TESTING THE METHANE HYDRATE HYPOTHESIS: EVIDENCE FROM THE NEOPROTEROZOIC DOUSHANTUO CAP CARBONATE IN SOUTH CHINA

The latest Neoproterozoic, from about 600 Ma to 543 Ma, has attracted considerable interest as a time of extreme glaciation and the onset of the greatest flurry of adaptive innovation in the history of life. Central to competing hypotheses for this interval is the interpretation of 1- to 5-m-thick deep water cap carbonates that in many places directly overlie the glacial deposits, and are characterized by an unusual carbon isotope anomaly (d¹³C values as low as Ð5ä PDB). The origin of these deposits remains controversial, with cap carbonates ascribed variously to the aftermath of an almost complete shutdown of ocean ecosystems for millions of years during the ice age (the snowball Earth hypothesis), to oceanic overturn accompanying climatic amelioration (the upwelling model), and to postglacial destabilization of methane hydrates (the methane hydrate hypothesis). Stable isotopic analysis at millimetric scale, petrography and field studies of Neoproterozoic cap carbonates overlying glaciogenic rocks of the Nantuo Formation in south China reveal d¹³C values ranging from +5ä to Ð41ä (PDB), along with sedimentary structures and textures that are consistent with carbonate precipitation at a cold (methane) seep. These data provide intriguing confirmation of a first-order prediction of the hypothesis that a prominent negative carbon isotopic excursion observed widely in cap carbonates relates to the postglacial degradation of methane hydrates. Variation of up to 35ä in d¹³C values over a distance of less than 1 mm in calcite cements with well-preserved original texture contrasts markedly across dolomitization fronts with texturally similar dolomitic cap carbonates that display limited variability and relatively heavy values (Ð2ä to Ð5ä). Isotopic homogenization by dolomitization may explain the limited isotopic variability found in microcrystalline dolomite seep-like facies in other examples of cap carbonates.