A carbon isotope and trace element analysis of Pennsylvanian rugose corals, North-Central Texas
Carbon and oxygen isotopes were analyzed to identify the process of algal symbiosis in Pennslyvanian rugose corals from the Graham Formation of north-central Texas. Because original oxygen and carbon isotopic compositions depend on the preservation of isotopic content during post-depositional diagenesis, trace element and petrographic analyses were used to determine the suitability of samples of Caninia, Lophophyllidum and the brachiopod Composita for analysis. Petrographic analysis of fossils from the Finis Shale indicate that these fossils have been subjected to four episodes of diagenetic cementation. Shell microstructures are partially preserved in Composita and Caninia. Recrystallization of the shell was more extensive in Lophophyllidum from the Finis Shale and Caninia from the Gunsight Limestone. Six episodes of cementation are recognized in Caninia from the Gunsight Limestone. Trace element analysis indicates that sodium and magnesium values are depleted and iron and manganese concentrations enriched in Caninia from the Finis Shale. Trace element analysis of Composita indicate that sodium, iron, manganese and strontium concentrations are within the observed range for Paleozoic brachiopods. Isotopic analyses of Caninia and Lophophyllidum exhibit 6 ^^C and 6 ^^0 values depleted compared to Composita and to the postulated range for carbonate precipitated in equilibrium with Pennsylvanian seawater. These findings suggest that an algal symbiont influenced the development of the coral skeleton.