbotina brevis, but since this species is very rare in this region, it cannot be used. For convenience, the LO of Globigerinatheka index is used as a proxy marker; this species is found between Samples 189-1168A-78X-CC and 79X-CC.
The LO of the nannofossil Discoaster saipanensis, which marks the top of Zone CP15, is dated at 34.2 Ma, which is located just below the Eocene/Oligocene boundary. This event is recorded between Samples 189-1168A-79X-CC and 82X-2, 21 cm.
The stratigraphic distribution of dinocysts and the few siliceous microfossils present in this interval are broadly in support of this age assessment. There is evidence that middle Eocene and Cretaceous dinocysts have been reworked into the upper Eocene.
Late Eocene to early Oligocene nannofossil assemblages exhibit distinctly warmer water characteristics than other comparable paleolatitude sites in the Southern Hemisphere. Dinocyst assemblages are dominated by relatively warm water, neritic, eutrophic forms; typical antarctic dinocyst species are conspicuously absent. Sporomorphs increase in relative abundance downhole, indicating decreasing terrigenous influence from the early to late Oligocene.
The upper Eocene planktonic foraminiferal assemblages continue to exhibit low species diversity, and their downhole distribution is patchy, with several samples being barren. In general, there is nannofossil and dinocyst evidence for a continued warm-water influence throughout the early Oligocene and Eocene. Dinocyst assemblages are poorly diversified and suggestive of eutrophic conditions and fluctuating salinity, from neritic open marine to brackish conditions. This is supported by the dominance of sporomorphs in the palynological associations in this part of the sequence.
The benthic foraminiferal record indicates an increase in paleodepth from neritic to upper bathyal in the late Eocene to lower bathyal to upper abyssal in the Neogene (Fig. F20). Changes coincide with all major stratigraphic boundaries. At the same time, the assemblages suggest a link between increasing water depth and improved bottom-water oxygenation.
All core-catcher samples plus additional samples from some critical intervals were examined for calcareous nannofossils at Site 1168. Calcareous nannofossils are generally abundant and well to moderately preserved throughout the cored interval (Table T2), with a few samples from the upper Eocene yielding few or no nannofossils. The nannofossil biostratigraphy (Table T3) established to date the upper Eocene through the Pliocene is believed to be the most detailed among the Southern Hemisphere sites of similar or higher paleolatitudes. The sequence will serve as an important reference section for the Southern Hemisphere.
The FO of Emiliania huxleyi, which defines the base of nannofossil Zone CN15 at 0.26 Ma, was found between Samples 189-1168B-1H-1, 55 cm, and 1H-1, 72 cm (Table T4). The LO of Calcidiscus macintyrei was found in Sample 189-1168A-2H-CC. This datum at 1.67 Ma can generally be used to approximate the Pliocene/Pleistocene boundary outside tropical temperate areas, where discoasters are generally rare or absent.
The LO of Discoaster surculus is located between Samples 189-1168A-2H-CC and 3H-CC. This provides a useful datum at 2.55 Ma for subdividing the late Pliocene nannofossil Zone CN12. Further subdividing this zone relies on other discoaster markers, which were not found in the samples examined. The LO of Reticulofenestra pseudoumbilicus (3.75 Ma) was found between Samples 189-1168A-6H-CC and 7H-CC, marking the lower Pliocene/upper Pliocene boundary.
Rare specimens of Amaurolithus were first observed in Sample 189-1168A-7H-CC. This suggests that the base of nannofossil Zone CN11 at 4.6 Ma is between Samples 189-1168A-6H-CC and 7H-CC. Ceratolithus acutus was recorded in Samples 189-1168A-8H-CC and 9H-CC. This indicates an age interval of 5.05-5.37 Ma for these two cores. The Miocene/Pliocene boundary is generally located slightly below the FO of C. acutus, that is, below Core 189-1168A-9H.
The FO of Amaurolithus (A. primus) is recorded between Samples 189-1168A-13X-CC and 14X-CC. This datum (7.2 Ma) marks the boundary between nannofossil Subzones CN9a and CN9b. The LO of Cyclicargolithus floridanus (11.9 Ma) is located between Samples 189-1168A-21X-CC and 22X-CC. This datum generally lies below the upper Miocene/middle Miocene boundary. A succession of nannofossil datums was recognized in the rest of the middle Miocene and lower Miocene—the LO of Sphenolithus heteromorphus (CN5/CN6 boundary at 13.6 Ma) between Samples 189-1168A-31X-CC and 32X-CC; the FO of Calcidiscus premacintyrei (17.4 Ma) between Samples 28X-CC and 29X-CC; the FO of S. heteromorphus (18.2 Ma) between Samples 31X-CC and 32X-CC; the LO of Sphenolithus belemnos (18.3 Ma) between Samples 31X-CC and 32X-CC; and the FO of S. belemnos (CN1/CN2 boundary at 20.6 Ma) between Samples 32X-CC and 33X-CC.
The LO of Reticulofenestra bisecta is located between Samples 189-1168A-46X-CC and 47X-CC. This marks the Oligocene/Miocene boundary. The following nannofossil datums were found that help date the Oligocene: the LO of Chiasmolithus altus (26.1 Ma) between Samples 189-1168A-54X-CC and 55X-CC; the LO of Sphenolithus distentus (27.5 Ma) between Samples 69X-CC and 70X-CC; the LO of Reticulofenestra umbilica (31.2 Ma) between Samples 77X-CC and 78X-1, 38 cm; and the LO of Isthmolithus recurvus (32.3 Ma) between Samples 78X-2, 10 cm, and 78X-2, 144 cm. The LO of Discoaster saipanensis (34.2 Ma) was recorded between Samples 189-1168A-79X-CC and 82X-2, 21 cm. This suggests that the Eocene/Oligocene boundary lies above this horizon. The last three datums span 3 m.y. and are within a 20-m sediment interval. This indicates a condensed section and/or one or more disconformities within this interval. The precise location of any disconformity within this interval, however, cannot be located or inferred based on the available data.
Samples 189-1168A-91X-CC through 95X-CC yielded generally rare specimens of nannofossils that do not provide useful age information. Examination of several samples from Core 189-1168A-94X, however, revealed abundant and diverse assemblages of nannofossils. They contain, among other Eocene nannofossils, I. recurvus and Reticulofenestra reticulata. The overlap of these two taxa suggests an age of ~36 Ma (Wei and Wise, 1992). This age inference is further supported by the fact that some specimens of the latter species are larger than 11 µm.
Nannofossil assemblages from the late Eocene through the early Oligocene show distinctly warmer water characteristics than those from comparable paleolatitude sites in the Southern Hemisphere, such as Sites 699, 703, 747, 748 (47°-58°S) in the South Atlantic and Indian Ocean sectors of the Southern Ocean. This is demonstrated by significantly higher abundance of low-latitude taxa, such as discoasters, Coccolithus formosus, and lower abundance of high-latitude taxa, such as chiasmoliths and Reticulofenestra daviesii, at Site 1168 than at the aforementioned Southern Ocean sites. Detailed quantitative analysis of the nannofossil assemblages from this interval should allow a detailed delineation of the changes through time in biogeographic gradients between Site 1168 and the other Southern Ocean sites and, thus, help paleoceanographic reconstruction.
Shipboard examination of all core-catcher samples revealed that a relatively complete marine sequence spanning the uppermost Eocene to the Holocene was recovered at Site 1168. The planktonic foraminiferal biostratigraphic succession seen at Site 1168 is typical of temperate sequences at these latitudes. All biozones belonging to the temperate latitude biostratigraphic scheme of Jenkins (1985, 1993a, 1993b) and Stott and Kennett (1990) are identified, with the notable exception of the Praeorbulina curva Zone (see Table T5). The distribution of species in these samples is given in Table T6.
Quaternary sediments appear to be confined within the uppermost core of all holes drilled at Site 1168 (Cores 189-1168A-1H, 189-1168B-1H, and 189-1168C-1H). The base of the Globorotalia truncatulinoides Zone (SN14), which approximates the Pleistocene lower boundary, is defined by the FO of the nominate taxon (1.96 Ma). This zone (SN14) was recorded only in Sample 189-1168B-1H-CC. Thus, the FO of G. truncatulinoides is constrained between the core depths of 3.89 and 7.24 mbsf at Site 1168. Rare specimens of Globorotalia tosaensis (0.65-2.9 Ma) were also found within two samples (Samples 189-1168B-1H-CC and 189-1168C-1H-CC).
The Pliocene is well represented in all three holes drilled at Site 1168. The base of the Globorotalia inflata Zone (SN13) is demarcated by the lowermost stratigraphic occurrences of the nominate taxon (3.2 Ma). This biostratigraphic datum was recorded in the following samples of each hole: Samples 189-1168A-5H-CC (45.96 mbsf), 189-1168B-6H-CC (51.51 mbsf), and 189-1168C-5H-CC (47.45 mbsf).
Continuing downsection, the G. inflata Zone is succeeded by the Globorotalia puncticulata Subzone (SN12b). The base of the G. puncticulata Zone is defined by the LO of Globorotalia pliozea (4.6 Ma). The G. puncticulata Zone is present in Samples 189-1168A-6H-CC (55.24 mbsf) and 189-1168C-6H-CC (56.26 mbsf). Subzone SN12b was not noted in Hole 1168B, but this is probably because of its probable position between Samples 189-1168B-5H-CC (51.51 mbsf) and 6H-CC (61.32 mbsf).
The base of the G. pliozea Zone, which coincides with the Miocene/Pliocene boundary, is defined by the FO of G. puncticulata (5.3 Ma). Of all three holes, the lowermost occurrences of G. puncticulata are in Sample 189-1168C-7H-CC at an average core depth of 63.09 mbsf. Planktonic foraminiferal assemblages are generally well preserved throughout the Pliocene, although intervals of shell fragmentation and moderate dissolution punctuate this portion of the record.
The absence of G. puncticulata combined with the presence of Globorotalia conomiozea defines the top of the uppermost planktonic foraminiferal zone of the late Miocene, the G. conomiozea Zone (SN11). This zone represents a long interval ranging from Samples 189-1168B-8H-CC through 189-1168A-14X-CC. The base of the G. conomiozea Zone is denoted by the FO of the nominate taxon (6.9 Ma). Specimens of G. conomiozea are first recorded at ~125 mbsf at Site 1168 (Samples 189-1168A-14X-CC and 189-1168C-14X-CC). Thus, the base of Zone SN11 is constrained to the stratigraphic interval bounded by Samples 189-1168A-14X-CC and 15X-CC. Hole 1168B terminates within the G. conomiozea Zone. It should also be noted that the stratigraphic position of the G. conomiozea FO is rendered somewhat subjective because of the morphological intergradation of this taxon with ancestral Globorotalia conoidea.
The Globorotalia miotumida Zone (SN10) is a gap zone bounded by the FO of G. conomiozea at its top (6.9 Ma) and the LO of Paragloborotalia continuosa at its base (8.0 Ma). The uppermost stratigraphic occurrence of P. continuosa was found to be within Sample 16X-CC in both Holes 1168A and 1168C. As a consequence, the base of this gap zone is constrained to the stratigraphic interval between Samples 189-1168A-15X-CC and 16X-CC.
The base of the P. continuosa Zone (SN9) is by definition the uppermost stratigraphic occurrence of Paragloborotalia nympha (10.1 Ma). The LO of P. nympha was identified in Samples 189-1168A-19X-CC and 189-1168C-20X-CC. This limits the possible stratigraphic range for the base of the P. continuosa Zone to the interval ranging from 173.37 to 175.23 mbsf.
The downhole stratigraphic sequence is continued with the lowermost zone of the late Miocene, the P. nympha Zone (SN8). The lower boundary of the P. nympha Zone is delimited by the LO of Paragloborotalia mayeri (11.4 Ma). The P. mayeri LO was identified in Samples 189-1168A-22X-CC and 189-1168C-22X-CC. Thus, the base of the P. nympha Zone is constrained to core depths spanning 194.62-203.67 mbsf. Preservation throughout the upper Miocene is generally good with only minor dissolution and etching of planktonic foraminiferal shells.
The uppermost zone of the middle Miocene is the P. mayeri Zone (SN7). The base of the P. mayeri Zone (12.1 Ma) is defined by FO of the nominate taxon. The lowermost stratigraphic occurrence of P. mayeri was recorded in Sample 189-1168A-23X-CC, whereas Hole 1168B terminated in sediments postdating the P. mayeri FO. Based on the section recovered from Hole 1168A, it appears that the base of the P. mayeri Zone is restricted to core depths of 214.69 to 224.27 mbsf.
The biostratigraphic succession continues uninterrupted downsection with the presence of the Orbulina suturalis Zone (SN6). The base of the O. suturalis Zone (15.1 Ma) is defined by the FO of the nominate taxon. The FO of O. suturalis was identified in Sample 189-1168A-28X-CC. Thus, the lower boundary of the O. suturalis Zone is constrained to the stratigraphic interval spanning 260.29 to 272.34 mbsf.
The Praeorbulina curva Zone (SN5) was not recognized in the series of core-catcher samples examined. However, specimens of P. curva were found co-occurring with O. suturalis, particularly in the lower part of the O. suturalis Zone. It is suspected that the FO of P. curva (16.3 Ma) was not identified because of the low sampling density. It is possible that the P. curva Zone is present within the stratigraphic interval bounded by Samples 189-1168A-28X-CC and 29X-CC (260.29-272.34 mbsf). Preservation throughout the middle Miocene is generally good, although chalky layers are present.
The later parts of the early Miocene are represented by the Globigerinoides trilobus Zone (SN4) at Site 1168. The lower boundary of this biozone is delimited by the FO of the nominate taxon (18.8 Ma). This zonal boundary was identified in Sample 189-1168A-30X-CC and not in 31X-CC. Thus, the precise stratigraphic position of this datum is confined to the 281.48-291.59 mbsf interval.
The Globoturborotalita connecta Zone (SN3), which predates the G. trilobus Zone, begins with the FO of the nominate taxon (20.9 Ma). The marker species G. connecta is present in Sample 189-1168A-34X-CC but not in 35X-CC. Therefore, the stratigraphic range for the lower boundary of the G. connecta Zone is constrained to 309.97-319.45 mbsf.
The downsection sequence continues uninterrupted with the presence of the Globoturborotalita woodi Zone (SN2). The lower boundary of this biozone is demarcated by the FO of the nominate taxon (22.6 Ma). The presence of G. woodi in Sample 189-1168A-40X-CC and its absence in 41X-CC restricts the lower boundary of the G. woodi Zone to the interval bounded by 376.54-387.07 mbsf.
The base of the Globoquadrina dehiscens Zone (SN1), which also corresponds to the Oligocene/Miocene boundary, is defined as the FO of the nominate taxon (23.2 Ma). The marker species G. dehiscens was recorded in Sample 189-186-1168A-42X-CC and absent in 43X-CC. Thus, the base of the G. dehiscens Zone, and Oligocene/Miocene boundary, is located within the 396.61-406.01 mbsf stratigraphic interval.
Preservation of planktonic foraminifers varies throughout the lower Miocene. In general, the assemblages are moderately to well preserved with restricted intervals of increased shell fragmentation and dissolution.
The Turborotalia euapertura Subzone (SP14b) is the only biozone of the late Oligocene recognized at this site. The lower boundary of this biozone is delimited by the LO of Chiloguembelina cubensis (28.5 Ma). This datum was difficult to determine with confidence because of the erratic stratigraphic distribution and relatively small size of C. cubensis. The uppermost stratigraphic occurrence of C. cubensis is tentatively placed within Sample 189-1168A-60X-CC. Thus, the lower boundary of the T. euapertura Zone is confined to the stratigraphic interval between 396.61 and 406.01 mbsf.
Planktonic foraminifers contained within the upper Oligocene section are only moderately preserved. Strong diagenetic overprinting is conspicuous at some sample horizons. Many specimens suffer from secondary calcite infilling and pyritization.
The uppermost biozone of the early Oligocene is the C. cubensis Subzone (SP14a). The base of the C. cubensis Subzone is delimited by the LO of Subbotina angiporoides (30 Ma). The S. angiporoides LO was identified within Sample 189-1168A-74X-CC. Therefore, the lower boundary of the C. cubensis Subzone lies within the 691.62-702.34 mbsf interval.
The downsection sequence is continued with the presence of the S. angiporoides Zone (SP13). The lower boundary of the S. angiporoides Zone is delimited by the LO of S. brevis (33 Ma). Poor preservation and unfavorable paleoenvironmental conditions complicated determination of this taxon's stratigraphic distribution. The uppermost stratigraphic occurrence of S. brevis is tentatively placed in Sample 189-1168A-77X-CC. The lack of S. brevis in the overlying sample (189-1168A-76X-CC) constrained the base of the S. angiporoides Zone to the 722.84-733.41 mbsf interval.
Preservation deteriorates downhole, decreasing from moderate to poor. Planktonic foraminifers are infilled with secondary calcite and pyritized. Planktonic foraminifers become rarer downsection, particularly below Sample 189-1168A-70X-CC.
The uppermost stratigraphic occurrence of the late Eocene marker species Globigerinatheka index (34.3 Ma) was identified in Sample 189-1168A-79X-CC. The absence of G. index in Sample 189-1168A-78X-CC suggests that the Eocene/Oligocene boundary is within the interval spanning 743.28-752.53 mbsf. Specimens of Subbotina linaperta and S. brevis were found to coexist with G. index in Sample 189-1168A-79X-CC. The overlapping stratigraphic ranges of these taxa suggest that the boundary between the S. brevis Zone (SP12) and the underlying S. linaperta Zone (SP11) is complete, although likely condensed (see Fig. F19). However, the degree of completeness of the Eocene/Oligocene boundary at Site 1168 remains equivocal and warrants more detailed investigation.
The presence of the calcareous nannofossil marker species, I. recurvus, in Sample 189-1168A-94X-CC suggests the possibility that rare Acarinina aculeata (the marker for the top of the Zone SP10) may also be present at the base of this hole, although paleoenvironmental conditions appear to have precluded its presence.
Preservation is generally poor and planktonic foraminifers are commonly infilled with secondary calcite. All samples below Sample 189-1168A-88X-CC were deemed barren of planktonic foraminifers.
Most of the benthic foraminiferal taxa preserved in the sediments are extant (Fig. F20). Their abundances, diversities, and preservation are extremely low in the upper Eocene. With improved bottom-water oxygenation, as indicated by changes in the assemblages, benthic foraminiferal numbers and diversities increase in the Oligocene. The Neogene is characterized by abundant and diverse assemblages of well preserved benthic foraminifers. The suggested oxygen levels increase from very low to well oxygenated at the same time as the paleodepth increases at the major stratigraphic boundaries (i.e., from neritic to upper bathyal; ~100-200 m) in the upper Eocene to lower bathyal/upper abyssal (~1000-2500 m) in the Neogene. Benthic foraminiferal assemblages suggest frequent reworking of the upper Pliocene to Holocene interval and in selected intervals of the Oligocene. The transition from articulated to disarticulated ostracodes at the Oligocene/Miocene boundary indicates an increase in bottom-water circulation at this time.
The lower Oligocene interval is almost entirely dominated by a low-oxygen fauna (Globobulimina spp. and Uvigerina spp.). Most Globobulimina spp. are badly preserved as their thin test shows signs of compaction. A transition from upper bathyal (~200-600 m) to bathyal (~400-2000 m) water depths is marked by the occurrence of Globocassidulina spp., indicating improved oxygenation of bottom waters. This interval is followed by an assemblage marked by the co-occurrence of increased numbers of Hoeglundina elegans and Globocassidulina subglobosa until ~520 mbsf (Sample 189-1168A-55X-CC). The remaining late Oligocene assemblages consist of calcareous benthic foraminifers together with high abundances of agglutinating foraminifers with a preference for using siliceous spicules in their tests. The entire Neogene interval contains typical open-marine benthic foraminiferal assemblages from bathyal depths (~1000-2500 m) with varying intervals when individual species show higher abundances (e.g., Vulvulina pennatula from ~320 mbsf to the lower/middle Miocene boundary at ~250 mbsf and Fontbotia wuellerstorfi from ~280-50 mbsf).
Ostracodes were observed in most core-catcher samples from 700 mbsf (189-1168A-73X-CC) to the surface. The lower part of the drilled section (950 to 700 mbsf) is marked by an increased amount of terrigenous input of varying nature and only infrequent traces of ostracodes. We observe a change at 400 mbsf (Sample 189-1168A-43X-CC) from articulated to disarticulated carapaces. This suggests a change from deposition in a tranquil environment with low or no bioturbation to an environment with increased bottom circulation and/or bioturbation. This coincides with an inferred change in sedimentation rate at the Oligocene/Miocene boundary (Fig. F19).
Bolboforma capsula was the only bolboformid positively identified. This species is present in Samples 189-1168A-16X-CC and 17X-CC, placing them in Zone NN9 (top of Zone NN8 to bottom of Zone NN10) (Spiegler and von Daniels, 1991) and, thus, being consistent with the nannofossil stratigraphy. Bolboforma antarctica, which strongly resembles a hexagonid Oolina, was possibly observed in Eocene samples. Bolboformids have been observed from a narrow temperature range of 4°-8°C, so their presence or absence may be a reflection of the sea-surface temperatures at the time.
Identifiable radiolarians were recovered from Samples 189-1168A-2H-CC and 11H-CC through 20X-CC; Samples 189-1168B-8H-CC, 11H-CC, and 12H-CC; and Samples 189-1168C-8H-CC, 10H-CC, 2H-CC through 20X-CC, and 22X-CC. These samples contain rare to abundant, poor- to well-preserved radiolarians. The radiolarian sequence in this hole cannot be correlated to the antarctic or tropical zonal schemes because of the absence of marker species. Three radiolarian datums were recognized from Hole 1168A (Table T7) and one from Hole 1168C (Table T8).
Samples 189-1168A-1H-CC through 10H-CC are nearly barren. Sample 189-1168A-2H-CC contains only one identifiable species, Stylacontarium aquilonarium.
Radiolarians are consistently present in Samples 189-1168A-11H-CC through 20X-CC. This fauna is characterized by an absence of antarctic taxa including Antarctissa and the subfamily Artiscinae. The fauna consist mainly of the actinommid genera Hexacontium, Thecosphaera, and Stylacontarium. Nassellarians are consistently present but are rare to common. Samples 189-1168A-12H-CC through 16H-CC commonly contain late Miocene radiolarians, including Carpocanistrum ob, Cycladophora antiqua, Dictyophimus splendens, Stylacontarium acquilonium, Stylatractus universus, and Theocorythium trachelium trachelium. The faunas of this interval consistently contain the deeper dwelling species Cornutella profunda, which in the modern ocean lives deeper than 400 m.
Useful datums were recognized from three horizons (Table T7): the FO of S. aquilonarium (Hays), estimated at 7.0 Ma in the North Pacific (Motoyama, 1999), is present between Samples 189-1168A-14X-CC and 15X-CC. The LO of Cyrtocapsella japonica is placed between Samples 189-1168A-16X-CC and 17X-CC. In the North Pacific this bioevent is dated as 9.9 Ma (Motoyama, 1999). The last abundant occurrence (LAO) of C. japonica is estimated at 10.1 Ma in the North Pacific and is placed between Samples 189-1168A-17X-CC and 18X-CC. Samples 189-1168A-21X-CC through 1168A-95X-CC are mostly barren.
The Oligocene samples occasionally contain poorly preserved radiolarian tests. These show dissolved surfaces and are infilled with opaque material. Only one specimen of Eucyrtidium spinosum, which appears at ~37.0 Ma, was found in Sample 189-1168A-65X-CC.
Samples 189-1168B-1H-CC through 12H-CC are barren or contain rare specimens. Only one species, S. aquilonarium, was found in Samples 189-1168B-11H-CC and 12H-CC.
Samples 189-1168B-1H-CC through 11H-CC are also barren or contain rare specimens. Of these, Samples 189-1168B-8H-CC and 10H-CC contain rare radiolarians, including S. aquilonarium.
Radiolarians are common in Samples 189-1168C-12H-CC through 16X-CC. The fauna of these samples is similar to that of Samples 189-1168A-11H-CC through 20X-CC but differ from the latter by lacking Botryostrobus and Cycladophora. This fauna consists mainly of Axoprunum angelinum, C. ob, the Eucyrtidium cienkowskii group, Lamprocyrtis hannai, Lithelius nautiloides, Sphaeropyle robusta, Stichocorys delmontensis, Stichocorys peregrina, S. acquilonarium, and S. neptunus.
Sample 189-1168C-17X-CC yielded few radiolarians, including S. aquilonarium. This sample contains several species that are not utilized for age determination. C. japonica, which becomes extinct at 9.9 Ma in the North Pacific (Motoyama, 1999), was found in Samples 189-1168C-18X-CC and 19X-CC. Sample 189-1168C-20X-CC is barren. Sample 189-1168C-21X-CC contains a few broken specimens of Cyrtocapsella tetrapera. Samples 189-1168C-22X through 31X-CC are mostly barren.
The LO of S. delmontensis is placed between Samples 189-1168C-12H-CC and 13X-CC (Table T8). The age of this datum has been determined as 5.18-6.9 Ma by Morley and Nigrini (1995).
All core-catcher samples from Holes 1168A, 1168B, and 1168C were analyzed for diatoms, silicoflagellates, and sponge spicules. Preparation followed the procedures outlined in "Biostratigraphy" in the "Explanatory Notes" chapter, although additional processing of sediment was necessary for most samples in order to concentrate the siliceous component. In this instance, 5-7 cm3 of core-catcher sample was treated with 40% HCl, washed and sieved through a 20-µm mesh. Strewn slides of the >20-µm fraction were prepared on a hot plate.
Diatoms are absent or in trace abundance throughout Site 1168 sediment except for two discrete sections downhole where they are common: Sections 189-1168A-12H-CC through 14X-CC and 61X-CC through 64X-CC (Table T9). Preservation is moderate and diversity is low. Their abundance appears to be controlled by preservation but probably is also a combination of factors including dilution by carbonate and overall low-diatom productivity (oligotrophic conditions?). Their occurrence downhole is directly proportional to the concentration of dissolved silica in the pore waters (sponge spicules comprise the bulk of the siliceous biogenic component, thereby contributing the most to dissolved silica; Fig. F21). Where diatoms are present, they are buffered from postdepositional dissolution where H4SiO40 concentration in pore waters exceeds ~500 µM.
The Thalassiothrix antarctica-longissima group dominates the assemblages in Samples 189-1168A-12H-CC and 13H-CC. Separation of the two species was not possible because of the poor preservation of valve poles. Sample 189-1168A-12H-CC contains a mixed signal assemblage: subantarctic taxa (Actinocyclus ingens, Proboscia barboi, and one specimen of the endemic antarctic diatom Eucampia antarctica), the temperate taxon Fragilariopsis reinholdii, and neritic planktonic diatoms (Chaetoceros resting spores and Thalassionema nitzschioides). Samples 189-1168A-13X-CC and 14X-CC contain a moderate abundance of the marker Actinocyclus ingens var. ovalis constraining these samples to the late Miocene. Samples 189-1168A-61X-CC through 64X-CC contain moderately preserved specimens of an apparently monospecific assemblage of an unidentified species of Arachnodiscus.
Sponge spicules comprise the bulk of the biogenic silica throughout most of Site 1168. However they are noticeably absent in Sections 189-1168A-31X-CC through 41X-CC and 69X-CC to the bottom of the hole. Sponge spicule abundance is directly related to the concentration of dissolved silica in interstitial pore waters (Fig. F21). Silicoflagellates are recorded in low abundance in Samples 189-1168A-13X-CC and 14X-CC only.
Onboard work included palynological analysis of every fourth core-catcher sample at Site 1168 in Hole 1168A. From the uppermost (Sample 189-1168A-1H-CC through 10H-CC) and the lowermost (Sample 189-1168A-74X-CC through 95X-CC) intervals, most available core-catcher samples were analyzed. Recovery of palynomorphs was quite variable. Dinoflagellate cysts (dinocysts) are the most prominent palynomorphs in the upper and middle parts of the succession in Hole 1168A (Table T10). Sporomorphs dominate the lower part of the succession.
Somewhat surprisingly, dinocysts were generally found to be more abundant in parts of the Neogene than in the Paleogene in Hole 1168A (although the interval between Samples 189-1168A-18X-CC to 30X-CC was found to be almost barren; Table T10). Preservation is good in the Neogene to Oligocene section in contrast to the older part of the succession (Sample 189-1168A-75X-CC to the base of the cored interval). Dinocyst species in Hole 1168A are largely cosmopolitan and rather long ranging. However, generally their stratigraphic distribution matches that known from the Northern Hemisphere and equatorial regions (calibrated against the calcareous plankton record of Hole 1168A). A few stratigraphically useful events were recorded (viz., the LO of Amiculosphaera umbracula and Invertocysta tabulata in Sample 189-1168A-6H-CC and the FO of Achomosphaera andalousiensis in Sample 189-1168A-14X-CC). The latter event may be taken to approximate the middle/lower Miocene boundary (Table T11). In addition, the LO of Pyxidinopsis fairhavenensis and Glaphyrocysta spp. appear to be stratigraphically useful events as well, recorded in Samples 189-1168A-34X-CC and 46X-CC, respectively. The available calibration of their tops/bases from the Northern Hemisphere fits well with the results from the onboard calcareous microfossil studies (Table T10; Fig. F19). Curiously, a single specimen of Diphyes ficusoides was found in Sample 189-1168A-82X-CC. This taxon is not known to occur outside of the Eocene of the North Sea Basin and surrounding areas, where its LO is near the top of the Lutetian (middle Eocene; e.g., Bujak and Mudge, 1994). The occurrence in Hole 1168A is interpreted to be the result of reworking of upper lower to lower middle Eocene strata into the upper Eocene. Similarly, the occurrence of Circulodinium distinctum in Sample 189-1168A-94X-CC is considered to be a result of reworking of (lower?) Cretaceous materials into the upper Eocene. Reworking of Paleogene material into the Pliocene is apparent by the occurrence of Wetzeliella articulata in Sample 189-1168A-6H-CC. Several undescribed species were recorded (e.g., new species of Cerebrocysta, Eocladopyxis, and Cannosphaeropsis); further taxonomic treatment awaits the examination of more specimens.
In the Neogene section, assemblages are marked by an abundance of warm-water species of Impagidinium, indicating relatively warm, oligotrophic (oceanic) surface water conditions throughout. In the uppermost samples, a few specimens of the arctic species Impagidinium pallidum were recorded. This indicates the influence, albeit limited, of colder water masses in this interval. Occasional abundance of other species in this part of the section, like that of Operculodinium echigoense, may be the result of downslope transport. The Oligocene assemblages are dominated by forms like Spiniferites ramosus, Operculodinium centrocarpum, and, in particular, Hystrichokolpoma rigaudiae, Thalassiphora pelagica and Systematophora placacantha, which are tentatively interpreted to reflect increasingly more eutrophic surface conditions, possibly associated with more shallow waters, and/or increased transportation from such settings. The presumed upper Eocene succession is marked by poorly diversified assemblages with sporadic occurrences of Deflandrea phosphoritica and Lejeunecysta spp. indicating eutrophic conditions with varying salinities, from normal marine to brackish conditions. The dinocyst assemblage in this part of the core is very similar to those recorded elsewhere from ancient delta front deposits.
Sporomorphs are frequent in the palynological associations, as are the organic linings of foraminifers. Quite appropriately, sporadic occurrences of representatives of the chlorophyte Tasmanites have been recorded in some samples. The percentage of terrestrial palynomorphs is given in Table T10.
The palynological associations of the lower part of Hole 1168A, notably from Sample 189-1168A-75X-CC downward (early Oligocene?-late Eocene), are dominated by sporomorphs. Long-ranging smooth and verrucate fern spores such as Verrucosisporites kopukuensis, Cyathidites minor, and Crassiretritriletes vanraadshovenii are particularly common. Pollen are represented by alete bisaccates and Nothofagidites spp., notably N. asperus. Few specimens of Acaciapollenites miocenicus are present in the lower sample suite; their presence may be taken to support the inferred late Eocene age of this interval on the basis of the calcareous microfossil record (following MacPhail, 1999). The abundance of sporomorphs in the lower part of the section (occasionally up to 100% of the palynomorph association), in conjunction with the massive abundance of plant tissue debris in this interval, further supports attribution to delta front environments. A more detailed study of the terrestrial palynomorph assemblages of the lower sequence of Hole 1168A may result in a more definite age assessment of these deposits.
The combined nannofossil, planktonic foraminifer, diatom, radiolarian, and dinocyst biostratigraphy at Site 1168 yielded 51 reliable bioevents with age significance. Principal trends through this essentially continuous section are shown in Figure F19. Datums are from the combined microfossil bioevents and 10 magnetic polarity datums. The bioevents are comprised of 23 FO events and 28 LO events and are listed in Table T12. All events are plotted according to their observed depths at Site 1168 and by their ages as defined in "Biostratigraphy" in the "Explanatory Notes" chapter. First occurrence events may have been estimated to be too shallow and LO events may have been estimated as being deep, based on the limited sampling interval. Stratigraphic position of these datums will be refined with further study. See individual microfossil group discussion for more detailed bioevent data.
Sedimentation rates increase downcore as biostratigraphic control decreases. The upper 380 mbsf are well constrained by 40 bioevents and 10 magnetic polarity events. The average sedimentation rate of 1.65 cm/k.y. from the Pleistocene to the mid-early Miocene may be low in part because of silica dissolution or other parameters. Silica concentration in pore-water samples (see "Inorganic Chemistry") does increase from ~100 to 225 mbsf and may be a result of increasing amounts of sponge spicules. Event levels between microfossil groups agree rather well, taking into consideration the large sampling interval. Datums that are well out of sequence, such as the FO of diatom Fragilariopsis reinholdii, have large ranges on the depth of the event (see Table T12 for more examples).
Biostratigraphic control degrades downcore, with six bioevents to date the lower Miocene through lowermost Oligocene interval from 380 to 700 mbsf. The average sedimentation rate increased to 4.3 cm/k.y. with an influx of siliciclastic material beginning in Core 189-1168A-44X, the beginning of lithostratigraphic Subunit IIB. Lower sedimentation rates mark the lower Oligocene, but the sequence appears to be continuous.
The uppermost Eocene section from 750 to 880 mbsf is delineated by only a single bioevent. Increasing dissolution of calcareous microfossils downcore coincided with a lithofacies change to sandy and organic-rich claystones (see "Lithostratigraphy"). Core-catcher samples 189-1168A-90X-CC through 95X-CC were barren of calcareous and siliceous microfossils. Toothpick samples from carbonate rich intervals of Samples 189-1168A-91X to 94X yielded a bottom-hole age of ~36 Ma based on the FO of I. recurvus at Sample 189-1168A-94X-3, 137 cm (see "Calcareous Nannofossils"). The bottom-hole age is overestimated, and detailed shore-based work may suggest an increase in the average sedimentation rate of 6.9 cm/k.y.