Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones; deglaciation, delta 13 C excursions, and carbonate precipitation

Abstract

Abstract Thin (< 15 m) laterally persistent carbonate units cap glacial deposits in Neoproterozoic successions (1000-544 Ma) on almost every continent. Because these enigmatic carbonate units are typically isolated within siliciclastic successions, recurrently overlie successive glacigenic units, and define one of the most pronounced delta 13 C excursions in the geologic record, they are interpreted to record a brief postglacial anomaly in ocean chemistry. In Australia, the Marinoan (Varanger equivalent nearly equal 600 Ma) cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe, including fine-grained dolomitic mineralogy, lateral persistence at basinal scales, thin laminae, graded beds, intervals with abundant marine cements, crystal fans, and a distinctive negative delta 13 C isotopic signature of up to -5 per thousand PDB. On the basis of these features, the Australian examples are interpreted here to be deeper-water deposits (below storm wave base) resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression. Detailed isotopic analysis of Australian cap dolostones indicates delta 13 C values ranging from -1 per thousand to -5 per thousand and generally becoming more depleted in 13 C upsection. Trace-element data indicate some diagenetic stabilization, but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event. The range in delta 13 C values between sections as well as more negative delta 13 C values appear to correlate with greater paleobathymetry within basins. This implies either (1) only partial preservation of the complete oceanic variation of delta 13 C or (2) precipitation of this peculiar facies owes its genesis to basinal-specific oceanographic processes such as proximity of a given section to a postglacial upwelling zone. In the later case, delta 13 C values would not represent whole ocean values. Stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression. This implies that the cause(s) of delta 13 C variation likely operated on time scales significantly less than the residence time of carbon in the oceans (10 5 yr). The continent-wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period. Such large-scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused (or have been the product of) profound changes in the carbon cycle and global climate at that time. Similar evidence for transgression and increased carbonate deposition in the Holocene are attributed to the changing basin shape and pH of the oceans (the Coral Reef Hypothesis of Berger 1982). Cap dolostones may record a Proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms.