Homogeneous Seawater Temperatures across the Early Ordovician Laurentian Carbonate Platform

During the early Ordovician, Laurentia (along with other continental landmasses) was largely submerged, and widespread deposition of platform carbonates is one of the characteristic features of this time. The interval was also one of apparent greenhouse conditions (warm temperatures and high atmospheric concentrations of CO₂). Most paleoclimatic and paleoceanographic interpretations for early Ordovician Laurentia have been based on facies distributions and faunal gradients. To test these interpretations, we have used oxygen isotopic measurements on conodont apatite to quantitatively estimate paleotemperatures including possible temperature gradients across the platform.  In a previous study we concluded the temperatures of early Ordovician seawater averaged 37.0°C during the Rossodus manitouensis biozone at the Lange Ranch section of the Tanyard Formation in central Texas assuming a δ_w value of -1‰_SMOW is appropriate for ice-free early Ordovician seawater. This result confirms warmth and argues against the proposition of large changes in δ_w during the early Paleozoic.  Using the same δ_w assumption and new data from 14 samples from the Rossodus manitouensis zone collected from the Missouri Gulch section of the Manitou Formation in Colorado and one sample from the Blue Earth Beds of the of the Prairie du Chien Group in Minnesota, we calculate temperatures that ranged from 35.1°C (Minnesota) to 37.9°C (Colorado).  That is, our preliminary results suggest only small temperature gradients across the carbonate platform.  Additional data are being generated from Nevada, Utah, and Oklahoma to improve the spatial resolution of this assessment.