Oxygen and carbon isotopes of Early Permian cold-water carbonates, Tasmania, Australia

Formed at a paleolatitude near 80 degrees S during the Gondwanan ice age, and associated with glaciomarine sediments. A shallow-marine carbonate depositional model has been proposed. Australian Permian brachiopods and molluscs have unusually light delta O ¹⁸ PDB values and heavier delta C ¹³ PDB values than modern cold-water carbonates. Tasmanian-Permian whole-rock delta O ¹⁸ PDB values fall at the edge of the "Normal Marine Limestone" and range towards lighter values (-16.9% PDB). The delta O ¹⁸ values of cements (-7.6 to -25.6% PDB) partly overlap with those delta O ¹⁸ 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. The delta O ¹⁸ values of fauna give unrealistic paleotemperatures because of melt-water dilution of the sea. However, calculated delta O ¹⁸ values, corresponding to marine delta C ¹³ values of brachiopods and Eurydesma and extrapolated from a model of the linear trend of delta C ¹³ -delta O ¹⁸ in modern and last-glacial cold-water carbonates, give reasonable estimates of Australian Permian temperatures of up to 15 degrees C with the coldest waters of less than 4 degrees C around Tasmania. The sequential deviation lines of delta O ¹⁸ -.C ¹³ of both cements and the fauna indicate that the original delta O ¹⁸ value of fauna was as high as +6% PDB. This indicates an average seawater temperature for Tasmania in the Early Permian of -1.8 degrees C, similar to the present average -1.9 degrees C water temperature near ice shelves around Antarctica. The delta O ¹⁸ 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 delta O ¹⁸ SMOW = -31% at 5 degrees C (-26% PDB). It is unlikely that the delta O ¹⁸ composition of the well-mixed open Permian sea ever reached a value as light as delta O ¹⁸ PDB = -6%. It is suggested that the Permian sea delta C ¹³ value was about +2% PDB, heavier than that of modern and Pleistocene seawater.--Modified journal abstract.