A PAIRED δ44/40CA AND Δ47 INVESTIGATION OF EARLY MISSISSIPPIAN PLATFORM CARBONATES. DID DIAGENESIS DRIVE THE KINDERHOOKIAN-OSAGEAN BOUNDARY EXCURSION?

Shallow-water platform carbonates are an invaluable record of climate change and interactions within the ocean-atmosphere-biosphere system as they comprise the majority of the pre-Jurassic sedimentary record. Carbon isotopes (δ¹³C) measured in carbonate rocks are thought to record a primary marine signal and are widely used as both a proxy for the ancient carbon cycle and chronostratigraphic correlations. However, recent work highlighting the impact of local factors on the δ¹³C record has called into question the fidelity of these records as processes like early marine diagenesis can alter their primary geochemical signal. New integrated studies combining elemental, isotopic, and petrographic data within a stratigraphic framework are needed to evaluate δ¹³C records in platform carbonates.

One of the largest positive δ¹³C isotope excursions of the Phanerozoic is recorded in the Early Mississippian and is coincident with the start of a long-term cooling trend near the onset of Late Palaeozoic Ice Age (LPIA). Known as the Kinderhookian-Osagean Boundary Excursion (KOBE), the KOBE reaches a peak value of +7‰ and has been documented globally across a variety of facies and depositional environments. As the large magnitude of the KOBE is difficult to reconcile through organic C burial alone, a robust geochemical test of the fidelity of the shallow-water δ¹³C record is needed.

Here we present paired calcium isotopes (δ^44/40Ca) and Sr/Ca from three sections in the United States to test the hypothesis that the KOBE platform carbonate record reflects a period of global diagenesis. All three localities (Briggs Woods, Iowa; Strawberry Creek, Wyoming; Funeral Mountain, California) preserve the KOBE within shallow marine limestones, and no significant covariation was recorded between δ^44/40Ca, Sr/Ca, and δ¹³C. At Briggs Woods, our shallowest section, a subset of samples was analyzed for clumped isotopes (Δ₄₇) which suggests primarily sediment-buffered diagenesis at relatively low temperatures. Our integrated δ^44/40Ca, Sr/Ca, and Δ₄₇ record shows that the δ¹³C signal of KOBE is likely not driven by early marine diagenesis. Instead, the shallow-water record of the KOBE is reflective of a primary marine signal consistent with a perturbation of the global C cycle near the onset of the LPIA.