Three holes were drilled at Site 1081 with a maximum penetration of 452.7 mbsf (Fig. 13). The APC was used down to Cores 175-1081A-16H, 175-1081B-21H, and 175-1081C-17H. Because of gas expansion and the fact that several intervals, each as much as 60 cm long, contained flow-in structures, the APC cores have an average recovery >100%. Hole 1081A was deepened to Core 175-1081A-49X (452.7 mbsf) using the XCB, with an average recovery of 78.9%. The XCB cores generally consist of 2- to 4-cm-thick sediment biscuits that are embedded in drill mud. The top 30–60 cm of both ACP and XCB cores are generally disturbed, and the core catcher contains a mixture of material from the whole core.
Sediments from Site 1081 form two lithostratigraphic units. Unit I is composed 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 amounts of diatoms, nannofossils, foraminifers, and radiolarians (Fig. 13). Unit I is moderately bioturbated, containing burrows of ~1 to 2 cm in diameter. Three subunits are defined based on variations in abundance and type of microfossils in the sediments. Unit II is composed of olive (5Y 4/3) and olive-gray (5Y 4/2) clayey nannofossil ooze.
The uppermost subunit is composed of bioturbated olive (5Y 4/3) and olive-gray (5Y 4/2), nannofossil- and foraminifer-rich clay. The contact between Subunits IA and IB is defined by an increase in the abundance of diatoms from bearing to rich and a corresponding decrease in nannofossil content. The contact is gradational over tens of meters, but the boundary is placed between Cores 175-1081A-9H and 10H (77 mbsf), Sections 175-1081B-9H-3 and 9H-4 (76 mbsf), and Sections 175-1081C-10H-4 and 10H-5 (90 mbsf). The subunit contains intervals with different-colored clays whose thicknesses range from 60 to 250 cm and that grade into one another over a distance of 20 to 40 cm. Subunit IA contains short intervals at Holes 1081A and 1081B in which the nannofossil and foraminiferal abundances are high enough for the sediments to be defined as nannofossil and foraminifer ooze (e.g., all of Core 175-1081A-2H and interval 175-1081B-6H-5, 50 cm, to 175-1081B-6H-7, 60 cm). Subunit IA has high carbonate (average of 30 wt%) and organic carbon (5 wt%) concentrations (see "Organic Geochemistry" section, this chapter).
Subunit IB is composed of intercalated intervals of black (5Y 2.5/1) and dark olive-gray (5Y 3/2) diatom-rich clay. Subunit IB is recognized in all three holes, but its contact with Subunit IC is observed only between Cores 175-1081A-26X and 27X (230 mbsf). The transition from Subunit IA to Subunit IB is gradational over several meters and is marked by an increase in the abundance of diatoms, a decrease of nannofossils, and a disappearance of planktonic foraminifers (see "Synthesis of Smear-Slide Analyses" and "Biostratigraphy and Sedimentation Rates" sections, this chapter). The same trend is reflected in the decrease of the carbonate concentration to an average of 6 wt%, with minimum values <1 wt%. Subunit IB contains intervals of different-colored clays ranging in thickness from 60 to 250 cm. Intervals grade into one another over a distance of 20 to 40 cm. Subunit IB is darker in color and has lower total reflectance than Subunit IA. Smear-slide examination indicates an increase in abundance of organic matter (see "Synthesis of Smear-Slide Analyses" section, this chapter), which is confirmed by an increase in the organic carbon concentration to a maximum of 8.2 wt% and an average of 6 wt%. Dolomitized clays are present in Subunit IB in intervals 175-1081A-17X-1, 0–24 cm (~137 mbsf), 175-1081A-19X-1, 0–25 cm (~154 mbsf), 175-1081A-20X-2, 60–110 cm (~166 mbsf), and 175-1081A-22X-3 (~183 mbsf; see Fig. 14).
Subunit IC is composed of olive (5Y 4/3) to olive-gray (5Y 4/2) nannofossil-rich clay. The transition between the diatom-rich, nannofossil-poor Subunit IB and the nannofossil-rich and diatom-poor Subunit IC is gradational and occurs between Cores 175-1081A-26X and 27X (230 mbsf). The transition from Subunit IC to Unit II is gradational and occurs within Section 175-1081A-43X-3 (390 mbsf). This boundary is marked by a significant decrease in the abundance of diatoms and an increase in the abundance of nannofossils. Subunit IC has high carbonate concentrations, which average 20 wt%, and low organic carbon concentrations, which average 2.5 wt%. No dolomitized clay intervals are found in Subunit IC; however, subhedral dolomite rhombs are observed in smear slides.
Unit II is composed of olive (5Y 4/3) to olive-gray (5Y 4/2) clayey nannofossil ooze. Unit II has high calcium carbonate content, which averages 40 wt%, but low organic carbon content, which averages 2 wt%. Foraminifers and diatoms are present in Unit II but only in Cores 175-1081A-44X and 45X.
Smear-slide analyses indicate that the detrital component of the sediment is a clay with rare silt-sized, angular and subangular, mono- and polycrystalline quartz grains, and feldspar. Muscovite and biotite are present in trace amounts. The biogenic component is represented by varying abundances of foraminifers (whole and fragments), nannofossils, and diatoms. Varying amounts of particulate organic matter and authigenic minerals, such as glauconite, framboidal pyrite, and dolomite, are observed in both trace and few abundances. Euhedral dolomite rhombs and fragments of sparry calcite cement are also common below Section 175-1081A-27X-2.
X-ray diffraction (XRD) analysis of sediments from Hole 1081A reveals that the clastic fraction is dominated by the clay minerals smectite, kaolinite and illite, muscovite (mica), quartz, and the feldspars microcline and albite. Pyrite was the only sulfide mineral identified as an accessory phase. In the lithified clay horizons, dolomite is identified, whereas external to these horizons, only minor amounts of dolomite are observed. The detected spacings of this mineral are slightly offset to larger lattices, which points to the formula Ca1.08Mg0.92(CO3)2. The detected biogenic components are calcite and opal. Calcite peak intensities are strongly correlated with measured calcium carbonate concentrations (r = 0.97). Opal was measured from the height of the amorphous opal bulge according to the method of Eisma and van der Gaast (1971); the opal data are semiquantitative because no standard sample was available for calibration on board. The opal results are corroborated by the trends in the diatom abundance curves (Fig. 15).
Based on the downcore variations of quartz, albite, and microcline, the core can be subdivided into five parts (Fig. 16). A major transition coincides with the boundary between lithologic Subunits IC and IB at ~230 mbsf. Below this boundary, all three minerals covary, both in the long-term increase with time and in shorter term, low-amplitude variations. The variations are also visible in the opal record. Above this boundary, the three minerals decrease and the shorter term variations show higher amplitudes and are different for albite compared with quartz and microcline. This is the interval where the diatom abundances (see "Biostratigraphy and Sedimentation Rates" section, this chapter) and opal increase rapidly. In the upper part of Subunit IB, between ~175 and 100 mbsf, all three minerals show different shorter term variations. For the long-term trend in this interval, quartz is high, as is opal. Between ~100 and 30 mbsf, the three minerals decrease in abundance, together with the opal counts and diatom abundance. In the shorter time scale, quartz and microcline show comparable variations, following the variations for opal. Above 30 mbsf, the three minerals show no correlation among one another or with opal.
The small grain size (silt sized) of quartz and feldspar, as observed from the smear slides, indicates an eolian origin. Their low-amplitude behavior and covariance with opal below ~235 mbsf suggest that the minerals reflect a relatively simple climatic system in which the force of the southeasterly trade winds controls the upwelling and the eolian input in late Miocene and early Pliocene times. The variations are too large to be explained solely by differences in dilution with calcium carbonate, although quartz and calcium carbonate are strongly negatively correlated (Fig. 17). Around the lower to upper Pliocene boundary (235 mbsf), the contribution of the eolian components starts to become decoupled, which implies that the character of the transport mechanisms or the source areas must have changed. A similar feature occurred around 2.2 Ma (175 mbsf), when the sources of quartz and microcline also became decoupled and the variations started to show higher amplitudes. The appearance and disappearance of the eolian transport mechanism may occur through the deplacement of the climatic system or by a change in the terrigenous source. Besides the southeasterly trade winds, the Namibian bergwinds (land winds that blow into the ocean particularly in the austral winter) are an important carrier of eolian sediments to the South Atlantic. These bergwinds may have become important during the early to late Pliocene transition either by their onset or because the southern African deserts originated during this period. The later, less well-defined transitions might then be caused by changes in the position and direction of the trade winds.
Color data were measured every 2 cm for Cores 175-1081A-1H through 9H. Cores 175-1081A-10H through 49X and all of Holes 1081B and 1081C were measured at 4-cm intervals. At Site 1081, total reflectance values range between 24% and 44%, and the red/blue (650 nm/450 nm) ratio varies between 1.1 and 1.6 (Fig. 18). This range is greater than that observed at Sites 1075–1080 (e.g., see "Lithostratigraphy" section, "Site 1075" chapter, this volume).
The general trends in red/blue ratio, total reflectance, magnetic susceptibility, and GRAPE density can be correlated to the litho-stratigraphic units and subunits defined for Hole 1081A (Fig. 18). The magnetic susceptibility record seems to follow clay abundance in these sediments. Changing concentrations of organic carbon and calcium carbonate influence total reflectance and red/blue ratio in sediments from Site 1081, as well as bulk density.
In lithostratigraphic Subunit IA of Holes 1081A, 1081B, and 1081C, red/blue ratios and total reflectance values are high compared with the other subunits of Unit I (Fig. 18). Subunit IA is characterized by high calcium carbonate and organic carbon contents. The red/blue ratio is positively correlated with calcium carbonate, whereas total reflectance shows a weak negative correlation with organic carbon content (Fig. 19A, B). There is no relationship between calcium carbonate and total reflectance (Fig. 19C). This suggests that in Subunit IA, carbonate dominates the red/blue ratio, whereas the concentration of organic matter may influence the total reflectance.
In lithostratigraphic Subunit IB, total reflectance, and red/blue ratio values are lower than in Subunit IA; GRAPE density and magnetic susceptibility also have low values (Fig. 18). Compared with Subunit IA, Subunit IB has higher abundances of diatoms (see "Description of Lithostratigraphic Units" section, this chapter) but similar organic carbon content (see "Organic Geochemistry" section, this chapter; also see Fig. 18). No specific relationship is evident between total reflectance and organic carbon. We suggest that the high abundances of diatoms in this subunit may attenuate the effect of organic carbon on total reflectance. Within Subunit IB, the highest red/blue ratios correspond to intervals containing dolomitized clays (see above), which is consistent with the high GRAPE density values.
Subunit IC has slightly higher red/blue ratios and total reflectance values than Subunit IB, as well as higher magnetic susceptibility and GRAPE density values (Fig. 18). Subunit IC contains more nannofossils and very few diatoms compared with Subunit IB. Over this interval, the organic carbon content decreases downcore, whereas the calcium carbonate content increases (see "Organic Geochemistry" section, this chapter). In this subunit, calcium carbonate content controls both total reflectance and red/blue ratios because the organic carbon content is so low (Fig. 19D, E). Similarly, in Unit II calcium carbonate content exerts the main control on color reflectance data (Fig. 19D, E).