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 CO (sub 2) levels. We used clumped isotope (Delta (sub 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 (sub 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 (sub 47) reordering (reequilibration of (super 13) C- (super 18) O bonding) at burial temperatures on T(Delta (sub 47) ), we applied a Monte Carlo simulation to a Delta (sub 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 (sub 47) ) and correct the reordering effect. Correcting for reordering decreased the mean T(Delta (sub 47) ) by only approximately 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 (super 18) O values derived from back-calculated T(Delta (sub 47) ) and brachiopod delta (super 18) O values average -0.3 + or - 0.6 ppm 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 (super 18) O model.