Assessing the paleoenvironmental significance of middle-late Pennsylvanian conodont apatite 18O values in the Illinois basin

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© 2014 SEPM (Society for Sedimentary Geology). Conodont apatite 18OV-SMOW values from Middle though Upper Pennsylvanian (Desmoinesian-Missourian) laminated, marine black shale units within cyclic deposits of intercalated terrestrial and marine strata (cyclothems) from the Illinois Basin (United States) were measured in order to evaluate their utility as a proxy for changes in the oxygen isotopic composition of the epicontinental Late Pennsylvanian Midcontinent Sea (LPMS). The average 18OV-SMOW values of well-preserved monogeneric (Idiognathodus) separates of conodont apatite from 12 lithologic units representing nine cyclothems range from 17.0‰ to 20.1‰ and average 19.0‰ ± 0.4‰ (1). Within the limits of analytical uncertainty of stable isotope measurements, the stratigraphic distribution of conodont apatite 18O values is nontrending; particularly, there is no significant shift in 18O values across the Desmoinesian-Missourian boundary, a period that has been interpreted to preserve a shift toward a warmer climate, increased seasonality, and shorter periods of wet conditions in the terrestrial record. Conodont apatite 18O values from stratigraphically equivalent black shale members across the Illinois Basin vary up to 2.6‰, which is nearly equivalent to the observed stratigraphic range of conodont apatite 18O values, and suggests differences in local (basin-scale) seawater 18O values affected the conodont apatite 18O values. Within analytical uncertainty, conodont apatite 18O values from the Illinois Basin and Midcontinent Basin (United States) are indistinguishable, suggesting a component of overarching broader regional to global controls on seawater 18O values. Nevertheless, if the large variability observed in stratigraphically equivalent black shale members in the Illinois Basin is attributed to regional factors, these results indicate caution should be used when attempting to interpret temporal shifts from single aliquots of conodonts in epicontinental settings.

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