ABSTRACT

Stable isotopic analyses have been performed on nearly 500 non-luminescent brachiopod shells from Kansas, New Mexico, and Texas to evaluate temporal and geographic variability in the carbon and oxygen isotopic record for the Late Pennsylvanian. 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 d¹⁸O and d¹³C values of non-luminescent brachiopod shells of the same genera. Average d¹⁸O 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 d¹⁸O of these genera appears minimal. d¹⁸O 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 d¹³C 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 d¹³C values. Average d¹³C 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 d¹³C average less than 1‰.

Data for non-luminescent brachiopods and marine cements reveal a Mid-Carboniferous d¹³C increase of 2‰ in Paleotethyan seawater. This increase is not seen in samples from the North American epicontinental seas which opened to the Panthalassa ocean. This regional difference in d¹³C appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of Pangea.