Seeps are typically considered to be soft sediment ecosystems, at least during initial stages of formation. Sediments may consist of quartz sand, carbonate sands, turbidites of terrestrial origin, fine grained muds or clays. However, carbonate precipitates are commonly associated with both active and fossil cold seeps and provide a source of hard substratum in an otherwise soft matrix (Bohrmann et al. 1998, Barbieri & Cavalazzi 2004). Methane-based cold seep communities are reported from exposed oceanic basement rock on the Gorda Escarpment at 1600 m (Stakes et al. 2002). In Monterey Bay, Stakes et al. (1999) have documented carbonate pavements (flat platforms), circular chimneys (cemented conduits), doughnut-shaped rings (cm to m in size) and veins in basement rock. Less structured carbonate pebbles, rocks and soft concretions are distributed haphazardly throughout sediments of many cold seep sites (e.g., Bohrmann et al. 1998) and are clearly visible in x-radiographs (Figure 2). Comparable interspersion of hard substrata with fine-grained sediments is evident on the Peru margin where phosphorite pebbles are common, and on seamounts where basalt fragments are common. Dense assemblages of crabs dwell at methane 'jacuzzis' on phosphorite hardgrounds on the upper Peru slope (R. Jahnke, personal communication).

Figure 2 X-radiograph of seep sediments from the Gulf of Alaska (2,200 m), showing carbonate concretions, which are higher density than surrounding sediments and appear as white reflectors. Image width = 9.5 cm.
Figure 3 Photograph of animals on carbonate outcrops on the Eel River margin (500 m). Anthomastus ritterii, Rathbunaster californicus and an unidentified sponge are the large taxa visible. Image width ~75 cm.

While a number of invertebrate taxa attach to carbonates (Figure 3), there have been no community descriptions of carbonate-associated or carbonate-burrowing seep taxa — either the epifauna or endolithofauna. In contrast, extensive programs have been developed to catalogue the species diversity associated with carbonate mounds and coral reefs in the North Atlantic and Gulf of Mexico. Sibuet et al. (1988) note the occurrence of Calyptogena species on a broad range of substrata in the Japan Trench, including sediments, mudstone, gravel, talus and vertical rock ledges. In surveying 50 sites, however, they observed that large colonies develop only on sediments and mudstones and suggest that these substrata promote greater lateral transport of rising pore fluids, enhancing the area suitable for the clams.

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