The aim of this synthesis is to explore the underlying lithologic control for the color of sediments recovered during Leg 175. At Sites 1075, 1076, and 1077, the major lithologic component is clay with very low calcium carbonate contents (on average <4 wt%). South of the Walvis Ridge, the sediments consist of calcareous oozes with clay abundances ranging between 10% and 30%. In this area, the calcium carbonate contents range between 40 and 70 wt%. Generally, the lithologic changes at each site were mostly gradual regarding the amounts of diatoms, nannofossils, and foraminifers. The largely continuous presence of authigenic minerals, such as pyrite and glauconite, downhole at all sites suggests reducing diagenetic conditions, which are typical for high-productivity areas (Wefer et al., Chap. 16, this volume). Iron oxyhydroxides were observed only at Sites 1085 and 1087 at the base of the cores near the Miocene/Oligocene boundary. The different lithologic environments encountered during Leg 175 enable us to evaluate the reflectance pattern of specific wavelengths in the visible band for each lithologic type and to compare the color data with preliminary shipboard measurements of calcium carbonate and organic carbon contents and magnetic susceptibility. To explore these relationships, individual reflectance patterns were selected based on carbonate contents.
Environments with carbonate contents <10 wt% were encountered at Sites 1075–1080. Here, sediments consist mostly of dark-colored terrigeneous clay with varying abundances of diatoms, nannofossils, and authigenic minerals, with very low calcium carbonate contents (<6 wt%) and relatively high organic carbon contents (4 wt% on average). The selected sediments have low reflectance levels and slightly enhanced reflectance values in the green-to-red spectrum (550–700 nm; Fig. 2A). In general, the r/b ratios are low. This pattern could be related to traces of reduced iron minerals such as glauconite and pyrite (see Wefer et al., Chap. 16, this volume). Relative changes in the biogenic component are not detectable; this is a possible consequence of the clay dominance, which leads to overall low reflectance values.
The reflectance intensity for individual wavelengths of the so-called "black" layers at Site 1084 (see "Lithostratigraphy" section, "Site 1084" chapter, this volume) was studied in greater detail. The major lithologic component of these layers is organic-rich clay with varying amounts of diatoms and nannofossils. Calcium carbonate contents are low (<10 wt%), whereas organic carbon contents are high (as much as 18 wt%). The black layers show low reflectance values, with highest values in the red spectrum (Fig. 2B). The reflectance spectrum shows an inflection point at 650 nm (Fig. 2B). This feature has also been observed in the reflectance spectrum of diatom oozes and siliceous clay (Mix et al., 1992). The authors related this reflectance pattern to the presence of biogenic silica or to the associated organic matter. Although the clay component is dominant in the black layers at Site 1084, smear-slide analyses indicate that diatoms are common to abundant. On the other hand, reflectance spectra from the Congo Basin sediments, where diatoms are common, do not show any inflection point at 650 nm (Fig. 2A). This characteristic seems to be typical for sediments containing abundant diatoms and high organic carbon contents. The color of these sediments is varying from olive, dark olive-gray, to black on the Munsell color chart. The same black layers were encountered at Site 1082 and also have high organic carbon contents (>10 wt%), but higher calcium carbonate contents (>10 wt%) and lower diatom abundances compared with Site 1084. The response to individual wavelengths is similar to that at Site 1084 and is characterized by an inflection point at 650 nm and a relatively high r/b ratio (Fig. 2C). At Site 1082, the diatom component is only minor, suggesting that the inflection point at 650 nm is related to high organic carbon contents.
For sediments with high carbonate contents (>20 wt%), the color reflectance is high regardless of the individual wavelength bands, but still shows a pronounced difference between the blue and green-to-red spectra (Fig. 2D). The diatom ooze interval at Site 1084 shows an inflection point at 650 nm. There are no organic carbon content measurements available for this layer. However, this site is characterized by overall high organic carbon contents (see "Geochemistry" section, "Site 1084" chapter, this volume). There is no clear explanation for the high r/b ratio characteristic of the nannofossil ooze. At Site 1085, the presence of pyrite throughout the core does not appear to influence the reflectance spectrum, for example, by lowering reflectance values in the red spectrum. The reflectance spectrum is flat at wavelengths from 500 to 700 nm in accordance with the study of Gaffey (1984).
Lithologic changes in sediments with low calcium carbonate contents and dominated by clay cannot be distinguished by relative differences in the reflectance of individual wavelengths (Fig. 2A). Also, crossplots of total reflectance and r/b ratio with calcium carbonate contents do not show any correlation that would suggest control on the color changes observed downcore (Fig. 3A, Fig. 3C). Comparison between organic carbon contents and total reflectance shows a weak negative correlation (Fig. 3B) and a weak positive correlation when compared with the r/b ratio (Fig. 3D). Despite the small number of measurements, the influence of organic carbon on the color reflectance (r/b ratio) is supported by the study of the black layers, which show high r/b ratios (Fig. 2B). Layers with high organic carbon contents have low percentage reflectance values even when calcium carbonate contents are >20 wt%, such as at Site 1082 (Fig. 2C), suggesting the control of organic carbon on total reflectance. Nevertheless, in such environments, in addition to the clay and organic matter components, the biogenic silica component has to be considered to understand the changes in total reflectance.
For sediments with calcium carbonate contents >10 wt%, such as at Sites 1082, 1083, and 1085, total reflectance correlates positively with calcium carbonate, indicating that the color changes are controlled by the changes in carbonate contents (Fig. 3E, Fig. 3G). A similar relationship is observed between total reflectance and calcium carbonate at Site 1084, although some differences exist between the lithologic subunits (Fig. 3F). The crossplots show larger scatter at Site 1084 than at Site 1082, especially for subunits containing diatomaceous clays. This may be because of the higher biogenic opal and organic carbon contents at Site 1084. Shore-based measurements of biogenic opal contents are required to quantify the biosiliceous component and to determine the competing influences of opal and carbonate on total reflectance.