THE SNOWBALL EARTH: MYTH OR METHANE?

Regionally persistent, thin intervals of carbonate rock directly and ubiquitously overlie Proterozoic glacial deposits on almost every continent, and are commonly referred to as cap carbonates. Their unusual facies, stratigraphically abrupt basal and upper contacts, and strongly negative carbon isotopic signature (d¹³C values between ~0 per mil and –5per mil) suggest a chemical oceanographic origin, the details of which remain unresolved. Here we propose that these enigmatic deposits are related to the destabilization of gas hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and interior basins. Supporting evidence for this hypothesis includes (1) the common occurrence within the cap carbonates of unusual fabrics, similar to those produced by cold methane seeps; (2) a distinctive time evolution for the carbon isotopic excursions indicative of a pulse addition of isotopically depleted carbon to the ocean-atmosphere system; and (3) agreement between mass-balance estimates of carbon released by hydrate destabilization and carbon buried in the cap carbonate. The amount of C buried in the cap carbonate is roughly comparable to the isotopic mass balance estimate of C released from methane to account for a whole ocean –5 per mil d¹³C excursion. It is also comparable to an independent estimate of methane released to the ocean based on the methane flux from flooded hydrates on the modern Arctic shelf expanded to the modern total shelf area. We infer that during times of low-latitude glaciation, characteristic of the Neoproterozoic, gas hydrates may have been in greater abundance than at any other time in Earth history. Given that cap carbonates and their associated isotopic excursions are limited to times in the Proterozoic during which conditions would have been highly favorable for gas hydrate formation, we pose a different question: How can consideration of changes in this pool of organic carbon be avoided?