Diagenesis, as indicated by cathodoluminescence and/or absence of microstructure, tends to enrich shells in Fe and Mn (X/Ca >= 0.7 mmol/mol) and deplete shells in Na and S. Mg content shows no consistent trend with diagenesis. In fabric-retentive, nonluminescent shell areas, Mg, Na, and S contents vary twofold to sevenfold depending on taxonomy, microstructure, and season. Overall, taxonomy is the dominant factor controlling MTE composition. Na and S concentrations are consistently highest in Crurithyris and Eridmatus, intermediate in Neospirifer, and lowest in Composita. In taxa with mixed microstructure (Composita, Neospirifer), secondary fibrous layer calcite contains 1.5 to 2 times more Na than does interlayer prismatic calcite. Thus whole-shell Na contents of these taxa depend on the proportion of fibrous and prismatic shell. Seasonal cycles are revealed in MTE transects across growth lines. Mg, Na, and S contents commonly vary by more than a factor of two between maxima (presumably summer) and minima (winter) within the same shell. Retention of taxonomic, microstructural, and seasonal effects in shell chemistry argues for preservation of original chemistry in fabric-retentive, nonluminescent Paleozoic brachiopod shells.