CERIUM ANOMALY ACROSS THE EARLY MISSISSIPPIAN KINDERHOOKIAN–OSAGEAN CARBON ISOTOPE EXCURSION

The Early Mississippian strata in the southern Great Basin (Utah and Nevada) host a prominent carbon isotope (δ¹³C) excursion with a magnitude of ≥+5‰ at the Kinderhookian–Osagean (K-O) transition. Sulfur isotope and sulfate oxygen isotope studies suggested increased oceanic anoxia at the peak of this δ¹³C excursion, but this interpretation has been challenged by the large spatial variations of organic carbon and sulfur isotopes observed in the mostly dolomitized Madison Limestone in Wyoming and Montana. To further test the potential redox change associated with the perturbation of the carbon cycle, we analyzed rare earth element (REE+Y) abundances of pure carbonates across the K-O δ¹³C excursion in two sections across a shallow-to-deep water transect in the southern Great Basin. Petrographic and geochemical data indicate that most samples are not significantly influenced by diagenetic alteration. In both sections, Ce anomaly (Ce/Ce*) values show large temporal variations across the δ¹³C excursion: 0.3–0.6 at the rise limb of the δ¹³C excursion; 0.7–0.9 with a few lower values at the peak of the d13C excursion; and 0.3–0.6 at the falling limb of the d13C excursion. The overall increase of Ce/Ce* values at the peak of the K-O δ¹³C excursion is consistent with the development of water column dysoxia/anoxia at least locally in the Antler foreland basin. However, the presence of a few lower Ce/Ce* values down to 0.4–0.5 within the Ce anomaly peak (0.7–0.9) implies either the input of oxic open seawater in a restricted basin associated with sea-level rise, episode(s) of ocean oxygenation after the maximum organic carbon/pyrite burial coincident with the δ¹³C peak, or increase of Fe-Mn oxide input and burial in the sedimentary basin. Such complexities require further test in broader paleogeographic settings of the Great Basin and globally.