Constraints on the origin and relative timing of the Trezona δ ¹³C anomaly below the end-Cryogenian glaciation

The Neoproterozoic Era was punctuated by the 'Sturtian' (~710million years ago) and 'Marinoan' (~635million years ago) low-latitude glaciations. Carbonates preceding the younger Marinoan glacial succession record an ~18‰ negative shift in the δ ¹³C of carbonate around the world. This 'Trezona' isotopic anomaly is the largest δ ¹³C shift in Earth history and its origin and timing remain controversial. The δ ¹³C anomaly could record a dramatic reorganization of Earth's carbon cycle and be linked causally to the initiation of Marinoan ice-house conditions. Alternatively, the δ ¹³C anomaly might record secondary fluid alteration following carbonate deposition. Here we document dropstones within the carbonate sediments immediately below the Marinoan glacial diamictite in South Australia. Advancing ice sheets caused soft-sediment deformation of the beds below the glacial diamictite, as well as subglacial erosion of the carbonates beneath, showing that the Trezona δ ¹³C values must have been acquired before glaciation. Although these stratigraphic relationships do not provide a specific mechanism to explain the Trezona δ ¹³C anomaly, they do require that the nadir of the Trezona δ ¹³C anomaly was recorded prior to local glacier advance and long before late-stage burial diagenesis could have occurred. Furthermore, the δ ¹³C recovery in the Trezona Formation toward 0‰ was synchronous with the appearance of icebergs in the tropics.