Hoffman et al. (2001) acknowledge that methane may have played an important role in unusual events associated with Neoproterozoic glaciation, but they question our permafrost gas hydrate hypothesis for 13C-depleted cap carbonate formation. They focus on three issues: (1) an interpretation for tube structures in cap carbonates unrelated to gas migration; (2) the absence of a suitable source for methane gas; and (3) the degree of 13C depletion in sheet-crack cements.

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Origin of tube structures. Hoffman et al. suggest that the tube structures represent the locus of 1- to 3-cm-wide dimples within a lattice of stromatolitic ridges on...

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