| The western Cordillera in the United States is arguably one of the best-characterized geologic terranes in the world. However records of paleotemperature and paleoelevation, two fundamental boundary conditions for climate and tectonic reconstruction, are sparse. In an effort to better understand the development of the western Cordillera during the Late Cretaceous and Paleogene, which are significant time periods both tectonically and climatically, I applied a new geochemical technique, clumped isotope (Delta47) thermometry, to carbonate samples from the western Cordillera.;The Delta47 temperatures from Paleogene paleosol carbonates from the Bighorn Basin in Wyoming suggest that summers were quite warm and that the seasonal range of temperature was similar to modern; these results match well with new high-resolution climate model data. Together, these results suggest that the long-standing paradox (suggested by earlier proxy records) that continental interiors were more equable during greenhouse climate conditions is a function of climate models that underestimated warmth and proxy data that inaccurately estimated seasonal temperatures. In contrast, Delta 47 temperatures from bivalve fossils from the same region and time are too warm to be physiologically reasonable and detailed textural analysis reveals the presence of diagenetic aragonite. Although these textural data and the Delta 47 data indicate the bivalve samples have been diagenetically altered, it is unclear whether the bulk oxygen isotope values of the shells have also been compromised. The Delta47 temperatures from Late Cretaceous paleosol carbonates from Utah and latest Cretaceous-Paleocene lacustrine carbonates also appear to reflect summer temperatures, and the temporal pattern of temperature change from these two datasets appear to match the pattern of global climate change (or lack thereof) implied by the marine foraminiferal oxygen isotope record. The average temperature difference between the samples closest in age from the two different sites suggests an elevation difference between Nevada and Utah of ∼2.5--3.6 km (Nevada = high, Utah = low). These results provide direct, independent evidence that a high-elevation "Nevadaplano" existed in Nevada during the Late Cretaceous. |
