Facies-dependent δ13C variation from a Cryogenian platform margin, South Australia: Evidence for stratified Neoproterozoic oceans?

Analysis of a Cryogenian interglacial platform margin in the Adelaide Geosyncline reveals a strong carbonate delta C-13-facies relationship. Detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 parts per thousand in time-equivalent strata. Shallow-water back-reef facies have delta C-13 values up to 8.2 parts per thousand, while equivalent basinal sediments have delta C-13 values between -3 and 0 parts per thousand. Allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy delta C-13 values (4-9 parts per thousand) and are surrounded by basinal calcareous shales with light delta C-13 values (ave. 0.8 parts per thousand). The regional and stratigraphic consistency of these delta C-13 trends suggests a primary marine origin. We interpret this delta C-13-facies correlation to be the result of ocean stratification/stagnation that significantly reduced the rate of deep-ocean ventilation and produced a large deep-water, organically derived carbon reservoir. We suggest that stratification may have been a persistent feature of the global ocean throughout much of Neoproterozoic time. Ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era, including large-scale delta C-13 variation, extreme climatic fluctuations, and the presence of cap carbonates. A highly variable climatic regime would be expected with the development of a large deep-water carbon reservoir. Small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface-ocean and atmospheric reservoirs, leading to intense greenhouse or icehouse conditions respectively. (C) 2008 Elsevier B.V. All tights reserved.