Mid-Neoproterozoic (∼ 830-750 Ma) isotope stratigraphy of Australia and global correlation

Carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual Australian Neoproterozoic basins, in a tectonic and sedimentologic framework, have allowed the construction of isotopic records for the interval 830-750 Ma. This, combined with the discovery of a restricted stratigraphic range for the acritarch Cerebrosphaera buickii in mid-Neoproterozoic strata of Australia, makes possible correlation with successions in Canada, Namibia and Spitsbergen. That part of the mid-Neoproterozoic succession in Spitsbergen which contains C. buickii was previously considered younger than the Sturtian glacial epoch, but this study has indicated that the Akademikerbreen Group is older than that. A correlation scheme is proposed along five tie lines, and is based on a correlation between Australia and Canada at similar to 830 Ma, and Australia, Spitsbergen and Namibia at similar to 760 Ma. The lowest ever recorded seawater Sr-87/Sr-86 ratios in the upper Shaler Supergroup of Canada (0.70561-0.70670) compare with ratios in evaporites of the similar to 830 Ma Bitter Springs Formation, Amadeus Basin, Australia (0.70568-0.70720). Comparable 613C(carb) records support a correlation. At about 760 Ma, a correlation is made between the heaviest mid-Neoproterozoic delta(13)C(carb) values of 7.2 parts per thousand (upper Burra Group, Adelaide Rift Complex) and 8.1 parts per thousand (Kanpa Formation, western Officer Basin) in Australia, 8.5 parts per thousand in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5 parts per thousand in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of C. buickii in Australia and Spitsbergen supports this correlation. A compilation of delta(13)C(carb) and Sr-87/Sr-86 records between about 830 and 750 Ma from Australia, Canada, Namibia and Spitsbergen reveals numerous negative excursions, which are not associated with preserved glacial deposits. It is necessary, therefore, to consider additional models to the 'Snowball Earth' to explain the large and rapid carbon isotopic excursions. (C) 2000 Elsevier Science B.V. All rights reserved.