Paper No. 4
Presentation Time: 9:00 AM


SNELL, Kathryn E., Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 100-23, Pasadena, CA 91125, PEPPE, Daniel J., Department of Geology, Baylor University, One Bear Place #97354, Waco, TX 76798-7354, EILER, John M., Division of Geology and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 E. California Blvd, Pasadena, CA 91125 and WERNICKE, Brian P., Division of Geological and Planetary Sciences, California Institute of Technology, Mail Stop 100-23, Pasadena, CA 91125,

The western USA evolved through the last 80 Ma in response to the transition from dominantly contractional to dominantly extensional tectonics, and from greenhouse to icehouse climate conditions with progressive aridification. Stable isotopes are used to reconstruct past climate and elevation by exploiting sensitivities of the isotopic composition of water, as recorded in authigenic minerals, to temperature and altitude. Assumptions are made to work around the uncertainties in interpretation of stable isotope data for these purposes. The carbonate clumped isotope thermometer (Δ47) provides estimates of paleotemperature and the oxygen isotope composition (δ18O) of water, in ways that remove some assumptions. We present our advances in an ongoing study of paleotemperatures for the western USA using Δ47 paleotemperature estimates as a record of summer temperatures as recorded in ancient lakes and soils, combined with compilation of previous paleobotanical records of MAT. With these data, we can address outstanding questions about the climatic and tectonic evolution of the western USA.

The long-term climate trends for western US terrestrial records roughly track trends in the marine δ18O record through the Cenozoic, for both high and low elevations. Also, temperature differences between the high and low elevation sites support tectonic models that prescribe high elevation for much of the western Interior from the Late Cretaceous to the Miocene, but do not show strong evidence for an increase in elevation during the Eocene and Oligocene, as has been previously suggested. This does not rule out the possibility of a pulse of uplift in the early Paleogene, but at the current resolution, the paleoelevation records in the USA cannot be used to support this event. More records from the Paleogene are required to test this theory. Finally, δ18O of water is highly dependent on the archive (e.g. lakes or soils); both archives generally show offsets between the high elevation and low elevation sites, but this is complicated because lakes show much greater variability in δ18O of water than soils. These data highlight the importance of independent temperature constraint for interpreting δ18O values from carbonates and underscore the importance of distinguishing among lithologies when interpreting large δ18O datasets.