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

This study establishes a high resolution delta(13)C-chemostratigraphic fram ework for the type Ediacarian (terminal Proterozoic) section in the Adelaid e Rift Complex, a framework that contributes to the emerging global chronos tratigraphic synthesis of a period that saw momentous changes in the Earth' s surface environment. Only 15m of the 3.5 km thick type section consists o f shallow-water carbonates of the kind that have formed the basis of most p revious studies of Neoproterozoic chemostratigraphy. However, the organic ( delta(13)C(org)) record reveals a surprising richness of information and sh ows excellent utility in correlation between sections in the Adelaide Rift Complex. Lateral variations in delta(13)C(org) profiles correspond to expec ted diachronous patterns of basin fill. With the addition of Sr-87/Sr-86 da ta, correlation can also be made to the Officer and Amadeus Basins, consist ent with known lithostratigraphic and sequence-stratigraphic constraints. S ublittoral micritic carbonates of the upper Nuccaleena Formation ('cap dolo stone') and lower and middle Wonoka Formation show strong basinwide C-13 de pletion (delta(13)C(carb) = -3 to -10 parts per thousand) despite compellin g evidence for preservation of depositional delta(13)C(carb) (such as good textural preservation, evidence for sea floor lithification, and in the Won oka Formation, pristine Sr-87/Sr-86 and low Mn/Sr). Associated with the dep leted carbonates are rare subaqueous evaporite, relatively heavy delta(13)C (org) and anomalously C-13-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 Med iterranean-like basin tenuously connected to the world ocean - is entirely consistent with previous suggestions of one or more Messinian-type desiccat ion events to explain very deep (1 km) incised valleys or canyons in the Ad elaide Rift Complex and Officer Basin, at horizons within the Wonoka Format ion and correlates. Positive excursions in delta(13)C(org) during basin str atification may in part be due to C-12 depletion of surface waters by photo synthetic carbon fixation. Despite the proposed semi-isolation of Australia n Ediacarian basins from the world ocean, major delta(13)C excursions - whi ch can to some extent be differentiated by the monotonic rise in Ediacarian marine Sr-87/Sr-86 - appear to be correlatable to the upper Windermere Sup ergroup, northwest Canada. (C) 2000 Elsevier Science B.V. All rights reserv ed.