TESTING THE DIAGENETIC MODEL FOR THE PRE-MARINOAN "TREZONA" d13C ANOMALY, SOUTH AUSTRALIA

The "Trezona" anomaly is an 18 ‰ negative shift in the δ¹³C of carbonate found worldwide preceding the younger low latitude Cryogenian (Marinoan) glacial succession. This δ¹³C excursion is the largest in Earth history, has been used as a global correlation tool and has been linked causally to the initiation of ice house conditions. However, recent modeling of the Ediacaran-age Shuram δ¹³C excursion of similar magnitude uses covarying δ¹³C-δ¹⁸O to suggest a secondary origin for highly negative δ¹³C values associated with burial diagenesis. We present detailed mapping, physical stratigraphy and paired oxygen, inorganic, and organic carbon isotope data to test the diagenesis models. We show that the Trezona Fm lithofacies range from stormy open shelf carbonate ribbonites to nearshore channelized grainstones and stromatolite reefs, all of which have variable primary porosity and have experienced variable degrees of recrystallization. However, the Trezona anomaly remains reproducible across both 1 and 100 km scales within these variable carbonate lithofacies and sedimentary environments across the Adelaide Rift Complex. In addition, δ¹³C and δ¹⁸O generally do not covary for the duration of the Trezona Fm negative δ¹³C excursion. Vertical truncation of this negative δ¹³C excursion by Elatina Fm glacial debris records the amount of subglacial erosion into the pre-Marinoan carbonate platform. Furthermore, extra-basinal ice-rafted debris incorporated in stromatolites immediately below the Elatina Fm suggest that the end of the "Trezona" anomaly may be synchronous with the initiation of glaciation. These stratigraphic relationships demonstrate that the "Trezona" δ¹³C anomaly must have been recorded in the carbonates during deposition or very early diagenesis prior to glaciation and thus before the carbonates could have experienced burial diagenesis at elevated temperatures.