High-resolution record of global cooling during a large Mississippian positive carbon isotope excursion

• High resolution clumped isotope record through Mississippian KOBE carbon isotope excursion. • Clumped isotopes reveal 5–7 °C cooling independent of seawater δ¹⁸O assumptions. • KOBE excursion initiated before global cooling, supporting CO₂ drawdown model. • Fluid δ¹⁸O reached −0.35 ‰ indicating ice volume approaching modern levels. The Kinderhookian-Osagean Boundary Excursion (KOBE) is among the largest positive carbon isotope excursions in the Phanerozoic Eon, but the temporal relationship between the KOBE and global cooling is not well understood. Previous attempts to establish connections between perturbations to the carbon cycle and global climate were hampered by the need to assume seawater δ 18 O values, a reliance on fossils with incomplete stratigraphic distribution, and disagreement between records from conodonts and brachiopods. Here, we present a high-resolution clumped isotope record from a Lower Mississippian section from north-central Iowa. When considered in the context of facies changes and evidence of diagenesis, these data show that a positive shift in δ 13 C—possible evidence of increased organic carbon burial—precedes cooling of 5–7 °C, and provides a maximum constraint on global ice volume of slightly less than modern quantities in the prelude to the main phase of the Late Paleozoic Ice Age. This evidence supports hypotheses that the KOBE was the result of a change in organic carbon burial dynamics (upland plant expansion, deep basin burial, expanded suboxia) rather than those that posit that initial cooling led to enhanced carbon burial. We demonstrate that clumped isotope analyses of bulk carbonate rocks from shallowly buried, open-marine environments that experienced early cementation provide complementary constraints to existing paleoclimate proxies.