13C-excursion,
marking the base of the Eocene (e.g., Bujak and Brinkhuis, 1998).Sediments ranging in age from late Paleocene (~56 Ma) to Quaternary were recovered at Site 1171. Triple APC coring of a relatively undisturbed upper Neogene sequence made it possible to construct a composite stratigraphy using biostratigraphic and physical properties data. Thus, stratigraphic coverage and resolution was greatly enhanced for Cores 189-1171A-1H through 14H. The age-depth model used to calculate sedimentation rates (Fig. F19) is plotted in meters composite depth (mcd).
The Neogene sequence is largely complete except for a hiatus at the Miocene/Pliocene boundary. The hiatus is evidenced by the absence of several planktonic foraminiferal zones, which indicate that the lowermost Pliocene and much of the upper Miocene is missing at Site 1171 (see "Planktonic Foraminifers"). A similar hiatus was identified across the Miocene/Pliocene boundary at Sites 1169 and 1170, but not at Site 1168. The lower upper and upper middle Miocene interval, by contrast, appears to be expanded, showing sedimentation rates approaching 4 cm/k.y.
Although sedimentation rates decrease, the biostratigraphic succession continues relatively uninterrupted across the Oligocene/Miocene boundary. Another hiatus is inferred farther downsection at the base of the upper Oligocene (~28.5 Ma). In all likelihood, the uppermost part of the lower Oligocene has been truncated as well (see "Planktonic Foraminifers"). Multiple lines of biostratigraphic evidence (planktonic foraminifers, calcareous nannofossils, and diatoms) corroborate the presence of a hiatus at the upper/lower Oligocene boundary (see Fig. F20).
The completeness of the Eocene-Oligocene transition, as recorded at Site 1171, remains unresolved. There is a sharp sedimentological break in Section 189-1171D-3R-3, 28 cm, which juxtaposes limestone and green siliciclastic sediments (see "Lithostratigraphy"). This abrupt lithologic change is well expressed as a precipitous decline in the percent CaCO3 record (see Fig. F30). Calcareous nannofossils from near the base of the overlying limestone (Sample 189-1171D-3R-3, 20 cm) yield a maximum age of ~32.3 Ma, placing the limestone firmly within the early Oligocene (see "Calcareous Nannofossils"). Furthermore, the stratigraphic ranges of three calcareous nannofossil taxa terminate within a relatively short interval just below the limestone (Samples 189-1171D-3R-3, 20 cm, and 3R-CC), suggesting that the sequence is condensed and/or the presence of a hiatus.
However, examination of smear slides taken from the upper meter of "green sands," directly beneath the limestone, revealed this interval to be barren of calcareous nannofossils. Study of the associated core-catcher sample (189-1171D-3R-CC) yielded a suite of palynomorphs containing the dinocysts Areophaeridium diktyoplokum and Enneadocysta partridgei. This association of dinocysts constrains the age of the core-catcher sample between 33 to 37 Ma (see "Palynology"). This finding has added significance because the last occurrence (LO) of A. diktyoplokum demarcates the Eocene/Oligocene boundary (Brinkhuis and Biffi, 1993). Sample 189-1171D-2R-CC was found to be barren of palynomorphs. Thus, the actual position of this important datum (the LO of A. diktyoplokum) is not precisely known but must fall somewhere between Sample 189-1171D-3R-CC and the abrupt lithologic change at Sample 189-1171D-3R-3, 28 cm. The precise stratigraphic position of the Eocene/Oligocene boundary and whether the upper stratigraphic range of A. diktyoplokum is truncated by the base of the upper Oligocene limestone needs further investigation.
Sedimentation resumes temporarily in the upper middle Eocene, but it is possible that yet another hiatus is present within the middle middle Eocene. This may be the oldest in a series of closely spaced unconformities and its presence is indicated by the coincidence of several calcareous nannofossil datums (see "Calcareous Nannofossils"). An alternative interpretation can be formulated using a biochronology based upon the middle Eocene palynological record (see "Palynology"). The sedimentation model yielded by dinocyst datums is equally viable and indicates that sedimentation resumes in the upper middle Eocene and continues uninterrupted until the lower middle Eocene. Finally, several lines of evidence (palynological, physical properties, organic geochemistry, and magnetostratigraphy) suggest the presence of a stratigraphic gap within the lower middle Eocene as well.
The lowermost part of the recovered section (Samples 189-1171D-70R-CC through 75R-CC) contains rare to few calcareous nannofossils and is barren of planktonic foraminifers, radiolarians, and diatoms. No age-diagnostic nannofossil taxa are present in this interval, although there are rare, possibly reworked, Paleocene taxa within the depauperate nannofloras. In contrast, the stratigraphic succession of palynological datums and bioevents observed over this same interval clearly indicate that sediments below Sample 189-1171D-73R-CC are of late Paleocene age. There is marked shift in the composition of these palynological assemblages between Samples 189-1171D-72R-CC and 74R-CC, suggesting that there may be a hiatus at, or near, the Paleocene/Eocene boundary.
Calcareous nannofossils and planktonic foraminifers dominate the Oligocene and Neogene carbonate sequences recovered at Site 1171. Siliceous microfossil groups (i.e., radiolarians and diatoms) are relatively abundant and well preserved throughout much of the same ~272 m of carbonates. Benthic foraminifers are well preserved throughout this interval but are sparse relative to their planktonic counterparts. Benthic foraminiferal faunas are characterized as having affinities for bathyal to abyssal water depths, reflecting an open-ocean environment with well-ventilated bottom waters. The Oligocene to Quaternary section is largely devoid of dinoflagellate cysts.
The thick (~670 m) Eocene sequence recovered at Site 1171 contrasts starkly with the younger carbonate sequences. The smectite-enriched glauconitic siltstones of the Eocene not only differ sedimentologically from the younger carbonates, but paleontologically as well. The Eocene sequence is punctuated by numerous intervals barren of calcareous microfossils. This holds true particularly for the planktonic foraminifers, which are absent across the Eocene-Oligocene transition. Siliceous microfossils are rare to absent throughout much of the Eocene as well, although benthic and neritic planktonic diatoms are common in the upper part of the Eocene section. Benthic foraminifers become significant contributors to sediment production in the Eocene, composing up to 50% of the >125-µm-size fraction in some samples. Another striking difference is that palynomorphs are very abundant throughout the Eocene, providing much needed biochronological control.
In general, the microfossil groups (planktonic foraminifers, radiolarians, dinocysts, diatoms, and calcareous nannofossils) are dominated by temperate to subantarctic species. Given that Site 1171 is south of the modern Subtropical Front and is the most southerly location drilled during Leg 189, this observation is expected. The decreases in abundance recorded in both the calcareous and siliceous microfossil groups in the Eocene sequence appear to have been caused by harsh paleoenvironmental conditions, rather than poor preservation. This interpretation is supported by various lines of independent evidence. Both the diatom floras and benthic foraminiferal faunas indicate that Eocene sediments were deposited in a neritic setting. This paleoenvironmental signal is consistent with the dinocyst biofacies. The palynological evidence also indicates that the Eocene "green sands" were deposited in a eutrophic, neritic environment (see "Palynology"). Furthermore, planktonic foraminifers, although rare in Eocene sediments, do not exhibit conspicuous signs of shell dissolution and/or fragmentation. Thus, it appears that marginal marine conditions, stemming from local and global variations in tectono-eustasy, prevailed during the Eocene.
All core-catcher samples, plus additional samples from some critical intervals, were examined for calcareous nannofossils at Site 1171. Calcareous nannofossil abundance decreases downcore with many barren samples below 770 mbsf (Table T3). Preservation is generally good to moderate. Nannofossil diversity is lower at Site 1171 than previous sites cored during Leg 189, reflecting its higher latitude. Some calcareous nannofossil zones/subzones could not be differentiated because of the lack of marker species. The nannofossil biostratigraphy for the middle Eocene through the Pleistocene presented in Table T4 was based on all the nannofossil bioevents recognized. Holes 1171A, 1171B, and 1171C had excellent core recovery and provide an overlap record down to ~11 Ma (Fig. F21).
The first occurrence (FO) of Emiliania huxleyi was not recorded in the first core-catcher sample at any of the three holes, indicating an age of older than 0.26 Ma for these samples. The LO of Pseudoemiliania lacunosa (0.46 Ma) was recognized, which helps subdivide the Pleistocene epoch (see Table T5 for occurrence intervals in Holes 1171A, 1171B, and 1171C).
The rarity of discoasters in all holes prevented the recognition of several subzones and necessitated the use of alternative markers to recognize epoch boundaries. The LO of Calcidiscus macintyrei (1.67 Ma) is used to approximate the Pliocene/Pleistocene boundary in the absence of Discoaster brouweri. The LO of Reticulofenestra psuedoumbilica (and its cooler water form Reticulofenestra gelida) (3.65 Ma) is used to mark the lower/upper Pliocene boundary.
The traditional marker for the Pliocene/Miocene boundary is Discoaster quinqueramus, which was not observed at this site. The LO of Triquetrorhabdulus rugosus (5.3 Ma) is used to approximate this boundary. The middle/late Miocene marker Discoaster hamatus was not encountered at any of the Leg 189 sites. The LO of Coccolithus miopelagicus (11.0 Ma) and the LO of Cyclicargolithus floridanus (11.9 Ma) are used to here to bracket the boundary.
The FO of Calcidiscus premacintyrei (17.4 Ma) is located between Samples 189-1171C-22X-CC and 23X-CC. The acme of Cyclicargolithus abisectus, observed between Samples 189-1171C-27X-CC and 28X-CC, marks the beginning of Subzone CN1b in the earliest Miocene (~23.2 Ma).
The LO of Reticulofenestra bisecta (23.9 Ma) was recognized between Samples 189-1171C-28X-CC and 29X-CC. The Oligocene/Miocene boundary is generally placed just above this datum. Another Oligocene nannofossil datum, the LO of Chiasmolithus altus (26.1 Ma), is one core lower between Samples 189-1171C-29X-CC and 30X-CC.
The first core from Hole 1171D stratigraphically overlaps with Core 189-1171C-29X. Both core-catcher samples contain similar nannofossil assemblages with an age range of 23.9-26.1 Ma. The LO of C. altus is located between Samples 189-1171D-2R-CC and 3R-CC.
Three nannofossil datums are truncated between Samples 189-1171D-3R-3, 20 cm, and 3R-CC. These datums are the LO of Reticulofenestra umbilica, Isthmolithus recurvus, and Reticulofenestra oamaruensis, indicating a hiatus that correlates to at least 32.3 to 33.7 Ma (Fig. F19) (i.e., the uppermost Eocene to the lowermost Oligocene is missing; Fig. F21).
Isthmolithus recurvus is present down to Sample 189-1171D-4R-CC and thus suggests an age of 36.0 Ma between Samples 189-1171D-4R-CC and 5R-CC. Both I. recurvus and R. oamaruensis, which are excellent stratigraphic markers in southern high latitudes, were easily recognized at Site 1171 but are missing at the previous Site 1170. This is most likely the result of missing more sections at Site 1170 than at Site 1171 over the Eocene-Oligocene transition.
Similar to Site 1170, the LO of Chiasmolithus solitus (40.4 Ma) coincides with the FO of Reticulofenestra reticulata (42.0 Ma) between Samples 189-1171D-6R-CC and 7R-1, 25 cm. A hiatus is therefore indicated here (Fig. F21). We note that this same middle Eocene hiatus at Site 1170 was called into question because of the lack of evidence for significant lithologic change. Further study is needed to resolve this issue.
The FO of R. umbilica (43.7 Ma) is located between Samples 189-1171D-26R-CC and 27R-CC. Assuming that the age for this datum is valid at this site, this would suggest a sedimentation rate of >10 cm/k.y. between Cores 189-1171D-7R and 26R.
Discoaster kuepperi was sporadic from Samples 189-1171D-45R-CC to 56R-CC. This suggests an age range of ~48-53 Ma for the samples. Because samples below this stratigraphic interval are generally barren of nannofossils, the true FO of D. kuepperi (~53 Ma) could not be determined.
Prinsius bisulcus and small Reticulofenestra were found in Samples 189-1171D-64R-CC and 67R-CC. The former species is generally limited to the Paleocene, whereas the latter taxon is generally limited to the Eocene or younger sediments. A consistent age cannot be assigned to these samples based on these nannofossil taxa. Furthermore, the co-occurrence of taxa of different ages suggests sediment reworking.
Sediments bearing planktonic foraminifers ranged in age from middle Eocene to Quaternary at Site 1171. The planktonic foraminiferal assemblages are typical of cool temperate to subantarctic regions, although they occasionally exhibit a temperate-water influence. Consequently, the subantarctic zonal scheme of Stott and Kennett (1990) rather than that of Jenkins (1993a, 1993b) is used to discuss the Eocene to lower Oligocene sediments at Site 1171. Most of the Neogene to the late Oligocene biozones are recognized at Site 1171, with the notable exception of the lower Pliocene/upper Miocene boundary where Zones SN10, SN11, and Subzone SN12a are missing. The absence of the late Oligocene Subzone SP14a indicates another stratigraphic break. Beneath this is the early Oligocene Zone AP13, which is followed by an upper to lower Eocene sequence. The sequence, including the hiatuses, is similar to that found at Site 1170.
Planktonic foraminifers provide little biostratigraphic control throughout much of the Paleogene sequence recovered at Site 1171. However, notable exceptions are found in the lower Oligocene and within discrete intervals in the upper Eocene to middle Eocene. In the lower Eocene, planktonic foraminiferal assemblages are often depauperate, but sporadic assemblages are present in the section. Within the Eocene, the assemblages are separated by a series of barren intervals, with all samples barren of planktonic foraminifers below Sample 189-1171D-65R-CC. The lower Eocene at this site contains the most diverse of the Eocene assemblages, but these assemblages are still relatively sparse. Furthermore, evidence derived from other microfossil groups (e.g., dinocysts and calcareous nannofossils) suggests the presence of an unconformity separating the middle and lower Eocene (Samples 189-1171D-32R-CC and 33R-CC). This complication, plus the ecological exclusion of key marker species such as Acarinina primitiva and Globigerinatheka index combined with the many barren intervals, will undoubtedly result in considerable refinement to the Eocene biostratigraphy discussed herein.
Stratigraphic distributions for Oligocene and Neogene species are given in Table T5. The core depths of the various planktonic foraminiferal datums at Site 1171 are given in Table T6. A brief discussion of the salient biostratigraphic findings is provided below.
The base of the Quaternary, as defined by the FO of Globorotalia truncatulinoides, is confined between the two uppermost cores in Holes 1171A and 1171B (Samples 189-1171A-1H-CC to 2H-CC). Curiously, the FO of G. truncatulinoides is not recognized in Hole 1171C (Sample 189-1171C-1H-CC; 9.65 mbsf). This appears to be another instance where the FO of G. truncatulinoides is an unreliable datum in the STR region (for further discussion see "Biostratigraphy" in the "Site 1170" chapter). The well-preserved assemblages are primarily subantarctic in character and dominated by such cool-water species as Globigerina bulloides, Globorotalia crassaformis, Globorotalia inflata, and Neogloboquadrina pachyderma (sinistral).
The late Pliocene Zone SN13, as well as the upper part of Subzone SN12b, are well represented at Site 1171 (see Table T5). The well-preserved faunas are dominated by specimens belonging to the Globorotalia puncticulata/Globorotalia inflata plexus. In contrast, the lower Pliocene (Subzone SN12a) and much of the upper Miocene (Zones SN10 and SN11) appear to be missing. There is a conspicuous break in the stratigraphic sequence at the Miocene/Pliocene boundary (Samples 189-1171A-6H-CC and 7H-CC). Preservation within Samples 189-1171A-6H-CC and 7H-CC deteriorates with increased carbonate dissolution and shell fragmentation.
Subzone SN12b corresponds to a succession ~25 to 30 m thick at Site 1171, which is relatively thin compared to the same subzone at Sites 1168 and 1169. Thus, the lower part of Subzone SN12b may also be missing given the evidence for carbonate dissolution. Subzone SN12b unconformably overlies the early late Miocene Zone SN9, which, in turn, is itself thin (10 to 20 m). The assemblages within Zone SN9 have been strongly affected by dissolution. Given that one of the missing zones (SN10) is by definition a gap zone, it is possible that its absence is, in part, a stratigraphic aberration caused by migratory shifts in the biogeographies of the marker taxa. A conservative estimate of the amount of time missing is ~2.5 m.y. This estimate should be treated as a minimum because it is likely that the uppermost part of the underlying Zone SN9 has also been removed. This same hiatus is present at Sites 1169 and 1170.
Zones SN5 to SN8, which are within the middle Miocene, are recognized with no conspicuous breaks. Planktonic foraminifers are typically abundant throughout the interval bounded by Zones SN7 to SN8. Warm-water species with subtropical affinities (e.g., Globorotalia limbata) are present in low abundances throughout much of Zone SN8 (see Table T5). Many foraminifers show varying degrees of dissolution with taxa, such as Orbulina universa being rare to absent. The lower zones (SN5 and SN6) have better-preserved assemblages, and foraminifers are abundant.
All early Miocene zones (SN1 to SN4) are recognized at Site 1171. Planktonic foraminifers are abundant and well preserved throughout the lower Miocene. However, determination of the base of Zone SN1, which coincides with the Oligocene/Miocene boundary, was problematic. The FO of Globoquadrina dehiscens (23.3 Ma) approximates the Oligocene/Miocene boundary. Unfortunately, the stratigraphic distribution of this marker species is quite erratic at Site 1171. The scarcity of this thermophilic species probably reflects the cold surface waters that bathed Site 1711 during the earliest Miocene. Consequently, the Oligocene/Miocene boundary is tentatively placed between Samples 189-1171C-26X-CC (237.56 mbsf) and 27X-CC (247.33 mbsf).
The transition from the lower Miocene (Zone SN1) to the upper Oligocene (Subzone SP14b) is confined to the interval between Samples 189-1171C-26X-CC (237.56 mbsf) and 27X-CC (247.33 mbsf). Much like at Sites 1168 and 1170, the biostratigraphic succession across the boundary appears to be relatively complete at Site 1171. However, Subzone SP14b is only ~26 m thick, as opposed to nearly 300 m at Site 1168. The presence of a hiatus at the base of Subzone SP14b is indicated by the appearance of Subbotina angiporoides in Sample 189-1171C-30X-CC (272.87 mbsf). Thus, the lower upper Oligocene (Subzone SP14a), which encompasses some 1.5 m.y., is missing at Site 1171. A similar hiatus was also recorded at Site 1170. Thus, Sample 189-1171C-30X-CC, and the bottom of Hole 1171C, is assigned to the early Oligocene Zone AP13. The uppermost core-catcher sample from Hole 1171D, Sample 189-1171D-1R-CC, overlaps with Hole 1171C in the late Oligocene Subzone SP14b.
The lower Oligocene at Site 1171 is much thicker (160 m) than at Site 1170 (40 m). The early Oligocene is represented by the S. angiporoides Zone (AP13), which is bounded by unconformities at Site 1171. The base of Zone AP13 appears to grade down into a barren interval in Sample 189-1171D-3R-CC (271.34 mbsf). The interval from Samples 189-1171D-2R-CC to 3R-CC is also associated with a precipitous decline in overall carbonate content (see "Organic Geochemistry"). In general, the overall character of the lower Oligocene stratigraphic succession, with its numerous barren intervals and with Zone AP13 directly overlying a barren interval, is similar to that at Site 1170. The uppermost occurrence of Chiloguembelina cubensis was in Sample 189-1171C-30X-CC (272.87 mbsf), where it coexists with S. angiporoides, indicating that these sediments belong to Zone AP13. The early Oligocene assemblages are characterized by low (trace) numbers of specimens to abundant planktonic foraminifers in the upper parts.
Much like the section at Site 1170, the Site 1171 Eocene-Oligocene transition (Sample 189-1171D-3R-CC) is barren of planktonic foraminifers. The absence of planktonic foraminifers is believed to be caused by unfavorable ecological conditions, as opposed to poor carbonate preservation. A few samples (Samples 189-1171D-4R-CC, 7R-CC, and 8R-CC) contain Subbotina angiporoides only and so can be no younger than Zone AP13. Sample 189-1171D-4R-CC contains the calcareous nannofossil I. recurvus (see "Calcareous Nannofossils") and is assigned a late Eocene age. Thus, Sample 189-1171D-4R-CC is believed to belong to Zone AP11. Samples 189-1171D-4R-CC, 7R-CC, and 8R-CC are older but still within Zone AP12, above the FO of S. linaperta.
Below the barren Eocene/Oligocene boundary interval, the stratigraphic sequence between Samples 189-1171D-14R-CC to 31R-CC (353.28-533.73 mbsf) is marked by numerous barren intervals that occasionally give way to discrete horizons containing impoverished assemblages. The co-occurrence of Pseudohastigerina micra and Subbotina linaperta, particularly in Sample 189-1171D-31R-CC, constrains the age of this interval to the late middle to early late Eocene (Zones AP9 to AP11).
There are sporadic assemblages between Samples 189-1171D-34R-CC and 46R-CC that contain such species as Acarinina collactea, Acarinina bullbrooki, Acarinina pentacamerata, and, in rare instances, Morozovella spinulosa. Similar faunas have been reported (Huber, 1991; Berggren, 1992) from the Kerguelen Plateau region and assigned to the early middle to late middle Eocene (Zones AP8 to AP10).
As with the rest of the Eocene, planktonic foraminifers are only sporadically present. The section between Samples 189-1171D-47R-CC and 65R-CC is within the stratigraphic range of Globanomalina australiformis, indicating the lower portion of this interval is no older than earliest Eocene (~55.5 Ma). The two lowermost samples (189-1171D-64R-CC and 65R-CC) contain specimens referable to Globanomalina ovalis and G. australiformis, placing these sediments into Zone AP6. Below this level, all samples are barren of planktonic foraminifers. Preservation in the lower Eocene section ranges from poor to moderate.
Similar to Site 1170, benthic foraminiferal assemblages at Site 1171 document a profound change from a shallow-water fauna in the Paleocene-Eocene to a bathyal to abyssal fauna in the Oligocene and Neogene (Fig. F22). The transition from water depths of <100 to >1000 m was accomplished by the end of the early Oligocene. Another aspect that distinguishes the intervals before and following the Eocene/Oligocene boundary is that for most of the Eocene, benthic foraminifers are abundant, accounting for as much as 50% of the material in the >125-µm fraction. In the Oligocene and Neogene sections, however, abundances are very low.
Samples 189-1171D-75R-CC to 70R-CC are dominated by the occurrence of the finely agglutinating Spiroplectammina spp., which is present together with a limited variety of coarsely agglutinating species. This interval includes the Paleocene with its upper limit possibly postdating the Paleocene/Eocene boundary. The lowest interval in the lowermost Eocene (Samples 189-1171D-69R-CC through 64R-CC) is marked by very low abundances of agglutinating foraminifers and occasional traces of calcareous species. The middle lower Eocene is an interval of alternating assemblages, the main feature of which is the presence or absence of Spiroplectammina spp. (Samples 189-1171D-63R-CC through 57R-CC). The remainder of the lower Eocene features low abundances of agglutinating foraminifers, such as Reophax spp. and of some calcareous species (Samples 189-1171D-56R-CC through 48R-CC). Most of the middle Eocene is characterized by highly abundant benthic foraminifers. Calcareous species were clearly dominant during this interval, with Elphidium and Lenticulina spp. being most important during the interval from Sample 189-1171D-47R-CC through 20R-CC. The interval 189-1171D-19R-CC to 7R-CC shows slightly lower abundances and is marked by the additional occurrence of calcareous Nonion deceptrix and agglutinating Eggerelloides spp. Faunal assemblages in the Paleocene and Eocene suggest neritic (~50-100 m) water depths. Preservation is moderate, with most tests showing signs of diagenetic alteration.
Samples 189-1171D-5R-CC and 3R-CC are glauconitic siltstones mainly barren of benthic foraminifers. But within this interval (Sample 189-1171D-4R-CC), rare and mostly calcareous foraminifers are present, suggesting upper- to middle-bathyal water depth (~200-1000 m) based on the presence of Cibicidoides spp. The top samples in Hole 1171D (189-1171D-2R-CC and 1R-CC) and the bottom samples in Hole 1171C (189-1171C-30X-CC through 27X-CC) show increased numbers of Karreriella bradyi, which was not observed at previous sites. The assemblages suggest middle to lower bathyal water depths (~600-1500 m) for the late Oligocene based on the presence of Bulimina and Melonis spp., and lower bathyal to abyssal water depths (~1500-2500 m) during the early Miocene as reflected by the presence of Cibicidoides mundulus and Nuttalides umbonifera.
By the middle Miocene, sediments were being deposited in upper abyssal depths (~2000-3000 m), as suggested by the presence of Epistominella exigua. The entire Neogene and Pleistocene section is marked by low abundances and reduced diversities of benthic foraminifers, when compared to all the previous sites. Both C. mundulus and Fontbotia wuellerstorfi disappear at the lower to upper Pliocene boundary (Sample 189-1171A-5H-CC). Melonis spp. are present in the Neogene, but they are never abundant. The topmost sample (189-1171A-1H-CC) shows traces of Chilostomella oolina, similar to Sites 1169 and 1170. Preservation of tests is generally good in the Neogene and Pleistocene section.
The benthic foraminiferal assemblages suggest that the most significant interval of subsidence occurred during the late Eocene-early Oligocene (i.e., the transition from water depths of <100 m to depths of >1500 m). This period is marked by numerous hiatuses interspersed with intervals of extremely low sedimentation, which makes a precise age control difficult. The lower Miocene might still have been shallower than the middle Miocene at this site, but the deepening to upper abyssal depths could have been concluded by the time of the early early Miocene.
Most samples in the biogenic carbonate sequence (189-1171C-30X-CC to 15X-CC and 189-1171A-14X-CC to 1H-CC) contain a few ostracode carapaces, whereas the Eocene interval is marked by only sporadic traces. Bolboforma spp. were identified in Samples 189-1171C-29X-CC and 21X-CC and were notably abundant in Samples 189-1171A-13X-CC through 11H-CC.
Radiolarians are generally common and well preserved in the Quaternary through lowermost Miocene, generally rare to common and poorly preserved in the Oligocene, whereas the Eocene is barren with a few exceptions. The datum, age, and sample intervals recognized at Site 1171 are shown in Table T7. Some remarks on selected radiolarian events are presented below.
Samples 189-1171A-1H-CC through 3H-CC, 189-1171B-1H-CC through 3H-CC, and 189-1171C-1H-CC through 3H-CC are Quaternary through early Pliocene in age. The boundary between the lower and upper Pliocene is placed between the LO and FO of Pseudocubus vema. The lower Pliocene is recognized in Samples 189-1171A-4H-CC through 6H-CC, 189-1171B-4H-CC through 6H-CC, and 189-1171C-4H-CC. The base of the Pliocene is approximated by the last consistent occurrence (LCO) of Stichocorys delmontensis (5.18-6.9 Ma).
The upper Miocene is recognized in Samples 189-1171A-7H-CC through 11H-CC, 189-1171B-7H-CC through 11H-CC, and 189-1171C-5H-CC through 11H-CC, respectively. Similar to Site 1170, the upper Miocene Stichocorys peregrina abundance zone at Site 1171 is recognized (Samples 189-1171A-7H-CC through 10H-CC, 189-1171B-7H-CC through 10H-CC, and 189-1171C-6H-CC through 9H-CC). Amphymenium challengerae has an age range of 6.8 to 6.1 Ma and is recovered from Sample 189-1171A-7H-CC only. The base of the upper Miocene is placed between Samples 189-1171A-11H-CC and 12H-CC, 189-1171B-11H-CC and 12H-CC, and 189-1171C-11H-CC and 12X-CC, based on the last abundant occurrence (LAO) of Cyrtocapsella japonica, for which a tentative age of 11.6 Ma has been derived from a cursory age-depth model at Site 1170.
The middle Miocene is recognized from Samples 189-1171A-12H-CC through 14X-CC, 189-1171B-12H-CC, and 189-1171C-12X-CC through 20X-CC. The boundary between middle and lower Miocene is placed between Samples 189-1171C-20X-CC and 23X-CC and is characterized by the FO of Lychnocanoma nipponica nipponica at 15.7 Ma and the LO of Cenosphaera coronata at 16.7 Ma. The lower Miocene radiolarian faunas at Site 1171 are marked by abundant C. coronata and Cenosphaera coronataformis. The FO of Cyrtocapsella tetrapera between Samples 189-1171C-25X-CC and 26X-CC and 189-1171D-2R-CC and 3R-CC approximates the base of lower Miocene.
Samples 189-1171C-26X-CC through 28X-CC are correlated to the latest Oligocene because the FO of Cenosphaera coronataformis at 24.4 Ma, calculated from the Site 1170 age-depth model, was recorded in Sample 189-1171C-28X-CC. Samples 189-1171D-1R-CC through 2R-CC are assigned to the early Oligocene as indicated by the presence of Lychnocanoma conica (FO between 32.6 and 33.1 Ma) and Eucyrtidium spinosum (LO between 32.6 and 33.1 Ma) in Sample 189-1171D-3R-CC. Samples 189-1171D-7R-CC through 10R-CC contain many well-preserved radiolarians but lack age-diagnostic radiolarians. Samples 189-1171D-11R-CC through 75R-CC are barren. Poorly preserved radiolarians are common in Samples 189-1171D-11R-CC, 17R-CC, 18R-CC, 23R-CC, 27R-CC, and 28R-CC.
All core-catcher material from Holes 1171A and 1171D, most samples from Holes 1171C, and selected samples from Hole 1171B were analyzed for diatoms, silicoflagellates, and sponge spicules. Smear slides were examined to assess overall relative abundance. For full assemblage analysis, additional material was treated with 40% HCl to remove the carbonate component.
Well-preserved diatoms are abundant throughout Hole 1171A, except for Samples 189-1171A-5H-CC through 7H-CC, which contain a poor to moderately preserved in situ flora along with reworked diatoms of Oligocene (e.g., Rocella vigilans) and middle Miocene (e.g., Actinocyclus ingens var. nodus) age. Reworking of shallow-water sediments (probably from the STR) is also implied in these three samples by the rare occurrences of neritic diatoms such as Paralia. Diversity generally decreases downhole. Likewise, diatoms are mostly abundant in every sample studied from Hole 1171C. Samples 189-1171D-1R-CC through 11R-CC contain common to abundant diatoms (except Sample 189-1171D-5R-CC, where diatoms are rare). Samples 189-1171D-12R-CC through 36R-CC contain trace to few diatoms, and below this level diatoms are mostly absent. The sharp decrease in abundance of diatoms between Samples 189-1171D-11R-CC and 12R-CC is reflected in a sharp decrease in the concentration of pore-water silica in Hole 1171D (see "Inorganic Geochemistry"). Relative abundance data for diatoms, sponge spicules, and silicoflagellates are presented in Tables T8 and T9.
Twenty-six age-diagnostic diatom bioevents are recognized in Holes 1171A, 1171C, and 1171D. These are presented in Table T10. The LO of Actinocyclus ingens var. ovalis (6.27 Ma) is recognized in Hole 1171C between Cores 189-1171C-6H and 7H (at a mean depth of 62.29 mbsf). This datum is difficult to place in Hole 1171A, however. As for Site 1170, the oldest diatom bioevents at Site 1171 are present above the Eocene/Oligocene boundary. At Site 1171, these represent the FO of Rocella vigilans (small form) (30.24 Ma) and the FO of Cavitatus (Synedra) jouseanus (30.62 Ma). Both events are between Samples 189-1171D-2R-CC and 3R-CC. The FO of R. gelida (26.50 Ma) also is between these samples, suggesting a hiatus representing ~4 m.y. across the early/late Oligocene boundary.
Below the Eocene/Oligocene boundary, diatoms are not age diagnostic (i.e., long-ranging taxa are present such as Stephanopyxis turris and benthics such as Actinoptychus). Eocene markers, such as those documented from the high southern latitudes (e.g., Fenner, 1984) and the eastern Indian Ocean (Fourtanier, 1991), are not encountered at Site 1171.
In terms of paleobathymetry, a similar geological setting to that inferred for Site 1170 is inferred at this site. The Eocene is marked by robust, neritic diatoms including eutrophic indicators such as resting spores of Chaetoceros. In the uppermost Eocene (Samples 189-1171D-3R-CC and 4R-CC), there is a low but noteworthy abundance (~<5% of the total assemblage) of more oceanic taxa (e.g., Coscinodiscus). The lowermost Oligocene yields mostly neritic diatoms along with some indicative of an increasingly open marine influence. Diversity also markedly increases in the lowermost Oligocene. Fully open ocean conditions are attained after the inferred early/late Oligocene hiatus. Neritic diatoms in the Eocene and lower Oligocene are considered to be in situ (i.e., not reworked) because of their high abundance and the absence of any other data to support major reworking. However, upper Oligocene through middle Miocene samples (189-1171C-27X-CC through 22X-CC) also contain common neritic diatoms. Reworking is inferred in this case since other data suggest an open marine setting for Site 1171 during this interval (e.g., see "Benthic Foraminifers, Ostracodes, and Bolboforma"). Such reworking is probably associated with rising sea levels. Samples 189-1171C-21X-CC and above contain almost exclusively open-ocean diatoms with only ~5% reworked neritic types (e.g., Paralia sulcata var. crenulata).
The Neogene section is marked largely by an endemic subantarctic flora. However, the common occurrence of warm-water diatoms (mainly Hemidiscus cuneiformis) suggests a warming or the influence of warmer waters during the early late Miocene (Sample 189-1171A-8H-CC) and middle to late Pliocene (Samples 189-1171A-4H-CC through 5H-CC). Assemblage diversity also markedly increases from the lower upper Miocene onward.
Onboard palynological analysis of a selection involved core-catcher samples from Site 1171, comprising a few from Holes 1171A and 1171C and a suite from Hole 1171D. Of the samples from Hole 1171A, only the topmost Sample 189-1171A-1H-CC yields a few (Pleistocene) dinoflagellate cysts, which are dominated by Spiniferites miriabilis. The remainder of the investigated samples from the Neogene and upper Paleogene are palynologically barren. Analysis of samples from Hole 1171C was therefore limited to the presumed Eocene-Oligocene transition and involved Samples 189-1171C-30X-CC and 31X-CC only. Results are similar to the correlative interval of 1171D (i.e., Samples 189-1171D-2R-CC and 3R-CC) (viz., Sample 189-1171C-30X-CC was found to be barren, whereas the palynological composition of Sample 189-1171C-31X-CC is almost identical to Sample 189-1171D-3R-CC; see "Hole 1171D" in "Operations").
Recovery of palynomorphs is generally excellent in Hole 1171D, and they are assigned to the upper Paleocene to lower Oligocene on the basis of combined biostratigraphic information. Only the uppermost two core-catcher samples of Hole 1171D (189-1171D-1R-CC and 2R-CC) proved to be devoid of acid-resistant organic matter.
Palynomorphs are consistently present in great abundance from Sample 189-1171D-3R-CC down. Unfortunately, the uppermost two core-catcher samples are completely barren (189-1171D-1R-CC and 2R-CC). This record is similar to the top of Hole 1170D and, thus, provides further evidence of the inception of the influence of well-oxygenated (bottom) water masses, which are responsible for the oxidation of organic matter in the early Oligocene apparently all along the Antarctic margin.
Dinocysts are generally the most abundant category of palynomorphs in productive samples from Hole 1171D, assigned to the upper Paleocene to lower Oligocene. In addition, pollen, spores, foraminifer organic linings, and acritarchs are present, albeit generally in low relative abundances. Only in the lowermost portion of Hole 1170D, in the upper Paleocene section, are sporomorphs quantitatively significant (Table T11). Paleocene assemblages throughout are indicative of (very) warm climates. The few identified sporomorph 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, as stated for previous sites, they are comparable to those reported by Mohr (1990) from the approximate age-equivalent sediments across Antarctica.
Palynomorphs are generally well preserved and dinocyst concentrations high, possibly on the order of 200,000 cysts/g in many samples, except for the interval between Samples 189-1171D-60R-CC to 50R-CC and in Sample 189-1171D-73R-CC. Recovered dinocyst assemblages are often of relatively low diversity for the Eocene and may be totally dominated by a single taxon. In terms of quantitative distribution patterns, the middle to upper Eocene interval is quite comparable to the correlative succession of Hole 1170D.
The dinocyst assemblages throughout are indicative of the latest Paleocene to earliest Oligocene (i.e., pre-O1b isotopic event of Zachos et al., 1996). Notably the middle and upper Eocene assemblages are composed of a mix of cosmopolitan and endemic taxa, with Enneadocysta partridgei, Deflandrea phosphoritica, and 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, possibly indicative of cool episodes, were found notably in the upper part of Hole 1171D (cf. Wrenn and Hart, 1988) (Table T11). Hence, the middle to upper Eocene interval is quite comparable to the correlative succession of Hole 1170D.
Middle to upper Eocene (and lowermost Oligocene?) Hole 1171D assemblages are quite comparable to those reported from age-equivalent records from nearby sites and even to nearly coeval successions recovered from the other side of Antarctica, as well as those from Northern Hemisphere high-latitude sites (see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter). Little is known from the upper Paleocene and lower Eocene in terms of southern high-latitude dinocysts. Most meaningful information has been derived from the New Zealand sites (e.g., Wilson, 1988). However, these dinocyst successions still await sound chronostratigraphic calibration. The dinocyst events applied are listed in Table T12. For the middle to upper Eocene, the dinocyst datums are reasonably consistent with results from the calcareous nannofossil results in Hole 1171D. For the upper Paleocene to middle middle Eocene interval, dinocysts are the sole source of biostratigraphic information (see also Fig. F19).
Somewhat differently from the situation encountered at Site 1170, the dinocyst and nannofossil biostratigraphies are in harmony for the Eocene-Oligocene transition (viz., the interval from Samples 189-1170D-6R-CC through 3R-CC). In the case of Hole 1171D, nannofossil distribution indicates a major hiatus between the youngest siliciclastic sediments and the inception of silica-rich limestones overlying them. This contrasts to the situation in Hole 1170D, where such a hiatus was inferred to be present, on nannofossil grounds, within the "green sand" (lithostratigraphic Unit III) just below the limestone interval. The youngest palynologically productive sample of Hole 1171D (189-1171D-3R-CC) still contains the duo Areosphaeridium diktyoplokum and Enneadocysta partridgei, as well as Alterbidinium distinctum and Stoveracysta ornata. The co-occurrence of these taxa constrains the age of the sample to 37.0-33.3 Ma. (see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter). Overlying samples are barren, and so no statements can be provided on possible missing sections. However, a conspicuous incoming taxon in the topmost productive sample from Hole 1170D is Stoveracysta kakanuiensis. This species has a FO in the regional New Zealand Runangan Stage (G.J. Wilson, pers. comm., 2000), which is equated to the uppermost part of the late Eocene. Since S. kakanuiensis is absent in the available (core catcher) material from this interval at Hole 1171D, it is possible that the (very) uppermost Eocene-lowermost Oligocene interval is extremely condensed or missing altogether. Obviously, as shown at other sites, the precise positioning of the onboard analyzed samples influences these kind of preliminary interpretations; postcruise work should provide more detail in this matter. Other leg sites consistently show that the "green sand episode" represents ~3 m.y., which may be as condensed as <1 m (e.g., see "Biostratigraphy" in the "Site 1172" chapter and "Palynology" in "Biostratigraphy" in the "Site 1170" chapter).
Comparison of the qualitative and quantitative dinocyst distribution in Sample 189-1171D-3R-CC with the uppermost samples from Hole 1170D would suggest correlation to Samples 189-1170D-7R-CC and/or 8R-CC. This is notable in view of the abundance of Vozzhennikovia spp. in all these samples.
The middle Eocene is recognized at the top by the FO of A. distinctum (between Samples 189-1171D-5R-CC and 4R-CC; ~37 Ma) and at the base by the LO of Charlesdowniea coleothrypta and Charlesdowniea edwardsii (between Samples 189-1171D-44R-CC and 40R-CC; ~48 Ma). Although sample spacing leaves much to be desired in this interval, there is no obvious gap in the succession between Samples 189-1171D-44R-CC and 5R-CC. There may well be a more condensed section and/or a hiatus between Samples 189-1171D-35R-CC and 29R-CC on the basis of the proximity of the LO of Membranophoridium perforatum and the inception of the Enneadocysta acme sensu Raine et al. (1997). The known/inferred ages of these events suggests that the larger part of the lower middle Eocene (Lutetian) interval is missing or condensed (cf. Fig. F19).
The Paleocene/Eocene boundary is recognized between the FO of A. homomorphum (between Samples 189-1171D-74R-CC and 73R-CC; ~56 Ma) and the FO of frequent Deflandrea phosphoritica group (in this case the D. antarctica morphotype, between Samples 189-1171D-73R-CC and 72R-CC; ~55 Ma following Wilson's [1988] correlation). It is believed to coincide with a minor hiatus, discussed below. Another important bioevent is the FO of Dracodinium waipawaense, part of the classic early Eocene Wetzeliella lineage, between Samples 189-1171D-71R-CC and 70R-CC. This event may be placed at ~53.5 Ma (middle early Eocene). The FO of the D. phosphoritica group is somewhat arbitrarily placed, since specimens are present in underlying samples. This is thought to be caused by contamination, as overlying samples yield massive concentrations of this taxon. The dinocyst succession from Samples 189-1171D-75R-CC through 73R-CC shows the established pre-late Paleocene suite from the LO of Spinidinium densispinatum and Cassidium fragile to the FO of A. homomorphum (Wilson, 1988). However, it should be noted that it is conceivable that the entire lower part of Hole 1171D is, in fact, early Eocene in age and that the latter represents reworked specimens.
A possible hiatus is
indicated by the abrupt compositional change of the palynological association
between Samples 189-1171D-74R-CC and 72R-CC, where dominating sporomorphs give
way to dominating dinocysts (Sample 189-1171D-73R-CC has only few palynomorphs).
This hiatus would also support the view that the underlying interval represents
the late Paleocene. In addition, the classic globally identified oldest Apectodinium
acme was not recorded in the available material (or any Apectodinium
acme). This event is indicative for the interval containing the
well-known negative 13C-excursion,
marking the base of the Eocene (e.g., Bujak and Brinkhuis, 1998).
The quantitative dinocyst distribution in the Paleocene-lower Oligocene 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 Eocene times), (2) the high dominance of a single taxon in most samples, (3) the frequent dominance of peridinioid dinocysts like D. phosphoritica and Vozzhennikovia spp. (considered to represent mainly heterotrophic dinoflagellates; Brinkhuis et al., 1992), and (4) the near absence of typically open-marine coastal/neritic cysts such as Glaphyrocysta spp., Cordosphaeridium spp., and so forth. The few open-marine taxa like Impagidinium and Spiniferites spp. are quite rare. For further discussion, see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter because the assemblages are comparable.
The rather abrupt change in palynological associations between Samples 189-1171D-74R-CC and 72R-CC, with downhole dominant sporomorphs, may reflect even more shallow, restricted marine conditions. This would also explain the poor palynological contents of Sample 189-1171D-73R-CC.
The combined nannofossil, foraminifer, diatom, radiolarian, and dinocyst biostratigraphy at Site 1171 yielded 91 bioevents with age significance. Principal trends through this section are shown in Figure F19. Datums are from the combined microfossil bioevents from Holes 1171A, 1171C, 1171D, and 25 magnetic polarity datums (see "Paleomagnetism"). The bioevents (Table T13) consist of 39 FO events, 49 LO events, 1 first abundant occurrence (FAO), and 2 last abundant occurrence (LAO) events. All events are plotted according to their observed composite depths at Site 1171 and by their ages as defined in "Biostratigraphy" in the "Explanatory Notes" chapter. The FO events may have been estimated to be too shallow, and the LO events may have been estimated 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.
A detailed age-depth curve was constructed for the last 12 m.y. at Site 1171 utilizing the composite section (see "Composite Depths") created from the top 14 cores (Fig. F23). Sedimentation rates of 1.3 cm/k.y. through the Pliocene-Pleistocene section are truncated by a hiatus of 1.6 m.y. at the Miocene/Pliocene boundary. The LO of foraminifer Paragloborotalia continuosa (8.0 Ma) at 65.2 mcd indicates a longer hiatus of 2.7 m.y. in duration. Physical properties measurements indicate a break at ~65 mcd of <2 m.y. (see "Physical Properties"). Paragloborotalia continuosa persists across the Zone SN9/SN10 boundary (Kennett and Srinivasan, 1983), and we suggest that the last common occurrence may be a more useful datum. Sedimentation rates for the late Miocene were quite low (0.5 cm/k.y.), increasing to 3.8 cm/k.y. across the middle/late Miocene boundary (10 Ma). Early and middle Miocene sedimentation rates fluctuated between 0.7 to 2.0 cm/k.y at the Oligocene/Miocene boundary.
Combined biostratigraphic datums may indicate up to four brief hiatuses (~2 m.y.) interspersed with brief periods of sedimentation (<1 cm/k.y.) through the late Oligocene to the middle Eocene. Nineteen foraminifer, nannofossil, radiolarian, diatom, and dinoflagellate cyst datums, spanning ~15 m.y., are within ~42 m of sediment (Fig. F19).
Sedimentation rates through the middle Eocene to Paleocene were dramatically higher compared to the Neogene portion of Site 1171, ranging from 4.4-12.5 cm/k.y. The age-depth curve through the bottom 600 m is based primarily on dinoflagellate cyst datums. Nannofossils and foraminifers are present down to ~870 mcd and provide an age datum. Radiolarians are sporadic below 340 mcd, and diatoms are present to 580 mcd but provide environmental data only. Dinoflagellate cyst datums between Cores 189-1171D-29X and 35X (Table T13) indicate a hiatus of ~2 m.y. at ~550 mcd. A spike in the hydrogen index (HI) and the percentage of total organic contents (see "Organic Geochemistry"), along with changes in density measurements from downhole logging (see "Downhole Measurements") indicate the hiatus at ~530 mcd. Paleomagnetic interpretations also indicate a gap in the middle Eocene (see "Paleomagnetism").
Sedimentation patterns from the Pleistocene through the Paleocene have been interpreted on the combined biostratigraphic data supplemented with information from other shipboard laboratories. The Eocene/Oligocene boundary interpretation and the middle Eocene hiatus are discussed in "Organic Geochemistry" and "Paleomagnetism".
