CONODONT OXYGEN ISOTOPE RECORDS OF LATE PENNSYLVANIAN PALEOENVIRONMENTAL VARIABILITY

Oxygen isotope data developed from conodont phosphate are proving to be an important paleoenvironmental tool. It has been suggested that shifts seen in Carboniferous oxygen isotope data may represent global changes in the isotopic composition of the ocean linked to varying global ice volume. Our work seeks to test this hypothesis through the development of a high-resolution oxygen isotope record across the various lithologies within cyclic deposits in Upper Pennsylvanian rocks of the midcontinent United States. This work also has the potential to resolve the longstanding debate over local vs. global controls on this apparent cyclicity. Eustatic variation linked to ice volume should yield conodonts with more negative oxygen isotope values in the deepest water facies, while conodonts in shallower water facies should yield more positive values. Potential temperature effects would be muted in the low latitude depositional setting. Preliminary data collected from the Upper Ft. Scott cycle, southwest Missouri, show a 1-2 per mil increase in δ¹⁸O across the stratigraphic transition from deep to shallow water sediments, which may indicate a glacio-eustatic control. Data from the overlying Pawnee cycle, however, yield less systematic trends across deepening and shallowing intervals. This complexity may be the result of dampened temporal resolution introduced by vertical lumping of samples, uncertain correlation among geographically separated sample locations, shorter-term paleoceanographic variability, and local overprints on global signals.

Our most recent work seeks to address these issues by studying the oxygen isotope record derived from three stacked cycles located near Kansas City, Missouri. Conodonts were obtained from 41 bulk samples collected through a 28 m exposure of the Swope, Dennis, and Cherryvale cycles. Samples were collected with a focus on minimizing the uncertainties described above. This record will allow us to further explore the relationships among conodont oxygen isotope signals, facies patterns, and global ice volume within a tight stratigraphic context.