ABSTRACT

To evaluate the potential for the study of seasonality in the Late Pennsylvanian, detailed isotopic and elemental analyses were performed on a single specimen of the brachiopod Neospirifer dunbari. Shell preservation was evaluated by petrographic and cathodoluminescence microscopy. Carbonate powders from 112 spots were collected from the sectioned shell for isotopic analyses, and 369 spots on the complementary thin section were analyzed by electron microprobe for chemical composition. In the nonluminescent part of the shell, Mg/Ca ratios were between 0.001 and 0.012, and Na/Ca and S/Ca ratios ranged from 0.003 to 0.012 and 0.003 to 0.021, respectively. Values of d¹⁸O varied between ~2.3‰ and ~1.1‰. Contours of ¹⁸O, 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 d¹⁸O. This is opposite to the expected diagenetic trend and is consistent with the temperature dependences of Mg content and d¹⁸O, thus implying preservation of shell chemistry. The ~1.2‰ range in d¹⁸O values suggests a seasonal temperature variation of 5 to 6 °C (assuming no change in the d¹⁸O of the water). This high seasonality for the tropical epicontinental sea of Kansas supports climate-model predictions of enhanced continentality in Pangean climate. Detailed stable isotope and element concentration profiles across growth bands of brachiopod shells can provide quantitative records of Paleozoic seasonality.