Oxygen Isotopic Composition of Biogenic Apatite of Early Permian Fossils (Wolfcampian; Bennett Shale, Kansas): Implications for Sea Surface Temperature and Ice Sheet Volume

The base of the Permian is defined by the first appearance of Streptognathodus isolatus. In Kansas, S. isolatus first occurs at the base of the Bennett Shale Member of the Red Eagle Limestone. The Bennett Shale is a black shale that contains abundant conodonts, orbiculoid brachiopods, and fish remains. The sample material for this study is from the lowermost part of the Bennett Shale. Therefore, the oxygen isotopic composition of biogenic apatite (δ¹⁸O_phos) from this interval represents a record of environmental conditions of the Earliest Permian in North America.

The δ¹⁸O_phos 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 (δ¹⁸O_sw) during the formation of the biogenic apatite. Thus, in order to estimate the sea surface temperature, one has to assume a δ¹⁸O_sw 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 δ¹⁸O_sw value of -1‰ VSMOW could be reasonable. Using this value, the δ¹⁸O_phos 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 δ¹⁸O_sw value of 0‰ of Permian seawater).