Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1084A (Table 13). Organic matter atomic carbon/nitrogen (C/N) ratios and Rock-Eval pyrolysis analyses were employed to determine the type of organic matter contained within the sediments. Elevated amounts of gas were encountered, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon in Site 1084 sediments range between 8.2 and <0.1 wt%, corresponding to 68.7 and 0.5 wt% CaCO3 (Table 13). The carbonate concentrations vary in two ways: (1) closely spaced changes related to light–dark color fluctuations and (2) more gradual downhole increases and decreases (Fig. 25). Sediments at this site are divided into four lithostratigraphic units (see "Lithostratigraphy" section, this chapter). Unit I is further divided in three subunits. Subunit IA comprises Pleistocene nannofossil clays and oozes and averages 39 wt% CaCO3. Subunit IB is a Pliocene–Pleistocene diatom-bearing clayey nannofossil ooze that averages 26 wt% CaCO3. Subunit IC is a Pliocene diatomaceous clay that contains an average of 14 wt% CaCO3. Unit II, a Pliocene clay-rich nannofossil diatom ooze, averages 29 wt% CaCO3. Unit III is a Pliocene nannofossil clay in which CaCO3 concentrations average 42 wt%. Unit IV is a Pliocene nannofossil ooze averaging 46 wt% CaCO3. The variations in concentrations reflect varying combinations of changes in delivery of calcareous material, dilution by noncalcareous components, and carbonate dissolution fueled by oxidation of organic matter.
TOC determinations were done on selected samples from Hole 1084A sediments to estimate the amounts of organic matter in the different lithostratigraphic units (Table 13). Like CaCO3 concentrations, TOC concentrations change in both short-term and longer term patterns (Fig. 26). Dark-colored sediments have higher TOC values than light-colored layers. TOC concentrations also differ in Hole 1084A lithostratigraphic units, averaging 8.21 wt% in Subunit IA, 7.00 wt% in Subunit IB, 4.92 wt% in Subunit IC, 3.36 wt% in Unit II, 4.55 wt% in Unit III, and 2.87 wt% in Unit IV. The high TOC concentrations in the subunits of Unit I are a consequence of the elevated paleoproductivity of the Benguela Current upwelling system, which has delivered abundant organic matter to the sediments, and the high accumulation rate of sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter), which enhances preservation of the organic matter.
Organic C/N ratios were calculated for sediment samples from the different Site 1084 lithostratigraphic units using TOC and total nitrogen concentrations (Table 13). The C/N ratios vary from 17.3 to 4.3 (Fig. 27). Most of these C/N ratios are intermediate between unaltered algal organic matter (5–8) and fresh land-plant material (25–35; e.g., Emerson and Hedges, 1988; Meyers, 1994). The means of the C/N ratios are Subunit IA, 13.1; Subunit IB, 14.0; Subunit IC, 14.0; Unit II, 11.5; Unit III, 12.3; and Unit IV, 8.9. Because of their setting under a major upwelling system and offshore from a coastal desert, it is likely that these organic carbon–rich sediments contain mostly marine-derived organic matter with only a minor contribution of detrital continental organic matter. The C/N ratios that are higher than fresh algal organic matter indicate that preferential loss of nitrogen-rich, proteinaceous matter and consequent elevation of C/N ratios occurred during settling of organic matter to the seafloor. Such early diagenetic alteration of C/N ratios is commonly seen under areas of elevated marine productivity such as upwelling systems (Meyers, 1997).
A Van Krevelen–type plot of the hydrogen index (HI) and oxygen index (OI) values indicates that the sediments contain type II (algal) organic matter (Fig. 28) that has been altered by microbial processing during early diagenesis. Well-preserved type II organic matter has high HI values (Peters, 1986); these values can be lowered by microbial oxidation (Meyers, 1997). Hole 1084A sediments deviate from the pattern observed at other Leg 175 sites in which lower HI values correlate with lower TOC values (Fig. 29). Because oxidation typically lowers both HI and TOC values (Meyers, 1997), the absence of this relationship suggests that, in addition to aerobic and anaerobic degradation, the marine organic matter has been subject to variable amounts of dilution. Evidence of substantial amounts of in situ organic matter degradation exists in the rapid depletion of sulfate and the exceptionally large increases in alkalinity and ammonia in the interstitial waters of Site 1084 sediments (see "Inorganic Geochemistry" section, this chapter).
The sediment samples have low Rock-Eval Tmax values (Table 14), showing that their organic matter is thermally immature with respect to petroleum generation (Peters, 1986) and therefore is unlikely to contain much detrital organic matter derived from erosion of ancient, thermally mature sediments from Africa.
High amounts of hydrogen sulfide, methane, and CO2 were found in sediments from Site 1084. The odor of hydrogen sulfide was noted through most of the sequence, and detectable concentrations of this gas were found in upper parts of Hole 1084A (Table 15). Much of the sedimentary sequence had an offensive odor, which may have resulted from microbial organo-sulfur gases such as dimethyl sulfide and CS2. Total gas pressures became great enough in sediments below Core 175-1084A-2H (6 mbsf) to require perforating the core liner to relieve the pressure and prevent excessive core expansion.
Methane (C1) concentrations increase rapidly with depth in headspace gas samples from Hole 1084A sediments. Concentrations become significant in sediments below 6 mbsf (Fig. 30). High methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 15) indicate that the gas is biogenic, as opposed to thermogenic, in origin. The origin of the methane is probably from in situ microbial fermentation of the marine organic matter present in the sediments. Similar microbial production of methane from marine organic matter has been inferred from high biogenic gas concentrations in Pliocene–Pleistocene sediments from DSDP Site 532 nearby on the Walvis Ridge (Meyers and Brassell, 1985), Sites 897 and 898 on the Iberian Abyssal Plain (Meyers and Shaw, 1996), and also in middle Miocene sediments from Site 767 in the Celebes Sea (Rangin, Silver, von Breymann, et al., 1990). A biogenic origin of the methane is supported by the disappearance of interstitial sulfate at approximately the same sub-bottom depth where methane concentrations begin to rise (see "Inorganic Geochemistry" section, this chapter), inasmuch as Claypool and Kvenvolden (1983) observe that the presence of interstitial sulfate inhibits microbial methanogenesis in marine sediments.
Natural gas analyses determined that the most abundant gas was CO2 and that headspace concentrations of this gas remained high deep in Hole 1084A (600 mbsf; Fig. 31). Cragg et al. (1992) report the existence of viable microbes to depths of ~500 mbsf in the sediments of the Japan Sea. The abundance of biogenic gases in sediments at Site 1084 suggests the presence of viable microbial communities to similar sub-bottom depths in the Walvis Basin.