Kennedy et al. (2001) propose a creative scenario to explain the unique occurrence of 13C-depleted “cap” carbonates on top of Neoproterozoic glacial deposits. It is roughly as follows: Organic-rich marine sediments are subaerially exposed by sea-level fall during glaciation. Methane produced within the sediments is sequestered as hydrate in the resulting permafrost. The sea-level rise accompanying deglaciation warms and destabilizes the hydrate, causing cold methane seeps on the seafloor. Kennedy et al. (2001) point to two anomalous cap facies in support of their cold methane seep hypothesis—vertical tube-like structures and isopachous sheet-crack cements formed on the seafloor. Both are...

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