Reassessment of ocean paleotemperatures during the Late Ordovician

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 (Delta(47)) 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(Delta(47))] ranging from 25 degrees C to 55 degrees C with a mode of 32 degrees C and a mean of 36 degrees C. To test the impact of Delta(47) reordering (reequilibration of C-13-O-18 bonding) at burial temperatures on T(Delta(47)), we applied a Monte Carlo simulation to a Delta(47) reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(Delta(47)) and correct the reordering effect. Correcting for reordering decreased the mean T(Delta(47)) by only similar to 1 degrees C, to 35 degrees C, and did not change the mode (32 degrees 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 degrees C approaching the Hirnantian glaciation. Seawater delta O-18 values derived from back-calculated T(Delta(47)) and brachiopod delta O-18 values average -0.3 parts per thousand +/- 0.6 parts per thousand 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-delta O-18 model.

Automated summary

The study indicates that the Late Ordovician climate in the Cincinnati Arch was warmer than modern subtropical seas, likely influenced by the upwelling of cool water. Though previous studies have reported higher temperatures, correcting for reordering effects did not significantly change the overall findings.