Sediments from Site 1082 form two lithostratigraphic units (Fig. 1). Unit I is composed of moderately bioturbated, intercalated intervals of olive (5Y 4/3), olive-gray (5Y 4/2), dark olive-gray (5Y 4/1), and black (5Y 2.5/1) clays, which contain varying abundances of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are distinguished based on microfossil type and abundance: Subunits IA, IB, and IC. Unit II is composed of homogeneous, greenish gray (5GY 6/1), light greenish gray (5GY 7/1), and olive-gray (5Y 4/2) nannofossil ooze. Units and subunits at Site 1082 are correlative to those identified at Site 1081. Differences between these two sites are the higher sedimentation rates and the better temporal resolution at Site 1082 compared with Site 1081. Sediments from Site 1082 also have higher abundances of nannofossils than those from Site 1081.
The uppermost subunit is composed of intercalated intervals of moderately bioturbated, olive (5Y 4/3) and olive-gray (5Y 4/2), nannofossil- and foraminifer-rich clay. Subunit IA contains intervals of different colored clays which range in thickness from 60 to 250 cm. They grade into each other over 20 to 30 cm (Fig. 2). Within Subunit IA some intervals contain nannofossil oozes (e.g., 175-1082A-6H-5, 90 cm, to 6H-6, 15 cm). Subunit IA generally has high carbonate and organic carbon contents, which average 42 and 6 wt%, respectively (see "Organic Geochemistry" section, this chapter). The contact between Subunits IA and IB is defined as the depth below which the diatom abundance shifts from diatom-bearing to diatom-rich. This transition is gradational and occurs between Cores 175-1082A-12H and 13H (112.3 mbsf), 175-1082B-13H and 14H, and 175-1082C-13H and 14H. The diatom abundance within the sediments begins to increase in Cores 175-1082A-8H, 175-1082B-8H, and 175-1082C-9H.
Subunit IB was intersected in all three holes, but its contact with Subunit IC is only observed between Cores 175-1082A-40X and 41X (359.9 mbsf). Subunit IB is composed of intercalated intervals of dark olive-gray (5Y 3/2) and olive (5Y 5/3) nannofossil- and diatom-rich clay and nannofossil-rich diatomaceous clay. Intervals of different colored clays range in thickness from 60 to 250 cm and grade into each other over 20 to 30 cm. Subunit IB contains several black (2.5Y 2.5/2) and olive-black (5Y 2.5/2), 35- to 150-cm-thick, clay intervals that have lower nannofossil abundances, higher abundances of organic matter, and coarse silt-sized, subangular mono- and polycrystalline quartz grains (see "Synthesis of Smear-Slide Analyses" section, this chapter). These intervals grade into the lighter colored clays over 20 to 40 cm (e.g., 175-1082A-16X-4, 90 cm, to 1082A-16X-CC, 50 cm; 175-1082A-18X-3, 90 cm, to 1082A-18X-4, 80 cm; 175-1082A-19X-4, 10 cm, to 1082A-19X-5, 15 cm; 175-1082A-19X-6, 23–59 cm; see visual core descriptions, Section 4, this volume). Subunit IB is darker in color and has a lower total reflectance than Subunit IA. Its color is thought to reflect higher organic carbon contents. Organic carbon concentrations range from 2 to 8 wt% and average 4 wt%. Calcium carbonate concentrations are lower than in Subunit IA, averaging 33 wt%.
The contact between Subunits IB and IC is marked by a change in the abundance of diatoms from diatom-rich to diatom-bearing clays, or clays with trace abundances of diatoms (see "Synthesis of Smear-Slide Analyses" and "Biostratigraphy and Sedimentation Rates" sections, this chapter). The boundary is located between Cores 175-1082A-40X and 41X (369.5 mbsf) and is gradational over several tens of meters. Nannofossils and foraminifers are present in higher abundances at Site 1082 than at Site 1081. Dolomitized clays are present in Subunit IB at intervals 175-1082A-15X-1, 0–13 cm (128.6 mbsf); 22X-3, 114–145 cm (200.7 mbsf); and 36X-1, 0–17 cm (327.5 mbsf).
Subunit IC is composed of intercalated intervals of pale yellow (5Y 7/4), olive (5Y 4/3), and dark olive-gray (5Y 3/2) nannofossil-rich clay and nannofossil clay. Concentrations of calcium carbonate are high and average 51 wt%, whereas concentrations of organic carbon are lower than in Subunit IB, averaging 4 wt%. The contact with Unit II is marked by a significant increase in nannofossil abundance and is gradational over tens of meters. The boundary between the nannofossil clays and nannofossil oozes of Unit II occurs between Cores 175-1082A-51X and 52X (475 mbsf).
Unit II is composed of olive (5Y 4/3) to olive-gray (5Y 4/2) nannofossil ooze. Unit II has high calcium carbonate contents, which average 67 wt%, but low organic carbon contents, which average 2 wt%.
Smear-slide analyses indicate that the detrital component of the sediments in all units and subunits consists of clay with rare silt-sized, angular and subangular feldspar and mono- and polycrystalline quartz grains. Muscovite and biotite are present in trace amounts. The grain size of identifiable detrital components is generally constant throughout all lithostratigraphic units. The relative proportion of detrital to biogenic particles is relatively constant throughout Unit I. The clastic fraction dominates Unit I, whereas biogenic carbonates are the most abundant component in Unit II. Authigenic minerals, such as glauconite and framboidal pyrite, are either rare or present in trace amounts only. Dolomite rhombs are observed in both Subunits IB and IC. The biogenic component is represented by varying abundances of foraminifers (whole and fragments), nannofossils, diatoms, radiolarians, sponge spicules, and silicoflagellates.
Smear-slide analyses of sediment from Subunit IA revealed abundant to common foraminifer fragments, common to very abundant nannofossils, rare siliceous sponge spicules, and trace amounts of radiolarians. Diatom abundances vary from common to barren. The relative abundances of the biogenic components change frequently within one core. Individual intervals are between 30 and 250 cm thick. The intercalated dark olive-brown and black clay intervals have distinctly lower abundances of biogenic components and occasionally show higher abundances of silt-sized mono- and polycrystalline quartz grains. The darkest layer shows amorphous brown aggregates of organic matter and is completely barren of microfossils.
In Cores 175-1082A-13H through 21X of Subunit IB, the abundances of diatoms are high, whereas nannofossils and foraminifer fragments are less abundant than those of Subunit IA. In Core 175-1082A-22X, nannofossil-rich sedimentary intervals are more frequent, and the abundances of nannofossils steadily increases in all the subsequent stratigraphically deeper intervals. Diatoms remain common down to Core 175-1082A-39H, below which they become rare. Foraminifer fragments are rare to common throughout Subunit IB but disappear at the bottom of this subunit. Radiolarians are present in trace amounts.
In Subunit IC, large carbonate aggregates become common components in all smear slides. These carbonates are most likely derived from the dissolution and recrystallization of foraminifer fragments and nannofossils. Nannofossils are the most abundant biogenic component in this subunit. Foraminifers are present in rare to trace abundances, whereas diatoms are largely absent. The transition to Unit II is marked by the depth below which nannofossils become the most abundant component in the smear slides.
Of all the sediments recovered from Site 1082, Unit II has the highest abundances of nannofossils. This unit also exhibits the smallest compositional variability. In addition to nannofossils, biogenic components include rare to frequent diatoms, siliceous spicules, foraminifers, and trace amounts of radiolarians. Recrystallized carbonate aggregates are common. In this unit, the detrital component is less abundant than the biogenic component, and angular quartz grains are present only in trace amounts.
X-ray diffraction (XRD) analysis of the upper 400 m of sediments from Hole 1082A reveals that the clastic fraction is dominated by the clay minerals smectite, kaolinite, illite, and muscovite (mica). Quartz, microcline, and albite are also major constituents in the clastic fraction. Pyrite is the only sulfide mineral identified as an accessory phase. Dolomite is identified in the lithified clay horizons. Detected spacings of dolomite indicate slightly larger crystal lattices and suggest a nonstoichiometric dolomite composition with slight relative calcium enrichment. The dolomite horizon at 327 mbsf (Sample 175-1082A-36X-1, 0–1 cm) contains 35 wt% dolomite (see "Organic Geochemistry" section, this chapter). Sediments adjacent to the dolomite horizon contain only trace amounts of dolomite. The detected biogenic components are calcite and opal. Calcite peak intensities are strongly correlated to measured calcium carbonate concentrations (r = 0.98). Opal concentrations were estimated from the height of the amorphous opal bulge according to the method of Eisma and van der Gaast (1971); the opal data are only qualitative because no standard opal sample for calibration was available on board. The general trends in opal abundances reflect the variation in diatom abundance curves in Hole 1082A (Fig. 3A).
The downcore variations in intensity of quartz and opal are weakly negatively correlated with calcium carbonate (Fig. 4), as is also true for the feldspars and muscovite. The variations, however, are too large to be explained by differences in dilution by carbonate alone. To exclude this dilution effect, we compared the ratios of the minerals relative to quartz. In the upper part of the core, the amplitudes increase in two phases, and the correlations decrease in two steps: the first around 150 mbsf and the second around 40 mbsf.
The fine silt size of quartz and feldspar, as observed from the smear slides, may indicate an eolian origin. Muscovite originates from the southern Namib Desert (Jansen and van der Gaast, unpubl. data). The correlation of quartz intensities with opal (Fig. 3C) suggest that the minerals reflect a climatic system in which the force of the southeast trade winds controls both the upwelling and the eolian input. In the vicinity of the Pliocene/Pleistocene boundary (~155 mbsf), the various contributions of the eolian components started to become decoupled, which implies that the character of the transport mechanisms or the source areas must have changed. A similar change occurred around 400 ka (40 mbsf) when a second increase in amplitude took place. Both transitions are also reflected in the sediments of Hole 1081A from the Walvis Ridge (see "Lithostratigraphy" section, "Site 1081" chapter, this volume).