LATE PENNSYLVANIAN SEASONALITY REFLECTED IN THE O-18 AND ELEMENTAL COMPOSITION OF A BRACHIOPOD SHELL

To evaluate the potential for the study of Late Pennsylvanian seasonal ity, detailed isotopic and elemental analyses were performed on a sing le specimen of the brachiopod Neospirifer dunbari. Shell preservation was evaluated by petrographic and cathodoluminescence microscopy. Carb onate powders from 112 spots were collected from the sectioned shell f or 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 deltaO-18 vary between -2.3 parts pe r thousand and -1.1 parts per thousand. Contours of O-18, 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 deltaO-18. Thi s trend is opposite to the expected diagenetic trend and is consistent with the temperature dependences of Mg content and deltaO-18, thus im plying preservation of shell chemistry. The approximately 1.2 parts pe r thousand range in deltaO-18 values suggests a seasonal temperature v ariation of 5 to 6-degrees-C (assuming no change in the deltaO-18 of t he water). This high seasonality for the tropical epicontinental sea o f 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 quantit ative records of Paleozoic seasonality.