Oxygen and Carbon Isotopes of Early Permian Cold-Water Carbonates, Tasmania, Australia

Abstract

ABSTRACT The Permian carbonates of Tasmania formed at a paleolatitude near 80°S during the Gondwanan Ice Age, contain abundant dropstones, and are associated with glaciomarine sediments. A shallow-marine carbonate depositional model with floating icebergs, calved from an adjacent ice shelf grading into an ice sheet, has been proposed. As the sea level rose during relatively warmer periods, large volumes of melt waters mixed with marine waters retarding limestone deposition and causing deposition of shales/siltstones. These phases are preserved in the stratigraphic record as interbedded units ( Australian Permian brachiopods and molluscs are characterized by unusually light O18 PDB values and heavier C13 PDB values than those that occur in many modern, cool-temperate to subpolar cold-water carbonates. Tasmanian Permian whole-rock O18 PDB values fall at the edge of the Normal Marine Limestone field of Keith and Weber (1964) and range towards lighter values (down to -16.9 PDB). The O18 values of cements (-7.6 to -25.6 PDB) in the Tasmanian Early Permian limestones partly overlap with those O18 values obtained for fresh-water cements in the Early Permian continental tillites from Antarctica and South Africa (Gondwanaland), indicating that the Early Permian sea was diluted by isotopically light melt waters during warming phases. The O18 values of Permian fauna, if one assumes no diagenetic equilibration, give unrealistic paleotemperatures because of this variable melt-water dilution of the Permian sea. However, calculated O18 values, corresponding to marine C13 values of brachiopods and Eurydesma and extrapolated from a model based on the linear trend of C13-O18 in modern and last-glacial cold-water carbonates, give reasonable estimates of Australian Permian temperatures of up t 15° C with the coldest waters of less than 4° C around Tasmania. The sequential deviation lines of O18-C13 of both cements and the fauna associated with these cements indicate that the original O18 value of fauna was as high as +6 PDB. This calculated original O18 value indicates an average seawater temperature for Tasmania in the Early Permian of -1.8° C, similar to the present average -1.9° C water temperature near ice shelves around Antarctica. The O18 values of ements suggest that the Tasmanian limestones reacted with subfreezing melt waters during early diagenesis. The O18 composition of the Early Permian sea is inferred to have been about + 1.2 PDB, similar to that observed during the Pleistocene glaciations, and was diluted by melt water as light as O18 SMOW = -31 at 5° C (-26 PDB) in variable amounts. It is unlikely that the O18 composition of the wellmixed open Permian sea ever reached a value as light as O18 PDB = -6. It is suggested that the Permian sea C13 value was about +2 PDB, heavier than that of modern and Pleistocene seawater.