The sediment recovered from Site 1083 represents a relatively continuous hemipelagic section spanning the last 2.6 m.y. The micropaleontological studies were carried out on core-catcher samples from Hole 1083A. 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. 3), resulting in a well-constrained age model for Site 1083. Sedimentation rates range from 6 to 14 cm/k.y. with highest values located within the last 260 k.y., as well as between 0.8 and 0.96 Ma (from the onset of the Jaramillo Chron to the onset of the Brunhes Chron).
In addition to Hole 1083A, the top part of Hole 1083B was studied to constrain the 0.26-Ma datum event (last occurrence [LO] of the Gephyrocapsa caribbeanica acme). This biohorizon could not be precisely located at Hole 1083A because of the poor recovery of some of the upper cores. Preservation of nannofossil specimens is good to very good. Calcareous nannofossils are abundant to very abundant throughout the entire section. Reworked specimens (essentially Pliocene) are common in Sample 175-1083A-7H-CC, then rare to common within every investigated sample from the lower half of Site 1083 (Cores 175-1083A-10H through 22H).
Site 1083 terminated within the upper Pliocene sediment. The scarcity of Discoaster index species in assemblages from the bottom part of Hole 1083A prevents us from assigning a zone to the deeper samples. Paleomagnetic data (see "Paleomagnetism" section, this chapter) suggest that Core 175-1083A-22H terminated near the NN17/NN16 zonal transition (~2.6 Ma). Ten nannofossil biohorizons, including five zonal boundary events, were recognized at Hole 1083A (Table 2; Fig. 3). Most datum events could be constrained within a range of ±4 m.
This zone only covers the top 3 mbsf of Hole 1083A. The Zone NN21b/NN21a boundary (0.09 Ma) is found between Samples 175-1083A-1H-CC and 2H-2, 90 cm.
Sedimentation rates estimates within this 170-k.y. interval should be used with caution. The base of Zone NN21a at Hole 1083A is located within an unrecovered sedimentary sequence (Core 175-1083A-4H). This datum event (LO of the Gephyrocapsa caribbeanica acme) was, however, recognized at Hole 1083B between Samples 175-1083B-3H-3, 130 cm, and 3H-5, 130 cm.
As documented in the more northern Leg 175 sites, this 0.2-m.y. interval is constrained within a narrow depth range (~13 m). The Zone NN20/NN19 boundary is identified within the lower part of Core 175-1083A-5H at a mean depth of 39 mbsf.
Most of the sedimentary sequence recovered from Site 1083 belongs to this stratigraphic interval (0.46–1.95 Ma). The stratigraphic resolution of the upper half of Zone NN19 is particularly well constrained, with five nannofossil biohorizons recognized between ~40 and 80 mbsf. Reworking of late and early Pliocene specimens is common throughout the lower half of this stratigraphic unit.
The Zone NN19/NN18 boundary event (LO of Discoaster brouweri) is recognized between Samples 175-1083A-17H-CC and 18H-CC. As discussed earlier, Site 1083 probably terminated near or passed the NN17/NN16 zonal boundary. Among the four Discoaster species commonly present in calcareous nannofossil assemblages representative of Zone NN17, D. brouweri, which extends from Zones NN10 to NN18, was the only species recognized in Samples 18H-CC through 22H-CC. This scarcity in late Pliocene index species is probably related to colder than average surface-water temperature (a combination of increased upwelling intensity and advection of subantarctic surface water) over the southwest African and Namibian continental margins during the last 500 k.y. of the Neogene (see "Diatoms" and "Planktonic Foraminifers" sections, this chapter).
The uppermost assemblage (Sample 175-1083A-1H-CC) is dominated by Globigerina bulloides, and Globorotalia inflata is abundant (Table 3). This pattern is maintained downcore. The remainder of the uppermost assemblage includes Orbulina universa, Neogloboquadrina pachyderma (dextral and sinistral), Hastigerina siphonifera, Globorotalia truncatulinoides, Globorotalia scitula, Globorotalia crassaformis, Globigerinoides sacculifer, Globigerinoides ruber, Globigerina falconensis, Globigerina umbilicata, and Beella digitata, and is consistent with the position within the Benguela Current.
Core 175-1083A-5H was sampled to identify faunas representative of possible glacial (green clay)/interglacial (white carbonate ooze) changes (see "Lithostratigraphy" section, this chapter, for a complete description of the sediments). The assemblages within the green clay (Sample 5H-1, 74–76 cm) are similar to the assemblage described above: they are dominated by Globigerina bulloides and G. inflata and also include G. umbilicata, O. universa, N. pachy-derma (sinistral and dextral), G. crassaformis, and G. truncatulinoides. The assemblage within the white mud (Sample 5H-3, 64–66 cm) is dominated by warm-water faunas, such as G. ruber, N. dutertrei, O. universa, and G. menardii, and indicates the influence of the warm-water Angola Current in the region.
The presence of G. menardii and G. dutertrei is used to trace a strong Angola Current contribution downcore (Fig. 4A). Samples 175-1083A-3H-CC, 11H-CC, and 15H-CC contain both Globorotalia menardii and N. dutertrei. G. menardii is present in Samples 175-1083A-2H-CC and 8H-CC, but N. dutertrei is not. These species are absent in Samples 175-1083A-5H-CC through 7H-CC and 17H-CC through 22H-CC, which is coincident with an influx of subantarctic diatom species (Fig. 4A). The change in fauna and flora would seem to indicate a northward shift in position of the Benguela Current. It is unlikely that the absence of these species is caused by dissolution, because G. menardii is quite resistant to dissolution.
The planktonic foraminifers are generally abundant and better preserved than at previous sites (Fig. 4B). The last-appearance datum of G. tosaensis defines the top of Zone Pt1a and constrains the age to the middle Pleistocene (LO = 0.65 Ma). The species last occurs in Sample 175-1083A-8H-CC (68.7 mbsf), but its absence in Sample 7H-CC may be the result of dissolution in that interval. The 0.65-Ma first-appearance datum (FAD) is therefore placed at (55 mbsf), between Samples 8H-CC and 6H-CC (42.4 mbsf), and is in agreement with datums from the other microfossil groups. The Pliocene/Pleistocene boundary (1.77 Ma) and the base of Zone Pt1a are marked by the FAD of G. truncatulinoides. It is present in Sample 175-1083A-15H-CC (134.8 mbsf), absent in Sample 16H-CC (144.5 mbsf), and placed at 139 mbsf. The species G. crassaformis viola (late Pliocene to early Pleistocene) is present in Samples 175-1083A-9H-CC through 18H-CC. The LO of G. crassaformis viola in Sample 9H-CC suggests an early Pleistocene age and is in agreement with the identification of the Brunhes/Matuyama boundary (early/middle Pleistocene) in the base of Core 8H. G. crassaformis viola is not present in Samples 175-1083A-19H-CC through 22H-CC, and its absence indicates an age equal to or older than the early late Pliocene Zone PL5. A position with Zone PL5 is in agreement with paleomagnetic analyses (see "Paleomagnetism" section, this chapter), which place the Matuyama/Gauss boundary (within Zone PL5) within Core 22X. The presence of G. pseudomiocenica in Sample 22H-CC confirms a Pliocene age for that sample.
The abundance of benthic foraminifers at Hole 1083A is high in the upper part of the investigated interval, but decreases downcore to few to rare.
The dominating benthic foraminiferal species in the uppermost core catchers (Sample 175-1083A-1H-CC; 1.74 mbsf) are Bulimina aculeata and Cassidulina laevigata (Table 4). The interval, comprising core-catchers 175-1083A-2H-CC through 9H-CC (9.22-77.84 mbsf), is dominated by Bulimina exilis and Cassidulina laevigata (Table 4). Other important species in this interval are Fursenkoina sp. 1 and Oridorsalis umbonatus. The lowermost interval, from core-catcher 175-1083A-11H-CC to the bottom of the hole (96.53–192.13 mbsf), is dominated by Cibicidoides wuellerstorfi, Melonis barleeanum, Uvigerina hispidocostata, and Uvigerina peregrina (Table 4).
Radiolarians are present in all of the core-catcher samples from Hole 1083A (Table 5). In most of the investigated samples, radiolarians are abundant, and preservation is good. The radiolarian fauna indicates a Quaternary to late Pliocene age for Hole 1083A. No apparent reworking has been identified.
The absence of Axoprunum angelinum indicates that the uppermost cores (Samples 175-1083A-1H-CC, 2H-CC, and 3H-CC) are within either the Pleistocene Collosphaera tuberosa Zone or the Pleistocene to Holocene Buccinosphaera invaginata Zone of Moore (1995). A finer zonal resolution could not be achieved because of the absence of B. invaginata.
Although the diagnostic species Anthocyrtidium angulare is absent throughout the core, Samples 175-1083A-5H-CC through 9H-CC are approximately assigned to the Pleistocene A. angelinum Zone or Amphirhopalum ypsilon Zone of Moore (1995) based on the presence of A. angelinum and the absence of Lamprocyrtis neoheteroporos. The diagnostic species C. tuberosa (which is used to recognize the A. angelinum and A. ypsilon Zones) is absent throughout the core.
Although the diagnostic species Anthocyrtidium angulare and P. prismatium are absent throughout the core, Samples 175-1083A-10H-CC through 18H-CC are 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 175-1083A-19H-CC, approximating the Pliocene/Pleistocene boundary between Samples 18H-CC and 19H-CC. The presence of Cycladophora davisiana indicates that the deepest sample (22H-CC) is younger than 2.7 Ma, and that the lowest four samples (19H-CC through 22H-CC) are within the P. prismatium Zone of Moore (1995).
Diatom counts and identification were carried out on smear slides and on acid-treated, sieved (20 µm) material from core-catcher samples from Hole 1083A (Table 6). Diatom preservation is moderate throughout Hole 1083A. The record of diatom abundance points to high deposition during the late Pliocene and early Pleistocene (from ~96 mbsf to the end of the hole; see Table 6).
Samples 175-1083A-1H-CC through 7H-CC are assigned to the Fragilariopsis ( = Pseudoeunotia) doliolus Zone NTD17 based on the occurrence of F. doliolus stratigraphically above the LO of Nitzschia reinholdii in Sample 175-1083A-8H-CC. The interval from Sample 8H-CC to the end of the hole is assigned to the Nitzschia rein-holdii Zone NTD16. As has been the case at Sites 1081 and 1082, we recorded two middle- to high-latitude cold-water indicator species, Proboscia ( = Simonseniella) curvirostris in Sample 8H-CC (at approximately the Brunhes/Matuyama boundary) and P. barboi in Samples 175-1083A-15H-CC through 22H-CC (Table 6). The occurrence of these species may indicate periods of intensified subantarctic input into the Benguela Oceanic Current system (see "Nannofossils" and "Planktonic Foraminifers" sections, this chapter).
The diatom assemblage is similar to that at Sites 1081 and 1082 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.).