OXYGEN ISOTOPES OF CONODONT BIOAPATITE FROM THE MIDDLE PENNSYLVANIAN (DESMOINESIAN) EXCELLO SHALE IN THE MIDCONTINENT US: IMPLICATIONS FOR CONODONT FACIES MODELS
The oxygen isotopic composition of conodonts should change in a predictable manner in relation to water depth changes: higher δ18OPO4 values represent colder (glacial) conditions and sea-level lowstands, and lower δ18OPO4 values represent the converse. Gondolella and Idioprioniodus have been interpreted as deepwater taxa and increases in their abundance to reflect deepening events. In the study sections, these taxa reach peak abundances in the middle of the shale interval, on the basis of which maximum water depths could be inferred. If so, the lowest δ18OPO4 values should coincide with peaks in abundance of Idioprioniodus and Gondolella.
We analyzed the oxygen isotope compositions of conodont apatite in specimens of Idiognathodus extracted from black and gray shales at 5-10 cm intervals from three geographically widely separated locations representing a ~500-km-long depositional transect across the Midcontinent Shelf. In each section, the lowest δ18OPO4 values are found in the lowermost 40-50 cm, averaging 19.2 ‰. Above this interval, δ18OPO4 values shift gradually toward heavier values in the upper part of the shale interval, reaching average peak values of 21.0 ‰.
The lower shale interval with minimum δ18OPO4 values includes the strong abundance peaks of Gondolella and Idioprioniodus. However, the sharply defined peak in Gondolella abundance is not accompanied by a corresponding decrease in δ18O values, as one would expect from a deepening event. The conodont biofacies model needs to be further refined to incorporate other paleoceanographic features, such as circulation patterns and nutrient distribution. More research on geochemical proxies is needed to fully understand the paleooceanographic processes that control the paleoecology of conodonts.