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The sedimentary record of carbonate carbon isotopes (δ13Ccarb) 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‰ positive δ13Ccarb 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 δ13Ccarb and δ18Ocarb greater than 2‰ across individual beds. Linear δ13Ccarb to δ18Ocarb 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 δ18Ocarb filter based on empirical δ18Ocarb values of well‐preserved carbonate mudstones allowed further sample discrimination. The resulting ‘least‐altered’ δ13Ccarb 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 δ13Ccarb values may result in part from local diagenetic overprinting; (ii) peak‐Guttenberg excursion δ13Ccarb values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental‐scale spatial gradient in δ13Ccarb (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 δ13Ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.

Mammoth Cave (Kentucky, USA) is the world's longest humannavigable cave system. Gypsum (CaSO 4 •2H 2 O) crystals line many dry passages of the limestone cave and the source of sulfur for the gypsum remains uncertain. Previous workers have suggested sulfate from pyrite oxidation, Mississippian-aged sulfate evaporites, and Pleistocene soil sulfate as possible sulfur sources. We use sulfur isotopes (d 34 S) to constrain the gypsum sulfur source by comparing the d 34 S values of different possible sources to that of gypsum throughout the cave. d 34 S gypsum values (n = 106) from 12 different locations within the Mammoth Cave system are along a continuum of d 34 S values from-12.0‰ to +12.8‰, with little variability along the crystal growth axis or between samples within the same cave chamber. Neither sulfur from coeval sedimentary evaporites (i.e., gypsum, anhydrite) nor from formations overlying the cave is required to explain the d 34 S gypsum data. Rather, the range of pyrite d 34 S in strata immediately surrounding the cave is sufficient to generate the spectrum of observed d 34 S gypsum. Modern water d 34 S SO 4 values are similar to the host formation d 34 S pyrite , suggesting that oxidized pyrite from the host formation continues to be a major sulfur source to this day. Together, these observations strongly suggest a significant local source of sulfur for Mammoth Cave gypsum, the majority (66%-100%) of which is derived from the oxidation of pyrite in strata adjacent to the cave. INTRODUCTION Mammoth Cave, located in Kentucky, USA (Fig. 1), is the longest human-navigable cave system in the world, and is more than 640 km in length (Mammoth Cave National Park Service, 2014). Aside from being an important karst terrain, Mammoth Cave is also a U.S. National Park and a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site and biosphere reserve. Despite nearly a century of research into the formation of Mammoth Cave and its interior deposits, the origin of the gypsum (CaSO 4 •2H 2 O) that lines cave walls remains uncertain (

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Table S1: U-Pb data for analyzed zircons from Ordovician K-bentonites, Missouri, United States.