METHANE HYDRATE DESTABILIZATION, DEGLACIATION AND OCEANIC ANOXIA, AND BIOLOGICAL INNOVATION IN THE LATE NEOPROTEROZOIC

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 suggest a chemical oceanographic origin, the details of which remain unresolved. These enigmatic facies have been related to at least three mechanisms, including an atmospheric buildup of CO₂ resulting from a frozen ocean (the snowball earth hypothesis), invigorated circulation of a stagnant ocean, and methane release from post-glacial destabilization of gas hydrates. Here we propose that abundant physical evidence for gas seepage in the form of cold seep facies in cap carbonates, and unambiguous carbon isotope evidence for the influence of methane (d¹³C ~ -40‰) superimposed on a global excursion indicative of a pulse addition of isotopically depleted carbon to the ocean-atmosphere system together suggest a major clathrate destabilization event coincident with cap carbonate deposition. 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‰ carbon isotope excursion. 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. Methane release during Neoproterozoic deglaciation may have led to a significant drawdown of oxygen and perhaps to oceanic anoxia, thereby influencing biological innovation during the Ediacaran Period (635-543 Ma).