Three holes with a maximum penetration of 205.1 mbsf were drilled at Site 1077. Core disturbances occurred at the top of the first five cores. The upper 30 cm of these cores were disturbed and soupy and thus unsuitable for sampling. The first four cores in all three holes had a strong hydrogen sulfide smell indicating the microbial reduction of sulfate to sulfide.
The lithostratigraphic description for the sedimentary sequence from Site 1077 is based on data from the following sources: (1) visual core description, (2) smear-slide examination, (3) color reflectance measurements, (4) bulk calcium carbonate measurements, and (5) XRD measurements.
Sediments from Site 1077 form one lithostratigraphic unit composed of intercalated, 40- to 150-cm-thick intervals of olive-gray (5Y 3/2) and greenish gray (5GY 5/1) diatom-rich, diatom-bearing, nannofossil-bearing, and nannofossil-rich clay (Fig. 1). The relative abundances of the biogenic components varied greatly downcore. Color changes occur gradually over 20 to 30 cm except in Core 175-1077A-13H, where very abrupt color changes are observed. Variations in color between these intervals may be a result of fluctuations in contents of organic carbon, diatoms, and diagenetic conditions (see "Synthesis of Smear-Slide Analyses" section, this chapter). Greenish gray intervals become thicker and more abundant down each hole. Most of the sediment is moderately bioturbated. Bioturbation is most clearly seen in mottled intervals of 15- to 30-cm thickness that are associated with a pronounced change in sediment color (Fig. 2). Burrows range in diameter from 1 to 2 cm. Pteropod shells are common in the first core from all three holes. Small shell fragments are present in many intervals throughout the sediment recovered from this site. The sediment progressively becomes more friable downcore as a result of compaction. Many cores contain gas expansion voids that were produced by the release of carbon dioxide and methane trapped in the sediment (see "Organic Geochemistry" section, this chapter). Rare, friable nodules are disseminated throughout certain intervals. Nodules may be phosphatic and range in diameter from 1 to 2 mm.
In Hole 1077A, calcium carbonate contents in sediments vary between 0.8 and 13.2 wt% (Fig. 1). Close sampling across a sharp color transition from greenish gray clay to olive-gray clay revealed that concentrations of CaCO3, organic carbon, and total sulfur sharply increase in the olive-gray layer. Figure 1 shows that sediments below 120 mbsf have, on average, lower concentrations of CaCO3 compared with sediments above 120 mbsf.
Smear-slide analyses indicate that the clastic component is dominated by clay minerals and minor amounts of quartz and feldspar. The biogenic portions of sediments contain rare to frequent diatoms, rare nannofossils, silicoflagellates, siliceous sponge spicules, phytoliths, and traces of radiolarian and foraminifer fragments. Variations in the abundances of diatoms are not directly related to color changes from greenish gray to olive-gray intervals.
Authigenic components are dominated by the presence of glauconite, dolomite, and iron sulfides. Worm casts, diatom frustules, radiolarians, and glauconite peloids often contain pyrite. Small dolomite rhombohedrons (6-100 µm) are found at all depths in the three holes. Iron sulfides are present primarily in the form of disseminated pyrite and framboidal pyrite, confirming the process of bacterial sulfate reduction. Framboidal pyrite is commonly observed in diatom tests. Euhedral pyrite crystals frequently replace glauconite peloids. In general, olive-gray (5Y 3/2) intervals contain more framboidal pyrite relative to glauconite than that found in the greenish gray (5GY 5/1) intervals.
XRD analysis of the sediments from Hole 1077A reveals that the clastic fraction is dominated by smectite, kaolinite/illite, and quartz. Pyrite is also present as an accessory mineral in all samples. Glauconite is not observed; it is probably masked in the X-ray patterns by other clay minerals. The smectites are generally poorly crystallized. Shipboard XRD spectra for Site 1077 are not precise enough to determine the smectite crystallinity. The clay-mineral association in the Congo Basin area is controlled mainly by the varying contribution of these poorly crystallized smectites (van der Gaast and Jansen, 1984). Similar to Sites 1075 and 1076, low smectite values suggest significant low-crystalline contributions to the overall mineral association. Comparison of the K/(K+Sm) ratios with the kaolinite and smectite intensities shows that the peaks in the ratio represent high kaolinite counts that are not diluted by large amounts of low-crystalline smectite (Fig. 3). Because kaolinite is a known product of chemical weathering of igneous rocks in the tropical rain forest (Singer, 1984), the high ratios suggest humid periods in the Congo drainage area, whereas the lower ratios represent more arid periods.
Color data were measured every 2 cm for Hole 1077A. Hole 1077B was measured at 4-cm intervals. The reflectance data range between 25% and 45% throughout the column recovered from Site 1077. The total reflectance (Fig. 4) and red/blue (650/450 nm) ratio (Fig. 5) data were smoothed over nine points for Hole 1077A and over five points for Hole 1077B to remove smaller scale variability. Total reflectance covaries with magnetic susceptibility (see Fig. 6), GRAPE density (see "Physical Properties" section, this chapter), and the downhole logging of the gamma-ray intensity at Hole 1077A (see "Downhole Logging" section, this chapter). Increased magnetic susceptibility, GRAPE density, and gamma-ray intensity suggest a relative increase in the clay component. If clay content were the only controlling parameter for total reflectance, then an increase in the clay component would decrease the total reflectance. The opposite is observed; therefore, total reflectance must be caused by variations in the other sediment components such as (1) the organic carbon content, (2) the biogenic component, which at Site 1077 is dominated by diatoms, and/or (3) the authigenic minerals. Organic matter is known to have a typically low reflectance in the visible domain and a high reflectance for the red to near-infrared domain (Mix et al., 1992). No clear relation, however, is observed between the red/blue ratio and organic carbon contents (Fig. 7C). There is less information available about the influence of biogenic silica. The carbonate might control the red/blue ratio for the sediments with calcium carbonate concentrations >4 wt% (Fig. 7B). These higher weight percentages are relatively rare in sediments from this area (Jansen et al., 1984). For samples with lower carbonate concentrations, the comparison between sulfur and total reflectance shows a slightly positive relationship that suggests the influence of authigenic minerals on total reflectance (Fig. 7B). No correlation with pyrite, as measured with XRD, is observed. Despite the weak correlation between sulfur and total reflectance, we suggest that the lower total reflectance values measured during periods of relative reduced abundance of clay may reflect a relative increase of authigenic minerals present in the sediments, which is the result of there originally having been high levels of organic matter in the sediment. Shore-based work is required to discover the mechanism controlling the trends in the color data.
According to the biozones (see "Biostratigraphy and Sedimentation Rates" section, this chapter), intervals with high values of total reflectance and magnetic susceptibility generally correspond to interglacial stages (Fig. 6). Conversely, low values of total reflectance and magnetic susceptibility correspond to glacial stages. Using the ages provided by the "Biostratigraphy and Sedimentation Rates" and "Paleomagnetism" sections (this chapter), the periodicity in the red/blue ratio seems to be dominated by the 23-k.y. period (see Fig. 1, Fig. 6), whereas total reflectance and magnetic susceptibility records show longer periodicities. We suspect, therefore, that color is dominated by mechanisms tied to wind-driven productivity changes, while total reflectance and magnetic susceptibility are linked to fluctuations in sea level.