Calcareous nannofossils, planktonic foraminifers, diatoms, and radiolarians were examined for biostratigraphic zonation. Benthic foraminifers were used to estimate paleobathymetry and paleoenvironment and to identify periods of downslope transport of shelf-derived material. Diatoms were used to reconstruct the coastal upwelling history in the Benguela Current system and riverine input at the Congo Fan. The presence of other siliceous groups was routinely scanned (silicoflagellates, opal phytoliths, and the dinoflagellate Actiniscus pentasterias). The abundance, preservation, and age or zone for each fossil group were incorporated into the JANUS database.
Preliminary ages were assigned based primarily on core-catcher samples. Samples from within the cores were examined when a refined age determination was necessary or when the core-catcher material was barren of planktonic foraminifers. Additional samples were examined for calcareous nannofossils on a routine basis.
Ages for calcareous nannofossil, planktonic foraminiferal, diatom, and radiolarian events and epoch boundaries are based mainly on the geomagnetic-polarity time scale of Berggren et al. (1995a, 1995b).
During Leg 175, we referred to the biohorizons listed in Table 2, which include most of Martini's (1971) and Okada and Bukry's (1980) zonal markers. Because recovery was mainly in the upper Neogene sediments, and to achieve a high-resolution biostratigraphy, we referred exclusively to these biohorizons rather than to the standard zonal boundaries. Ages for most of the calcareous nannofossil datums employed to construct the Leg 175 age model come from Berggren (1995b). For the Pliocene-Pleistocene interval (Fig. 8A), however, we chose the updated ages used during Leg 172 (Shipboard Scientific Party, in press), which are derived, in part, from a conversion to the astronomical time scale of Lourens et al. (1996).
In addition to the classical concept of first/last occurrences (FO/LO) of index species, we used dominance intervals of single-species/taxonomical categories to improve the stratigraphic resolution of the Pleistocene interval. This includes the commonly used Emiliania huxleyi acme Zone, which roughly covers the last 90 k.y. (Thierstein et al., 1977), and the "Small Gephyrocapsa Zone" of Gartner (1977), an interval which defines the last 300 k.y. of Okada and Bukry's CN13b Biozone. The top of the Gephyrocapsa caribbeanica acme Zone, dated at 260 ka, is synchronous with the FO of Emiliania huxleyi (Pujos, 1988) and thus provides a useful alternative for identifying the base of Martini's NN21 Zone. In addition to these classical intervals of species dominance, we used the formal sequence of the isotopically calibrated Gephyrocapsa acme Zones built by Weaver (1993) for the last 1.2 m.y. to further improve the stratigraphic resolution of this time period (Fig. 8B). Based on updated ages (Raffi et al., 1993), we recalibrated the Small Gephyrocapsa acme Zone of Gartner (1977) as the interval spanning isotope Stages 30 to 44.
Besides biostratigraphical information, calcareous nannofossils can be used as tracers of upwelling dynamics and trophic domains in the Benguela upwelling system, as documented from studies of the water column (Giraudeau and Bailey, 1995) and surface sediments (Giraudeau, 1992) off Namibia and South Africa. Recent proxy species of eu-, meso-, and oligotrophic domains have already been identified from these pilot studies (Giraudeau and Rogers, 1994); their relative abundances were used in Quaternary sediments recovered from the southernmost sites of Leg 175 to infer the past intensity of the Benguela upwelling process.
Standard smear slides were made for all samples using Canada Balsam as a mounting medium. Calcareous nannofossils were examined by means of standard light microscope techniques under crossed nicols and transmitted light at 1000x magnification. Unless otherwise noted, we followed taxonomic concepts summarized in Perch-Nielsen (1985).
For description of preservational states and estimation of species relative abundances, we followed the system used during Leg 165 (Shipboard Scientific Party, 1997a) as follows:
Preservation:
Abundance:
Total abundance of calcareous nannofossils for each sample was estimated as follows:
Range charts including relative abundance estimates were constructed only for the stratigraphically significant taxa listed in Table 2.
The tropical Neogene planktonic foraminiferal "N-zonation" scheme used during Leg 175 is based on Blow (1969) and Kennett and Srinivasan (1983). A list of planktonic foraminiferal datums used in this study is presented in Table 3. The datum ages are based on Berggren et al. (1995a, 1995b) and Berggren et al. (1985) with various modifications. Estimates of changes in the position of the Benguela Current are based on six foraminiferal assemblages defined for the region by Jansen et al. (1996; Table 4).
Unlithified ooze was either washed directly in tap water or soaked briefly in a weak (10%) hydrogen peroxide (H2O2) solution, then washed over a 63-µm mesh sieve. Semilithified ooze was first partially broken up by hand and then soaked in a weak H2O2 solution before washing and sieving. All samples were dried at ~50°C on a hotplate.
Planktonic foraminiferal abundance was defined as follows:
Approximately 10 cm3 of sediment were washed through a 63-µm sieve, and the larger fraction was dried. The larger fraction was thereafter dry-sieved through a 125-µm sieve, and the benthic foraminifers were picked from the larger fraction. The counting procedure consisted of splitting the >125-µm fraction into an aliquot containing 300–400 specimens. Chang (1967) showed that identification of 300 randomly selected specimens from a larger assemblage provides a valid database for statistical analysis and that the results are not significantly improved by examining a greater number of specimens. Where available, specimens were identified to species level, counted, and the relative abundance for the species found was calculated.
Radiolarian zones are given in the tropical zonation of Moore (1995) wherever possible. Because some of the Leg 175 sites are located under cool current conditions, southern high-latitude zonations (e.g., Caulet, 1991; Lazarus, 1992) or northern mid- to high-latitude ones (e.g., Bjørklund, 1976; Motoyama, 1996) can be adopted. Applicability of the Indian low-latitude zonation (Johnson et al., 1989), as well as those mid- to high-latitude zonations, to the eastern South Atlantic are not yet confirmed. Some of the zones and datums presented by Sanfilippo et al. (1985) and Johnson et al. (1989) were tied to paleomagnetic polarities in the eastern equatorial Pacific sedimentary sequences of ODP Leg 138 (Moore, 1995). Caulet (1991) and Lazarus (1992) gave numerical ages to the Antarctic Neogene radiolarian datums. Table 5 lists late Miocene to Pleistocene tropical and Antarctic radiolarian datums selected from Caulet (1991), Moore (1995), and Motoyama (1996).
Sample preparation for microscopic examination during Leg 175 followed the standard techniques described by Sanfilippo et al. (1985). All samples were treated with acid and sieved at 63 µm, with the coarse fraction retained for slide preparation. When the acid-treated residues contained large clumps of clay aggregates, the coarse fraction was further treated with a strong base (NaOH) for several minutes, then briefly immersed in an ultrasonic bath and resieved.
For each sample examined, qualitative estimates of radiolarian abundance and preservation were made.
Radiolarian assemblage abundance was assessed as follows:
Preservation of the radiolarian assemblage was based on the following categories:
The diatom assemblages preserved in Leg 175 sediments allow recognition of the low-latitude diatom zonations of Barron (1985a, 1985b). Because of the mixture of warm, temperate, and Southern Ocean species and the occasional lack of biostratigraphic markers at the southern sites (Mid-Cape Basin and Southern Cape Basin), diatom biostratigraphic zonations are difficult to apply.
Biostratigraphic events, such as the FO or LO of a species (Table 6), have been assigned to the sample containing the first or last observed specimens following Cieselski (1983), Baldauf and Barron (1991), and Baldauf and Iwai (1995). For silicoflagellates, the LO of Bachmannocena quadrangula (0.8 Ma; Locker, 1996) was a useful marker at several sites. Because onboard biostratigraphy and magnetostratigraphy was based on the Berggren (1995b) time scale, published diatom datum ages were converted to the time scale of Shackleton et al. (1995), which approximates the Berggren (1995b) time scale. This conversion was done using the conversion table published by Wei (1994), which includes Shackleton et al.'s (1995) Leg 138 results.
The original geographic locations for which these zonations were established are not the same as that of this leg. Diatom data from previous DSDP legs (40 and 75) in the area of the Benguela Current between the Guinea Basin and South Africa are sparse. Diatom data for more recent sediments (<100,000 yr) of the Benguela Current area are readily found in the literature (e.g., Summerhayes et al., 1995). Recently, results from sediment trap studies have shown that diatom sedimentation is markedly seasonal and episodic in the area, and that these short-term variations may not be preserved in the geological record (Lange et al., 1994; Treppke et al., 1996a, 1996b). This considerable loss of information suggests caution in interpreting the sedimentary record. Nevertheless, typical diatom assemblages preserved in the sedimentary record can be used as tracers of the corresponding hydrographic conditions of the surface waters. In addition, occurrences of freshwater diatoms in the sediments are used as a link to continental climate recording eolian and/or riverine input (e.g., Pokras and Mix, 1985; Gasse et al., 1989).
In summary, the diatom assemblages observed in sediments recovered during Leg 175 provide environmental information (through marker species for upwelling, oceanic conditions, riverine input, etc.), and this has been the focus of diatom research during this leg.
Two types of slides were prepared for diatom analysis, depending on overall abundance. For areas of high abundance (e.g., Congo Basin, Walvis Ridge, and Walvis Basin), smear slides were prepared from a small amount of raw material in a core catcher. When dictated by a low concentration of diatom valves and/or abundant clay (e.g., Mid-Angola and Mid-Cape Basins), selected core-catcher samples were processed by boiling them in a solution of H2O2 and sodium pyrophosphate to remove organic matter and to disperse the clay-sized material, followed by treatment with hydrochloric acid to remove CaCO3. The treated samples were then washed with distilled water and sieved through a 20-µm sieve. Although this procedure biases assemblages toward larger diatoms, it improves the chances of successful biostratigraphy on diatom-poor sediments. In each case, aliquots of raw and cleaned samples were mounted on microslides using Hyrax mounting medium or Norland optical adhesive. All slides were examined in their entirety with phase-contrast illumination at a magnification of 400x for stratigraphic markers and paleoenvironmentally sensitive taxa. The counting convention of Schrader and Gersonde (1978) was adopted.
Overall diatom abundance and species relative abundances were determined based on smear-slide evaluation at 400x, using the following convention:
For computing purposes, a number was assigned to each abundance category (1 = T, 2 = R, 3 = F, 4 = C, and 5 = A). A new category, very abundant (VA = 6), was applied only at Site 1084 to reflect the very high abundances of diatoms encountered when each field of view was filled with pennate diatoms of the family Thalassionemataceae.
Preservation of diatoms was determined qualitatively as follows:
A number was assigned to each category: 1 = poor, 2 = moderate, and 3 = good.