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

Abstract

Analysis of a Cryogenian interglacial platform margin in the Adelaide Geosyncline reveals a strong carbonate δ 13C-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‰ in time-equivalent strata. Shallow-water back-reef facies have δ 13C values up to 8.2‰, while equivalent basinal sediments have δ 13C values between − 3 and 0‰. Allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy δ 13C values (4–9‰) and are surrounded by basinal calcareous shales with light δ 13C values (ave. 0.8‰). The regional and stratigraphic consistency of these δ 13C trends suggests a primary marine origin. We interpret this δ 13C-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 δ 13C 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.