STRATIGRAPHIC TESTS FOR THE ORIGIN OF THE DEEPEST CARBON-ISOTOPE ANOMALY IN EARTH HISTORY - THE WONOKA FORMATION OF SOUTH AUSTRALIA

The deepest carbon isotope excursion in Earth history is recorded in Ediacaran carbonates from Oman, South Australia, south China, southern Siberia, southwest USA, Namibia and Scotland. Called the ‘Shuram’ excursion, two end member hypotheses seek to explain the observed trend. The first interprets the carbonate isotopes as a primary seawater signal, and postulates the existence of a large, relict DOC pool (much larger than today) based upon remarkably invariant δ¹³C_org during the δ¹³C_carb excursion. Remineralization of this pool during the Ediacaran would push global DIC to very low δ¹³C values. The second holds that the isotopic signal is not primary, but rather is a diagenetic overprint. The depleted δ¹³C values and observed covariation between δ¹³C and δ¹⁸O might have resulted from migrating, high pCO₂ fluids that interacted with the sediments during burial diagenesis at elevated temperatures. The second hypothesis is of critical importance, as δ¹³C has become the backbone for the correlation of pre-fossil stratigraphy. In order to test these hypotheses, we present a program of integrated mapping, physical stratigraphy, and paired δ¹³C_carb - δ¹⁸O_carb - δ¹³C_org analyses of the Wonoka Formation of South Australia. The Wonoka outcrops over 100s of kilometers, enabling us to map the carbon isotopic profiles in a range of paleodepositional environments - from stormy open shelf and microbialite reef to deep submarine canyon. We are thus able to assess the relationship between δ¹³C and sedimentary environment, geographic location, and primary porosity. We find covariation between δ¹³C and δ¹⁸O only in the lower part of the Wonoka. Furthermore, at some localities, the base of the Wonoka is marked by deep (>1km) paleocanyons; a possible relationship between canyon formation, local sea level change and the Shuram excursion has never been explored. We find that the Wonoka was a prograding carbonate ramp that cut into itself (and recycled itself into the canyon fill) towards the end of its deposition, and that canyon formation postdated the nadir of the Shuram excursion. As canyon incision occurred during Wonoka deposition, this observation requires that the morphology of the deep negative δ¹³C excursion was acquired early, and must have preceded any significant heating or burial diagenesis.