Concentrations of methane and CO2 inferred from headspace analyses are significant to high in sub-bottom sediments in the four depositional regimes on the southwest African margin that are encompassed by the 13 Leg 175 drill sites. Residual CO2 concentrations generally increased quickly with depth in the cores, whereas methane concentrations did not begin to increase until interstitial sulfate was depleted, usually within a few tens of meters below the seafloor. The origins of these gases appear to be from in situ microbial production. Carbon dioxide is released during the degradation of organic matter; methane is produced primarily by microbial reduction of CO2. No evidence of migration of hydrocarbon gases into the drilled sequences from deeper, thermally mature sediments was found.
Little correspondence exists between concentrations of the microbial gases and the availability of either organic carbon or carbonate carbon in these sedimentary sequences. This lack of correspondence indicates that only a small fraction of the available carbon is converted to gaseous components by microbial processes.
Despite the widespread occurrence of strong bottom-simulating seismic reflectors along the Congo and Angola margins, physical evidence of gas hydrate layers was not found. Dolostone lenses may be the cause of the seismic reflectors.
Results of analyses of headspace gases and of expansion-pocket gases generally agreed well, but systematic differences were observed at Sites 1082, 1083, and 1084. The greater mobility of methane is probably responsible for the elevated concentrations of this gas that were measured in expansion pockets from these three sites.