Chemical variation in Pennsylvanian brachiopod shells - Diagenetic, taxonomic, microstructural, and seasonal effects
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abstract
To improve our ability to use minor and trace element (MTE) variation in biotic carbonates as diagenetic and paleoenvironmental indicators, we performed electron probe microanalysis on more than 100 Late Pennsylvanian brachiopod shells from Texas, Kansas, Missouri, and New Mexico. Texturally preserved specimens of the genera Crurithyris, Composite, and Neospirifer from all three regions were analyzed, as were Eridmatus specimens from Texas. Twenty measurements were made in two transects across each shell. Shell microstructure and cathodoluminescence were described for each spot analyzed. Three modern shells were analyzed for comparison. 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.
