Rare earth element and 87Sr/86Sr step-leaching geochemistry of central Australian Neoproterozoic carbonate

Abstract

The Cryogenian-Ediacaran boundary marks a profound change in the Earth environment from preceding glacial conditions to a subsequent period of shallow marine carbonate deposition. Earliest Ediacaran carbonate (i.e., “cap carbonate”) serves as a record of the chemical and physical conditions of the world’s ocean immediately following this transition. We present new step-leached trace element, rare earth element plus yttrium (REY), and Sr isotope (87Sr/86Sr) results from mid-Cryogenian, earliest Ediacaran, and late Ediacaran carbonate of the Amadeus Basin in central Australia that bracket the Cryogenian-Ediacaran boundary. Both REY and 87Sr/86Sr from a sample of mid-Cryogenian carbonate in the Amadeus Basin (the Ringwood Mbr. of the Aralka Fm.) are consistent with deposition in an anoxic marine environment. Subsequent basal Ediacaran cap carbonate (the Olympic cap) has REY compositions that document an up-section change from non-seawater to oxic marine conditions during cap dolostone deposition in the wake of Cryogenian deglaciation, presumably reflecting an important early contribution from glacial meltwater. Overlying cap limestone of the Olympic cap is marked by a strong negative Ce anomaly and, at early and intermediate leaching steps, elevated Y/Ho, attributes that point toward deposition in oxic seawater. Some step-leaching REY results from a sample of mid-Ediacaran carbonate of the Julie Fm. are also suggestive of a marine origin, though the remainder of leaching steps from this sample are consistent with pervasive “contamination” by detrital clay. Cumulatively, these data are consistent with key Cryogenian-Ediacaran transitions from largely anoxic marine conditions during the Cryogenian Period, to earliest Ediacaran meltwater-dominated conditions, to a subsequent Ediacaran oxic marine environment.