STABLE ISOTOPES IN LATE PENNSYLVANIAN BRACHIOPODS FROM THE UNITED-STATES - IMPLICATIONS FOR CARBONIFEROUS PALEOCEANOGRAPHY

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 deltaO-18 and deltaC-13 values of nonluminescent brachiopod shells of the same genera. Average deltaO-18 values for the three genera analyzed (Crurithyris, Composita, and Neospirifer) are highest in Kansas (almost-equal-to -1.9 parts per thousand), intermediate in Texas (almost-equal-to -2.3 parts per thousand), and lowest in New Mexico (almost-equal-to -3.6 parts per thousand). Vital effect on the deltaO-18 of these genera appears minimal. DeltaO-18 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 deltaC-13 values of Composita average about 1 parts per thousand higher than those of ring Crurithyris and Neospirifer, suggesting microhabitat differences or vital effects. Preservation of this species effect argues for preservation of original deltaC-13 values. Average deltaC-13 values are highest in Texas, intermediate in Kansas, and lowest in New Mexico. Although these values range from 2.6 parts per thousand to 4.9 parts per thousand, for individual genera the regional variation in deltaC-13 averages less than 1 parts per thousand. Data for nonluminescent brachiopods and marine cements reveal a mid-Carboniferous deltaC-13 increase of 2 parts per thousand 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 deltaC-13 appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of Pangea.