As previously mentioned, much of what we infer about the distribution of dolomitic layers is based on logging results. Cores and core catchers did contain samples from some of the layers seen in the logging profiles, but many of these samples consisted of small fragments. In addition to being present in solid layers, dolomite and calcite are also present as concretions and as disseminated small crystals.
Dolomite is commonly present together with phosphorite; presumably, the precipitation of both is stimulated by the effects of high productivity. Apparently, productivity must be sufficiently high to generate a favorable environment, but other factors also are important. Many questions are open at this point:
These questions cannot be properly addressed by shipboard results. Here, we can only give some preliminary discussions. Whether disseminated dolomite (or calcite) crystals are associated with dolomite layers can be checked to some extent with smear-slide data. It is reported that dolomite crystals can form during early diagenesis in organic-rich sediments already a few centimeters or decimeters below the sediment surface (e.g., Shimmield and Price, 1984; Suess, von Huene, et al., 1988, Leg 112). However, isotopic analyses showed that most of the disseminated dolomite crystals form later at greater depths in the sediment. Presumably, the more conspicuous layers and concretions form within unconsolidated sediments by precipitation and replacement in depths of tens to hundreds of meters below the seafloor.
The abundance patterns of dolomite rhombs at our sites show that rhombs are much more common at some sites than at others (Fig. 9; the categories from 1 to 5 are semi-quantitative and denote trace, rare, frequent, common, and abundant). Sites near the coast (Site 1076 off the Congo, Sites 1078 and 1079 off Lobita, and Sites 1081 and 1082 on the slope of the Walvis Basin) seem to show the higher values. There seems to be no obvious relationship to the presence of opal or organic matter or carbonate content.
Carbonate particles show a pattern that is entirely different from that of the dolomite rhombs (Fig. 10). They are most abundant in the sites far from the coast (1082–1085), more or less independently of the composition of the sediment and of organic matter content. Except for Site 1083, there seems to be a minimum near 2.5 Ma at the sites south of the Walvis Ridge.
A comparison of Fig. 9 and Fig. 10 suggests that dolomite rhombs and carbonate particles may be mutually exclusive to some degree. This may indicate geochemical preferences for dolomite formation, or it may (at least in part) reflect a difficulty in identifying rhombs among abundant carbonate particles.
The patterns do not readily lead to hypotheses about the formation of dolomite. At the first three sites, off the Congo, no dolomite concretions or layer were found. They were first encountered in the organic-rich sediments off Angola (Site 1078). This site is strongly characterized by terrigenous influx; sediments are carbonate rich and contain little diatom debris. Several dolomite layers ~3 to 7 cm thick were found in Sections 175-1078C-13H-1 (111.7 m), 15X-3 (131.4 mbsf), and 175-1078D-10H-3 (82.9 mbsf). Smear-slide determinations show trace (0–120 m) and trace to frequent (135–140 mbsf) abundances (Fig. 9). For carbonate, there is a modest maximum between 112 and 130 mbsf (Fig. 10). Again, there is no evidence for an association among these three abundance observations. No dolomite layers were found at Site 1079; possibly, the holes drilled were too shallow, although at the adjacent Sites 1078 and 1080, dolomite was detected at depths of <100 m. These findings demonstrate that dolomite formed within the last million years.
The presence of a hard dolomite layer at rather shallow depths in the two holes drilled at Site 1080 (52 and 38 mbsf, respectively) was one reason drilling was terminated at this site; another was the fact that the late Quaternary record is largely missing. Dolomite rhombs (not shown in Fig. 9) were seen in trace abundances between 0 and 26 mbsf and near 35 mbsf. Carbonate was frequent near 20 mbsf and rare near 35 mbsf.
Dolomite layers, as previously mentioned, were found to be quite abundant at Site 1081, mainly on the basis of logging, but also verified through samples in cores and in core catchers (Fig. 8). It should be possible by comparison with Ocean Drilling Program (ODP) Sites 1082 and 1083 and DSDP Sites 532 (1331 m) and 362 (1325 m) to study dolomite formation under similar conditions, but with slightly different levels of productivity and interstitial water composition. Although the comparison sites are seaward of the upwelling center, they show higher organic carbon content because of transport of upwelled water to these locations by the Benguela Current and its filaments. As far as abundances at Site 1081, there is no evidence that dolomite layers and dolomite rhombs have a tendency to be present together; the opposite is more likely (Fig. 11A). Also, where carbonate is recorded in smear slides, dolomite layers are no more likely (perhaps less likely) to be found than elsewhere. Similar observations may be made at Site 1082 (Fig. 11B). In fact, dolomite rhombs and dolomite layers were never reported together at any depths. Regarding the general pattern, the presence of diatoms, foraminifers, or nannofossils allows no statement regarding the likelihood of finding dolomites at either site.
At Site 1083, located only 25 km to the northwest of Site 1082, but at a greater water depth (2200 m), no dolomite was found. Carbonate contents are rather high (50 to 70 wt%) and Corg values are distinctly lower than those at Site 1082.
At Site 1084, the extreme-productivity site, sulfate is completely consumed at <6 mbsf. Many lithified carbonate layers were identified in the logging profiles based on density, velocity, and resistivity. In Hole 1084A, a thick hard layer (apparently underlying soft nannofossil ooze) was hit with the APC at ~120 mbsf. Lithified layers do not seem to be present at all holes at the same depth, suggesting isolated concretions perhaps arranged in horizons (as familiar from outcrops on land).
No dolomite layers or concretions were seen at Site 1085. Disseminated dolomites are described in places (in the smear-slide analyses), especially in sediments older than Pliocene in age.