Previous studies of stable isotopic paleoclimate proxies found in intermontane basins and adjacent metamorphic core complexes suggest that the topography of western North America developed diachronously, obtaining high elevations first in British Columbia at about 50 million years ago and sweeping into Nevada by about 40 million years ago. The stable isotopic studies show that there are rapid and large isotopic shifts that cannot be due to surface uplift alone and call for climatic controls. This research aims to test the hypothesis that relief development and possibly regional scale surface elevation (driven by tectonics) attained threshold values that caused rapid climate and precipitation shifts by actively interfering with atmospheric vapor transport and/or stability. To test this hypothesis, the research team is using a multi-disciplinary approach that involves: (1) collection of stable isotopic data from intermontane basins over discreet time intervals and over a wide geographic area so as to compare with isotope results from climate models; (2) measurement of cooling ages of detrital minerals in an effort to constrain relief and mountain building development within the basin catchments; (3) detailed sedimetological and high-resolution geochronologic studies in basins in order to place the detrital thermochronology and stable isotopic analyses in proper geologic context; and (4) simulation of climate conditions and isotopes of precipitation under different topographic/elevational scenarios using global and regional climate models as a way to interpret the observed stable isotope signals. The goal is to discriminate between two markedly contrasting tectonic models both of which are consistent with current data sets. One calls for the construction of dynamic topography from a moderate elevation low-relief landscape to a north-to-south swell of a high elevation landscape in the Eocene to Oligocene. The other is the north-to-south collapse of a low-relief, high elevation so-called Nevadaplano into region of similar to lower mean elevation but with significantly higher-relief.
This proposal addresses a fundamental problem in paleoclimate analysis ? the cause for rapid climatic shifts. It has been proposed that with increased global warming the Earth may undergo rapid reorganization of climate regimes once critical thresholds are reached. Identifying these rapid climate changes during times when the Earth was significantly warmer and had higher concentrations of carbon dioxide is essential for our understanding of how the Earth?s climate behaves during warming episodes. The research team has identified areas in the American West through stable isotope analysis that record rapid climatic shifts when the Earth was significantly warmer (50 to 40 million years ago). What causes these climatic shifts is unknown, however. By combining global climate models with isotope paleo-precipitation measurements it is possible to assess what may have caused these rapid climate shifts. Specifically, the project will test whether they represent regional responses to the rise of mountains or large-scale reorganization of climate.
