Stable isotopic analyses have been performed on nearly 500 nonluminescent brachiopod shells from Kansas, New Mexico, and Texas to evaluate temporal and geographic variability in the carbon and oxygen isotopic record for late Pennsylvanian time. Brachiopod specimens were collected from Missourian and Virgilian shales and examined in thin section for preservation of microstructure and absence of cathodoluminescence as a primary test for shell preservation.
Regional variations are observed in the δ18O and δ13C values of nonluminescent brachiopod shells of the same genera. Average δ18O values for the three genera analyzed (Crurithyris, Composita, and Neospirifer) are highest in Kansas (≈-1.9‰), intermediate in Texas (≈-2.3‰), and lowest in New Mexico (≈-3.6‰). Vital effect on the δ18O of these genera appears minimal. δ18O data and other evidence suggest warmer temperatures for the shallow New Mexico localities and slightly higher salinity for the Kansas sea relative to the Texas sea. The δ13C values of Composita average about 1‰ higher than those of co-occurring Crurithyris and Neospirifer, suggesting microhabitat differences or vital effects. Preservation of this species effect argues for preservation of original δ13C values. Average δ13C values are highest in Texas, intermediate in Kansas, and lowest in New Mexico. Although these values range from 2.6‰ to 4.9‰, for individual genera the regional variation in δ13C averages less than 1‰.
Data for nonluminescent brachiopods and marine cements reveal a mid-Carboniferous δ13C increase of 2‰ in Paleotethyan sea water. This increase is not seen in samples from the North American epicontinental seas, which opened to the Panthalassa ocean. This regional difference in δ13C appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of Pangea.