Isotope stratigraphy of the Ediacarian (Neoproterozoic III) of the Adelaide Rift Complex, Australia, and the overprint of water column stratification

Abstract

This study establishes a high resolution δ 13C-chemostratigraphic framework for the type Ediacarian (terminal Proterozoic) section in the Adelaide Rift Complex, a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the Earth's surface environment. Only 15 m of the 3.5 km thick type section consists of shallow-water carbonates of the kind that have formed the basis of most previous studies of Neoproterozoic chemostratigraphy. However, the organic (δ 13C org) record reveals a surprising richness of information and shows excellent utility in correlation between sections in the Adelaide Rift Complex. Lateral variations in δ 13C org profiles correspond to expected diachronous patterns of basin fill. With the addition of 87Sr/ 86Sr data, correlation can also be made to the Officer and Amadeus Basins, consistent with known lithostratigraphic and sequence-stratigraphic constraints. Sublittoral micritic carbonates of the upper Nuccaleena Formation (‘cap dolostone’) and lower and middle Wonoka Formation show strong basinwide 13C depletion (δ 13C carb=−3 to −10‰) despite compelling evidence for preservation of depositional δ 13C carb (such as good textural preservation, evidence for sea floor lithification, and in the Wonoka Formation, pristine 87Sr/ 86Sr and low Mn/Sr). Associated with the depleted carbonates are rare subaqueous evaporite, relatively heavy δ 13C org and anomalously 13C-depleted organic carbon of benthic microbial mat origin, and a common cause — salinity stratification of the water column — is postulated. The indicated palaeogeographic setting — an evaporative Mediterranean-like basin tenuously connected to the world ocean — is entirely consistent with previous suggestions of one or more Messinian-type desiccation events to explain very deep (1 km) incised valleys or canyons in the Adelaide Rift Complex and Officer Basin, at horizons within the Wonoka Formation and correlates. Positive excursions in δ 13C org during basin stratification may in part be due to 12C depletion of surface waters by photosynthetic carbon fixation. Despite the proposed semi-isolation of Australian Ediacarian basins from the world ocean, major δ 13C excursions — which can to some extent be differentiated by the monotonic rise in Ediacarian marine 87Sr/ 86Sr — appear to be correlatable to the upper Windermere Supergroup, northwest Canada.