The Late Ordovician is a paradox, with a greenhouse climate shifting to an icehouse climate during a time of presumably high CO2 levels. We used clumped isotope (Δ47) microanalysis of fossil brachiopod shells to determine paleotemperatures of North America (Cincinnati Arch) during the Katian (453–443 Ma). Microanalyses of fossil brachiopods yielded a right-skewed distribution of clumped isotope temperatures [T(Δ47)] ranging from 25 °C to 55 °C with a mode of 32 °C and a mean of 36 °C. To test the impact of Δ47 reordering (reequilibration of 13C-18O bonding) at burial temperatures on T(Δ47), we applied a Monte Carlo simulation to a Δ47 reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(Δ47) and correct the reordering effect. Correcting for reordering decreased the mean T(Δ47) by only ~1 °C, to 35 °C, and did not change the mode (32 °C). These temperatures are lower than those of previous studies, but still suggest that the Late Ordovician climate of the Cincinnati Arch was warmer than the modern subtropical seas. However, carbonate facies in these strata argue for upwelling of cool water; thus, typical subtropical surface waters may have been as warm as 35 °C approaching the Hirnantian glaciation. Seawater δ18O values derived from back-calculated T(Δ47) and brachiopod δ18O values average –0.3‰ ± 0.6‰ relative to Vienna standard mean ocean water (VSMOW), consistent with the value expected for subtropical surface waters in a greenhouse Earth, assuming a constant seawater-δ18O model.

Throughout the Phanerozoic, δ 18 O of carbonate fossils displays an increasing trend of -8‰ VPDB in the Cambrian to approximately -1‰ VPDB in the modern. This trend has led to three different hypotheses: 1) sea surface temperatures decreased through the Phanerozoic; 2) δ 18 O of seawater rose through the Phanerozoic, or 3) diagenesis has altered older samples more than younger samples. To test these Cincinnati Natural History Museum & Science for allowing access to their fossil collections. Also, thanks to Dr. Brenda Hunda from the Cincinnati Museum of Natural History & Science and Don Bissett from the Cincinnati Dry Dredgers for their help with fossil collection in the field.

Methods, data (Tables S1 and S2), Figures S1–S6, and Python Code.<br>

Methods, data (Tables S1 and S2), Figures S1–S6, and Python Code.<br>