Three holes with a maximum penetration of 205 mbsf were drilled at Site 1076 (Fig. 1). The upper 30 cm of the first five cores at all three holes were very disturbed and soupy and unsuitable for sampling. Severe core disturbances and large voids from gas release and core cutting of friable sediments occurred at the top and bottom sections of Cores 175-1076A-9H through 22H and 175-1076C-9H through 22H.
The lithostratigraphic description for the sedimentary sequence from Site 1076 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) X-ray diffraction (XRD) measurements.
Sediments from Site 1076 form one lithostratigraphic unit composed of organic carbon-rich olive-gray (5Y 3/2) to greenish gray (5GY 5/1) clay. The variation in color between olive-gray and greenish gray intervals may result from variations in several parameters: (1) organic carbon contents, (2) the relative contribution of the clay vs. the biogenic component, and (3) changes in the abundances of glauconitic clays and pyrite. Most of the sediments are bioturbated, which is most clearly seen in intervals with a pronounced change in sediment color. The clays progressively become more friable with depth as a result of compaction. Many cores contain gas expansion voids that were produced by the release of carbon dioxide trapped in the sediment (see "Organic Geochemistry" section, this chapter). A 3-cm-thick package of interlayered laminae of shell fragments and olive-gray clay is present in interval 175-1076C-14H-5, 45-47 cm. Laminae range in thickness from 2 to 3 mm. Shell-rich laminae are composed of broken pteropod shells and are discontinuous. The package has sharp lower and upper contacts (Fig. 2) and is coincident with the paraconformity described in the "Biostratigraphy and Sedimentation Rates" section (this chapter). In addition, small shell fragments are present in many intervals throughout the sediment recovered from this site. Above 150 mbsf, calcium carbonate in sediments alternates from a low of ~3 wt% to a maximum of 16 wt%. Below 150 mbsf, calcium carbonate preservation is poor and values are limited to a maximum of 3.5 wt%.
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 abundant diatoms with rare nannofossils, silicoflagellates, siliceous sponge spicules, phyto-liths, and traces of radiolarian and foraminifer fragments. Diatoms are abundant in both greenish gray and olive-gray intervals independently of organic carbon contents.
Authigenic components are dominated by the presence of glauconite, dolomite, and iron sulfides. Glauconite peloids, along with pyritized-worm casts, -diatoms and -radiolarians, are observed. Small dolomite rhombohedrons (6-100 µm) exist throughout. Iron sulfides are present primarily in the form of disseminated pyrite and framboidal pyrite, confirming the process of bacterial sulfate reduction. Rare, friable nodules, possibly phosphatic, are disseminated throughout certain intervals. Nodules range in diameter from 1 to 2 mm.
The XRD patterns of Hole 1076A document five major minerals: smectite, kaolinite/illite, quartz, calcite, and pyrite. The smectites are generally poorly crystallized. Shipboard XRD spectra for Site 1076 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). As at Hole 1075A, low measured smectite values may represent large contributions of this mineral to the mineral association at this hole.
Comparison of the K/(K+Sm) ratios with the original 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). There is only one clear exception: at 96.5 mbsf, high kaolinite intensity coincides with high smectite intensity. Because kaolinite is a known product of chemical weathering of igneous rocks in the tropical rain forest (Singer, 1984) the high ratios, except the one at 96.5 mbsf, 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 1076A. Holes 1076C and 1076D were measured at 4-cm intervals. The light reflectance data are in the range of 30% to 45% throughout the column recovered from Site 1076. The total reflectance (Fig. 4) and red/blue (650 nm/450 nm) ratio (Fig. 5) were smoothed over nine points for Hole 1076A and over five points for Holes 1076C and 1076D to remove smaller scale variability. Light reflectance records for Hole 1076A show marked intervals with higher reflectance values. The significance of these intervals is not readily apparent. Presumably, clay-rich sediments have a lower total reflectance (Mix et al., 1992), while carbonate-rich and diatom-rich sediments are more highly reflective. Thus, a high proportion of biogenous components may be indicated. The red/blue ratio presents no correlation with calcium carbonate and shows a weak correlation with organic carbon (see Fig. 6). Using the age model provided by the "Biostratigraphy and Sedimentation Rates" and "Paleomagnetism" sections (this chapter; see Fig. 1), periodicity in the red/blue ratio seems to be dominated by the 23-k.y. period.