The original stable isotopic composition of Paleozoic marine carbonate rocks is important to establish a baseline for evaluating diagenetic alteration and providing a reliable data-base for modeling exogenic geochemical cycles of the elements. We have compared the δ18O and δ13C values in the nonluminescent portions of brachiopods with similar published data of whole brachiopods, other whole fossils, and estimates of original isotopic composition of marine cements.
The oxygen and carbon isotopic records for each of these Paleozoic components show generally good internal consistency. Significant discrepancies, however, were noted between results on the different components. Isotopic and trace-element data presented in this study suggest that Permo-Carboniferous whole brachiopods are little altered chemically. Nonetheless, a more reliable (but not fool-proof) approach for obtaining pristine isotopic values is microsampling of “least-altered” regions based on cathodoluminescence in conjunction with staining techniques and elemental analyses.
The carbon and oxygen isotopic compositions of all components decrease in δ18O and δ13C in geologically older material. Neither temporal trend is monotonic, however. Oxygen isotopic results suggest an important positive shift in δ18O of about 2‰ during the early Carboniferous, whereas the carbon isotope trend shows a major positive shift (≈2%–3‰) during the mid-Carboniferous.