Oxygen Isotopic Composition of Biogenic Apatite of Early Permian Fossils (Wolfcampian; Bennett Shale, Kansas): Implications for Sea Surface Temperature and Ice Sheet Volume
The δ18Ophos values of conodonts (Streptognathodus sp.), brachiopods (Orbiculoidea sp.), and sharks (Cladodus sp.) range from 22.1 to 19.8‰ V-SMOW. The oxygen isotopic composition of the PO4 component of invertebrate tooth and shell material depends on the temperature and the isotopic composition of the seawater (δ18Osw) during the formation of the biogenic apatite. Thus, in order to estimate the sea surface temperature, one has to assume a δ18Osw value. Since the lower part of the Bennett Shale has been interpreted as a maximum flooding surface, caused by deglaciation of Gondwanan ice sheets, an ice-free δ18Osw value of -1‰ VSMOW could be reasonable. Using this value, the δ18Ophos values would indicate an Early Permian subtropical seawater temperature of ~17.2°C. This temperature estimate is fairly low for a low-latitude region and lower than sea temperature estimates for other Paleozoic epicontinental seas.
The sea surface temperature estimate could be explained by upwelling of cold deep water during the transgression. Alternatively, if Gondwanan ice sheets did not completely melt, and ice sheets were comparable to modern ice sheets in volume, than the sea surface temperature estimate would be ~21.5°C (assuming a δ18Osw value of 0‰ of Permian seawater).