Mid-Neoproterozoic isotope stratigraphy of Australia

Systematic isotopic variations in carbon and strontium in the mid-Neoproterozoic (~830-750 Ma) of Australia have improved stratigraphic resolution within Australian basins and allowed correlation with successions from Canada, Namibia and Spitsbergen. The use of isotopes of carbon, oxygen, sulfur and strontium, and biomarkers, all in a sedimentologic, tectonic and palaeobiological framework has allowed for the confident recognition of marine and non-marine environments in the ~830 Ma Bitter Springs Formation, Amadeus Basin, at a time when the fossil record is sparse. It has also led to one of the most comprehensive documentations of a Neoproterozoic carbonate anywhere. Little variation in δ¹³Ccarb across the Amadeus Basin allowed the construction of a composite isotopic curve of the Bitter Springs Formation (Supersequence 1), which could then be correlated with other Australian basins. δ¹³Ccarb curves and comparable stromatolite assemblages in Supersequence 1 of the eastern Officer Basin and Arkaroola Subgroup (lower Callanna Group) of the Adelaide Rift Complex allow correlation with the Amadeus Basin. These are the earliest Australian Neoproterozoic sediments, and they were deposited in a shallow epicontinental sea that extended at least into the northern and eastern areas of the Centralian Superbasin, and northern areas of the Adelaide Rift Complex at about 830 Ma. The isotopic study of successions from various basins highlights the importance of interbasinal correlation in confirming marine deposition, which once established allows correlations with global successions possible. In the Amadeus and eastern Officer Basins, and Adelaide Rift Complex a transition to nonmarine deposition then occurred, after which mid-Neoproterozoic sediments are not seen in the Amadeus and eastern Officer Basins. In the Adelaide Rift Complex, this period of nonmarine deposition, represented by the Curdimurka Subgroup, is supported by extremely depleted and erratic δ¹³Ccarb values. Periods where δ¹³Ccarb values are less erratic and closer to 0‰ may indicate brief periods of marine deposition. But the inability to correlate carbon isotopic curves across the Adelaide Rift Complex suggests marine incursions were diachronous, and thus sedimentation was dominated by local tectonics in the developing rift basin. This period of dominantly non-marine deposition prevailed from about 800 to 780 Ma. The approximately 780 Ma and younger Burra Group of the Adelaide Rift Complex was mostly deposited under marine conditions. This is shown by correlation of δ¹³Ccarb curves from successions in the Peake and Denison, and southern Flinders Ranges. Correlations between these two successions have allowed for greater stratigraphic resolution within the Burra Group. An Australian Mid-Neoproterozoic (~830-750 Ma) carbon and strontium isotopic record was then compiled from the Bitter Springs Formation (~830 Ma) and Burra Group (~780 Ma), with a gap of about 50 m.y. between 830 and 780 Ma. A correlation scheme is proposed along seven tie lines, and is based on a correlation between Australia and Canada at ~830 Ma, and Australia, Spitsbergen and Namibia at ~760 Ma. The lowest ever recorded seawater ⁸⁷Sr/⁸⁶Sr ratios in the upper Shaler Supergroup of Canada compare with ratios in evaporites of the ~830 Ma Bitter Springs Formation, Amadeus Basin, Australia. Comparable δ¹³Ccarb records support correlation. At about 760 Ma a correlation can be made between the heaviest mid- Neoproterozoic δ¹³Ccarb values of 7.2‰ in Australia (Burra Group, Adelaide Rift Complex), 8.5‰ in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5‰ in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of Cerebrosphaera buickii, which is restricted to the upper mid-Neopoterozoic (~760 Ma) in Australia, supports correlation with Spitsbergen and necessitates a reinterpretation of the Neoproterozoic succession there. There are numerous negative δ¹³Ccarb excursions in a compilation of δ¹³Ccarb and ⁸⁷Sr/⁸⁶Sr records between about 830 and 750 Ma in Australia, Canada, Namibia and Spitsbergen, which are not associated with known glacial deposits. Apart from models involving the release of methane, explanations for these rapid and large carbon isotopic excursions in the Neoproterozoic involve ice sheets that cover most of the Earth's surface. Future models to explain Neoproterozoic Earth history must therefore include mid-Neoproterozoic isotopic records, which differ substantially from late Neoproterozoic records.

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