A relatively continuous section spanning the Pleistocene and penetrating into the upper Pliocene was recovered from Site 1075. Core-catcher samples were examined for all microfossil groups, and additional smear slides from within the cores were examined for calcareous nannofossils and diatoms. Biostratigraphic analysis focused on Hole 1075A, although samples from Holes 1075B and 1075C were used to supplement interpretations for Hole 1075A.
Calcareous microfossil floras and faunas are poorly preserved in much of the section, particularly in the lower portion. Siliceous microfossils show good preservation at Site 1075 and are abundant throughout Hole 1075A. Diatoms are represented by marine and freshwater taxa. Fluctuations in freshwater diatom and phytolith assemblages reflect changes in the continental supply. Pyrite is common throughout the core in the form of pyritized borrows and pyrite grains in foraminifers and diatoms, and this affects the stratigraphic resolution. Thus, both datums and environmental interpretations of the calcareous microfossil groups should be used with caution. Nevertheless, we were able to develop an integrated, high-resolution biostratigraphy for the site that is in agreement with paleomagnetic interpretations (Fig. 18). No apparent reworking has been identified.
Because of the low overall abundance and generally poor preservation of the nannofossil assemblages contained within the bottom part of Hole 1075A, additional core-catcher samples from Holes 1075B and 1075C were studied in an attempt to provide stratigraphic information for the base of Site 1075.
Nannofossil assemblages generally show low diversity and are mostly poorly preserved and sparse, especially toward the lower part of the section. Most of the samples investigated from Core 175-1075A-15H through 22H (bottom of Hole 1075A) are barren, which affects the estimates of the mean depth of the datums within Hole 1075A (Table 2). The range over which the datums could exist varies from ~3 m at the Zone NN21a/NN20 boundary to 20 m for the last occurrence (LO) of Helicosphaera sellii.
Within the resolution of the datums, the nannofossil biostratigraphy suggests that drilling at Site 1075 recovered a continuous stratigraphic record from the upper part of the upper Pliocene to the Holo-cene (Zone NN18/NN19 boundary to upper Zone NN21b). The oldest recognized datums were identified at Hole 1075B (LO of Calcidiscus macintyrei) and Hole 1075C (LO of Discoaster brouweri, zonal marker for the Zone NN18/NN19 boundary).
The acme interval of Emiliania huxleyi defines the top 90 k.y. of the Pleistocene and Holocene periods. Nannofossil assemblages from Cores 175-1075A-1H and 2H are dominated by this species.
The lower boundary of this zone is defined as the first occurrence (FO) of E. huxleyi. This event is difficult to recognize with light microscopy, particularly in poorly preserved assemblages. Since the LO of the Gephyrocapsa caribbeanica acme is synchronous with the E. huxleyi datum, the LO of G. caribbeanica was used instead to locate the Zone NN21a/NN20 boundary at 41.3 mbsf (Sample 175-1075A-5H-CC to Sample 6H-3, 1 cm). Nannofossil assemblages representative of this zone are characterized by the overall dominance of small Gephyrocapsa spp. (G. aperta acme Zone; Weaver, 1993).
The LO of Pseudoemiliania lacunosa, the lower boundary marker for this zone, was identified within Core 175-1075A-7H at a mean depth of 55.6 mbsf. G. caribbeanica is the dominant calcareous nannofossil species within Zone NN20, but this acme interval also extends to the top of Zone NN19. The Zone NN20/NN19 boundary and its associated datum event occur within isotope Stage 12, as shown by Thierstein et al. (1977).
This zone spans the upper part of the upper Pliocene and the lower and middle Pleistocene sequences. Site 1075 did penetrate the Zone NN19/NN18 boundary. The LO of Discoaster brouweri, the boundary marker for this event, was identified at the base of Hole 1075C (Sample 175-1075C-22H-CC). Poorly preserved (and sometimes barren) assemblages made it impossible to recognize this datum event at Holes 1075A and 1075B. The short age range of Reticulofenestra asanoi was used to further constrain the stratigraphy of Zone NN19 and to compare the nannofossil chronology to the paleomagnetic time frame (see Fig. 18 and the "Paleomagnetism" section, this chapter). The top and bottom boundaries of the Small Gephyrocapsa acme Zone (Gartner, 1977) could not be identified, probably because of the generally poor preservation of the nannofossil assemblages within the bottom cores at Site 1075. The LO of Helico-sphaera sellii was found at Holes 1075A and 1075B at mean depths of 142.1 and 138.7 mbsf, respectively. The presence of C. macintyrei in Sample 175-1075B-18H-CC provides an additional datum (1.67 Ma) for the lower part of Site 1075.
The planktonic foraminifers have limited biostratigraphic utility in the Pleistocene. The only datum for the Pleistocene, the LO of Globorotalia tosaensis, occurs above the Matuyama/Brunhes boundary at 0.65 Ma (Berggren et al., 1995). One specimen of G. tosaensis was identified at Hole 175-1075A (5H-CC), although the depth of this datum is not in agreement with the detailed age model developed using calcareous nannofossil datums (Fig. 18). The species was not found in the overlying or underlying core-catcher samples. The specimen in question may be reworked. In any case, high abundances of G. tosaensis are not anticipated at Site 1075 because the species is ancestral to Globorotalia truncatulinoides (Blow, 1969), and G. truncatulinoides is not a significant component of the fauna in this region (Parker, 1971). Zonation schemes based on faunal changes within the Pleistocene (e.g., changes in the presence/absence of Globorotalia menardii and the coiling direction of G. truncatulinoides) are not readily applicable at Site 1075 because of dissolution. Dissolution is selective, which makes it difficult to identify faunal changes caused by hydrographic changes (Berger, 1970). Samples from 175-1075A-17H-CC through 22H-CC are barren of planktonic foraminifers, and Samples 175-1075A-11H-CC, 14H-CC, and 15H-CC show low abundances.
The uppermost core-catcher (175-1075A-1H-CC) assemblage is dominated in the coarse fraction (>250 µm) by Globigerinoides ruber (pink and white), Orbulina universa, and Globigerina bulloides. Other common species include Neogloboquadrina pachyderma (dextral), Hastigerina siphonifera, Globorotalia crassaformis, and the Globigerinoides immaturus-G. sacculifer-G. quadrilobatus series. Globorotalia tumida, Globorotalia scitula, G. truncatulinoides, Pulleniatina obliquiloculata, and Globigerinita glutinata are present but not common. This faunal distribution may be related to the presence of two water masses in the study area: the Equatorial Atlantic Undercurrent (EUC), which flows south at depth, and the overlying Angola Current. G. ruber and G. crassaformis are common components of the EUC fauna, and G. crassaformis is also characteristic of deeper water (Bé and Tolderlund, 1971). The high relative abundances of G. bulloides presumably are related to the enhanced primary productivity induced by inputs from the Congo River and seasonal coastal upwelling.
There is a downcore shift in the assemblage toward dominance of more temperate species (e.g., N. pachyderma and G. inflata), particularly in Samples 175-1075A-5H-CC and deeper. This may reflect climatic influences associated with glaciation (e.g., the northward migration of cooler surface waters of the Benguela Current). For example, there is a change to a more temperate fauna in Sample 175-1075A-2H-CC, which is, in turn, underlain by a fauna in Sample 175-1075A-3H-CC that is similar to the uppermost assemblage (Sample 175-1075A-1H-CC). Alternatively, the faunal shift may be a function of the greater resistance of these species to dissolution rather than to a hydrographic change. For example, Sample 175-1075A-14H-CC is poorly preserved and dominated by species that are very resistant to dissolution: G. crassaformis and N. pachyderma (dextral). Table 3 lists the dominant species for Site 1075 according to decreasing susceptibility to dissolution. The codominance of dissolution-resistant species, such as N. pachyderma, and dissolution susceptible species, such as G. ruber, indicates that the faunal response to the multiple factors controlling distribution (e.g., glacially induced changes in riverine input, hydrographic variation, and resuspension and deposition of shelf sediments) results in a complex assemblage.
The benthic foraminiferal fauna in the upper part of Site 1075 is characterized by low abundance and relatively low diversity. Below Sample 175-1075A-9H-CC, the benthic foraminiferal fauna is very sparse, and a marked decrease in the abundance of benthic foraminifers occurs. Below Sample 175-1075A-14H-CC, the core catchers are essentially barren, probably a result of dissolution. The preservation is good to moderate in the upper part of Site 1075 with the exception of Sample 175-1075A-4H-CC, which is essentially barren. Farther down the hole, preservation deteriorates.
The relative abundance of the benthic foraminifers found at Site 1075 is presented in Table 4. The dominant species in the uppermost two core catchers (175-1075A-1H-CC and 2H-CC) are the Melonis barleeanum/M. pompilioides group and Uvigerina peregrina; in the uppermost sample, the fragile Chilostomella ovoidea predominates. Farther downcore, in the interval 175-1075A-3H-CC through 9H-CC, the Praeglobobulimina/Globobulimina group, together with Melonis barleeanum and M. pompilioides, are the dominant species, with strong contributions from Bulimina exilis and various uvigerinids. Below Sample 175-1075A-10H-CC, there is a marked decrease in total abundance of benthic foraminifers, which makes comments on the assemblages unreliable. The species Melonis barleeanum is not present in the lower part of the hole, and Epistominella exigua is restricted to the lower part. The uvigerinids as well as Melonis pompilioides and the Praeglobobulimina/Globobulimina group are present throughout the hole, with the exception of those core catchers that are barren.
An important factor to consider is the variation in susceptibility to dissolution of different benthic foraminiferal species. The overall downcore increase in dissolution not only decreases the abundance but also modifies the composition of the benthic foraminiferal assemblages.
Hole 1075A contains abundant, well-preserved radiolarians (Table 5). The absence of Axoprunum angelinum indicates that the uppermost cores (Samples 175-1075A-1H-CC through 5H-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 obtained because of the absence of B. invaginata.
Although the diagnostic species Anthocyrtidium angulare is absent from the core, Samples 175-1075A-6H-CC through 12H-CC are approximately assigned to the Pleistocene A. angelinum Zone or the Amphirhopalum ypsilon Zone of Moore (1995) based on the presence of A. angelinum and the absence of Lamprocyrtis neoheteroporos. The diagnostic species C. tuberosa used to recognize the A. angelinum and A. ypsilon Zones are absent. The LO of L. neoheteroporos is found in Sample 175-1075A-13H-CC, indicating an age older than 1.07 Ma for Samples 13H-CC through 22H-CC.
The last consistent occurrence of Eucyrtidium calvertense in Sample 19H-CC is correlative to the extinction of E. calvertense at the base of the Olduvai magnetic polarity event in the southern high-latitude oceans (Hays, 1965; Hays and Opdyke, 1967). This indicates that regardless of the absence of the diagnostic species Pteropcanium prismatium, Samples 13H-CC through 18H-CC are approximately assigned to the A. angulare Zone of Moore (1995) and that the lowermost cores (Samples 19H-CC through 22H-CC) belong to the Pliocene Phi Zone of Hays (1965). The presence of Cycladophora davisiana throughout the core indicates an age of <2.71 Ma for the lowermost cores.
The rare sporadic occurrences following the last consistent occurrence of E. calvertense may not be evidence of reworking. Instead, this species may have a longer range in this region, similar to its sporadic occurrence through the Pleistocene in the North Pacific (Kling, 1973).
A preliminary examination of core-catcher samples indicates the presence of faunal fluctuations. The ratio of upwelling index species to circum-tropical warm-water index species shows significant variation from sample to sample.
The upwelling index consists of five species: Acrosphaera murrayana, Cycladophora davisiana, Botryostrobus auritus/australis, Lamprocyrtis neoheteroporos, and L. nigriniae. This upwelling index has been established in the Peru margin upwelling region and the Oman margin upwelling region (Nigrini, 1991). The species Didymocyrtis tetrathalamus, Dictyocoryne spp., Euchitonia spp., Octopyle stenozona, Tetrapyle octacantha, and Acanthodesmia viniculata (= Giraffospyris angulata) have been selected as species representing the warm-water index. All of these species, except for Euchitonia spp., are species with well-known ecologies (Lombari and Boden, 1985). The ratio of the upwelling species to warm-water species was estimated by counting 100 specimens of the index species in core-catcher samples. Figure 19 shows the variations in relative abundances of the upwelling index species at Hole 1075A. The relative abundance of upwelling species ranges from 3% (Samples 175-1075A-3H-CC and 10H-CC) to 64% (Sample 2H-CC). This suggests that the strength of upwelling in the water column above Hole 1075A has changed significantly throughout the Pleistocene.
Diatoms were the dominant microfossil group at Hole 1075A. Species counts and identification were carried out on smear slides. In addition, opaline phytoliths and silicoflagellates were also counted without distinction of species or morphotypes. Diatoms are abundant and well preserved throughout Hole 1075A, except for Sample 175-1075A-10H-CC (Table 6; Fig. 20). Examination of the core-catcher samples indicates a Pleistocene age for this hole. Samples 175-1075A-1H-CC through 12H-CC are assigned to the Pseudoeunotia doliolus Zone, and Samples 175-1075A-13H-CC through 21H-CC to the Nitzschia rheinholdii Zone. A biostratigraphic marker species is lacking from Sample 175-1075A-22H-CC. Pliocene species are not found in Hole 1075A.
The flora is dominated by upwelling-indicator species (>50% of total diatom assemblage consists of Thalassionema nitzschioides var. nitzschioides and Chaetoceros resting spores and setae; Table 6), accompanied by freshwater taxa (e.g., Aulacoseira granulata, A. islandica, and Cyclotella spp.), neritic species (e.g., Actinoptychus senarius), and species characteristic of oceanic conditions (e.g., Alveus [= Nitzschia] marinus and Rhizosolenia robusta). In general, indicator species characterize Hole 1075A as upwelling-dominated with variable freshwater input (Fig. 20).
The presence of freshwater diatoms at Hole 1075A 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 contribution of the freshwater assemblage is moderately high in Samples 175-1075A-4H-CC (~5%), 10H-CC (~10%), and 15H-CC (~6%) and is high in Sample 22H-CC (~27%). Opaline phytoliths, the second continental signal, are present in low numbers in almost all core-catcher samples; highest relative abundances are seen in Sample 10H-CC.