Along with further dissolution of the hydrates, the signal left by methane seep communities comes in the form of crusts of calcium carbonate, as long as the sites sit above the CCD.  The subsurface carbonates at this site yield δ¹³C values as light as –40.8% (Paull et al., 1995), indicating derivation of the carbonate carbon from microbial anaerobic methane oxidation. These carbonates were once thought to have been gas-hydrate-rafted dropstones; however, submersible observations suggest instead that the large carbonate blocks within the depression are likely to have formed at the sediment-water interface (Van Dover et al. 2003).  The carbonates were abundant with fossilized clams and mussels, similar in structure to those found at the site. 

Picture of calcium carbonate with fossilized clams and mussels recovered on an Alvin dive at the gas hydrate site at the intersection of the Blake Ridge and Carolina Rise.  Ruler is added for scale.  (Van Dover et al. 2003).