OXYGEN ISOTOPES OF CONODONT BIOAPATITE FROM THE MIDDLE PENNSYLVANIAN (DESMOINESIAN) EXCELLO SHALE IN THE MIDCONTINENT US: IMPLICATIONS FOR CONODONT FACIES MODELS

Cyclothems in Midcontinent North America represent important archives of global climatic and glacio-eustatic information for the Late Paleozoic Ice Age. Although it is recognized that cyclothems are the result of waxing and waning of Gondwanan glaciers that produced widespread transgressive-regressive events, the relationship of conodont biofacies to changes in water depth remains in dispute. The purpose of this study is to test the relationship of eustatic changes, as inferred from conodont bioapatite δ¹⁸O_PO4 data, to the conodont biofacies model.

The oxygen isotopic composition of conodonts should change in a predictable manner in relation to water depth changes: higher δ¹⁸O_PO4 values represent colder (glacial) conditions and sea-level lowstands, and lower δ¹⁸O_PO4 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 δ¹⁸O_PO4 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 δ¹⁸O_PO4 values are found in the lowermost 40-50 cm, averaging 19.2 ‰. Above this interval, δ¹⁸O_PO4 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 δ¹⁸O_PO4 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.