Sediment recovered from Site 1078 represented a relatively continuous hemipelagic section spanning the last ~360 k.y. of the Pleistocene. The micropaleontological study was carried out on core-catcher samples. Additional samples from within the cores were examined for calcareous nannofossil biostratigraphy. Two laminated intervals were sampled at high resolution to examine their diatom content. With the exception of the laminated intervals, diatoms, silicoflagellates, and radiolarians are absent from this site. Calcareous microfossils are abundant and well preserved in all of the examined samples except for the laminated intervals, which are barren of nannofossils. Both calcareous nannofossils and planktonic foraminifers show evidence of reworking within the middle part of Hole 1078B.
Calcareous nannofossils were studied in core-catcher samples from Holes 1078A, 1078B, and 1078C. Additional samples from within cores of Holes 1078A and 1078B were examined to improve the biostratigraphic resolution. Preservation of nannofossil specimens is good to moderate. The overall abundance ranges from very abundant to abundant throughout the entire section of Site 1078, except within two short (~ 40-cm-long) laminated intervals (see "Diatoms," section, this chapter) in Cores 175-1078A-5H, 175-1078B-5H, 175-1078C-5H, and 175-1078C-15X, which are barren.
The nannofossil-based biostratigraphy (Table 2; Fig. 18) suggests that Site 1078 terminated within the upper half of Zone NN20, between 0.26 and ~0.36 Ma. The last occurrence (LO) of Pseudoemiliania lacunosa, the datum event for the Zone NN20/NN19 boundary (0.46 Ma), is not recognized in bottom samples of the deepest drilled Hole 1078C. Hole 1078A probably terminated close to 0.18 Ma, as predicted from applying the sedimentation rate pattern inferred from Holes 1078B and 1078C.
The stratigraphic record is rather continuous, except for two stratigraphically anomalous intervals. Sample 175-1078C-15X-CC, which is placed within Zone NN20 immediately below the Zone NN21a/NN20 boundary, is contaminated with abundant reworked flora of the Zone NN21a-aged interval. This core-catcher sample belongs to the top XCB core collected at Hole 1078C, therefore confirming the problem of contamination induced by the XCB coring device. Samples 175-1078B-9H-6, 120 cm, and 9H-CC contain a reworked nannofossil assemblage typical of Zone NN20 within the Zone NN21a part of the section. Both samples are part of an interval of highly disturbed material (see "Lithostratigraphy" section, this chapter). Oblique migration of older material through gas expansion could be one possible mechanism responsible for this stratigraphic anomaly.
Assuming a linear sedimentation rate between the two available datum events, sediments accumulated at a rate close to 60 cm/k.y. for the interval between 0.09 and 0.26 Ma (Zone NN21a).
The first occurrence of the Emiliania huxleyi acme which defines the Zone NN21b/NN21a boundary was recognized in all three investigated holes. The depth difference of this datum event between Hole 1078C and Holes 1078A and 1078B is essentially caused by low sampling resolution of Hole 1078C.
This stratigraphic interval spans most of the section of Site 1078. The LO of the Gephyrocapsa caribbeanica acme (Weaver, 1993), which marks the Zone NN21a/NN20 boundary, was identified both at Hole 1078B (Samples 175-1078B-12H-CC to 13H-6, 51-56 cm) and at Hole 1078C (Samples 175-1078C-13H-CC through 14H-CC).
Drilling at Site 1078 recovered only the upper part of this zone.
Core-catcher samples were analyzed from Cores 175-1078A-1H and 2H, Cores 175-1078B-2H through 14H, and Cores 175-1078C-15X through 18X. Planktonic foraminifers are common throughout the section, with the exception of Samples 175-1078B-12H-CC and 13H-CC, in which they are rare. Samples 175-1078C-16X-CC through 18X-CC contain only trace amounts of planktonic foraminifers.
The uppermost assemblage at Hole 1078A (7.4 mbsf) is domi-nated by high abundances of Globigerinoides ruber (pink), Orbulina universa, Globigerina bulloides, Globigerinella siphonifera, Globigerinoides sacculifer, and Globorotalia inflata (Table 3). Other species present but not abundant are Globigerinoides ruber (white), Neogloboquadrina pachyderma (dextral and sinistral), and N. dutertrei (Table 3).
Downcore faunal variations are based on analysis of Hole 1078B and suggest a change in surface-water conditions in Sample 175-1078B-12H-CC (Table 3). Below this level (109 mbsf), the fauna is dominated by N. pachyderma (Fig. 19). The high abundances of N. pachyderma in the lower part of the core may indicate a greater penetration of cool surface waters at that time (within Stage 8; i.e., when N. pachyderma is abundant, the Benguela Current penetrated the region). There is an overall increase in abundance of G. ruber above Sample 12H-CC (Table 3), which indicates that the system was dominated by warm surface water from Stage 8 and younger. The reduction in abundance of N. pachyderma may also indicate greater influence of a warmer surface water current such as the Angola Current, rather than an alteration in the position of the Benguela Current. It is possible that the dominance of N. pachyderma in the lower part of the section is the result of aliasing (i.e., because of the large sampling interval, only glacial periods were sampled below Sample 12H-CC and the interglacial above). Another factor to be considered is differential dissolution.
The two planktonic foraminiferal groups that provide biostratigraphic information for the Pleistocene, G. tosaensis-G. truncatulinoides and G. tumida flexuosa, are absent. Possibly, abundances of G. inflata, which has been used as a tracer of the Angola Benguela Front in this region (Jansen et al., 1996), could be used to erect a climatologic stratigraphy, based on more detailed postcruise work.
Benthic foraminifers were analyzed from Holes 1078A, 1078B, and 1078C (partly) to obtain information on faunal changes through time (Table 4).
The benthic foraminiferal fauna at Site 1078 is well preserved and displays low diversity. The total abundance is very high, and the faunal assemblage is strongly dominated by Bolivina sp. 1 and Bolivina sp. 2. In all samples, except for the uppermost ones (Samples 175-1078A-1H-CC and 1H-CC through 2H-CC), these two species make up >50% of the total assemblage (~53%-99%). Additional species are Cassidulina laevigata, the Praeglobobulimina/Globobulimina group, and to some extent Rectuvigerina cf. multicostata. Besides high abundances of the mentioned species, the uppermost core catchers also exhibit high abundance of Bolivina sp. 1 (Sample 175-1078B-2H-CC, 27%) Bulimina exilis (Samples 175-1078A-1H-CC, 18%, and 175-1078B-1H-CC, 17%) and Cancris auriculus (Samples 175-1078A-1H-CC, 21% and 175-1078B-1H-CC, 4%). The species Uvigerina peregrina constitutes only 5% or less of the faunal assemblage, but in a few samples, this species becomes an important component (Samples 175-1078B-6H-CC, 21%; 175-1078C-14H-CC, 29%; and 175-1078C-17X-CC, 16%; Fig. 19).
The dominance of Bolivina sp. 1 and Bolivina sp. 2 and the overall low faunal diversity suggest low oxygen concentrations at the sea-floor. Bolivina (and its close relatives Suggrunda and Brizalina) has been reported to be the most tolerant genus to low oxygen levels (Smith, 1963, 1964; Sellier de Civrieux and Bonilla, 1971; Phleger and Soutar, 1973). In a study of the Santa Barbara Basin, Phleger and Soutar (1973) found that although faunal diversity was low because of oxygen-depleted waters, the number of living individuals was larger than in nearby areas with higher oxygen concentrations. In oxygen-deficient areas off the western coast of South America, several studies have shown depletion in the benthic foraminiferal fauna along with a strong dominance of small-sized Bolivina, Suggrunda, and Brizalina, whose ornamentation was poorly developed (Sellier de Civrieux and Bonilla, 1971; Boltovskoy, 1972).
Core-catcher samples from Holes 1078A, 1078B, and 1078C were analyzed for their diatom and radiolarian content. Samples were prepared as smear slides (for diatoms) and acid-cleaned (for both microfossil groups). The treated samples were washed with distilled water and sieved through 20-, 38-, or 63-µm sieves. Diatoms and radiolarians are absent in almost all core-catcher samples. Only a few diatoms and radiolarians are present in Sample 175-1078A-9X-CC. The presence of the radiolarian Lamprocyrtis nigriniae indicates a Quaternary age for Sample 175-1078A-9X-CC. Silicoflagellates are absent.
A laminated interval composed of alternating lighter olive and darker olive layers is seen in Core 175-1078A-5H, spanning Section 5H-2, 124 cm, to 5H-3, 14 cm. This interval is also found at Hole 1078B (from Section 5H-3, 140 cm, to 5H-4, 30 cm), at Hole 1078C (from Section 5H-4, 10 cm, to 5H-4, 50 cm), and at Hole 1078D (from Section 5H-6, 14 cm, to 5H-6, 64 cm). These laminated intervals were sampled at high resolution, and smear slides were analyzed for their diatom content (Table 5). Diatom abundance ranges from common to rare; preservation is poor to moderate. The diatom assemblage is mixed and composed of the following:
Light and dark layers show similarity in species composition. The composition of the upwelling and freshwater flora is the same as that at Sites 1075, 1076, and 1077.
For radiolarians, a scrape sample from the laminated interval 175-1078B-5H-4, 0-30 cm, was taken. It contains common and well-preserved radiolarians. The assemblage is dominated by Didymocyrtis tetrathalamus, Dictyocoryne spp., Octopyle stenozona, Tetrapyle octacantha, and Acanthodesmia vinculata, indicating warm-water conditions. Upwelling species are present in low relative abundances. Silicoflagellates and Actiniscus pentasterias are also present.
A second laminated interval is seen in Section 175-1078C-15X-2, at 114-140 cm. Smear-slide analysis of light and dark layers indicate that the species content is similar to the one mentioned above, except that freshwater diatoms are absent (Table 5). Calcareous nannofossils are absent in both laminated intervals.