Reassessment of ocean paleotemperatures during the Late Ordovician
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AbstractThe 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 (47) microanalysis of fossil brachiopod shells to determine paleotemperatures of North America (Cincinnati Arch) during the Katian (453443 Ma). Microanalyses of fossil brachiopods yielded a right-skewed distribution of clumped isotope temperatures [T(47)] ranging from 25 C to 55 C with a mode of 32 C and a mean of 36 C. To test the impact of 47 reordering (reequilibration of 13C-18O bonding) at burial temperatures on T(47), we applied a Monte Carlo simulation to a 47 reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(47) and correct the reordering effect. Correcting for reordering decreased the mean T(47) 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 18O values derived from back-calculated T(47) and brachiopod 18O 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-18O model.
