Significant to high concentrations of residual headspace methane and CO2 were found in sub-bottom sediments at all of the Leg 175 sites. Carbon dioxide concentrations generally increased quickly with depth in the drill holes, whereas methane concentrations did not begin to increase until interstitial sulfate was depleted. In addition, the odor of H2S was often noted, especially in the upper parts of the sedimentary sequences, but detectable concentrations (>1ppmv) of this gas were rarely found. Total gas pressures typically became great enough in sediments cored below 10 to 20 mbsf to require perforating core liners to relieve the pressure and prevent excessive core expansion. Nevertheless, gas-expansion pockets developed in many cores. Headspace gas profiles from representative sites in each of the four depositional regimes are described below.
Concentrations of CaCO3 are low in Site 1076 sediments. The maximum is 16 wt%, and most sediment samples contain <5 wt% CaCO3. These generally low concentrations reflect the paucity of coccolith microfossils and the high abundances of opaline and continental clastic material in these sediments. In contrast, total organic carbon (TOC) values are elevated; they range from 0.9 to 4.3 wt% (Fig. 2) and average 2.6 wt%. The organic matter appears to be predominantly marine in origin (see "Organic Geochemistry" section, "Site 1076" chapter, this volume).
A succession of relatively abundant gases exists in sediments from Hole 1076A. Hydrogen sulfide could be detected by its odor, but not by H2S sensors having a sensitivity of ~1 ppmv, in Cores 175-1076A-3H through 5H (18–39 mbsf). Methane first appears in headspace gas samples from Hole 1076A sediments at 12.5 mbsf. Concentrations rapidly increase and become significant in sediments below 30 mbsf (Fig. 2). Gas pressures became great enough in sediments below Core 175-1076A-15H (138 mbsf) to require perforating the core liner to relieve the pressure and to alleviate core expansion. Analyses of gas in expansion pockets showed that much of this gas was CO2 (see "Organic Geochemistry" section, "Site 1076" chapter, this volume).
As in the Lower Congo Basin, concentrations of CaCO3 are low (Fig. 3). They vary between 11 and 25 wt% in Site 1078 sediments. TOC values range from 1.1 to 5.3 wt% (Fig. 3) and average 2.5 wt%. The organic matter appears to be predominantly marine in origin (see "Summary" section, "Site 1078" chapter, this volume).
As found at Site 1076, a succession of relatively abundant gases exists in sediments from Site 1078. Although hydrogen sulfide could be detected by its odor, it remained below the instrumental detection limit of ~1 ppmv in Cores 175-1078A-1H through 3H (1.5–26.5 mbsf). Methane first appears in headspace gas samples from Site 1078 sediments at 6 mbsf. Concentrations rapidly increase and become significant in sediments between 20 and 35 mbsf, below which they decrease (Fig. 3). Gas pressures became great enough in sediments below Core 175-1078A-4H (36 mbsf) to require perforating the core liner to relieve the pressure and alleviate core expansion. Analyses of expansion pockets showed that most of this gas was CO2.
Concentrations of CaCO3 in Site 1084 sediments fluctuate between 0.5 and 69 wt% (Fig. 4). The fluctuations consist of (1) closely spaced changes related to light–dark color changes and (2) more gradual downhole increases and decreases in concentration. TOC concentrations change in both short-term and longer term patterns (Fig. 4). Dark-colored sediments have higher TOC values than light-colored layers. TOC concentrations also differ in Hole 1084A lithostratigraphic units, averaging 8.2 wt% in Subunit IA, 7.0 wt% in Subunit IB, 4.9 wt% in Subunit IC, 3.4 wt% in Unit II, 4.5 wt% in Unit III, and 2.9 wt% in Unit IV. The organic matter appears to be predominantly marine in origin (see "Organic Geochemistry" section, "Site 1084" chapter, this volume). The high TOC concentrations in the upper 200 mbsf (Fig. 4) are a consequence of the elevated paleoproductivity of the Benguela Current upwelling system.
High concentrations of H2S, methane, and CO2 were found in sediments from Site 1084. The odor of H2S was noted throughout most of the sequence, and detectable concentrations of this gas were found in upper parts of Hole 1084A (see Table 15 in the "Site 1084" chapter, this volume). Much of the sedimentary sequence had an offensive odor, which may have resulted from microbial production of or-gano-sulfur gases, such as dimethyl sulfide and carbon disulfide, in this sulfur-rich sequence. Methane (C1) concentrations increase rapidly with depth in headspace gas samples from Hole 1084A sediments. Concentrations become significant in sediments below 6 mbsf (Fig. 4). High methane/ethane (C1/C2) ratios (Fig. 4) and the absence of major contributions of higher molecular weight hydrocarbon gases (see Table 15 in the "Site 1084" chapter, this volume) indicate that the gas is microbial, as opposed to thermal, in origin. 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. The most abundant headspace gas was CO2; concentrations of this gas remained high throughout the 600-m-deep Hole 1084A (Fig. 4).
Concentrations of CaCO3 are generally high in Site 1085 sediments, varying between 28 and 85 wt% (Fig. 5). The concentrations vary in two ways: (1) closely spaced changes related to light–dark color fluctuations and (2) more gradual downhole increases and decreases. TOC amounts change in both short-term and longer term patterns (Fig. 5). TOC concentrations are low, averaging ~1 wt% over the sedimentary sequence. These TOC values reflect a history of moderate-to-low productivity in the southern portion of the Benguela Current system.
Moderately high amounts of methane and CO2 were found in sediments from Site 1085 (Fig. 5). Methane first appears in headspace gas samples from Hole 1085A sediments at 17.7 mbsf. Concentrations become significant in sediments below 46 mbsf. High C1/C2 ratios (Fig. 5) and the absence of major contributions of higher molecular weight hydrocarbon gases (see "Organic Geochemistry" section, "Site 1085" chapter, this volume) indicate that the gas is microbial, rather than thermal, in origin. The most abundant gas was methane, not CO2 as in the other depositional regimes. Headspace concentrations of methane remained high, whereas those of CO2 gradually diminished with depth in sediments from Hole 1085A (Fig. 5).