Local paleoenvironmental controls on the carbon-isotope record defining the Bitter Springs Anomaly

Large magnitude (>10 parts per thousand) carbon-isotope (delta C-13) excursions recorded in carbonate-bearing sediments are increasingly used to monitor environmental change and constrain the chronology of the critical interval in the Neoproterozoic stratigraphic record that is timed with the first appearance and radiation of metazoan life. The similar to 10 parts per thousand Bitter Springs Anomaly preserved in Tonian-aged (1000-720 Ma) carbonate rocks in the Amadeus Basin of central Australia has been offered as one of the best preserved examples of a primary marine delta C-13 excursion because it is regionally reproducible and delta C-13 values covary in organic and carbonate carbon arguing against diagenetic exchange. However, here we show that delta C-13 values defining the excursion coincide with abrupt lithofacies changes between regularly cyclic grainstone and microbial carbonates, and desiccated red bed mudstones with interbedded evaporite and dolomite deposits, recording local environmental shifts from restricted marine conditions to alkaline lacustrine and playa settings that preserve negative (-4 parts per thousand) and positive (+6 parts per thousand) delta C-13 values, respectively. The stratigraphic delta C-13 pattern in both organic and carbonate carbon recurs within the basin in a similar way to associated sedimentary facies, reflecting the linkage of local paleoenvironmental conditions and delta C-13 values. These local excursions may be time transgressive or record a relative sea-level influence manifest through exposure of sub-basins isolated by sea-level fall below shallow sills, but are independent of secular seawater variation. As the shallow intracratonic setting of the Bitter Springs Formation is typical of other Neoproterozoic carbonate successions used to construct the present delta C-13 seawater record, it identifies the potential for local influences on delta C-13 excursions that are neither diagenetic nor representative of the global exogenic cycle.