Stratigraphic expression of Earth's deepest δ¹³C excursion in the Wonoka Formation of South Australia

Abstract

The most negative carbon isotope excursion in Earth history is found in carbonate rocks of the Ediacaran Period (635-541 Ma). Known colloquially as the “Shuram” excursion, workers have long noted its broad concordance with the rise of abundant macro-scale fossils in the rock record, collectively known as the “Ediacaran Biota.” Thus, the Shuram excursion has been interpreted by many in the context of a dramatically changing redox state of the Ediacaran oceans—for example, a result of methane cycling in a low O₂ atmosphere, the final destruction of a large pool of recalcitrant dissolved organic carbon (DOC), and the step-wise oxidation of the Ediacaran oceans. More recently, diagenetic interpretations of the Shuram excursion have challenged the various redox models, with the very negative δ¹³C values of Ediacaran carbonates explained via sedimentary in-growth of very δ¹³C depleted authigenic carbonates, meteoric alteration or late-stage burial diagenesis. A stratigraphic and sedimentological context is required to discriminate between these explanatory models, and to determine whether the Shuram excursion can be used to evaluate oxygenation in terminal Neoproterozoic oceans. Here we present chemo-stratigraphic data (δ¹³C, δ¹⁸O, and trace element abundances) from 15 measured sections of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia. In some locations, the Wonoka Fm. is ∼700 meters (m) of mixed shelf limestones and siliciclastics that record a 17 permil δ¹³C excursion (−12 to +5‰). Further north in the basin, the Wonoka Fm. is host to deep (∼1 km) paleocanyons, which are partly filled by tabular-clast carbonate breccias. Canyon-filling ceased during ongoing sedimentation on the shelf interior (the “canyon-shoulder”), as evidenced by upper canyon-shoulder units that overlie and cap certain canyon-fill sequences. The unprecedented size of the chemostratigraphic dataset presented here (2671 δ¹³C-δ¹⁸O measurements from the canyon-shoulder, 1393 δ¹³C-δ¹⁸O measurements from canyon clasts, and 11 different trace elements measured on 247 Wonoka Fm. carbonate samples), when coupled with the unique canyon-shoulder to canyon-fill depositional system of the Wonoka Fm., allows for new insights into Ediacaran carbon isotope systematics. The excursion is preserved in a remarkably consistent fashion across 12,000 km² of basin area; fabric-altering diagenesis, where present, occurs at the sub-meter vertical scale, results in < 1 permil offsets in δ¹³C and cannot be used to explain the full δ¹³C excursion. Multi-variate analysis of the dataset allows for rigorous assessment of different potential carbonate sources for the Wonoka canyon-fill breccias. Eroded and transported canyon-shoulder carbonates are the most likely source, thus requiring a syn-depositional age for the extraordinary range of δ¹³C values (−12 to +5‰) observed in both the Wonoka Fm. canyon-shoulder and canyon-fill breccias. Geological observations (for example, excellent preservation of sedimentary structures in Wonoka carbonates, absence of top-down alteration profiles associated with exposure surfaces) do not provide first-order evidence for the meteoric or authigenic carbonate models. Thus, the balance of evidence supports either a primary origin or syn-depositional, fabric-retentive diagenesis for the deep negative δ¹³C excursion hosted in the Wonoka Formation of South Australia.