Site 1080 was drilled in 2800 m water depth in the Angola Basin. Sediment recovered from Site 1080 represents a disturbed and incomplete hemipelagic section, which terminates within the early–middle Pleistocene. The micropaleontological studies were carried out on core-catcher samples from Holes 1080A and 1080B. Additional samples from within the cores were examined for calcareous nannofossil-, diatom-, and silicoflagellate-based biostratigraphy.
Calcareous nannofossils were examined in core-catcher samples from Holes 1080A and 1080B. Additional samples within a disturbed sequence of Section 175-1080A-2H-5 were analyzed for identifying a possible sedimentary hiatus.
The downhole succession of nannofossil assemblages from both Holes 1080A and 1080B suggests that the sedimentary sequence is incomplete and disturbed by possible turbiditic deposition. Site 1080 terminated within the middle part of Zone NN19 (between 0.6 and 1 Ma), as shown by the dominance of Small Gephyrocapsa spp., the presence of Pseudoemiliania lacunosa, and the absence of Helico-sphaera sellii in assemblages from the deepest samples.
Samples 175-1080A-1H-CC and 175-1080B-1H-CC are placed within Zone NN21b (younger than 0.09 Ma). The Zone NN21b/NN21a boundary lies between Samples 175-1080A-1H-CC and 2H-5, 44 cm. Zone NN21a is interrupted by a disturbed sequence located at Hole 1080B between the lower half of Core 175-1080B-2H and the upper half of Core 3H, and at Hole 1080A from Sample 175-1080A-2H-5, 49 cm, down to the upper part of Core 4H. Nannofossil assemblages within this disturbed sequence show abundant Pleistocene reworking. Abundant aragonite needles within the upper part of this interval suggest rapid burial linked with turbiditic deposition. The lower part of Site 1080, immediately below the disturbed sequence, is placed in the lower part of Zone NN19. Hole 1080B terminated between 0.6 and 0.83 Ma (i.e., within the Small Gephyrocapsa acme Zone sensu Weaver [1993]). Barren samples at the bottom of the deeper Hole 1080A prevented us from estimating a maximum age for this hole. Paleomagnetism suggests penetration to the Jaramillo Chron (see "Paleomagnetism" section, this chapter).
Planktonic foraminifers were examined from Holes 1080A and 1080B. Sample 175-1080A-1H-CC (3.25 mbsf) is dominated by Globigerina bulloides (an upwelling, high-productivity indicator) and also contains abundant Neogloboquadrina pachyderma. Globorotalia inflata, Globorotalia crassaformis, Globorotalia scitula, and Globigerinita glutinata are common, although not abundant, components of the fauna. N. pachyderma is an important species in the modern Benguela Current system (Little et al., 1997), and the sinistral form is found in association with coastal upwelling centers. G. bulloides is found in abundance in the fringes of the upwelling cells (Little et al., 1997). G. inflata is common in the transitional zone between upwelled water and subtropical waters and may function as an indicator of this boundary downcore. Unfortunately, dissolution affects the abundance of the planktonic foraminifers downcore. Samples 175-1080A-2H-CC and 4H-CC through 6H-CC are barren of planktonic foraminifers. Sample 175-1080A-3H-CC has rare foraminifers: G. bulloides and G. crassaformis are dominant, and Globigerinoides ruber is abundant. The absence of N. pachyderma may represent a change to warmer surface-water conditions, but it may also be an artifact of dissolution. There are no planktonic foraminifers in the 150- to 250-µm fraction.
A lithologic change associated with the Brunhes/Matuyama boundary in Core 175-1080A-2H-5 was subsampled and analyzed (Samples 175-1080A-2H-5, 43–45, 62–64, 70–72, and 103–105 cm). The presence of clay layers between the samples at 43 and 64 cm, 64 and 70 cm, and 70 and 103 cm suggests turbiditic deposition. There is a low planktonic/benthic foraminiferal ratio in the sample above the interval (Sample 2H-5, 43–45 cm). G. inflata and G. bulloides are dominant, and G. crassaformis is present. Sample 175-1080A-2H-5, 62–64 cm, is dominated by G. inflata and N. pachyderma (sinistral), with common G. crassaformis. G. bulloides is present but not abundant. This assemblage is different from others observed in the region and may represent reworking (i.e., mixed assemblages representing turbidite deposition). Sample 175-1080A-2H-5, 70 cm, is dominated by G. bulloides and G. inflata. G. crassaformis is present, but N. pachyderma (sinistral) is not. This fauna is similar to the sample immediately above the turbidite layer (Sample 175-1080A-2H-5, 43–45 cm) and is probably in situ. The lowermost sample (2H-5, 102 cm) is strongly affected by dissolution.
The benthic foraminifers from Site 1080 were studied at both Holes 1080A and 1080B. The abundance is high in the first core catchers (Samples 175-1080A-1H-CC, 175-1080B-1H-CC, and 1080B-2H-CC). The subsequent core catchers are barren or contain very few benthic foraminifers. The preservation is good in the uppermost core catchers but deteriorates downhole.
The uppermost core catchers (Samples 175-1080A-1H-CC and 175-1080B-1H-CC) are dominated by Nonion sp. 1 (Table 2). Additional species are Melonis barleeanum and Oridorsalis umbonatus. The subsequent core catchers are dominated by Bulimina exilis with additional contribution from fissurinas, stilostomellas, uvigerinids, and the Praeglobobulimina/Globobulimina group. The lowermost two core catchers in each hole are barren (Samples 175-1080A-5H-CC and 6H-CC, and 175-1080B-3H-CC and 4H-CC, respectively).
Radiolarians are present in all core-catcher samples from Hole 1080A (Table 3). Radiolarian abundance ranges from few to abundant, and preservation is good in all investigated samples. No apparent reworking has been identified.
Radiolarian fauna indicates a Quaternary age for Hole 1080A. The absence of Axoprunum angelinum suggests that the uppermost core (175-1080A-1H-CC) is within either the Pleistocene Collo-sphaera tuberosa Zone or the Pleistocene to Holocene Buccinosphaera invaginata Zone of Moore (1995).
Although the diagnostic species Anthocyrtidium angulare is absent throughout Hole 1080A, Samples 175-1080A-2H-CC, 3H-CC, and 4H-CC are 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 from the samples investigated. A last occurrence (LO) of L. neoheteroporos is found in Sample 175-1080A-5H-CC, indicating an age older than 1.07 Ma for the deeper two Samples 175-1080A-5H-CC and 6H-CC (Fig. 8).
Diatom analysis was carried out on smear slides of core-catcher samples from Holes 1080A and 1080B. Diatom abundance ranges from few to abundant. In general, preservation is poor, and diatom valves are fragmented. Silicoflagellates, opaline phytoliths, and sponge spicules are also present. Reworking is not evident. However, the presence of nonplanktonic diatoms in all core-catcher samples points to material advected from the shelf. The upwelling assemblage dominates. Chaetoceros resting spores are abundant in all samples and are accompanied by neritic (rare–few) and open-ocean species (trace–rare).
Samples 175-1080A-1H-CC and 175-1080B-1H-CC are assigned to the diatom Pseudoeunotia doliolus Zone; all other samples are assigned to the Nitzschia rheinholdii Zone.The silicoflagellate bio-stratigraphic marker, Bachmannocena quadrangula (LO, 0.8 Ma.; Locker, 1996), is found in Samples 175-1080A-2H-6, 80 cm, and 175-1080B-2H-3, 30 cm, which agrees with paleomagnetic data (Fig. 8; see "Paleomagnetism" section, this chapter).