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.
The purpose of this research was to document how the surface topography of the western U.S. Cordillera developed during the last 65 million years. If we are able to understand how surface elevation developed we can place serious constraints on the drivers behind the tectonics and climate of this important mountain belt. That said, however, until recently it has been difficult to reconstruct past topography from the geologic record. Using a recently developed technique, that one of the PIs (Chamberlain - Stanford) helped to develop - called stable isotope paleoaltimetry - we can now reconstruct past elevations of mountain belts. This technique works under the principle that the heavy isotopes of oxygen and hydrogen in rainwater are preferentially removed during precipitation and the amount they are removed is dependent upon the surface elevation. To recover past precipitation we can look at the isotopes of minerals that contain water and use the isotopes of these mienrals to recover the isotopes of past precipitation. For the American West there are abundant fossil soils (called palesols) and fossil lakes that have ancient water-bearing minerals. These were the targets of our study. Using this isotopic method we were able to show the following. First, the topography of the west developed as a "topographic wave" that swept down from Canada at around 50 Ma reaching northern Nevada at about 40 Ma and sourthern Nevada at 24 Ma. This "wave" was synchronous with the formation of volcanic rocks and the onset of extensional faults in the western U.S. Our best guess scenario is that this surface uplift formed a large Andean-like plateau that was some 4 kilometers high that was punctured with volcanoes. The western border of the plateau was the Sierra Nevada and the eastern edge were the Rocky Mountains of Wyoming and Montana. Rivers drained west across the Sierra Nevada and east into ancient lakes in western Wyoming. The ancient continetal divide lay somewhere near cental Nevada. We can no longer see this plateau because since the mid-Miocene (~20 million yeats ago) it has collapsed forming the modern Basin and Range and Sierra Nevada. Second, we have shown that this surface uplift was extremely rapid. The rapid surface uplift and north to south uplift strongly suggests that it was driven by some deep processes occuring in the upper mantle. We suggest that the removal of the Farallon plate that was being subducted under North America during this time is the cause of the surface uplift. The removal of the lithospheric plate and its replacement by hot mantle material would cause both a rise in elevation and the production of volcanoes as the mantle melted the overlying crust. If the plate was removed in a north to south manner it would appear as a topographic wave - which is what we see in the past. This research has had a number of spin-offs that have influenced other fields as well - most notably in paleobiology. There has long been an interest in the evolutionary drivers behind mammalian evolution - and the American West stands as one of the key study areas. The debate has been whether climate or tectonics (i.e.; the formation of mountain chains) is the driver behind speciation of mammals. By using our new topographic map of ancient mountains it is now possible to test the underlying causes of mammalian evolution of this area on Earth. The initial findings show that the mamalian evolution of the western U.S. was largely driven by the formation of mountain barriers created by the rise of the Andean-like plateau. Interestingly, it may also explain why at the Eocene-Oligocene boundary (which is when the Earth globally cooled) the mammals in Europe evolved during this cooling event but the mammals of western U.S. did not. We argue that because the mammals of the western U.S. were already adaped to living at the cold high altitudes of the ancient plateau they were preadapted to global cooling. Finally, as part of this project we needed to develop better quantitative models to understand our isotopic results. As such, we built one dimensional models that allow the surface of the Earth (its soils and plants) to interact with the atmosphere. These vapor transport models can then be used to understand how the recharge of water vapor through evaporation and transpiration affect the isotopes of precipitation. They do in profound ways. We have used these models to show that the replacement of forests by grasses, which occurred globally around 15 million years ago changed the isotopic values of precipitation. Thus, many of the results used in stable isotope plaeoaltimetry may record this ecosystem change rather than just tectonics. That siad, the new model is a powerful way to inform our isotopic studies and to understand what are tectonic and what are climate controls.
