Results obtained from samples collected from coring diapirs are listed in Table 2. Samples from Sites 991, 992, and 993, on the crest and flank of the Cape Fear Diapir, contain residual hydrocarbon gases, with methane dominant. In general, methane concentrations increase with depth (Fig. 2; see Table 2 for details); methane concentrations at Site 993 are more complex. Maximum depth sampled was 51.5 meters below seafloor (mbsf). At all sites, the residual methane distribution is controlled by the availability of sulfate (for sulfate distributions, see Paull, Matsumoto, Wallace, et al., 1996) and follow the classical model of Claypool and Kaplan (1974). At Sites 991 and 993, the rapid increase in methane concentrations near 40 m and 25 m, respectively, corresponds to the intervals where sulfate values, rapidly depleting with depth, approach 0 mM. In contrast, at Site 992 the methane concentrations are relatively low, and sulfate concentrations in the same stratigraphic interval range from ~30 mM near the surface to ~5 mM at a depth of ~50 mbsf. The presence of sulfate apparently limits the generation of methane in sediment at Site 992. Higher molecular weight hydrocarbon gases are also present at these sites, with C1/(C2 + C3) ratios <100 in many cases (Table 2).
Samples from Site 996, located on the crest of the Blake Ridge Diapir, also contain residual hydrocarbon gases (Table 2), but in this case concentrations of methane are very high (between 10,000 and 40,000 µL/L) within the sampled interval of 3.6 to 63.0 mbsf. Sulfate concentrations in this same stratigraphic interval are depleted to 0 mM, and massive and vein-filling gas hydrate was recovered to a depth of ~60 mbsf (Paull, Matsumoto, Wallace, et al., 1996). Absolute concentrations of residual higher molecular weight hydrocarbons are larger in samples from Site 996 than at the three sites on the Cape Fear Diapir (Table 2), but the concentrations relative to methane are lower as reflected in C1/(C2 + C3) ratios ranging from 1100 to 1600.