TESTING WHETHER LATE ORDOVICIAN CARBON ISOTOPE EXCURSIONS RECORD RAPID VARIATIONS IN THE GLOBAL CARBON CYCLE

Carbonate rocks are used to study changes in the global carbon cycle throughout Earth history by measuring their carbon isotopic composition (δ¹³C) as a proxy for seawater chemistry. Positive δ¹³C excursions are thought to result when ¹²C is preferentially removed from the global oceanic carbon reservoir from elevated organic burial rates. If age-equivalent δ¹³C excursions are correlative and are not alteration products, it suggests that perturbations to the global carbon cycle may be caused by paleoenvironmental change, such as marine anoxia. δ¹³C values are commonly measured from a single drilled spot, but drawing interpretations from these isolated data points could be tenuous without fully documenting that alteration does not play a role.

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.