332-2 Late Cretaceous (Campanian) Clumped Isotope (Δ47) Paleothermometry Based on Inoceramid Prisms from the Pierre Shale Formation: “The Ugly, The Bad and The Good(?)”

Abstract:

The Western Interior Seaway (WIS) of North America is an iconic feature of the Late Cretaceous world. However, use of traditional oxygen isotopes (δ¹⁸O) in biogenic carbonates for paleoenvironmental reconstruction in the Late Cretaceous WIS has been plagued by uncertainty stemming from overprinting by spatiotemporal variation in seawater δ¹⁸O_w and/or diagenetic alteration. Here, we evaluate the potential of clumped isotope (Δ₄₇) measurements on calcite prisms from inoceramid clams for reconstructing Campanian bottom-water temperatures and δ¹⁸O_w in the WIS. Inoceramid prisms were taken from cores drilled through the Pierre Shale (Gregory, Crow Creek, Degrey Members) in the town of Presho and Aurora County, South Dakota. We first report the ugly/bad results from the Presho section where Δ₄₇ analyses yielded implausibly hot bottom-water temperatures (~40-60⁰C). By contrast, the “good” news is that inoceramid Δ₄₇ analyses for the Aurora section yielded more plausible temperatures that fall within the range of published Δ₄₇-based temperatures derived from Maastrichtian/Campanian molluscan (bivalve, gastropod) shells in the WIS (Petersen et al., 2016). Comparative analyses of frosty (opaque white) versus translucent prisms taken from the same Aurora samples revealed that the former returned implausible Δ₄₇-temperatures (~50⁰C) while the latter registered tenable Δ₄₇-temperatures (15-35°C). We suspect that deeper burial depths (thermal reordering of isotopologues) may be the cause of the erroneous Δ₄₇-temperatures in the Presho record. In the well-preserved Aurora samples, we find that bottom-water temperatures and δ¹⁸O_w rapidly decrease during the Late Campanian, suggesting a southerly incursion of cooler, δ¹⁸O_w depleted boreal waters sourced from the circum-Arctic region. These results imply that, given the availability of well-preserved specimen, Δ₄₇ paleothermometry has the potential to reveal short time-scale oceanographic changes.

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