Gas hydrates form when methane-rich fluids enter the GHSZ. At SHR, the distribution of gas hydrates is highly heterogeneous (Tréhu et al., 2004a). Generally, gas hydrate comprises <5% of the pore space, averaged over the GHSZ. However, massive gas hydrate deposits, estimated to comprise ~25% of the total volume, are present in the upper 20–30 m at the summit of the ridge. Very little hydrate (<2% of pore space) was found within the sediments of the eastern basin, except for immediately above the base of the GHSZ. We note that no drilling was permitted at the crest of Anticline B. In this section, we discuss how the geologic history outline in the previous sections has influenced large-scale patterns in gas hydrate distribution.
The BSR is a conspicuous reflection with negative amplitude through most of the 3-D survey. Its amplitude, however, varies greatly. Figure F11 shows the variation in relative amplitude of the BSR overlain by the boundaries between the stratigraphic units on the BSR surface. It shows several distinct zones of high amplitude.
The most prominent high-amplitude anomaly is associated with the buried Anticline B and lies within Unit S.VII. We note that a drill site that was originally located on this anomaly was moved to Site 1252 by the ODP Pollution Prevention and Safety Panel. We interpret the relatively uniform character of this amplitude anomaly to indicate the widespread presence of free gas throughout these older, seismically incoherent, and presumably pervasively fractured sediments and relatively high and heterogeneous gas hydrate content within the GHSZ.
Another pronounced bright spot is located in sediments of Unit S.VI on the western flank of SHR. This bright spot is associated with the local circular high that was active prior to deposition of Horizon A. This relict structure, which may have been a mud volcano in the trench, as is observed in subduction zones elsewhere, appears to remain as a preferential methane migration path ~1 Ma since it was active. No drilling data are available to test this hypothesis.
The summit region is characterized by a relatively large zone containing numerous linear and patchy bright spots in the tightly folded and deformed sediments of Unit S.VI. We interpret these patterns to reflect variable permeability controlled by discrete stratigraphic horizons and faults.
Where the base of the GHSZ falls in the younger sediments of Units S.III, S.II, and S.I, the BSR is generally faint or absent except where coarse-grained horizons like A, B, and B´ exist to provide paths for gas migration. Where these horizons intersect the BSR, strong anomalies are observed. These may, at least in part, indicate constructive interference between two negative polarity reflections. The numerous small normal faults that offset strata in Unit S.II may also contribute to the bright BSR and to relatively high and heterogeneous (1%–8%) gas hydrate content within the GHSZ (Tréhu et al., 2004a; Weinberger et al., 2005).
In summary, the strongest BSR anomalies are associated with the oldest sediments. We infer that these have the potential to deliver free gas to the GHSZ more readily than the younger strata for at least two reasons. First, these older sediments have generally undergone greater uplift, leading to a large decrease in the solubility of methane in the pore fluids because of depressurization. Based on the methane content of abyssal plain sediments drilled at Deep Sea Drilling Project (DSDP) Site 174, these sediments should be supersaturated in methane when uplifted to the depth at which they are now found (Claypool and Kaplan, 1974; Claypool et al., this volume; Tréhu et al., this volume). Second, pervasive fracturing of recovered samples, the variable character of geophysical logs from these zones, and very low gradients in chloride concentration (Shipboard Scientific Party, 2003b, 2003d, 2003e; Torres et al., 2004b) suggest that the accretionary complex sediments are permeable and that the pore fluid within them is well mixed, allowing efficient transport of gas into the GHSZ. We note that at northern Hydrate Ridge, where venting and authigenic carbonate development is widespread over the summit of the structure, sediments of Units S.VII and S.VI are present very near the surface, consistent with the association we document here of older sediments with venting. Our results also demonstrate the value of high-resolution seismic data in identifying relict buried structures within the older sediments that focus fluid flow.