PHASE- AND FACIES-SPECIFIC CARBON, CLUMPED, AND CALCIUM ISOTOPES OF THE KINDERHOOKIAN-OSAGEAN BOUNDARY EXCURSION IN NORTH-CENTRAL IOWA

The Kinderhookian-Osagean Boundary Excursion (KOBE) is one of the largest positive carbon isotope excursions in the Phanerozoic. Interpretations of these and other Paleozoic carbon isotope excursions include burial diagenesis, meteoric diagenesis associated with the initiation of the Late Paleozoic Ice Age, phase-specific isotope effects, local variations in the δ¹³C of DIC, or a global perturbation to the carbon cycle. Exceptionally preserved Early Mississippian carbonates from two drillcore sections in north-central Iowa provide an opportunity to integrate facies analyses, petrography, and geochemistry of the excursion interval.

Significant burial diagenesis is highly unlikely given the muted burial history in north-central Iowa, low clumped isotope (Δ₄₇) temperatures (median = 32°C), and excellent preservation of original carbonate fabrics at the petrographic scale. Evidence of meteoric diagenesis, including silica cements, potential karst breccia, and low fluid δ¹⁸O values exists at the tops of measured sections, but does not extend to the positive excursion interval. Several hundred Δ₄₇ and dozens of calcium isotope measurements (δ^44/40Ca) suggest the dominance of sediment-buffered diagenesis, consistent with KOBE as an original depositional signal.

We do not observe a strong dependence of δ¹³C composition on facies in our measured sections. The peloidal to fossiliferous grainstone facies that carries the peak of the KOBE is also present above and below the excursion. Algal cortoids are present at the peak of the excursion (~6‰) as well as on the falling limb (~3‰). Trends in δ¹³C persist across parasequence boundaries with only minor shifts, indicating a subordinate role for facies changes in the overall excursion.

Phase-specific isotope analyses of crinoid ossicles, abundant during the excursion peak, show a median +0.8‰ δ¹³C offset (maximum +1.7‰) from coeval carbonate matrix. While this cannot explain the full magnitude of the excursion (~3‰ above baseline) shown in bulk-rock analyses, particularly considering that crinoids never comprise >50% of bulk samples, this positive offset must still be accounted for. Preliminary measurements of crinoids, rugose corals, and brachiopods tend to have cooler Δ₄₇ temperatures and limited to no δ^44/40Ca offset compared to coeval carbonate matrix.