Reassessment of ocean paleotemperatures during the Late Ordovician

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