Techniques for assessing spatial heterogeneity of carbonate δ13C values: Implications for craton‐wide isotope gradients

The sedimentary record of carbonate carbon isotopes (delta C-13(carb)) provides one of the best methods for correlating marine strata and understanding the long-term evolution of the global carbon cycle. This work focuses on the Late Ordovician Guttenberg isotopic carbon excursion, a ca 2.5 parts per thousand positive delta C-13(carb) excursion that is found in strata globally. Substantial variability in the apparent magnitude and stratigraphic morphology of the Guttenberg excursion at different localities has hampered high-resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle. This work investigates the magnitude, spatial scale and sources of isotopic variability of the Guttenberg excursion in two sections from Missouri, USA. Centimetre-scale isotope transects revealed variations in delta C-13(carb) and delta O-18(carb) greater than 2 parts per thousand across individual beds. Linear delta C-13(carb) to delta O-18(carb) mixing lines, together with petrographic and elemental abundance data, demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components. These patterns facilitated objective sample screening to determine the least-altered' data. A delta O-18(carb) filter based on empirical delta O-18(carb) values of well-preserved carbonate mudstones allowed further sample discrimination. The resulting least-altered' delta C-13(carb) profile improves the understanding of regional as well as continental-scale stratigraphic relations in this interval. Correlations with other Laurentian sections strongly suggest that: (i) small-scale variability in Guttenberg excursion delta C-13(carb) values may result in part from local diagenetic overprinting; (ii) peak-Guttenberg excursion delta C-13(carb) values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental-scale spatial gradient in delta C-13(carb) (for example, arising from chemically distinct aquafacies') is required during Guttenberg excursion-time. This study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for delta C-13(carb) chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.