Abstract

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. 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 δ18O vary between -2.3‰ and -1.1‰. Contours of 18O, Mg, Na, and S concentrations parallel growth bands and reveal a record of 1½ to 2 cycles. Mg, S, and Na contents varied nversely with δ18O. This trend is opposite to the expected diagenetic trend and is consistent with the temperature dependences of Mg content and δ18O, thus implying preservation of shell chemistry. The ∼1.2‰ range in δ18O values suggests a seasonal temperature variation of 5 to 6 °C (assuming no change in the δ18O 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.

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