Sediments recovered from Site 1082 represent a relatively continuous hemipelagic section spanning approximately the last 5.7 m.y. The micropaleontological studies were carried out on core-catcher samples from Hole 1082A. Additional samples from within the cores were examined to improve the biostratigraphic resolution. An integrated biostratigraphic framework composed of both calcareous and siliceous microfossils was established (Fig. 8, Table 2), resulting in a well-constrained age model for Site 1082. Sedimentation rates range from 7 to 20 cm/k.y. They are highest within the upper part of the upper Pliocene sediments as well as the lower part of the lower Pliocene sediments. A disturbed sequence resulting in an altered stratigraphic interval is identified between ~70 and 110 mbsf. Unlike at previous sites, preservation of calcareous microfossils is good throughout most of Site 1082. Siliceous microfossils indicate a subantarctic influence off Namibia during the late Pliocene.
Calcareous nannofossils were studied in core-catcher samples from Hole 1082A. Additional samples from within the cores were examined close to datum events to improve the stratigraphic resolution. Overall abundance ranges from abundant to very abundant within the entire section. A few samples are barren or poor in nannofossils either near an epoch boundary (Sample 175-1082A-19X-CC, close to the Pliocene/Pleistocene boundary) or within dolomite-rich layers (e.g., Sample 175-1082A-42X-CC). Preservation is moderate to poor between 170 and 250 mbsf in the upper part of the upper Pliocene sediment. Below and above this sequence, preservation is good to very good.
Site 1082 terminated in the uppermost upper Miocene sediment within Zone NN11. Based on the oldest identified datum, the bottom age is estimated at 5.8 ± 0.2 Ma. Within the sampling resolution, the sedimentation appears continuous throughout the entire section (Fig. 8). However, the absence of the index assemblage for the Small Gephyrocapsa acme interval (Gartner, 1977) between the last occurrences (LOs) of Reticulofenestra asanoi and Helicosphaera sellii probably points to a disturbed sequence from 70 to 115 mbsf.
Because of the scarcity of the Neogene Discoaster, Amaurolithus, Ceratolithus, and Triquetrorhabdulus index species, the nannofossil-based biostratigraphy for the Pliocene and upper Miocene parts of Site 1082 is constrained by three biohorizons only. Consequently, Zones NN18 down to NN16, as well as Zones NN15 down to NN12, are lumped into two coarse biostratigraphic intervals.
The Subzone NN21b, which describes the last 0.09 m.y., was not recognized in the uppermost core of Site 1082. The 0.26 Ma datum event (Zone NN21/NN20 boundary) was identified at the mean depth of 25.5 mbsf between Samples 175-1082A-3H-5, 100 cm, and 3H-CC.
This interval of 0.2-m.y. duration terminates at 41.1 mbsf (between Samples 175-1082A-5H-3, 10 cm, and 5H-5, 120 cm), which is the mean depth of the LO of the Pseudoemiliania lacunosa datum event.
In addition to the zonal boundary events, three biohorizons were identified within this interval. These are the LO of Reticulofenestra asanoi (0.83 Ma) at 67.5 mbsf; LO of Helicosphaera sellii (1.25 Ma) at 117 mbsf; and LO of Calcidiscus macintyrei (1.67 Ma) at 149 mbsf. The highest Pleistocene accumulation rates (>12 cm/k.y.) for Site 1082A are found within the upper part of this zone. As discussed above, the suggested presence of a disturbed sequence between 70 and 115 mbsf might explain this change in accumulation rate pattern.
The top of Zone NN18 is defined by the disappearance of the last Discoaster species, D. brouweri (between Samples 175-1082A-22X-CC and 23X-CC). This species is found consistently within the Neogene interval of Hole 1082. A single specimen of D. pentaradiatus, the marker species for the Zone NN18/NN17 boundary, was identified in Sample 28X-CC, therefore providing a minimum age of 2.45 Ma for the associated core depth (262 mbsf). Reticulofenestra pseudoumbilica, whose LO defines the Zone NN16/NN15 boundary (3.82 Ma), is commonly found as reworked within the lower half of the Zone NN18-NN17-NN16 part of Site 1082A. The LO of Sphenolithus spp.(between Samples 175-1082A-43X-CC and 44X-CC), a synchronous event dated at 3.66 Ma (lowermost part of Zone NN16), is therefore used at Site 1082 to approximate the Zone NN16/NN15 boundary.
This interval extends from 3.84 to 5.54 Ma. Although the sparse occurrence of index species prevents us from identifying biohorizons, some investigated samples can be confidently placed within more restricted stratigraphic intervals. Samples 175-1082A-50X-CC through 52X-CC contain common specimens of A. delicatus (LO within Zone NN13), with sparse occurrences of D. pansus and A. tricorniculatus (range: Zones NN14, NN13, and NN12). These samples, therefore, belong to Zones NN13 and/or NN12 (5.02–5.54 Ma). Sample 175-1082A-53X-CC contains sparse C. armatus, a species with a range restricted to Zone NN12 (5.23–5.54 Ma).
The Zone NN12/NN11 boundary is defined by the LO of D. quinqueramus, a datum identified near the base of Hole 1082A between Samples 175-1082A-61X-CC and 62X-CC. D. berggrenii, the synonym species to D. quinqueramus, is present in Sample 175-1082A-63X-CC, thereby confirming the stratigraphic position of the bottom cores of Hole 1082A.
Analysis of planktonic foraminifers indicates that a relatively continuous uppermost Miocene to Pleistocene section was recovered. Foraminifers are generally common to abundant in the Pleistocene sediments and common in the lower Pliocene sediments; however, the upper Pliocene sediments have low abundance or are barren. Trace amounts of Globorotalia inflata dominate the fauna between Samples 175-1082A-15X-CC and 40X-CC. Although dissolution affects abundance at Hole 1082A, the overall foraminiferal abundances are higher than at Site 1081, which contains long intervals that are either barren or contain only pyritized planktonic foraminifers. The zonation is coarse because of the generally low abundance of tropical to subtropical species that are used to define the Pliocene zonations and because of dissolution.
The uppermost sample (175-1082A-1H-CC, 7.8 mbsf) is dominated by Globigerina bulloides and Globorotalia inflata. Orbulina universa is abundant, and Globoratalia truncatulinoides, Globigerinella siphonifera, Globigerina quinqueloba, and Neogloboquadrina pachyderma (sinistral and dextral) are present (Table 3). G. bulloides and G. inflata are the dominant components of the assemblage within Sample 175-1082A-8H-CC and indicate upwelling, although downcore faunal variations indicate that warm surface water (Angola Current) penetrated the region in the past. For example, the tropical-warm subtropical species Globorotalia crassaformis and Globorotalia menardii are present in Sample 175-1082A-4H-CC (Table 3). This surface-water warming is associated with a decrease in upwelling, as indicated by a reduced abundance of G. bulloides in this sample.
The detection of the upper Pliocene/lower Pleistocene and lower/upper Pliocene boundaries was complicated by dissolution. The samples at the boundaries (as determined by other fossil groups) are barren of foraminifers; however, the samples above and below the boundaries, although affected by dissolution, contain sufficient specimens to evaluate the zonations of the other microfossil groups.
The Pliocene/Pleistocene boundary is generally recognized by the first-appearance datum (FAD) of G. truncatulinoides. This FAD occurs in Sample 175-1082A-14H-CC (128.6 mbsf); however, the overall abundance is low in Samples 175-1082A-15X-CC through 20X-CC, and the fauna is dominated by the dissolution-resistant foraminifer G. inflata. The absence of G. truncatulinoides in Samples 15X-CC through 20X-CC, therefore, may be the result of dissolution, and so G. truncatulinoides is not a reliable marker for the boundary at this site. Calcareous nannofossil and paleomagnetic data place the boundary in Core 19H.
Abundance is low in Samples 175-1082A-15X-CC through 26X-CC, but it increases in Sample 175-1082A-27X-CC (252.1 mbsf) in association with a lithologic change. G. bulloides is present within a warm-water fauna (e.g., G. ruber and O. universa). Abundance decreases again in Sample 175-1082A-28X-CC, as does diversity. The fauna in Samples 28X-CC through 30X-CC is nearly monospecific in the >250-µm fraction (G. inflata dominates) and barren in the 150- to 250-µm fraction, suggesting dissolution. Sample 175-1082A-31X-CC is barren.
Pliocene sediments from Site 1082 are difficult to zone because many of the index species are not present. Samples 175-1082A-27X-CC through 34X-CC (252.1–317.7 mbsf) are middle to upper Pliocene sediments, based on the presence of G. crassaformis viola and the absence of G. truncatulinoides. G. crassaformis viola is present in Sample 27X-CC through 34X-CC and ranges from Zone N21 to early Zone N22 (Kennett and Srinivasan, 1983). Zones N21–N22 of Blow (1969) are approximately correlative with Zones PL5, PL6, and early Pt1 of Berggren et al. (1995). The evolutionary first appearance (FO) of G. inflata occurs in the North Atlantic at 2.3 Ma, although it has been recorded earlier in the Southern Ocean (at ~3.4 Ma; see Brunner, 1991), and provides a possible means of further differentiating the zonation. The FO of G. inflata occurs in Sample 34X-CC (317.7 mbsf). If the FAD is at 2.3 Ma, this limits Sample 34X-CC to Zone PL6. If the earlier FO in the Southern Ocean is, however, at 3.4 Ma and is not caused by drilling disturbance or some other form of reworking (Brunner, 1991), then Sample 34X-CC can be constrained to Zones PL5 or PL6 based on the presence of G. crassaformis viola. Other species present in this interval include N. pachy-derma, G. acostaensis, G. puncticulata, and G. crassaformis crassa-formis.
The lower/upper Pliocene boundary is difficult to identify at this site. Dentoglobigerina altispira, Globorotaloides hexagona, Gs. extremus, and Gs. apertura are present in Sample 175-1082A-40X-CC (366.5 mbsf), which constrains the age to late Miocene–late Pliocene. The first definite appearance of G. margaritae (lower Pliocene) is in Sample 175-1082A-41X-CC (378.3 mbsf), although there is a transitional form in Sample 40X-CC (366.5 mbsf). Thus, the boundary between the early (Zones PL1 and PL2) and late (Zones PL3–PL6) Pliocene occurs above Sample 41X-CC (378.3 mbsf). The occurrence of G. theyeri (early Pliocene) provides further evidence that Sample 41X-CC is restricted to early Pliocene Zones PL1 and PL2. The FO of G. margaritae occurs in Sample 175-1082A-61X-CC (570.48 mbsf), indicating that the sample is of early Pliocene age. G. cibaoensis (late Miocene to early Pliocene) is also present in that sample.
The Pliocene/Miocene boundary is placed between Samples 175-1082A-62X-CC and 61X-CC. Sample 62X-CC is assigned to the Mio-cene Zones N16–N19 based on the presence of G. nepenthes (Zones N14–N19), G. extremus (Zones N16–N21), and N. acostaensis (Zones N16–N20). Although Zones N16–19 of Blow (1969) are correlative with Zones Mt 9, Mt10, and PL1 of Berggren et al. (1995), the absence of G. margaritae (ZOne PL1) restricts Samples 62X-CC and 175-1082A-63X-CC to Zones Mt9 and Mt10. The presence of G. conoidea in Sample 63X-CC is in agreement with the late Miocene zonation.
Sediments recovered from Hole 1082A contain abundant benthic foraminifers throughout (Table 4, back-pocket foldout, this volume). The preservation is good throughout, except for a few samples that exhibit moderate to good preservation. Two samples (175-1082A-42X-CC (386.97 mbsf) and 48X-CC (442.70 mbsf)) were lithified; consequently, the benthic foraminifers could not be counted.
The benthic foraminiferal fauna shows high diversity throughout Hole 1082A. The uppermost core catcher (Sample 175-1082A-1H-CC; 7.84 mbsf) is dominated by Bulimina aculeata (–40%) and contains Bulimina exilis, Cassidulina laevigata, and Uvigerina auberiana (Table 4; Fig. 9). Bulimina aculeata shows very low abundance farther downhole, except for two peaks (Samples 175-1082A-15X-CC [135.30 mbsf; –18%] and 20X-CC [183.51 mbsf; –32%]).
The uppermost lithostratigraphic Subunit IA (nannofossil and foraminifer-rich clay; see "Lithostratigraphy" section, this chapter) between 0 and 112 mbsf is characterized by high relative abundances of Bulimina marginata, Uvigerina hispidocostata, Cassidulinoides cf. bradyi, Cassidulina laevigata, and the Praeglobobulimina/Globobulimina group (Table 4; Fig. 9). These species are more or less restricted to lithostratigraphic Subunit IA, whereas Bulimina exilis, which is a significant contributor to this faunal assemblage, also occurs farther down in the section.
Lithostratigraphic Subunit IB (diatom-rich clay) covers the interval between 369 and 112 mbsf and is dominated by Bulimina exilis and Uvigerina auberiana. Other significant species are Globocassidulina subglobosa, Pullenia bulloides, Bulimina mexicana, Cibicidoides wuellerstorfi, and Gavelinopsis lobatulus (Table 4; Fig. 9). Most of these species are more or less absent in the lithostratigraphic unit above but present in the underlying lithostratigraphic units—except for Globocassidulina subglobosa, which seems to be restricted to lithologic Subunit IB. Some species have short-term peaks in their relative abundance within this lithostratigraphic unit and are more or less absent elsewhere: Bulimina truncana shows a single peak (–32%) right at the top of this unit (128.58 mbsf); the genus Stilostomella has peaks at 112.84 mbsf (–73%) and 280.53 mbsf (–59%); and Uvigerina hispida has a peak at 328.60 mbsf (–8%; see Table 4; Fig. 9).
Lithostratigraphic Subunit IC (nannofossil clay, 475–369 mbsf) and Unit II (nannofossil ooze; bottomhole, 475 mbsf) contain similar benthic foraminiferal assemblages. The dominant species is Bolivina subaenarensis with contributions from Sigmoilinopsis schlumbergeri, Melonis barleeanum, Sphaeroidina bulloides, Bulimina mexicana, and Cibicidoides bradyi (Table 4; Fig. 9). The species Cibicidoides pachyderma and Epistominella exigua are essentially restricted to lithostratigraphic Unit II.
Core-catcher Samples 175-1082A-18X-CC through 64X-CC were examined for radiolarians to assess the Pliocene to Miocene biostratigraphy. Radiolarians are present in most of the samples from Hole 1082A (Table 5). In the upper sequence (18X-CC through 40X-CC), radiolarians are generally abundant and well preserved. In the lower sequence (41X-CC through 64X-CC), radiolarians are rare and show signs of dissolution, although it was possible by the sample preparation to concentrate the sample enough to produce high abundances of radiolarians in the assemblage slides. Radiolarian fauna indicates an early Pleistocene to late Miocene age for the investigated sequence. No apparent reworking has been identified.
The radiolarian zones used for this hole are those of Moore (1995) and Motoyama (1996). There are some difficulties in applying the established tropical and Antarctic zonations (Sanfilippo et al., 1985; Lazarus, 1992) because of the absence of index species such as Pterocanium prismatium, Spongaster pentas, Diartus hughesi, Helotholus vema, and Amphymenium challengerae. The radiolarian faunas are similar to those from time-equivalent samples from Hole 1081A.
Although the diagnostic species Anthocyrtidium angulare and P. prismatium are absent throughout the core, Samples 175-1082A-18H-CC, 19X-CC, and 20X-CC are approximately assigned to the Pleistocene A. angulare Zone of Moore (1995) based on the presence of Lamprocyrtis neoheteroporos and the absence of Cycladophora pliocenica, which became extinct at 1.78 Ma in the Antarctic Ocean (Caulet, 1991).
The LO of C. pliocenica is placed in Sample 175-1082A-21X-CC, approximating the Pliocene/Pleistocene boundary between Samples 20X-CC and 21X-CC; thus, the upper boundary of the A. angulare Zone, originally defined by the LO of P. prismatium, should be placed at about Sample 21X-CC.
The FO of Cycladophora davisiana, in Sample 175-1082A-29X-CC, indicates an age of 2.7 Ma and the lower boundary of the P. prismatium Zone.
Spongorus pylomaticus first occurs in Sample 175-1082A-54X-CC, giving an age of 5.2 Ma and a Miocene/Pliocene boundary for the horizon just below this sample. Samples 175-1082A-21X-CC through 54X-CC, which are below the FO of C. davisiana and above the FO of S. pylomaticus, can be correlated to a zonal sequence from the lower part of the Cycladophora sakaii Zone to the S. pylomaticus in the North Pacific (Motoyama, 1996).
In the sequence below Sample 54X-CC, the fauna consists mainly of Spumellaria, and age-diagnostic forms are sparse. Therefore, no zones were defined for Samples 175-1082A-55X-CC through 64X-CC. The presence of Stichocorys peregrina probably indicates that the deeper sample (62X-CC) is no older than ~9 Ma in age because its FO approximates that age in the high-latitude Southern Ocean (Lazarus, 1992); however, the FO of this species is at ~7 Ma in the tropical region.
Stichocorys peregrina seems to prefer tropical to temperate oceanographic condition (see "Biostratigraphy and Sedimentation Rates" section, "Site 1081" chapter, this volume). In Hole 1082A, S. peregrina disappears in Sample 175-1082A-39X-CC, far below the FO of C. davisiana (2.7 Ma). The presence of S. peregrina through the lower sequence indicates temperate oceanographic conditions, and its terminal event indicates a cessation of temperate conditions from the Walvis Basin region. Simultaneously, the Antarctic species C. pliocenica first appears in Sample 39X-CC; it ranges up to Sample 21X-CC, indicating invasion of cooler waters into the study region from higher southern latitudes, which agrees with diatom observations (see below).
Diatom counts and identification were carried out using smear slides and acid-treated, sieved (63 µm) material from core-catcher samples from Hole 1082A. Additional samples from within the cores were examined close to datum events to improve the stratigraphic resolution and refine floral changes (Table 6). Diatom preservation is moderate throughout Hole 1082A. As was the case at Site 1081, the record of diatom abundance points to a substantial increase in deposition during the late Pliocene and early Pleistocene (from ~366 to 110 mbsf), reaching a maximum in the late Pliocene, followed by a decrease within the Pleistocene about 1 Ma (~100 mbsf). Overall abundances are low (trace to few) or diatoms are absent in upper Mio-cene and lower Pliocene sediments (Table 6). We may assume that the diatom content at Hole 1082A reflects a varying nutrient supply that could be related to upwelling of nutrient-rich deeper waters and high biological productivity, especially in the late Pliocene (Fig. 10).
The diatom biostratigraphic zones used for this hole are those of Barron (1985). Zones could not be applied to Samples 175-1082A-40X-CC through the end of the hole because of the scarcity of diatom valves and concomitant lack of biostratigraphic markers (Table 6). As was the case at Site 1081, we recorded two middle- to high-latitude cold-water indicator species, Proboscia ( = Simonseniella) curvirostris in Sample 175-1082A-8H-CC (at approximately the Brunhes/Matuyama boundary) and P. barboi in Samples 175-1082A-20X-CC through 30X-CC (~1.8–2.8 Ma; see Fig. 10). The presence of these species may indicate periods of intensified Benguela Current transport, an assumption that is also supported by the presence of the Antarctic radiolarian C. pliocenica (see above).
The diatom assemblage is similar to that at Site 1081 and consists mainly of a mixture of upwelling-indicator (Chaetoceros resting spores and Thalassionema nitzschioides var. nitzschioides) and oceanic species (e.g., Azpeitia nodulifer, A. tabularis, Hemidiscus cuneiformis, and Thalassiothrix spp.). Upwelling-related species are common during highest abundance times in the upper Pliocene sediment within Subunit IB (see "Lithostratigraphy" section, this chapter; (Table 6).