Oxygen and carbon isotopes of Early Permian cold-water carbonates, Tasmania, Australia
Oxygen and carbon isotopes of Early Permian cold-water carbonates, Tasmania, Australia
Journal of Sedimentary Petrology (December 1982) 52 (4): 1111-1125
- Australasia
- Australia
- Brachiopoda
- C-13/C-12
- carbon
- carbonate rocks
- cement
- diagenesis
- environment
- geochemistry
- glacial environment
- glaciomarine environment
- Invertebrata
- isotopes
- Lower Permian
- marine environment
- Mollusca
- O-18/O-16
- oxygen
- paleo-oceanography
- paleoenvironment
- Paleozoic
- Permian
- sedimentary rocks
- sedimentation
- stable isotopes
- Tasmania Australia
- Berriedale Limestone
- Maria Island
- Darlington Limestone
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 (super 18) PDB values and heavier delta C (super 13) PDB values than modern cold-water carbonates. Tasmanian-Permian whole-rock delta O (super 18) PDB values fall at the edge of the "Normal Marine Limestone" and range towards lighter values (-16.9% PDB). The delta O (super 18) values of cements (-7.6 to -25.6% PDB) partly overlap with those delta O (super 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 (super 18) values of fauna give unrealistic paleotemperatures because of melt-water dilution of the sea. However, calculated delta O (super 18) values, corresponding to marine delta C (super 13) values of brachiopods and Eurydesma and extrapolated from a model of the linear trend of delta C (super 13) -delta O (super 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 (super 18) -.C (super 13) of both cements and the fauna indicate that the original delta O (super 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 (super 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 (super 18) SMOW = -31% at 5 degrees C (-26% PDB). It is unlikely that the delta O (super 18) composition of the well-mixed open Permian sea ever reached a value as light as delta O (super 18) PDB = -6%. It is suggested that the Permian sea delta C (super 13) value was about +2% PDB, heavier than that of modern and Pleistocene seawater.--Modified journal abstract.