TESTING WHETHER LATE ORDOVICIAN CARBON ISOTOPE EXCURSIONS RECORD RAPID VARIATIONS IN THE GLOBAL CARBON CYCLE
A published δ13C trend measured from Upper Ordovician carbonates in drill core records several positive δ13C excursions interpreted to represent carbon cycle perturbations. High-resolution re-sampling with multiple spot analyses of carbonate rocks was done to test whether these reported δ13C excursions are reproducible or artifacts of alteration, which can be checked using paired δ18O values. Re-evaluation of six carbonate rock samples, which range from lime mudstone to calcareous shale lithologies, was made on samples previously measured for δ13C and δ18O from the Elkhorn drill core of Ohio. Comparison to published values shows δ13C values differ by 0.48–2.40‰. Replicate spot analyses (2–5 values per sample) differ from each other by 0.07–0.21‰, with standard deviations between 0.05–0.11. Petrographic analysis of thin sections using transmitted light and cathodoluminescence microscopy shows evidence of post-depositional alteration (dolomitization) in all studied samples with several stages of cementation, which may have different δ13C values compared to bulk carbonate material. Inadvertent sampling of these phases could explain apparent rapid fluctuations in δ13C, ultimately questioning whether some published positive δ13C excursions are representative of Late Ordovician seawater. Results suggest that alteration can produce variable δ13C from a single succession, which has implications for studies that correlate rapid δ13C excursions based on a few data points without adequate screening to characterize alteration effects.