CONODONT GEOCHEMICAL RECORDS OF LATE PALEOZOIC PALEOENVIRONMENTAL VARIABILITY IN MIDCONTINENT NORTH AMERICA

Conodont apatite is known to yield oxygen isotope data that are suitable for oceanic temperature estimates. It has been suggested that oxygen isotope variability in Pennsylvanian Midcontinent conodonts records the waxing and waning of Gondwanan ice storage. Potential temperature effects would have been dampened in the inferred low-latitude depositional setting. Our work seeks to develop a more comprehensive oxygen isotope record for Pennsylvanian cyclothem-type deposits to test these and other hypotheses. Specimens were evaluated for lateral variation across a single shale sampled in the Illinois Basin over a relatively short distance (<200 km); no systematic variation was observed. Similarly, our comparison of data from Missouri and Kansas with those of Indiana of roughly the same geologic age revealed comparable narrow oxygen isotope ranges of 18.3‰ to 20.6‰ and 17.6‰ to 21.0‰ V-SMOW, respectively. These data closely match the range of 18.6 to 22.3‰ reported by Horacek et al. and preliminarily argue against secondary alteration and regional differences in the primary composition of the marine reservoir. For example, our comparison nominally eliminates the possibility of reservoir differences that might have existed between relatively deep- and shallow-water shales. Furthermore, we observed no meaningful relationship between taxonomy and isotopic value in our assessment of vital effects through analysis of varying genera collected from single samples. Also, no systematic isotopic variation was observed between juvenile and mature forms of a species collected from a single sample, although this data set is still very small.

The primary focus of this research remains the development of a high-resolution oxygen isotope record for Pennsylvanian cyclic sequences of midcontinent North America, which should prove useful in resolving longstanding debates centered on global versus regional controls on the cyclic patterns on Pennsylvanian deposition.