Combined microfossil biostratigraphy indicates that the recovered sequence at Site 1170 represents the Paleocene to Quaternary. Calcareous nannofossils are abundant from the Quaternary to the upper Oligocene, absent in the Eocene/Oligocene boundary transition, and generally few to common in most of the samples downhole through the middle Eocene. Planktonic foraminifers are abundant from the Quaternary to the Oligocene but rare or absent below the lower Oligocene. The paucity or absence of calcareous microfossils in the Paleogene is not attributed to strong carbonate dissolution because nannofossils, where present, show generally good preservation with some delicate features intact. Diatoms and radiolarians are generally abundant in the upper Neogene, and their abundances diminish toward the Oligocene. Dinoflagellate cysts are present in the upper Pliocene and Pleistocene, abundant in the Eocene, and absent in the lower Pliocene through the Oligocene samples analyzed.
The late Neogene has the best biostratigraphic resolution, as a large number of bioevents from various microfossil groups can be used. Biostratigraphic data from this site show a generally uniform sedimentation rate for the late Neogene (see "Age Model and Sedimentation Rates"; Fig. F10), in contrast with the extremely high sedimentation rate (~24 cm/k.y.) for the early Pliocene at Site 1169. This suggests that the high sedimentation rate at Site 1169 was a local rather than a regional phenomenon.
A hiatus representing the late Miocene (between ~7 and 11 Ma) was recognized. This hiatus was also recognized at Site 1169 (where it has a longer duration) and thus appears to be of regional significance. Another hiatus was revealed near the lower Oligocene/upper Oligocene boundary.
A questionable hiatus is indicated in the Eocene-Oligocene transition interval based on nannofossil data. However, the interpretation hinges heavily on the use of one nannofossil datum (the last occurrence [LO] of Reticulofenestra umbilica) with the exclusion of dinocyst datums (the LOs of Areosphaeridium diktyoplokum and Enneadocysta partidgei). The use of the dinoflagellate datums in the exclusion of the nannofossil datum will result in a significantly different age model interpretation. The available data do not allow for an unambiguous conclusion as there are a number of uncertainties, including whether the biochronological ages calibrated elsewhere can be applied to the nannofossil and dinoflagellate datums at this site (cf. Figs. F11, F12). Shore-based studies of additional samples, detailed comparison of results from this site with those from other southern high-latitude sites, and examination of some samples from those sites should shed more light on the age assignments for the critical Eocene-Oligocene transition.
Another questionable hiatus is indicated in the middle Eocene based on the co-occurrence of two nannofossil datums—the LO of Chiasmolithus solitus (38.2 Ma) and the first occurrence (FO) of Reticulofenestra reticulata (41.2 Ma). The inference of a hiatus, however, appears to be in conflict with dinocyst datums and sedimentological studies. Further work is needed to clarify this problem as it is critical for other studies requiring reliable age information.
Benthic foraminiferal assemblages change dramatically between the shallow-water middle-upper Eocene and the deeper, well-oxygenated Oligocene and Neogene sediments. This transition in water depth, from neritic to bathyal/abyssal, coincides with a sequence of glauconitic siltstone (Cores 189-1170D-7R through 9R) that is devoid of benthic foraminifers. Furthermore, there are distinctly neritic diatom floras in Cores 189-1170D-9R through 7R (Eocene), mixed diatom floras from Cores 189-1170D-5R through 1R, which suggest a period of deepening in the earliest Oligocene, and fully open-ocean diatom floras in the late early Oligocene.
Noticeable reworking of dinocysts and radiolarians was observed in the middle Eocene sediments. In the case of radiolarians, the reworked specimens are dominantly of early Eocene age.
The quantitative dinocyst distribution in the lowermost Oligocene to middle middle Eocene generally indicates somewhat restricted, overall eutrophic, neritic marine conditions throughout the succession. Cross-correlation with the sedimentological (smear slide) data, and the diatom, radiolarian, and calcareous nannofossil distribution suggests (1) a positive correlation between maxima in Deflandrea spp. (dinocysts) with diatom maxima and most radiolarian maxima and (2) a positive correlation between maxima in calcareous nannoplankton content and E. partridgei (dinocysts) maxima. Optima in Thalassiphora pelagica (dinocysts) immediately follow an E. partridgei maximum. These correlations are tentatively interpreted to reflect changes in trophic levels of the water masses and are shown to have a cyclic nature.
Terrestrial palynomorphs are consistent in the Eocene, but in low relative abundances. The few identified taxa may be compared with established records from New Zealand and Australia and indicate cooling and increasing humidity from the middle middle Eocene into the late Eocene.
A discussion of each microfossil group studied is provided below.
All core-catcher samples from the four holes at Site 1170 were examined for calcareous nannofossils. Additional samples from some critical intervals were also examined to refine biostratigraphic resolution. Calcareous nannofossils are generally abundant and well preserved in the Neogene. Nannofossil abundance decreases downhole from generally abundant in the upper Oligocene to generally common or few in the middle Eocene, with a number of samples barren of nannofossils. Nannofossil datums recognized, and their ages, sample intervals, and depth intervals are listed in Table T3. The stratigraphic distribution of selected species is presented in Table T4. Most of this nannofossil biostratigraphic information in these five tables is self-explanatory. A few comments are made below for several nannofossil datums.
Ceratoliths are very rare at this site. The FO and LO of Ceratolithus acutus (Table T3) were based on the rare occurrence of the species in one sample (189-1170A-12H-CC). The true FO is most likely to be stratigraphically lower and the true LO stratigraphically higher than recorded. Similarly, amauroliths are also very rare and sporadic at the site. The FO of Amaurolithus delicatus recorded between Samples 189-1170C-16H-CC and 17H-CC (Table T3) is likely to be stratigraphically higher than its true FO.
Sample 189-1170D-6R-CC contains an Oligocene nannofossil assemblage without Cyclicargolithus abisectus and R. umbilica. This suggests an age of ~31.3 Ma for this sample. Core 189-1170D-7R and the upper part of Core 189-1170D-8R are barren of calcareous nannofossils. Sample 189-1170D-8R-5, 50 cm, immediately below this barren interval, contains R. umbilica and R. reticulata. An age of 35.8 Ma can be assigned to this sample. The interval from Samples 189-1170D-8R-5, 50 cm, through 7R-CC thus appears to represent an ~4-m.y. duration. This would suggest a condensed section or, more likely, a major hiatus over the Eocene/Oligocene boundary.
Another hiatus is tentatively placed in Core 189-1170D-13R based on co-occurrence of two nannofossil datums—the LO of Chiasmolithus solitus (40.4 Ma) and the FO of R. reticulata (42.0 Ma). The duration of the hiatus would be >1.6 m.y. if the above ages are used. The inference of a hiatus in this core, however, appears to be in conflict with a dinocyst datum in Core 189-1170D-12R. Shore-based studies of additional samples, detailed comparison of results from this site with those from other southern high-latitude sites, and examination of samples from those sites should help clarify whether a hiatus exists and its potential duration.
The FO of R. umbilica (43.7 Ma) was recorded between Samples 189-1170D-37R-CC and 38R-CC. This suggests a sedimentation rate of >30 cm/k.y. between ~529 and 774 mbsf. As the deepest sample from the site (189-1170D-38R-CC) contains only rare specimens of nannofossils, the FO of R. umbilica is considered tentative, and its true FO may be stratigraphically lower. This would indicate possibly even higher sedimentation rates assuming that the numerical age can be applied to this site.
The generally low abundance (and sometimes absence) of nannofossils in the Eocene interval is attributed to dilution by abundant clays and to a very shallow and restricted water environment rather than dissolution of carbonate, as nannofossils, where present, are generally well preserved with very fine structures intact.
As at Site 1168, the Eocene through lower Oligocene nannofossil assemblages at this site show warmer water characteristics than those from sites of comparable paleolatitude (e.g., Sites 689, 690, 744, and 748, which range from 52° to 65°S) as there are fewer high-latitude taxa (e.g., chiasmoliths and Reticulofenestra daviesii) at Site 1170.
Shipboard examination of planktonic foraminifers from all core-catcher samples disclosed that sediments ranging in age from middle Eocene to Quaternary were recovered at Site 1170 (see Table T5 for datum ages). The planktonic foraminiferal assemblages are typical of cool-temperate to subantarctic regions. The Neogene section is punctuated by two significant hiatuses, one in the upper Miocene and another in the lower middle Miocene. Given the low sample density, the absence of some of the planktonic foraminiferal zones may reflect condensed intervals because of slow sedimentation rates, as opposed to actual hiatuses.
Planktonic foraminifers provide little biostratigraphic control throughout much of the Paleogene sequence recovered at Site 1170. There are several intervals within the upper Eocene to lowermost Oligocene sequence in Hole 1170A that are barren of planktonic foraminifers (e.g., samples below Sample 189-1170A-52X-CC, 464.25 mbsf). In Hole 1170D, planktonic foraminiferal assemblages are depauperate and sporadic throughout the section spanning the middle Eocene to lowermost Oligocene (Samples 189-1170D-6R-CC to 38R-CC). Nonetheless, the middle Eocene to upper Oligocene record still appears incomplete with the lower upper Oligocene missing. The stratigraphic distributions of species are given in Table T6.
Identification of the FO of Globorotalia truncatulinoides (~1.98 Ma), the datum that approximates the lower boundary of the Quaternary, has been problematic. Typically, specimens of G. truncatulinoides are relatively large and morphologically distinct. However, at Sites 1168, 1169, and 1170 only diminutive, juvenile G. truncatulinoides with thin, pregametogenic shells comprise the lowermost occurrences of this marker species. Matters are further complicated by the fact that these small G. truncatulinoides are rare. The sporadic stratigraphic occurrence of early G. truncatulinoides has given rise to apparent ages that are discordant with those derived from calcareous nannoplankton datums. Kennett (1970) has reported that G. truncatulinoides first immigrated into subantarctic waters as recently as 200 ka. Thus, it appears that environmental conditions during the early Pleistocene were unfavorable for supporting viable populations of G. truncatulinoides in the region. These observations suggest that the FO of G. truncatulinoides is an unreliable datum in the STR area. The FO of G. truncatulinoides is tentatively placed within the interval between Samples 189-1170A-3H-CC (20.75 mbsf) and 4H-CC (29.07 mbsf).
Significant changes in faunal compositions were also noted in the series of Quaternary core catchers examined. For instance, in Sample 189-1170B-3H-CC warm temperate-water species such as Orbulina universa and Globigerinella aequilateralis are rare to absent, whereas the cold-water species Neogloboquadrina pachyderma (sinistral) is quite abundant. The opposite is the case for Sample 189-1170B-4H-CC in which temperate species increase in abundance. These faunal responses likely reflect geographic shifts in water-mass boundaries and concomitant changes in water temperatures during the Quaternary.
The base of the Globorotalia inflata Zone (SN13) is demarcated by the FO of the nominate taxon. This datum is used to recognize the base of the upper Pliocene (3.2 Ma) and is constrained to the interval between Samples 189-1170A-7H-CC (58.25 mbsf) and 8H-CC (68.07 mbsf). Upper Pliocene assemblages are mostly temperate in character containing such species as Globigerina bulloides, Globigerina quinqueloba, Globorotalia crassaformis, Globorotalia puncticulata, and Globorotalia puncticuloides.
The early Pliocene is composed of two subzones, the G. puncticulata (SN12b) and the Globorotalia pliozea (SN12a) subzones. The boundary between the subzones is defined by the LO of G. pliozea (4.60 Ma), which is restricted to the interval bounded by Samples 189-1170A-14H-CC (123.73 mbsf) and 15H-CC (133.94 mbsf).
The lower boundary of Subzone SN12a coincides with the Miocene/Pliocene boundary and is delimited by the FO of G. puncticulata (5.30 Ma). The base of Subzone SN12a was constrained to the interval between Samples 189-1170A-16H-CC (143.75 mbsf) and 17H-CC (153.47). It is curious that Subzone SN12a, which is defined as the interval in which the ranges of G. puncticulata and G. pliozea overlap, was not recognized over the same depth interval in the other holes at Site 1170. This inconsistency is attributed to the rarity of G. pliozea.
The uppermost zone of the upper Miocene, the Globorotalia conomiozea Zone (SN11), is delimited by the lower portion of the nominate taxon's stratigraphic range that predates the FO of G. puncticulata. The lowermost core catcher containing typical populations of G. conomiozea was determined to be Sample 189-1170A-18H-CC (163.70 mbsf). The next sample down examined (189-1170A-19X-CC, 167.13 mbsf) contains no specimens of G. conomiozea, but abundant Paragloborotalia continuosa, indicating Zone SN9. This pattern of faunal succession suggests that the base of Zone SN11 is truncated by a hiatus and/or that the 3.43-m section separating Samples 189-1170A-18H-CC and 19X-CC is stratigraphically condensed. The Globorotalia miotumida Zone (SN10), as defined as the stratigraphic gap between the ranges of P. continuosa and G. conomiozea, was not recognized. In fact, Zone SN10 was not identified in any of the holes at Site 1170. A similar, but stratigraphically more extensive, hiatus was documented at Site 1169.
The upper portion of the P. continuosa Zone is most likely truncated by the hiatus at the Miocene/Pliocene boundary as well. The base of this zone, which is defined by the LO of Paragloborotalia nympha (10.10 Ma), is confined to the interval between Samples 189-1170A-19X-CC (167.13 mbsf) and 20X-CC (174.54 mbsf).
The downhole stratigraphic sequence continues with presence of the P. nympha Zone (SN8), which straddles the upper/middle Miocene boundary. The base of Zone SN8 is demarcated by the LO of Paragloborotalia mayeri (11.4 Ma). This datum has been constrained to the interval bounded by Samples 189-1170A-21X-CC (180.91 mbsf) and 22X-CC (192.05 mbsf). As at Site 1168, assemblages spanning the upper Miocene to Pliocene are dominated by temperate species belonging to the G. conomiozea lineage, as well as the Globoturborotalita woodi/Globoturborotalita decoraperta/Globoturborotalita apertura plexus.
The P. nympha Zone (SN8) is followed by the P. mayeri Zone (SN7). The base of Zone SN7 is defined by the FO of P. mayeri (12.1 Ma). Sediments within the interval spanning Samples 189-1170A-22X-CC and 29X-CC are assigned to Zone SN7. Given that the P. mayeri Zone encompasses only 700 k.y., its thickness (~80 m) at Site 1170 is remarkable.
The Orbulina suturalis Zone (SN6) is defined as the interval that contains the nominate taxon but predates the FO of P. mayeri. The base of Zone SN6 is delineated by the FO of O. suturalis (15.1 Ma). The O. suturalis Zone was not recognized at Site 1170, which differs from the more temperate Site 1168 where this same zone was nearly 47 m thick. The absence of Zone SN6 at Site 1170 suggests the presence of a hiatus or a sharp reduction in sedimentation rates so that the O. suturalis Zone is confined within a single core and therefore not sampled. Alternatively, the paleogeographic position of Site 1170 may have been at, or near, the edge of the biogeographic range of O. suturalis; therefore, Zone SN6 is not well represented at this location.
The Praeorbulina curva Zone (SN5) was identified only in Sample 189-1170A-30X-CC, so its maximum thickness cannot exceed 22.3 m. This biozone (SN5) was not recognized at Site 1168, yet another difference between Site 1170 and the more northerly Site 1168. The lower/middle Miocene boundary coincides with the base of Zone SN5, which, in turn, is denoted by the FO of P. curva (16.30 Ma).
The uppermost part of the lower Miocene appears to be an expanded sequence as reflected by the estimated thickness (57.03 m) of the Globigerinoides trilobus Zone (SN4). The lower boundary of Zone SN4 is defined by the FO of its nominate taxon (18.80 Ma). This datum is placed between Samples 189-1170A-36X-CC (327.22 mbsf) and 37X-CC (338.80 mbsf).
The Globoturborotalita connecta Zone (SN3) is defined as that part of the nominate taxon's range, which predates the FO of G. trilobus. Thus, the base of Zone SN3 is marked by the FO of G. connecta (20.90 Ma). This datum has been constrained to the interval embraced by Samples 189-1170A-37X-CC (338.80 mbsf) and 38X-CC (347.18 mbsf).
The downhole succession of early Miocene biozones continues uninterrupted with the identification of the G. woodi Zone (SN2) in Sample 189-1170A-38X-CC. Zone SN2 is defined as the interval in which G. woodi is present prior to the FO of G. connecta. The lower boundary of the G. woodi Zone (22.60 Ma) is therefore restricted between Samples 189-1170A-38X-CC (347.18 mbsf) and 39X-CC (356.53 mbsf).
The Globoquadrina dehiscens Zone (SN1) is the lowermost biozone of the early Miocene. The base of Zone SN1 is by definition the FO of G. dehiscens (23.20 Ma). For practical purposes, this datum is also used to approximate the Oligocene/Miocene boundary. The lowermost occurrence of G. dehiscens was observed in Sample 189-1170A-41X-CC, thereby confining this datum between Samples 189-1170A-41X-CC (376.53 mbsf) and 42X-CC (387.17 mbsf).
The presence of the Turborotalia euapertura Subzone (SP14b), the uppermost biozone of the late Oligocene, suggests that the transition from the lower Miocene to the upper Oligocene is relatively complete. Foraminiferal assemblages lacking both G. dehiscens and Chiloguembelina cubensis in Samples 189-1170A-42X-CC (387.17 mbsf) to 44X-CC (405.83 mbsf) are indicative of this gap zone. The presence of Subzone SP14b constrains the age of this series of samples from 23.20 to 28.50 Ma, so the actual completeness of the uppermost Oligocene is difficult to assess.
It is noteworthy that, at Site 1170, Subzone SP14b is very thin in comparison with Site 1168. Furthermore, Subzone SP14b directly overlies Zone SP13. The absence of the C. cubensis Subzone (SP14a) is consistent with the presence of a significant hiatus separating the upper from the lower Oligocene. The moderately diverse assemblages are well preserved, with little or no evidence of secondary infilling.
Relative to the late Oligocene, the early Oligocene faunas at Site 1170 are marked by a reduction in diversity. The C. cubensis Subzone (SP14a) straddles the boundary between the lower and upper Oligocene. It is defined as the stratigraphic interval below the LO of C. cubensis (28.5 Ma), but above the LO of Subbotina angiporoides (30.0 Ma). However, at Site 1170, C. cubensis was invariably found to coexist with S. angiporoides, making Subzone SP14a unrecognizable. This faunal pattern, combined with the observed thinness of Subzone SP14b, suggests the presence of a hiatus separating the upper lower Oligocene from the lower upper Oligocene.
The upper boundary of the S. angiporoides Zone (SP13) is delimited by the LO of the nominate taxon. The highest stratigraphic occurrence of S. angiporoides is in Sample 189-1170A-45X-CC (410.05 mbsf), marking the top of the lower Oligocene at this site. Thus, sediments assigned to the late Oligocene Subzone SP14b unconformably overlie those of the early Oligocene Zone SP13. Specimens ascribed to the taxon S. angiporoides range down to Sample 189-1170A-51X-CC. Hole 1170A terminated at 464.27 mbsf. This sample (189-1170A-52X-CC) contained no S. angiporoides but was still assigned to Zone SP13 based on the presence of Globigerina labiacrassata, a species that has not been recorded below Zone SP13 elsewhere (Hornibrook et al., 1989).
Coring in Hole 1170D commenced within the S. angiporoides Zone (~427.27 mbsf) and overlaps with the lower Oligocene record recovered at the bottom of Hole 1170A. The S. angiporoides Zone ranges down into Sample 189-1170D-5R-CC (464.21 mbsf). The stratigraphic interval spanning Samples 189-1170D-6R-CC to 14R-CC is barren of planktonic foraminifers. Calcareous nannofossil assemblages indicate that the Eocene/Oligocene boundary falls within this same barren interval (472.72-548.46 mbsf). This would account for the absence of the Subbotina brevis Zone (SP12) at Site 1170. Zone SP12 straddles the Eocene/Oligocene boundary. Confident delineation of Zone SP12 is further hampered by the nondescript shell morphology and general paucity of S. brevis in the region. Thus, planktonic foraminifers were of little value to studying the Eocene/Oligocene boundary at Site 1170.
Much of the middle and upper Eocene is characterized by poor preservation and a general scarcity of planktonic foraminifers. The sporadic record appears to be largely a function of paleoenvironmental conditions. Only rare specimens of Subbotina linaperta and S. angiporoides are found in Samples 189-1170D-15R-CC (558.49 mbsf) and 17R-CC (577.77 mbsf). The LO of S. linaperta has been dated at 37.7 Ma (Berggren et al., 1995), although Hornibrook et al. (1989) report this species as ranging up to the topmost Eocene. In either case, the presence of S. linaperta is clear evidence for assigning these samples to the late Eocene, tentatively to Zone SP11.
Sample 189-1170D-18R-CC (585.10 mbsf) contains Acarinina aculeata, which ranges from the top of Zone SP10 to within Zone SP9; this supports the notion that the section immediately below the upper barren interval is no older than late middle Eocene. The interval between Sample 189-1170D-20R-CC (606.55 mbsf) and 32R-CC (722.15 mbsf) is barren with the exception of a single specimen of S. linaperta in Sample 189-1170D-28R-CC. Rare specimens of S. linaperta are also found from Samples 189-1170D-33R-CC to 38R-CC. Finally, Sample 189-1170D-37R-CC (768.73 mbsf) was found to contain small specimens of Turborotalia pomeroli, suggesting a maximum age of Zone SP8 (middle Eocene). A striking difference between the middle to upper Eocene assemblages at Site 1170 and those at other Southern Ocean locales is the conspicuous absence of Globigerinatheka index. As with the lower Oligocene, the low-diversity assemblages are only moderately preserved, showing signs of recrystallization in the lowermost samples.
Benthic foraminiferal assemblages at this site change dramatically between the shallow-water Eocene and the deeper Oligocene and Neogene (Fig. F13). This transition in paleodepth, from neritic to bathyal/abyssal, coincides with a sequence of glauconitic siltstone at the Eocene/Oligocene boundary, which is devoid of benthic foraminifers. Diatoms from this sequence, however, indicate neritic depths also for the bottom part of this interval and open-ocean conditions for the topmost part (see "Diatoms, Silicoflagellates, and Sponge Spicules"). This indicates that subsidence at this site is a much more rapid event than at Site 1168. Whereas the Neogene assemblages at both of these sites are rather similar, there are clearly strong differences in the Paleogene. At Site 1168, the Oligocene assemblages exhibit distinctly changing depth zonations and clearly were still shallower than the Neogene assemblages; whereas at Site 1170, the Oligocene assemblages are similar to those of the lower Neogene and are inferred to represent similar water depths.
The Eocene exhibits a transition from less restricted faunas, which are represented by a diverse assemblage of calcareous (e.g., Lenticulina, Vaginulina, Guttulina, Cibicidoides, and Bulimina spp.) and agglutinating species (Samples 189-1170D-38R-CC to 33R-CC) via a nondiverse, mostly agglutinated assemblage (Samples 189-1170D-32R-CC to 19R-CC) to an extremely restricted assemblage of traces of calcareous (e.g., Elphidium and Lenticulina spp.) and agglutinating species (Samples 189-1170D-17R-CC to 12R-CC, plus 10R-CC). Sample 189-1170D-18R-CC forms an exception to this trend with relatively high abundances of nodosariids and lenticulinids. The presence of the extinct species Elphidium saginatum in most of these samples indicates neritic water depths (50-100 m). Both intervals of glauconitic siltstone over the Eocene-Oligocene transition (Samples 189-1170D-11R-CC and 9R-CC to 5R-CC) are devoid of benthic foraminifers. The second, prominent interval of glauconitic siltstone is followed by an open-marine, biogenic carbonate sequence of abundant planktonic and benthic foraminifers of early Oligocene age. Preservation in the Eocene varies. Although the preservation of surface structures, particularly in the calcareous tests may appear good, these are generally only retained as siliceous infillings. Signs of breakage and diagenetic staining are observed.
The top samples in Hole 1170D, the earliest carbonate sequence, are characterized by the presence of Bolivinopsis and Nonion spp., as well as increased numbers of unilocular species (Samples 189-1170D-4R-CC to 1R-CC). The faunal composition, together with the absence of Cibicidoides mundulus, indicates intermediate water depths (~600-1000 m). This observation stands in contrast to the suggested bathyal to abyssal paleodepth for the bottom samples recovered from Hole 1170A, which include C. mundulus. This discrepancy implies that the top section of Hole 1170D predates the bottom section of Hole 1170A. Sediment depth in mbsf, however, suggests an overlap between the two sections. Recovery at both sites during the interval in question is low, and only a high-resolution postcruise study of the recovered sediment will help solve the question of whether there is a faunal transition with a potential gap between the two holes or an overlap with alternating assemblages.
The entire Oligocene sequence in Hole 1170A is marked by high numbers of C. mundulus, suggesting bathyal to abyssal paleodepths (~1500-2000 m) and well-oxygenated bottom waters. In fact, this species is abundant throughout Samples 189-1170A-51X-CC to 16H-CC. Sample 189-1170D-43X-CC and, to a lesser extent, 42X-CC are additionally marked by the presence of Stilostomella abyssorum. The presence of larger numbers of unilocular species in the calcareous sequence at the top of Hole 1170D suggests equally good oxygenation. Preservation in the Oligocene and Neogene sections is generally good.
By the middle Miocene, paleodepths had deepened to 2000-3000 m (upper abyssal). Fontbotia wuellerstorfi is present in Samples 189-1170A-31X-CC (middle Miocene) to 11H-CC (early Pliocene) and only in traces thereafter. The interval 189-1170A-27X-CC to 11H-CC contains abundant Melonis barleeanus and Bolivina huneri, indicating increased organic carbon flux. The remaining Samples 189-1170A-10H-CC to 1H-CC in the Pliocene-Pleistocene are marked by both M. barleeanus and Melonis pompilioides, with additional Chilostomella oolina in Sample 189-1170A-1H-CC, resembling the assemblage pattern at Site 1169. At Site 1169, however, abundances of C. oolina were much higher. Organic carbon flux during this interval is most likely high. The occurrence of Sigmoilopsis schlumbergeri is restricted to Samples 189-1170A-4H-CC to 1H-CC. The general trends in the Neogene sections at Sites 1168 and 1170 show similarities but different timing based on the age-depth models. Detailed study of the sediments has potential in developing regional trends.
Ostracodes were recorded from most core-catcher samples of Hole 1170A, with the exception of Samples 189-1170A-50X-CC to 52X-CC. They are also present in Samples 189-1170D-1R-CC to 3R-CC. In the Eocene section of Hole 1170D, they sporadically are present as trace occurrences. Traces of Bolboformids were found in the middle and upper Miocene. Bolboforma aculeata is the only species to reach significant numbers in one of the samples (189-1170A-25X-CC).
Radiolarians are generally common and well preserved in the Quaternary through Oligocene and generally rare to common in the Eocene, where species cannot be identified because of the strong recrystallization. The faunas at Site 1170 are characterized by the lack of index species of the traditional zonations. Consequently, no standard zonations can be recognized at this site. All datums applied herein were tentatively selected from the species with consistent occurrences at this site. The radiolarian sequence of the site is potentially useful for establishing a new zonation for the subantarctic. The datums, ages, and sample intervals recognized at Site 1170 are shown in Table T7. Selected datums and radiolarian faunas are discussed below.
Samples 189-1170A-1H-CC through 15H-CC, 189-1170B-1H-CC through 15H-CC, and 189-1170C-1H-CC through 15H-CC are assigned to the Quaternary through Pliocene. The base of the Pliocene is generally approximated by the last abundant occurrence (LAO) of Stichocorys delmontensis at 5.18-6.9 Ma.
The upper Miocene Stichocorys peregrina abundance zone was recognized in Samples 189-1170A-15H-CC, 189-1170B-18H-CC, and 189-1170C-19H-CC. The boundary between the upper and middle Miocene is placed between Samples 189-1170A-19X-CC and 20X-CC based on the FO of Dictyophimus splendens at 11.8 Ma. The base of the middle Miocene is approximated between Cores 189-1170A-32X-CC and 36X-CC based on the LO of Cenosphaera coronata at 16.7 Ma (Cenosphaera sp. of Morley and Nigrini, 1995). The early Miocene radiolarian faunas in Hole 1170A are marked by an abundance of C. coronata and Cenosphaera coronataformis. These two species have been reported only from the high-latitude North Pacific (Shilov, 1995). The FO of Cyrtocapsella tetrapera (23.6 Ma) between Samples 189-1170A-41X-CC and 42X-CC approximates the base of the lower Miocene.
Sample 189-1170D-1R-CC is assigned to the Oligocene on the basis of the absence of C. tetrapera (FO at 23.6 Ma) and the presence of its unnamed ancestor. Previous studies have reported no useful datums within the Oligocene in the temperate through antarctic oceans in the Southern Hemisphere so that the Oligocene cannot be divided into early and late Oligocene on radiolarian evidence. However, the abundance of Stylosphaera radiosa may be useful for dividing the Oligocene.
The Oligocene faunas in Hole 1170D are characterized by the presence of Theocorys robusta, Lychnocanoma conica, and Lophophaena tekopua. The Eocene/Oligocene boundary cannot be delineated using radiolarians because of the absence of marker species.
Radiolarians are poorly preserved and sometimes absent in Samples 189-1170D-5R-CC through 28R-CC, which are dated as Eocene age by nannofossils. Radiolarians extracted from Samples 189-1170D-12R-CC through 28R-CC are filled with silica, fragmented, and highly recrystallized. Samples 189-1170D-29R-CC through the deepest one, 38R-CC, are nearly barren. Some samples from the middle Eocene contain early Eocene reworked radiolarians such as Spongatractus balbis with an age range of 49.0-52.9 Ma.
All core-catcher material from Holes 1170A and 1170D was analyzed for diatoms, silicoflagellates, and sponge spicules. Well-preserved, diverse, and abundant diatom assemblages are present in Samples 189-1170A-1H-CC through 11H-CC. Samples 189-1170A-18H-CC through 41X-CC contain generally less abundant and diverse assemblages of moderately to poorly preserved specimens. Within this interval, however, Samples 189-1170A-19X-CC (late Miocene) and 34X-CC (middle Miocene) contain notably abundant and well-preserved diatoms. In addition, abundant well-preserved Oligocene diatoms are present in Samples 189-1170A-42X-CC through 49X-CC. Thin-section analysis of the silicified limestone Sample 189-1170A-50X-CC (see "Lithostratigraphy") revealed few diatoms that could not be identified. The lowermost samples from Hole 1170A (189-1170A-51X-CC and 52X-CC) are barren of diatoms.
For Hole 1170D, Samples 189-1170D-1R-CC through 9R-CC contain few to abundant diatoms, except Sample 189-1170D-6R-CC where diatoms are present only in trace amounts. Samples below Sample 189-1170D-9R-CC generally contain trace to rare diatoms or are barren. Relative abundance data for diatoms, sponge spicules, and silicoflagellates from Hole 1170A are presented in Table T8. The relative abundance of diatoms in Hole 1170D are graphically presented in Figure F14 along with the abundance of the dinocyst Deflandrea phosphoritica group (see "Palynology").
Twenty-one diatom bioevents are recognized in Holes 1170A and 1170D (see Table T9). As many as three bioevents within the same depth interval (e.g., the LO of Thalassiosira complicata [2.5 Ma], Thalassiosira inura [2.5 Ma], and Fragilariopsis weaveri [2.65 Ma] are present between Samples 189-1170A-7H-CC and 8H-CC). Onboard analysis of core material to constrain events was undertaken only for the FO of Cavitatus jouseanus (30.62 Ma), which is placed at 454.43 mbsf within an accuracy of 0.53 m (see Table T9). Reworking is not strongly apparent throughout Holes 1170A and 1170D.
In terms of paleobathymetry, Samples 189-1170A-1H-CC through 46X-CC contain wholly open-ocean floras. Samples 189-1170A-47X-CC through 49X-CC and Samples 189-1170D-1R-CC through 5R-CC contain moderately diverse early Oligocene mixed floras of dominantly open-ocean diatoms with a common component of neritic taxa. Samples 189-1170D-7R-CC through 9R-CC contain a distinctly more neritic flora. Below Sample 189-1170D-9R-CC, diatom abundance markedly falls; however, specimens recovered from these samples likewise suggest neritic water depths for the Eocene at Site 1170. The mixed diatom floras from Samples 189-1170D-5R-CC to 1R-CC suggest a period of deepening in the earliest Oligocene. Fully open-ocean conditions were attained in the late early Oligocene. Neritic diatoms at Site 1170 are considered to be in situ (i.e., not reworked) because of their high relative abundance and mostly good to moderate preservation. Such assemblages include heavily silicified resting spores of Chaetoceros, Xanthiopyxis, and Stephanopyxis plus benthic genera such as Diploneis. Species of the tychopelagic genus Paralia are also present.
Significantly high abundances of Chaetoceros resting spores in Samples 189-1170D-7R through 9R (the highest amounts for this site) strongly suggest enhanced eutrophic conditions for this interval. In a broad sense, the overall diatom signal throughout Hole 1170D positively correlates to that for the diatom-scavenging Deflandrea phosphoritica dinocyst group (Figs. F14, F15; see "Paleoenvironment" for discussion).
Onboard palynological analysis included every fourth core-catcher sample taken from the cores of Hole 1170A and most of the core-catcher samples from Hole 1170D. In Hole 1170A, recovery of palynomorphs is good down to Sample 189-1170A-8H-CC, which is assigned to the lower Pliocene on the basis of calcareous microfossils. Unfortunately, below this horizon samples are palynologically barren. Recovery of palynomorphs is, in general, excellent in Hole 1170D, which is attributed to the middle Eocene to lower Oligocene on the basis of combined biostratigraphic information. Only the uppermost five core-catcher samples of Hole 1170D (189-1170D-1R-CC to 5R-CC) proved to be devoid of acid-resistant organic matter.
Dinoflagellate cysts (dinocysts) are the most abundant palynomorphs in Hole 1170A. Together with the record from Hole 1169A, these occurrences represent the southernmost late Neogene dinocysts found to date. Sporomorphs are also present in Samples 189-1170A-1H-CC and 4H-CC (Table T10). Foraminifer organic linings, common in age-equivalent samples from Holes 1168A and 1169A, are only present in trace abundances in productive samples from Hole 1170A.
Pleistocene/uppermost Pliocene Sample 1169A-1H-CC yields a dinocyst assemblage dominated by Nematosphaeropsis labyrinthea, a cosmopolitan oceanic species. The few coexisting taxa are either indicative of the influence of relatively warm, oligotrophic water masses or of colder and more eutrophic water masses. The latter includes Dalella chathamense, a species endemic to the antarctic region. The underlying lower Pliocene Samples 189-1170A-4H-CC and 8H-CC, in contrast, yield poorly diversified dinocyst assemblages generally indicative of warm oligotrophic surface waters (abundant Impagidinium aculeatum and I. paradoxum). In Sample 189-1170A-8H-CC, some indications of the influence of colder water masses can be found in the occurrence of the cold to temperate Corrudinium harlandii. The range top of Invertocysta tabulata (2.65 Ma) between Samples 189-1170A-1H-CC and 4H-CC may assist the age assessment of this hole (Table T11). The consistent occurrence of C. harlandii in the lower Pliocene in the region (cf. results from Sites 1168 and 1169), but distinct absence in the younger section, may prove to be a stratigraphically and palaeoenvironmentally useful event. The "mixed" assemblages, providing evidence for the varying influence of warmer and colder water masses, indicate a potential for future paleoceanographic studies involving dinocyst analysis; these may indicate former shifts in position of the subtropical convergence.
Palynomorphs are consistently present in great abundance from Sample 189-1170D-6R-CC down. Unfortunately, the uppermost three core-catcher samples are completely barren (189-1170D-1R-CC to 3R-CC), whereas samples from the silicified hard limestone of Samples 189-1170D-4R-CC and 5R-CC were not processed. Available smear slides and thin sections of these samples did not reveal the presence of palynomorphs. This aspect may indicate the inception of influence of well-oxygenated (bottom) water masses responsible for the oxidation of organic matter.
Dinocysts are the most abundant category of palynomorphs in productive samples from Hole 1170D and are assigned to the late middle Eocene to early Oligocene. In addition, pollen and spores, foraminifer organic linings, and acritarchs are present, albeit in low relative abundances, throughout Hole 1170D (Table T11). Palynomorphs are generally well preserved, and dinocyst concentrations are high, possibly in the order of >200,000 cysts/g in most samples. However, recovered assemblages are of relatively low diversity and are usually totally dominated by a single taxon. Terrestrial palynomorphs consistently are present in the middle Paleogene section (but in low relative abundances). The few identified taxa may be compared with established records from New Zealand and Australia and indicate cooling and increasing humidity from the middle middle Eocene into the late Eocene. Moreover, they are comparable to those reported by Mohr (1990) from ~age-equivalent sediments from across Antarctica.
The dinocyst assemblages throughout these sediments are indicative of the middle Eocene to earliest Oligocene (i.e., pre-O1b isotopic event of Zachos et al., 1996) and are composed of a mix of cosmopolitan and endemic taxa, with Enneadocysta partridgei, Deflandrea phosphoritica, allied morphotypes (including the presumed endemic Deflandrea antarctica and, occasionally, Deflandrea cygniformis), and/or Thalassiphora pelagica dominating the assemblages. High abundances of typical high-latitude Eocene representatives of Vozzhennikovia, Alterbidinium, and/or Spinidinium were increasingly found in the younger Eocene samples (cf. Wrenn and Hart, 1988) (Table T11). Such pulses of the latter taxa may indicate stepwise cooling of surface waters toward the Eocene-Oligocene transition.
As may be expected, Hole 1170D assemblages are quite comparable to those reported from the nearby upper middle to lower upper Eocene of Leg 29, Sites 280 and 281, and other sites on the STR (Haskell and Wilson, 1975; Crouch and Hollis, 1996; Truswell, 1997) and of western Tasmania continental margin sites (Truswell, 1997). They are also similar to those recorded from roughly age-equivalent sections from New Zealand (e.g., Wilson, 1985, 1988) and comparable to largely reworked assemblages recorded from presumed Oligocene deposits in the CIROS-1 drill hole, McMurdo Sound, Antarctica (Wilson, 1989; Hannah, 1993). More surprisingly, the Hole 1170D assemblages are virtually identical to the approximate age-equivalent assemblages recorded by Mohr (1990) from Leg 113 drill sites from the opposite side of Antarctica (Weddell Sea) and from the outcrops in the Seymour Island region (Wrenn and Hart, 1988; Mao and Mohr, 1995). Moreover, they are similar to those reported by Goodman and Ford (1983) from the Eocene-Oligocene transitional strata from Leg 71 drill sites (Falkland Plateau). Furthermore, they bear a strong resemblance to coeval assemblages from Northern Hemisphere high-latitude sites like the Greenland Sea (Firth, 1996), Labrador Shelf, and Barents Sea (H. Brinkhuis, pers. observation).
Comparison of the dinocyst distribution in Hole 1170D with previous (Southern Ocean) studies, combined with the biostratigraphic compilation of Raine et al. (1997), indicates that the middle Eocene to lower(most) Oligocene succession is essentially continuous. The few age-diagnostic events in the relevant interval include the top and bottom of the Enneadocysta acme sensu Raine et al. (1997) (Samples 189-1170D-17R-CC and 36R-CC respectively; see Table T11). In addition, the LO of Cerebrocysta bartonensis and the first consistent occurrence of Alterbidinium distinctum appear to be stratigraphically useful events, occurring in Samples 189-1170D-12R-CC and 8R-CC, respectively. Age calibration of these events is, however, poor being based solely on high-latitude nannofossils. Despite this notion of stratigraphic usefulness, dinocyst datums are inconsistent with results from the calcareous nannofossil results in Hole 1170D.
On the basis of calcareous nannofossils, a hiatus of ~3.3 m.y. is inferred between Samples 189-1170D-12R-CC and 13R-CC. No breaks in the palynological record are apparent here; the horizon does coincide with the LO of Pyxidinopsis waipawaense (Sample 189-1170D-13R-CC), but otherwise, no change, either qualitative or quantitative, is recorded between these two samples. The range of P. waipawaense is poorly known; in its type area it first is present near the base of the middle Eocene (~49.5 Ma), whereas its extinction was not recorded (Wilson, 1988). Outside the type area, it has thus far only been recorded from Site 280 from the middle middle Eocene sediment; no calibration is available (Crouch and Hollis, 1996). In contrast to the nannofossils, the dinocysts indicate a nearly complete section from Sample 189-1170D-7R-CC downward. A minor hiatus may be recognized between Samples 189-1170D-6R-CC and 7R-CC, as discussed below.
Because the combined biostratigraphy of Hole 1170D indicates that the recovered succession is not older than ~43 Ma, the occurrences of Wilsonidinium ornatum and Hystrichosphaeridium tubiferum (including H. truswelliae morphotypes) are of interest. The (limited) available data indicate that these taxa have last occurrences at the base of the middle Eocene (at ~49.5 Ma; Wilson, 1988) or in the lower middle Eocene (at ~45 Ma), respectively (Bujak and Mudge, 1994). Occurrences of the former are, hence, taken to indicate a reworking of earliest Eocene materials into the middle Eocene. Records of H. tubiferum may indicate reworking of Upper Cretaceous to lower middle Eocene strata, because it first appears in the Upper Cretaceous. However, it may indicate that the taxon is longer ranging in the Southern Ocean than elsewhere. In addition, scattered occurrences of reworked lower and Upper Cretaceous dinocysts are recorded in samples from Hole 1170D (Table T12).
The combined shipboard biostratigraphies are also not in harmony where they come to the Eocene-Oligocene transition (viz., the interval from Samples 189-1170D-8R-CC to 6R-CC and higher [palynologically barren] samples [Figs. F11, F12]). In this case, potentially stratigraphically useful dinocyst criteria are the consistent occurrence throughout productive samples of Hole 1170D (and range tops) of Enneadocysta partridgei and the morphologically (extremely) similar Areosphaeridium diktyoplokum. It is surmised here that E. partridgei is, in fact, conspecific with A. diktyoplokum, which has a well-calibrated range top of 33.3 Ma (Brinkhuis and Biffi, 1993). Enneadocysta partridgei is here considered to represent a dorso-ventrally compressed variety of the latter (i.e., not partiform, as suggested by Stover and Williams , but "compressed sexiform" as in Areoligera ["Gv"-cysts of Evitt, 1985]). The form appears to reflect an adaptation to conditions at higher latitudes since it also is present in Northern Hemisphere high-latitude Eocene strata (e.g., in the Barents Sea, H. Brinkhuis pers. observation). The present material has E. partridgei as the dominant morphotype. However, consistently, albeit in low abundances, the related taxa A. diktyoplokum and Enneadocysta harrisii are recorded as well. In the youngest productive samples from Hole 1170D, this group is joined by Enneadocysta pectiniforme, yet another related taxon, more typical for the lower Oligocene. The topmost productive sample yields representatives of all these taxa.
Since the LO of A. diktyoplokum is documented to be associated with the O1b isotopic event of Zachos et al. (1996) and calibrated to occur in Chron C13N in central Italy (Brinkhuis and Biffi, 1993), its demise was suggested to be linked to the onset of major antarctic glaciation (e.g., Brinkhuis and Visscher, 1995). The limited available data indicate that the E. partridgei morphotype has the same stratigraphic range as A. diktyoplokum. Reported post-33.3 Ma occurrences may be a result of reworking or should otherwise be suspect. Both bioevents may be useful to discriminate pre-O1b strata from younger sediments. Sample 189-1170D-6R-CC contains a few specimens of E. partridgei, E. pectiniforme, and A. diktyoplokum, which thus suggests either an age not younger than 33.3 Ma or that the specimens are reworked. This age (~33.3 Ma) contrasts with an age of ~31 Ma inferred from nannofossil assemblages in the same sample (see "Calcareous Nannofossils").
There is a minor break in the palynological succession between Samples 189-1170D-6R-CC and 7R-CC. Vozzhennikovia spp. and Spinidinium macmurdoense, frequent in Sample 189-1170D-7R-CC, are not as abundant in Sample 189-1170D-6R-CC, whereas Stoveracysta kakanuiensis first appears in the latter sample. Stoveracysta kakanuiensis was described from the lower Oligocene of New Zealand (Clowes, 1985) but was suggested to range from the upper Eocene to lower Oligocene.
In summary, the present palynological information is difficult to reconcile with a presumed ~3-m.y. hiatus (including the marked cooling event of O1b over the Eocene-Oligocene transition). Furthermore, in sites around the Southern Ocean, where both nannofossil and dinocyst biostratigraphic information is available, the former consistently indicates an early Oligocene (implying post-O1b) age, whereas the palynology, including records of A. diktyoplokum and E. partridgei, would indicate an earliest Oligocene age at the youngest (cf. Goodman and Ford, 1983, with Wise, 1983, from Leg 29 and Mohr, 1990, with Wei and Wise, 1990, from Leg 113). Moreover, in all of these samples, and in other Southern Ocean cases, the overlying samples are conspicuously barren of palynomorphs. It is proposed here that this absence of (acid resistant) organic matter is the result of the effects of well-oxygenated (bottom) waters associated with the development of the Antarctic Circumpolar Current, the cryosphere ("Icehouse" conditions), and the psychrosphere.
The quantitative dinocyst distribution in the Eocene generally indicates somewhat restricted, eutrophic, neritic conditions throughout the succession. A generally eutrophic, restricted setting (possibly related to freshwater influences and/or coastal upwelling) may be indicated by (1) the relatively low species diversity (for middle Eocene times), (2) the high dominance of a single taxon in most samples, (3) the frequent dominance of peridinioid dinocysts like Deflandrea phosphoritica, Vozzhennikovia spp. (considered to represent mainly heterotrophic dinoflagellates; Brinkhuis et al., 1992), and (4) the near absence of typically open-marine coastal/neritic cysts like Glaphyrocysta spp., Cordosphaeridium spp., and so forth. The few open-marine taxa such as Impagidinium and Spiniferites spp. are quite rare.
From the combined onboard biostratigraphy, it appears that the succession below ~520 mbsf (from approximately Sample 189-1170D-12R-CC down) is unequivocally continuous and represents the interval between ~40/41? to 43 Ma. On the basis of the available core-catcher samples, an alternating pattern in the dominances of E. partridgei and D. phosphoritica group and common T. pelagica is apparent in this interval (Fig. F15). Interestingly, cross-correlation with the sedimentological (smear slide) data and the diatom, radiolarian, and calcareous nannofossil distribution suggests (1) a positive correlation between maxima in Deflandrea spp. with diatom maxima (see Figs. F14, F15) and most radiolarian maxima and (2) a positive correlation between maxima in calcareous nannoplankton content and E. partridgei maxima. Optima in T. pelagica immediately follow an E. partridgei maximum.
In addition, the Peridinioid/Gonyaulacoid (P/G) ratio (numbers of peridinioid cysts/numbers of gonyaulacoid cysts; not shown) mimics to a large extent the Deflandrea spp. percentage curve (Figs. F14, F15). The P/G ratio may be regarded to reflect the relative abundance of presumed heterotrophic dinoflagellates vs. the phototrophic ones: Extant Protoperidinium spp. feed primarily on diatoms; only few extant peridinioids have phototrophic lifestyles. The broadly positive correlation between the Deflandrea percentage maxima with the diatom abundance appears to support such a relationship. A similar result was obtained by Firth (1996), integrating dinocyst and diatom information from Hole 913B (Leg 151, Greenland Sea). In addition, all, except the lowermost, optima in radiolarian abundances also broadly correlate to these horizons (Fig. F15). Many representatives of this group also feed on diatoms. Most diatoms are noncolonial forms (C. Stickley, pers. comm., 2000).
The "E. partridgei-calcareous nannofossil" correlation may possibly be understood in terms of less eutrophic conditions, contrasting with highly eutrophic conditions associated with diatom and peridinioid maxima. Thalassiphora pelagica may be placed in an intermediate position in this scenario. There is evidence that T. pelagica represents dinoflagellates tolerant of strongly eutrophic conditions, often being found in conjunction with dysoxic and anoxic conditions ("black shales"; Köthe, 1990; Vonhof et al., 2000).
The record is likely to have a "third order" (~400-200 k.y.) resolution; the initial work demonstrates the potential for high-resolution quantitative palynology toward the paleoenvironmental interpretations of the middle to late Eocene of Hole 1170D and associated correlative strata in the area. Strong cyclicities are recorded as well in the physical properties record of this interval of Hole 1170D; these may well be compared and integrated with a future quantitative palynological record.
The combined microfossil biostratigraphy at Site 1170 yielded 71 bioevents with age significance. Principal trends through this section are shown in Figure F10. Datums are from the combined microfossil bioevents from Holes 1170A and 1170D and 37 magnetic polarity datums (see "Paleomagnetism"). The bioevents (Table T13) are comprised of 31 FO events, 39 LO events, and abundance events. All events are plotted according to their observed depths at Site 1170 and by their ages as defined in "Biostratigraphy" in the "Explanatory Notes" chapter. The FO events may have been placed too shallow and the LO events too deep because of the limited sampling interval. The stratigraphic positions of these datums may be refined with further study. More detailed bioevent data is provided in individual microfossil group discussions.
Sedimentation at Site 1171 is punctuated by several hiatuses. Five clear hiatuses or condensed sections were encountered in the Neogene portion, and two possible hiatuses were seen in the Paleogene portion. The first 165 mbsf are well constrained by 29 bioevents and 19 magnetic polarity events. The average sedimentation rate is 2.6 cm/k.y. from the Pleistocene to the early late Miocene. Neogene event levels between microfossil groups agree fairly well, taking into consideration the large sampling interval. There are seven microfossil bioevents from three microfossil groups between 165 to 186 mbsf, indicating a condensed section in the upper Miocene. A condensed section was also present at Site 1169 in the upper Miocene and may represent a regional event.
High sedimentation rates (11.7 cm/k.y.) in the upper middle Miocene are truncated by a brief hiatus (~60 k.y.) marked by five closely spaced bioevents. Sedimentation through the remainder of the Miocene ranges from 1.0-2.6 cm/k.y. The sedimentation curve was based on the close agreement of 23 diatom, radiolarian, foraminifer, and paleomagnetic datums.
The Oligocene/Miocene boundary is marked by a 1-m.y. hiatus. The upper Oligocene is marked by a low sedimentation rate (0.88 cm/k.y.). A condensed section/hiatus encompasses the lower Oligocene/middle Oligocene boundary and is delineated by seven bioevents from three microfossil groups.
Sedimentation patterns from the lower Oligocene through the middle Eocene at the bottom of the hole have been interpreted in two ways. One interpretation (Figs. F11, F12) has two hiatuses, one within glauconitic clayey siltstones (lithostratigraphic Unit IV; see "Lithostratigraphy") across the Eocene/Oligocene boundary and one in the middle Eocene. The Eocene/Oligocene boundary interpretation is discussed in "Calcareous Nannofossils" and "Palynology". The middle Eocene hiatus is placed between Cores 189-1170D-12R to 14R based on calcareous nannofossil and dinocyst biostratigraphy. This hiatus corresponds to the subdivision between lithostratigraphic Subunits VA and VB (see "Lithostratigraphy"), and changes in physical properties are also seen in this interval (see "Physical Properties"). Sedimentation rates, based on the nannofossil interpretation, through the middle Eocene section are ~18.7 cm/k.y. The second interpretation of the dinoflagellate cyst biostratigraphy indicates a fairly continuous sequence through the lower Oligocene to middle Eocene (Figs. F11, F12). Sedimentation rates through the late Eocene-early Oligocene are 1.2 cm/k.y., increasing to 6.0 cm/k.y. in the middle Eocene (Fig. F10).