Three holes with a maximum penetration of 201 mbsf were drilled at Site 1075 (Fig. 13). In the upper 50 m of all three holes, the sediments had a distinct hydrogen sulfide smell, suggesting bacterial oxidation of organic matter. Below 50 mbsf, the hydrogen sulfide smell gradually disappeared. Many cores contain gas voids that were produced by the release of carbon dioxide trapped in the sediment (see "Organic Geochemistry" section, this chapter).
The lithostratigraphic description for the sedimentary sequence from Site 1075 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 1075 form one lithostratigraphic unit composed entirely of greenish gray (5GY 5/1) to olive-gray (5Y 3/2) nannofossil-bearing diatomaceous clay and diatomaceous clay (Fig. 13). Most of the sediment is moderately to extensively bioturbated. Bioturbation is most clearly seen as mottled intervals in which there is a change in sediment color. Visual examination of the sediments revealed that subtle color changes are common throughout the core. A sharp color change in Section 175-1075C-5H-3 accompanies the appearance of nannofossils (Fig. 14). The sediments become increasingly compacted downhole. No change in lithology was observed. The sediments have overall low calcium carbonate contents of generally <2.5 wt% CaCO3 (see "Organic Geochemistry" section, this chapter). Between 40 and 100 mbsf, narrow intervals have calcium carbonate contents as high as 17.3 wt%. Evidence from smear-slide and micropaleontological analyses suggests that increases in the abundance of nannofossils and/or foraminifers are responsible for the increase in CaCO3 (see "Biostratigraphy and Sedimentation Rates" section, this chapter).
Smear-slide analyses indicate that the clastic component is dominated by clay minerals with trace amounts of quartz and feldspar. Biogenic portions of sediments contain abundant diatoms (10%-35%) with variable amounts of nannofossils (0%-10%), rare silicoflagellates, siliceous sponge spicules, phytoliths, and traces of radiolarian and foraminiferal fragments.
Authigenic components are dominated by the ubiquitous presence of iron sulfides, primarily in the form of disseminated pyrite and framboidal pyrite, confirming the process of bacterial sulfate reduction (see "Inorganic Geochemistry" section, this chapter). Framboidal pyrite is also frequently found in the tests of diatom shells. Rare, friable whitish gray nodules, possibly phosphatic, are disseminated throughout the core. Nodules range in diameter from 1 to 3 mm and envelope the surrounding sediment. Small shell fragments are present in many intervals.
The XRD patterns of Hole 1075A represent five major minerals: smectite, kaolinite/illite, quartz, calcite, and pyrite. The smectites are generally poorly crystallized. Shipboard XRD spectra for Site 1075 are not precise enough to determine the smectite crystallinity. Van der Gaast and Jansen (1984) found that in the Congo Basin area, the peak height of smectite covaries with its crystallinity. High and low values correspond to higher and lower crystallinities, respectively. They inferred that the main source of variations in clay mineral associations is the varying contribution of poorly crystallized smectites. Consequently, low measured smectite values may represent a large contribution of this mineral to the mineral association.
In the upper 100 m of Hole 1075A, most kaolinite peaks coincide with low smectite counts, suggesting little dilution by low-crystallinity smectite and large contributions of kaolinite. There is one clear exception: at 119 m, high kaolinite intensity coincides with a high smectite intensity. Most kaolinite low counts correspond to relatively well-crystallized smectite. The relative contributions of poorly crystallized smectite (Sm) and kaolinite (K) can be expressed by their K/(K+Sm) ratio. Kaolinite is a known product of chemical weathering of igneous rocks in the tropical rain forest (Singer, 1984). High ratios, except the one at 119 m, may suggest humid periods in the Congo drainage area. Lower ratios between 30 and 130 m represent more arid periods (Fig. 15).
Visible light reflectance was measured every 2 cm for Hole 1075A and in the upper 12 cores from Hole 1075B. The lower sections of Holes 1075B and 1075C were measured at 4-cm intervals. This decreased resolution was matched to the stratigraphic resolution used for the MST data. The variations in total visible light reflectance levels are low (20%-50%) and differ slightly among the three holes. The red/blue ratio displays a pronounced periodicity in all three holes (Fig. 16). Preliminary biostratigraphic analysis suggests that this periodicity corresponds to the 23-k.y. precession cycle (see "Biostratigraphy and Sedimentation Rates" section, this chapter). Variations in the ratio of the red-to-blue wavelengths are weakly correlated with the concentration of organic carbon but show no covariation with concentrations of CaCO3 or total sulfur (Fig. 17). The ratio between the red (650 nm) and blue (450 nm) end-member reflectance levels at this site may be attributed to the presence of iron sulfides (e.g., pyrite) in the sediment. High abundances of pyrite in clays tend to decrease the reflectance of sediment at 650 nm and would lower the red/blue ratios (Mix et al., 1992). By contrast, a greater amount of diatoms would enhance the reflectance at 650 nm and would thus lead to higher red/blue ratios.