MULTIPROXY TEMPERATURE ESTIMATES AND DIAGENETIC CONSTRAINTS SUGGEST COOLER TROPICAL TEMPERATURES DURING THE APEX OF LATE PALEOZOIC GLACIATION: INSIGHTS FROM THE US MIDCONTINENT

Late Paleozoic Ice Age (LPIA; ~255–355 Mya.) climate trends have been compiled for decades using brachiopod oxygen isotope (δ¹⁸O) thermometry from the US Midcontinent. To evaluate the fidelity of these records and establish better constraints for LPIA climate, we combined petrographic analyses and dual proxy paleothermometers—δ¹⁸O and carbonate clumped isotopes (Δ₄₇). Brachiopods and bulk-rock carbonates from three stratigraphic successions in eastern Kansas, spanning Gzhelian (uppermost Pennsylvanian) to Asselian (Lower Permian) strata, yielded δ¹⁸O temperatures ranging from 10–29 °C with a mean of ~20 °C. Clumped isotope measurements from the same interval produced apparent temperatures of 35–79 °C with a mean of ~50 °C, indicating all study sites were influenced by some degree of post-depositional alteration. This alteration resulted in compromised Δ₄₇ values and distinct δ¹⁸O and Δ₄₇ populations between locations in northeastern and southeastern KS. These isotopic differences, paired with brachiopod textural evidence, are interpreted to reflect different alteration styles between regions. Isotopic evidence and good textural preservation from northeastern KS indicate its deposits were primarily impacted by rock-buffered diagenesis with little water-rock interaction. Southeastern KS samples, in contrast, display poorer textural preservation and isotopic alteration through burial and hydrothermal activity likely related to southern Midcontinent tectonism. Because fluid alteration overprints original geochemical signatures, carbonate deposits traditionally employed to construct LPIA climate records, such as those in southern KS, OK, and TX, should be reconsidered or used with caution. Based on our results, samples from southern KS yield estimated temperatures 6–12 °C higher than northern KS and likely record overprint from nearby hydrothermal interaction. Previous LPIA isotopic analyses from the US Midcontinent exhibit similar estimated temperature differences between northern and southern locations, suggesting that northern deposits are more representative of LPIA climate. Our findings suggest that tropical temperatures during the onset of peak LPIA glaciation were lower than previously indicated, with a temperature range of 10–26 °C and an average temperature of ~18 °C.