THE VERACITY AND IMPLICATIONS OF EDIACARAN NEGATIVE CARBON ISOTOPE EXCURSIONS IN THE WONOKA AND SHURAM FORMATIONS

The carbon isotope record obtained from carbonates in the Shuram and Wonoka Formations of Oman and south Australia indicate a systematic and reproducible drop of values to –10‰ PDB that remain low through a thick succession (>2000m) of stratigraphically complex rocks with multiple sequence boundaries representing tens of million of year. Both the magnitude and duration of these perturbations are unique to this interval of Earth history, and immediately precede or even coincide with the first appearance of metazoans in the fossil record. If these values record seawater constrained by the global carbon isotope mass balance, then this excursion provides a fundamental challenge to the current understanding of carbon isotopic systematics, which should not be capable of sustained (steady state) marine values of < -5‰. By comparison, negative carbon isotope excursions in the Phanerozoic are modest (~4‰) and short lived (<1 million years) resulting from abrupt ecosystem perturbations and recovery. Alternatives include a) the oxidation of dissolved organic carbon concentration in a series of restricted marine or lacustrine basins or upwelling margins from long term stratified basins b) alteration of initial marine carbonate values from deep basinal CO2 formed by decarboxylation or meteoric stabilization, or c) mechanical erosion and redeposition of carbonate grains. Wave ripples and swaley cross stratification indicate deposition on a storm-dominated, high-energy (well mixed) shelf with values sustained through multiple sea level falls, arguing against models invoking sustained stratification. Stratigraphicaly reproducible and systematic variation of values argues for a primary marine origin, however, strong covariance between oxygen and carbon isotope values along with facies dependence of isotopic values, concretionary textures, and systematic shifts associated with exposure surfaces argue for a diagenetic origin. In both Formations, negative isotope values are associated with a mixed-silisiclastic carbonate grainstone dominated by microcrystalline carbonate cemented quartz silt grains- a texture uncommon in Phanerzoic rocks and with no obvious marine source (carbonate factory).