Calcareous nannofossils and planktonic foraminifers indicate an expanded Pleistocene section (>415 m thick) underlain conformably by a 52-m interval that could be either Pleistocene or upper Pliocene. The Pliocene-Pleistocene unit is underlain disconformably by a relatively thin Miocene section. The lowermost core from Hole 1127B yielded lower to middle Miocene nannofossils.
Benthic foraminifers registered two assemblage changes, at ~405 mbsf and at the Pleistocene/Miocene disconformity, but indicate upper bathyal paleodepths throughout. The youngest and oldest assemblages include a redeposited neritic component. A cool temperate Pleistocene planktonic foraminifer fauna is found accompanied by warm-water species at various intervals, probably reflecting global climatic fluctuations and regional paleoceanographic variations, especially in relation to the flow of the Leeuwin Current. In the three groups of microfossils studied, the Pleistocene assemblages are characterized by concentrations of smaller forms. In the case of benthic foraminifers, this indicates sediment transport and redeposition from the shallow shelf to the upper slope.
Sediments recovered at Site 1127 are generally rich in calcareous nannofossils, except for intervals at 289-296 mbsf, 328-337 mbsf, 441.7-453 mbsf, and at 501 mbsf, where the abundance of nannofossils is greatly reduced. Preservation is moderate to good but deteriorates rapidly below the Unit II/Unit III lithostratigraphic boundary at 467-468.5 mbsf (Core 182-1127B-50X). An expanded Pleistocene section (~460 m thick) is indicated by the calcareous nannofossil assemblages recovered from the first 50 cores. A mixed assemblage immediately below the sharp lithostratigraphic contact in Core 182-1127B-50X indicates a relatively thin Miocene section.
Samples 182-1127B-1H-CC, 26-29 cm (5.83 mbsf), through 17X-CC, 23-26 cm (155.20 mbsf), yielded assemblages readily assignable to the combined Zones NN21-NN20. These assemblages contain Gephyrocapsa caribbeanica, Helicosphaera carteri, Helicosphaera hyalina, Calcidiscus leptoporus, and small Gephyrocapsa spp. Emiliania huxleyi, the key species for Zone NN21, was identified down to Sample 182-1127A-2H-CC, 12-15 cm (15.43 mbsf).
Zone NN19 assemblages occur in Samples 182-1127B-18X-CC, 29-32 cm (162.24 mbsf), through 50X-3, 144-146 cm (466.94 mbsf). In addition to the key species Pseudoemiliania lacunosa, these assemblages include Braarudosphaera bigelowii, C. leptoporus, Coccolithus pelagicus, Dictyococcites productus, H. carteri, Scyphosphaera spp., Reticulofenestra minutula, and Reticulofenestra minuta. At several levels, the assemblages are dominated by small taxa, such as Gephyrocapsa spp., R. minuta, and D. productus. Small taxa were also noted for the associated benthic and planktonic foraminifers (see relevant sections below). The highest occurrence of Helicosphaera sellii is recorded in Sample 182-1127B-17X-CC, 23-26 cm (155.20 mbsf), and that of Calcidiscus macintyrei in Sample 182-1127B-38X-CC, 31-34 cm (357.16 mbsf), a depth difference of ~202 m. These two events occur at the same level at Site 1126, where the Pleistocene section is relatively condensed. The downhole appearance of C. macintyrei occurs at 357.16 mbsf.
The Zone NN19 assemblages from Samples 182-1127B-49X-3, 64-68 cm (456.54 mbsf), and 49X-CC, 34-37 cm (459.62 mbsf), are dominated by B. bigelowii, suggesting a short event marking a drastic change in surface-water conditions. Other species present in these assemblages include small Gephyrocapsa spp., C. macintyrei, and D. productus.
Mixed assemblages are recorded from the highly bioturbated Samples 182-1127B-50X-4, 54-56 cm (467.54 mbsf), through 53X-CC, 23-26 cm (493.69 mbsf). These mixed assemblages suggest a middle-late Miocene age. Sample 182-1127B-50X-4, 54-56 cm (467.54 mbsf), came from below the sharp Unit II/Unit III lithostratigraphic boundary between 467 and 468.5 mbsf (see "Lithostratigraphy"). This sample contains upper Miocene taxa (probably Zone NN10), including few Scyphosphaera spp. and Sphenolithus neoabies, and rare Discoaster bellus, Discoaster brouweri, Discoaster variabilis, Minylitha convallis, and Reticulo-fenestra pseudoumbilicus, as well as older species such as Calcidiscus premacintyrei. The Miocene taxa indicate that the lithostratigraphic boundary in Core 182-1127B-50X represents a major break in sedimentation, with much of the Pliocene and a part of the upper Miocene missing.
A few pieces of chert with a calcareous "rind" recovered in the core catcher of the lowest cores, Samples 182-1127B-54X-CC, 0-1 cm (501.10 mbsf), and 55X-CC, 5-6 cm (507.75 mbsf), yielded several age-diagnostic nannofossils. In Sample 182-1127B-54X-CC, 0-1 cm (501.10 mbsf), the presence of C. macintyrei suggests an age younger than Zone NN5. Sample 182-1127B-55X-CC, 5-6 cm (507.75 mbsf), contains Sphenolithus heteromorphus, Coronocyclus nitiscens, Cyclicargolithus floridanus, and Sphenolithus moriformis. These nannofossils indicate the presence of combined Zones NN4 and NN5.
Planktonic foraminifers in all core-catcher samples were studied from Holes 1127A (only one core recovered) and Hole 1127B, except for Samples 182-1127B-54X-CC, 0-1 cm, and 55X-CC, 5-6 cm, that were primarily taken for nannofossil analysis. Three additional samples from Cores 182-1127B-44X and 45X were also examined. Preliminary results indicate that an extremely expanded Pleistocene sequence was recovered from Hole 1127B, extending into the upper Pliocene. The sequence unconformably overlies a thin Miocene section that contains strongly mixed planktonic foraminifer assemblages.
Samples 182-1127A-1H-CC, 23-26 cm, and 182-1127B-1H-CC, 26-29 cm, through 8H-CC, 27-29 cm, contain numerous small ostracodes, bivalves, gastropods, and sponge spicules. Planktonic foraminifers are common, forming 5%-10% of the >63-µm residue, and are beautifully preserved. The low-diversity (~10 species) assemblage is mainly composed of Globorotalia inflata, Globorotalia truncatulinoides, Globigerina bulloides, and Globigerinoides ruber. In the 63- to 150-µm fractions, Globigerina falconensis, Globigerina quinqueloba, and Neogloboquadrina pachyderma are dominant. Minor constituents include Globigerinoides rubescens, Globigerinoides tenellus, Orbulina universa, and Neogloboquadrina dutertrei. Except for having mostly small specimens, this assemblage is similar in composition to those from recent sediments in southern mid-latitudes (Bé, 1977). The first occurrence of Globorotalia hirsuta was detected in Sample 182-1127B-7H-CC, 24-27 cm. In the southwest Atlantic, this datum level lies within the middle Brunhes, at marine oxygen isotope Stage 12 (Pujol and Duprat, 1983). Almond et al. (1993), however, found G. hirsuta from marine isotope Substage 5e and younger sediments in the Great Australian Bight, suggesting that this species may have arrived in this region much later.
From Cores 182-1127B-9H through 43X, the common planktonic foraminifers are moderately to poorly preserved, especially in samples from firmgrounds. The consistent occurrence of G. truncatulinoides down to 404.42 mbsf (Sample 182-1127B-43X-CC, 36-39 cm) indicates Pleistocene Zone Pt1 (Fig. F5). Within this interval, the last occurrence of Globorotalia tosaensis in Sample 182-1127B-26X-CC, 67-70 cm, can be used to define the top of Subzone Pt1a (Berggren et al., 1995). Pleistocene sediments in Hole 1127B exceed 400 mbsf.
Throughout this Pt1 interval, the planktonic foraminifer assemblage as a whole is of the southern temperate type, and only at a few levels did we observe rare specimens of (sub)tropical species. For example, Globigerinoides sacculifer s.l. was found in Samples 182-1127B-6H-CC, 2-4 cm; 12H-CC, 17-20 cm; and 28X-CC, 43-46 cm; and Globorotalia tumida in Samples 182-1127B-15H-CC, 9-12 cm; 18X-CC, 29-32 cm; 20X-CC, 12-15 cm; and 26X-CC, 67-70 cm. Samples 182-1127B-28X-CC, 43-46 cm, and 39X-CC, 22-25 cm, contain Sphaeroidinella dehiscens and Pulleniatina finalis, respectively. These typical low-latitude species signal warmer climatic conditions and/or stronger flows of the Leeuwin Current (Almond et al., 1993). The timing and duration of these events are believed to reflect global climatic cycles. A high-resolution postcruise study of this much-expanded section will contribute to a more accurate dating of the Pleistocene cycles, as well as providing an improved understanding of regional climatic variations in the Great Australian Bight.
The occurrence of numerous small (mainly 100-180 µm) specimens from this expanded Pleistocene sequence could have resulted from preferential sorting during sediment redeposition events, so that only small specimens accumulated on this gentle slope setting. Mixing between the warmer-water dwellers, such as G. falconensis and Globigerinoides rubesens, and subantarctic representatives, especially G. quinqueloba and N. pachyderma, argues also for different sources for these sediment particles: warm species from the west via the Leeuwin Current and cold species from the southwest via the West Wind Drift. During the sorting process, a relatively cooler part of the strongly mixed upper water column may have developed, perhaps caused by upwelling, in which small tests flourished (Li et al., 1999).
In Cores 182-1127B-44X through 49X, planktonic foraminifers are characterized by many poorly preserved globorotaliids (Globorotalia crassaformis, Globorotalia crassula, and Globorotalia puncticulata), together with species that are common also in cores above (Globorotalia inflata, Globigerina bulloides, and G. ruber). This assemblage is similar to the upper Pliocene fauna of New Zealand (Hornibrook et al., 1989), but the age conflicts with nannofossils from this interval (the "G. crassaformis interval" in Fig. F5) that are characteristic of Pleistocene Zone NN19 (see "Calcareous Nannofossils"). Further study of nannofossil and foraminifer datum levels is necessary to clarify the age of this interval.
The G. crassaformis biofacies was succeeded downhole at 472.25 mbsf (Samples 182-1127B-50X-CC, 32-35 cm) by a moderately preserved assemblage indicating probable latest Miocene age. If the G. crassaformis biofacies was indeed from the Pleistocene, this faunal change should indicate a hiatus of at least 3 m.y. within Core 182-1127B-50X, at a disconformity between lithostratigraphic Units II and III (see "Lithostratigraphy"). The association of Globorotalia menardii, Globorotalia plesiotumida, G. tumida, and Globigerina nepenthes indicates Subzone Pl1a, which, according to Berggren et al. (1995), is a zone straddling the Pliocene/Miocene boundary. Further downhole, typical middle-lower Miocene species are found mixed together with upper Miocene taxa in Samples 182-1127B-51X-CC, 30-33 cm; 52X-CC, 20-23 cm; and 53X-CC, 23-26 cm (479.94-493.69 mbsf). Jenkinsella mayeri, Globoconella conoidea, and Globoquadrina dehiscens represent the middle-late Miocene; Zeaglobigerina connecta, the early Miocene. Also occurring are species with ranges from the upper Miocene-Pliocene, such as Zeaglobigerina nepenthes, Zeaglobigerina woodi, and G. falconensis. This mixed biofacies from an interval of ~10 m apparently indicates reworking during the middle and upper Miocene, when the sedimentation rate was markedly low (Fig. F6).
Benthic foraminifers were studied from every fourth core-catcher sample at Hole 1127B. However, all core-catcher samples were studied in intervals where the benthic foraminifer assemblages showed major changes in composition (e.g., Cores 182-1127B-43X through 53X). Benthic foraminifers are generally abundant and well preserved, except toward the base of Hole 1127B (below Core 44X), where abundance fluctuates markedly and large numbers of abraded and corroded tests are found in some of the samples. Approximately 300 benthic foraminifers were picked from the >63-µm fraction, except in samples in which abundance was low. The benthic foraminifer assemblages studied include mainly calcareous taxa, and only few species and specimens of agglutinated taxa. Pleistocene, Pleistocene/Pliocene, and Miocene benthic foraminifer asssemblages are recognized in the Neogene succession of Hole 1127B.
This Pleistocene assemblage is characterized by the common to abundant occurrence of small specimens of Triloculina spp., Spiroloculina spp., Quinqueloculina spp., Elphidium spp., Patellina corrugata, Spirillina spp., Palliolatella spp., and Bolivina spp. Also present as rare to few constituents of the assemblage are Bulimina mexicana, Neolenticulina peregrina, Hoeglundina elegans, Uvigerina hispidocostata, Astrononion pusillum, Cancris spp., Rectuvigerina spp., Fissurina spp., Cibicides spp., Cibicidoides spp., Ehrenbergina sp., and various nodosariids. The assemblage is strikingly dominated by small specimens of benthic foraminifers (63-150 µm). The uniformity in test size and the relative rarity of medium to large tests in many of the samples examined from this interval suggest grain-size sorting during downslope redeposition. The composition of the assemblage provides further evidence that the assemblage was partly derived from the shelf and redeposited downslope, as it includes a large proportion of shallow-water taxa (Triloculina spp., Spiroloculina spp., Quinqueloculina spp., Elphidium spp., and Patellina spp.), which are typically found in inner to middle neritic environments. Apparent fluctuations in the relative proportions of these neritic taxa and upper bathyal taxa such as B. mexicana, N. peregrina, H. elegans, U. hispidocostata, and A. pusillum indicate that the intensity of downslope influx probably varied in time. These fluctuations may be related to changes in climate and oceanic circulation patterns occurring on a global or regional scale during the Pleistocene (McGowran et al., 1997). Postcruise studies will be necessary to resolve the timing and scale of this cyclicity.
This assemblage is characterized by occurrence of few to common Heterolepa dutemplei, Stilostomella spp., and Loxostomum spp. Also present are B. mexicana, U. hispidocostata, Uvigerina proboscidea, Cibicidoides mundulus, Cibicidoides spp., Triloculina spp., Bolivina spp., and various nodosariids. The composition of the assemblage indicates that the benthic foraminifers originated from an upper bathyal environment. Abundance fluctuates markedly within this assemblage, but remains generally low. There is no evidence of preferential test-size sorting within this interval. However, most samples contain a high proportion of abraded and corroded tests, suggesting extensive reworking or mixing. This is consistent with observations of slumped beds in Cores 48H to 49X and of disturbed beds in Cores 45X to 47X.
This assemblage typically contains small specimens of Bolivina spp., Elphidium spp., Loxostomum spp., Stilostomella spp., Loxostomoides spp., Uvigerina spp., Cibicidoides spp., and Patellina spp., as well as various nodosariids. Abundance is generally low and preservation is poor. The uniformity in test size and the mixing of shallow-water indicators such as Elphidium spp. and Patellina spp. with upper bathyal taxa suggest that a large proportion of the tests within this assemblage originated from the shelf, before being sorted and reworked downslope. The nannofossil and planktonic foraminifer assemblages from this interval include mixed Miocene taxa, providing further evidence of reworking.
Sediment accumulation rates shown in Figure F6 were calculated from preliminary biostratigraphic and paleomagnetic results from Site 1127 (see "Paleomagnetism"). The biostratigraphic datum levels and relevant paleomagnetic data used to calculate sedimentation rates are listed in Table T2. Identification of the onset of the Jaramillo is regarded with some uncertainty (see "Paleomagnetism").
A very high sedimentation rate averaging 240 m/m.y. is recorded for the Pleistocene section. Paleomagnetic data indicate that accumulation in the 240-280 mbsf interval was significantly lower, at 170-180 m/m.y. Accordingly, microfossil datum levels recorded from 300 to 400 mbsf appear to be slightly displaced, probably by slumps and/or bioturbation. In contrast, the sedimentation rate for the underlying Miocene section was significantly lower, varying between 2 and 8 m/m.y. from poorly constrained datums. A major disconformity at the base of the Pleistocene section is indicated by this sharp change in sedimentation rate. Biostratigraphic data suggest that the lower Pliocene, perhaps the upper Pliocene, and part of the upper Miocene are missing. The duration of this hiatus is likely to be >3 m.y.