Late Pennsylvanian seasonality reflected in the ¹⁸ O and elemental composition of a brachiopod shell

To evaluate the potential for the study of Late Pennsylvanian seasonality, detailed isotopic and elemental analyses were performed on a single specimen of the brachiopod Neospirifer dunbari. Contours of ¹⁸ O, Mg, Na, and S concentrations parallel growth bands and reveal a record of 1 1/2 to 2 cycles. Mg, S, and Na contents varied inversely with δ ¹⁸ O. This trend is opposite to the expected diagenetic trend and is consistent with the temperature dependences of Mg content and δ ¹⁸ O, thus implying preservation of shell chemistry. The ~1.2‰ range in δ ¹⁸ O values suggests a seasonal temperature variation of 5 to 6°C. This high seasonality for the tropical epicontinental sea of Kansas supports climate-model predictions of enhanced continentality in Pangean climate. -Authors