Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals?

Regionally persistent, thin intervals of carbonate rock directly and ubiqui tously overlie Proterozoic glacial deposits on almost every continent, and are commonly referred to as cap carbonates, Their unusual facies, stratigra phically abrupt basal and upper contacts, and strongly negative carbon isot opic signature (delta C-13 values between similar to0 parts per thousand an d -5 parts per thousand) suggest a chemical oceanographic origin, the detai ls of which remain unresolved. Here we propose that these enigmatic deposit s are related to the destabilization of gas hydrate in terrestrial permafro st following rapid postglacial warming and flooding of widely exposed conti nental shelves and interior basins, Supporting evidence for this hypothesis includes (1) the common occurrence within the cap carbonates of unusual fa brics, similar to those produced by cold methane seeps; (2) a distinctive t ime evolution for the carbon isotopic excursions indicative of a pulse addi tion of isotopically depleted carbon to the ocean-atmosphere system; and (3 ) agreement between mass-balance estimates of carbon released by hydrate de stabilization and carbon buried in the cap carbonate. 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 Eart h history.