Carbon and strontium isotope chemostratigraphy of the Neoproterozoic carbonates from the Amadeus Basin, NT.

Abstract

Stratigraphic sections that include Bitter Springs Group, Wallara Formation and Ringwood Member were selected from two cores in the Amadeus Basin, which are BR05DD01 and Wallara-1 to be examined, and evaluated using stable carbon isotopes (δ13Ccarb) and radiogenic strontium (87Sr/86Sr) isotopes, together with major and trace elements, measured in carbonates. Correlation of acquired δ13CC and 87Sr/86Sr data with local and global C and Sr isotope records were explored and evaluated, and our results suggest that δ13Ccarb trend from studied cores showed strong correlation with presumably coeval global δ13C records, however most of our 87Sr/86Sr data appeared to be more radiogenic than global marine Sr isotope record. This in turn suggests that the depositional environment of the Amadeus Basin was predominantly restricted with respect to a coeval Neoproterozoic open ocean. Also, this proposed basin restriction became more enhanced with time (from Tonian to Cryogenian/Ediacaran), however some 87Sr/86Sr data from the Tonian Bitter Springs Group are very close to the expected global marine Sr isotope trend. This, on the other hand, is not the case for younger (Cryogenian, Ediacaran) carbonates from the Amadeus Basin (e.g., Ringwood Member, Olympic Formation) as these have systematically much more radiogenic 87Sr/86Sr signatures compared to the expected Neoproterozoic paleo-seawater 87Sr/86Sr trend. We also found a high abundance of rubidium (Rb) in studied carbonate rocks, which will cause higher production of radiogenic strontium (87Sr) within the bulk carbonates since their deposition. All our 87Sr/86Sr data were thus corrected for in-situ 87Rb decay, which caused the measured strontium isotopes to be additionally more radiogenic. Nevertheless, even after this correction for Rb decay, our 87Sr/86Sr trends from most of the studied carbonates are systematically more radiogenic than the global marine 87Sr/86Sr trend, further corroborating the above progressive restriction of the Amadeus Basin throughout the Neoproterozoic.

Furthermore, based on C isotope constraints, we also argue that the formation carbonates in the top part of the BR05DD01 core correspond to the Aralka Formation and its carbonate-rich Ringwood Member. The paleo-redox conditions were constrained using rare earth element (REE) patterns, specifically based on the Ce anomaly (Ce/Ce*), wherein most of the results from studied Neoproterozoic carbonates showed a positive Ce anomaly. This in turn indicates that the seawater/local basin waters in the Amadeus Basin were predominantly anoxic during the deposition of both Bitter Springs Group (Tonian) and the Ringwood Member (Cryogenian). However, selected carbonates from Bitter Springs Group sampled from the Wallara-1 core showed a true negative Ce anomaly, and also have less radiogenic 87Sr/86Sr values (close to expected global ocean) which thus might suggest transient more oxic shallow-water conditions in marine-dominated evaporitic settings, compared to the rest of the studied Neoproterozoic carbonate records from the Amadeus Basin.