Sediment recovered from Site 1076 represents a relatively continuous hemipelagic section spanning the last 1.5-1.6 m.y. of the Pleistocene. The micropaleontological study was carried out on core-catcher samples from Hole 1076A. Additional samples from within the cores were examined for calcareous nannofossil- and silicoflagellate-based biostratigraphy.
Calcareous microfossil abundance and preservation deteriorates gradually from Core 175-1076A-11H through 22H. All three calcareous groups show evidence for common reworking in the majority of the samples. In contrast, siliceous microfossil are abundant and well preserved throughout Hole 1076A. The calcareous nannofossil-based biostratigraphy disagrees with the paleomagnetic time frame for the lower half of Hole 1076A. We tentatively explain this discrepancy as a result of both poor preservation and reworking of calcareous nanno-fossils.
Both calcareous nannofossil and benthic foraminiferal assemblages suggest a discontinuity within the sedimentary record at an approximate depth of 120 mbsf. Downcore changes in planktonic foraminiferal and diatom assemblages are used as indices of variable surface and subsurface hydrography, as well as proxies of coastal upwelling and fluvial input.
Nannofossil-derived biostratigraphy of Site 1076 is based on a high-resolution study of Hole 1076A. Table 2 indicates that most locations of datum events at Hole 1076A were constrained within a maximum range of 8 m. Because of frequent occurrences of calcareous nannofossil-barren samples toward the bottom part of Hole 1076A, there is a broader depth range (~11 m) in which the oldest datum (last occurrence [LO] of Helicosphaera sellii) can occur.
Nannofossil assemblages are generally characterized by low diversity and moderate to poor preservation. Reworked specimens are common throughout the entire section. The overall abundance ranges from abundant for Cores 175-1076A-1H through 10H and within Cores 175-1076A-13H and 14H, to poor or barren for Cores 175-1076A-11H and 12H and from Cores 175-1076A-15H through 22H.
The nannofossil biostratigraphy suggests that drilling at Site 1076 recovered a continuous stratigraphic record from the lower Pleistocene (lower part of Zone NN19) to the Holocene (upper Zone NN21b). Based on the identification of the LO of H. sellii datum near the base of Hole 1076A, the oldest sediment recovered from Site 1076 has an approximate age of 1.3 Ma.
The biostratigraphic framework disagrees with the paleomagnetic interpretations from 90 to 120 mbsf. Sedimentation rates calculated from nannofossil datums are lower than those estimated from paleomagnetic boundary events (Fig. 7). Reasons for this discrepancy probably include poor preservation and frequent reworking of nannofossil assemblages within Cores 175-1076A-11H and 12H, which affect primarily the LO datum events of individual species (i.e., LO of Pseudoemiliania lacunosa and LO of Reticulofenestra asanoi). However, we do not believe that the depth locations of datum events based on acme intervals (LO of Small Gephyrocapsa acme [sen. str. Weaver, 1993]; LO of Small Gephyrocapsa acme [sen. str. Gartner, 1977]) are affected by poor preservation or common reworked specimens. In any case, nannofossil-based estimates of sedimentation rates should be taken with extreme caution.
A major paraconformity was identified within the top part of Core 175-1076A-14H. This feature (see "Lithostratigraphy" section, this chapter) has the effect of duplicating similar nannofossil assemblage successions and datums above and below ~120 mbsf.
The uppermost three cores down to the mean depth of 17.3 mbsf (Sample 175-1076-3H-1, 80 cm, to Sample 175-1076-3H-4, 60 cm) are ranged within Zone NN21b. Nannofossil assemblages within this interval are dominated by Emiliania huxleyi.
Most of this interval ranges within the Gephyrocapsa aperta acme Zone sen. str. Weaver (1993) and extends throughout isotope Stages 6, 7, and upper 8. The lower boundary marker for Zone NN21a was identified within Core 175-1076A-8H at a mean depth of 70 mbsf.
The LO P. lacunosa event (which marks the lowermost boundary of this zone) was difficult to constrain because of frequent reworking within Cores 175-1076A-9H and 10H. We tentatively located this event between Samples 175-1076A-9H-CC and 10H-5, 34 cm.
Site 1076 did not penetrate the Zone NN19/NN18 boundary (uppermost Pliocene) but terminated slightly below the LO H. sellii datum event (Samples 175-1076A-20H-CC to 21H-7, 10 cm). Other datums identified within the Zone NN19 interval were the LO of R. asanoi at 102.9 mbsf and the LO of Small Gephyrocapsa acme sen. str. Gartner (1977) at both 115.8 and 127.7 mbsf (see above discussion on paraconformity).
The planktonic foraminifers at Site 1076 are affected by dissolution. In addition, there is evidence for reworking in the uppermost sample. One specimen of Globorotalia tosaensis (LO at 0.65 Ma) was identified in Sample 175-1076A-1H-CC. It is not present in the underlying samples and agrees with calcareous nannofossil floral distributions that indicate reworking.
Zonation schemes based on faunal changes within the Pleistocene (e.g., Globorotalia menardii and G. truncatulinoides zonations based on the presence/absence and the coiling direction, respectively) are not readily applicable because of dissolution and the broad sampling interval (core catchers).
The uppermost assemblage is dominated in the coarse fraction (>250 µm) by Orbulina universa (Fig. 8). Table 3 lists the dominant species for Site 1076 by decreasing susceptibility to dissolution. Globigerina bulloides and Globorotalia crassaformis are abundant components. Other common species include Neogloboquadrina pachyderma (sinistral and dextral), Globorotalia inflata, and G. scitula. This assemblage differs from the uppermost assemblage at Site 1075 in that Globigerinoides ruber (pink) is not abundant. O. universa has been found in very high abundances south of the study area, in the Benguela Current region (Bé and Tolderlund, 1971), but it was not found in high abundances in a study performed in the Congo Basin (Ufkes et al., in press). The high abundance of O. universa (Fig. 8), in conjunction with the low abundance of G. ruber (pink), may be representative of the surface-water conditions at the eastern edge of the gyre. The dominance of O. universa and absence of G. ruber (pink) occurs in Samples 175-1076A-1H-CC through 5H-CC, as well as in Samples 175-1076A-13H-CC, 17H-CC, 19H-CC, and 21H-CC. The absence of G. ruber (pink) in these samples may be the result of dissolution, however, and be unrelated to surface-water changes.
G. crassaformis is abundant in the upper part of the section but is absent below Core 175-1076A-8H-CC (71.84 mbsf). Although these samples are within the dissolution interval (Samples 175-1076A-10H-CC through 22H-CC), dissolution-susceptible species such as G. ruber are present, suggesting that the absence of G. crassaformis may be related to a hydrographic change and not just to dissolution. G. crassaformis is commonly found at depths >100 m in this region, in the oxygen minimum zone, and below the Equatorial Undercurrent (Bé and Tolderlund, 1971).
The presence of G. bulloides presumably indicates elevated productivity, possibly related to upwelling. The consistent downcore presence of both G. bulloides and O. universa suggests that conditions similar to modern ones prevailed downcore in Samples 175-1076A-1H-CC, 4H-CC, 5H-CC, 7H-CC, 13H-CC, 17H-CC, and 19H-CC.
The upper 10 core-catcher samples of Hole 1076A (Samples 175-1076A-1H-CC through 10H-CC) contain well-preserved benthic foraminifers in relatively high abundance. Farther downcore, the abundance is lower and the preservation moderate to poor. The preservation displays a dual character in the interval 175-1076A-13H-CC through 14H-CC (perhaps down to Sample 175-1076A-16H-CC, but the abundance of benthic foraminifers in those samples is very low). Here well-preserved specimens of Hoeglundina elegans are present with heavily eroded specimens of the same species. The upper part of that interval is equivalent with the "paraconformity," as suggested by the calcareous nannofossil assemblages.
The diversity of benthic foraminifers deteriorates downcore as a result of lower absolute abundance. This impedes a relevant analysis of faunal changes through time, even though some general comments can be made. Bulimina aculeata is continuously present in high relative abundance throughout Hole 1076A. Its presence is associated with high relative abundance of B. exilis and Uvigerina auberiana. Other species that are present in high relative abundance throughout Hole 1076A are Bulimina mexicana, Cassidulina laevigata, Cibicidoides pachyderma, fissurids, and the Praeglobobulimina/Globobulimina group (Fig. 9). The species Epistominella sp. 1 and Quadrimorphina allomorphoides are restricted to the upper part of Hole 1075A, whereas Stilostomella spp. occurs in the lower part only. Relative abundances of the benthic foraminiferal species found at Hole 1076A are presented in Table 4.
Most of the benthic foraminiferal species found at Site 1076 (Table 4) are long-ranging, cosmopolitan species that have been reported from the southeastern South Atlantic (Boersma, 1984a, 1986b). The faunal composition is indicative of upwelling and enhanced productivity; important species such as Bulimina aculeata and B. exilis are both reported to prefer such conditions. The species B. aculeata is reported to have high tolerance to oxygen deficiency and to prefer high nutrient abundance (Van der Zwaan, 1982); B. exilis is often associated with sapropels and diatomites formed under conditions of significant oxygen depletion (Jonkers, 1984). The species B. mexicana is present in high relative abundances at Hole 1076A, and it is reported to be common in regions of coastal upwelling and high productivity (Boersma, 1984b). This species is a major component of the benthic foraminiferal assemblages at Deep Sea Drilling Project (DSDP) Site 532, which was drilled on the southern margin of the Angola Basin at approximately the same depth as Site 1076.
Radiolarians are present in all of the core-catcher samples from Hole 1076A (Table 5). They are common to abundant in most of the samples, but rare to few in some samples from the lower part of the hole. Good preservation in all investigated samples suggests that the low abundances are probably caused by low productivity and/or dilution by terrigenous sediments, rather than by enhanced dissolution. No apparent reworking has been identified.
The radiolarian fauna indicates a Quaternary age for Site 1076. The absence of Axoprunum angelinum indicates that the uppermost cores (175-1076A-1H-CC through 8H-CC) are within either the Pleistocene Collosphaera tuberosa Zone or the Pleistocene to Holo-cene Buccinosphaera invaginata Zone of Moore (1995). A finer zonal resolution could not be achieved because of the absence of B. invaginata.
The lower part of the section (Cores 175-1076A-9H-CC through 22H-CC) is 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, regardless of the absence of the diagnostic species Anthocyrtidium angulare throughout Hole 1076A. The diagnostic species C. tuberosa, used to recognize the A. angelinum and A. ypsilon Zones, is extremely rare. The age-diagnostic species L. neoheteroporos, which becomes extinct at 1.07 Ma in tropical oceans, was not found. The oldest sediment could, thus, be younger than 1.07 Ma.
Diatom counts and identification were carried out on smear slides. Opaline phytoliths and silicoflagellates also were counted. Diatoms show marked fluctuations in abundance and preservation (Table 6). In general, diatoms are abundant and well preserved throughout Hole 1076A, except for Samples 175-1076A-3H-CC, 4H-CC, 11H-CC, 14H-CC, 15H-CC, and 16H-CC (Table 6; Fig. 10). Examination of the core-catcher samples from Hole 1076A indicates a Pleistocene age for this hole. Samples 175-1076A-1H-CC through 10H-CC are assigned to the Pseudoeunotia doliolus Zone. Diatom biostratigraphic marker species are lacking downhole. However, the silicoflagellate Mesocena quadrangula is present in Samples 175-1076A-17H-CC (LO at 0.8 Ma) through 20H-CC (first common occurrence at 1.0 Ma; Locker, 1996) and may be used as a biostratigraphic marker instead.
The diatom flora is dominated by upwelling-indicator species (>60% of total diatom assemblage Thalassionema nitzschioides var. nitzschioides and Chaetoceros resting spores and setae; see Table 6), accompanied by freshwater taxa (e.g., Aulacoseira granulata, A. islandica, and Cyclotella spp.), and neritic species (e.g., Actinoptychus senarius). Two distinct pulses of the coastal species Skeletonma costatum are observed in Samples 175-1076A-6H-CC (~21%) and 10H-CC (~12%), respectively. Species characteristic of open-ocean conditions (e.g., Alveus [= Nitzschia] marinus, Azpeitia spp., and Rhizosolenia robusta) were less abundant than here than at Hole 1075A. In general, diatom assemblages characterize Hole 1076A as a coastal upwelling site with variable freshwater input (Fig. 7; also see "Site 1075" chapter, this volume).
As at Hole 1075A, the presence of freshwater diatoms at Hole 1076A is attributed to supply by the Congo River, and high abundances may be interpreted as signals for humid intervals on the African continent (e.g., Jansen et al., 1989). The average contribution of the freshwater assemblage is somewhat higher than at Hole 1075A (6.4% vs. 3.5%). Peaks are seen in the upper 40 m (>15%) and in Samples 175-1076A-8H-CC through 10H-CC (8%-10%) and 175-1076A-17H-CC. Opaline phytoliths are present only sporadically and in low numbers (Table 6); therefore, the ratio PhFD (phytoliths/ [freshwater diatoms + phytoliths] x 100) is not given.