Mountain glaciers were numerous in the Rocky Mountains of western North America during the last Pleistocene glaciation, and the geologic record of these glaciers provides dramatic evidence of climatic changes during and following the Last Glacial Maximum. This research will build upon and augment the Pleistocene mountain glacial chronologies in the U.S. Rocky Mountain region, and use these chronologies to model past climate changes. The project uses both field and analytical methods to address two broad objectives: (1) to determine the changes from modern climate that would be necessary to sustain Rocky Mountain glaciers at their last Pleistocene maximum extents, along a transect from northern Montana to south-central New Mexico, and (2) to determine the timing and rate of subsequent ice recession, and the magnitudes and rates of climate change necessary to drive that recession. The first objective involves additional glacial mapping and chronology in eight study areas, making use of cosmogenic beryllium-10 surface-exposure dating to constrain the timing and extent of glaciers during Last Glacial Maximum. A numerical model of glacier mass balance and ice flow will allow determination of the character and magnitudes of climatic change sufficient to produce the observed glacial extents. The second objective involves additional surface-exposure dating in four of the eight study areas to constrain the timing and rates of ice recession following its maximum stand, and to allow modeling of the climate changes that caused the recession. The results of this research will include a regional synthesis of glacier-climate reconstructions for the last Pleistocene glaciation and the subsequent deglaciation, an assessment of the climatic significance of the apparent variability in the timing of the Last Glacial Maximum, and an improvement in the chronology of mountain glacier movements.
This application of high-resolution glacier modeling and cosmogenic-radionuclide dating methods will provide insight into the history of climate change in the Rocky Mountain region. The time interval of interest is the Last Glacial Maximum (the interval of maximum global ice volume) and the subsequent transition to the present warm period, the latter occurring over approximately 6000 years. This transition was an interval of major global warming, during which substantial changes in atmospheric circulation occurred in the western United States in response to shrinking ice sheets, increased incoming solar radiation, and rising atmospheric carbon dioxide levels. Such changes in airflow undoubtedly brought about changes in regional precipitation patterns, as is suggested by paleoclimate models. The results of this research will assess the accuracy of such models in representing precipitation patterns of the past, and may also provide insight into the accuracy of model predictions of future precipitation changes in the Rocky Mountains region, an area where demand for limited water resources continues to grow.
The last glaciation of the Pleistocene Epoch featured the expansion of glacier ice in both hemispheres, culminating during the interval 26,500-19,000 years ago. Accompanying the growth of large ice sheets in North America was the expansion of smaller glaciers in the U.S. Rocky Mountains. Because of their small size, mountain glaciers were more sensitive to climate change than larger ice sheets. From northern Montana to New Mexico, the sediments and landforms delimiting the maximum size of these now-vanished glaciers are well preserved and provide an opportunity to improve the understanding of how climate changed during and immediately following the last glaciation in the Rocky Mountains. This project used field and analytical research methods to attain two fundamental goals. 1. Assess the changes from modern climate that would be necessary to sustain Rocky Mountain glaciers at their maximum extent along a latitudinal transect from northern Montana to south-central New Mexico. Focusing on ten settings along the transect, this study applied a well-established method of modeling the response of glaciers to changes in climate that affect the rate at which they gain mass by the accumulation of snow and lose mass by melting. The modeling method characterizes the modern climate in each setting, and then computes the size of glaciers based on prescribed changes in temperature and precipitation. A key outcome of this research is a set of possible temperature and precipitation combinations that accompanied the last glaciation in each setting of interest. 2. Determine the timing and rate at which glaciers retreated after the culmination of the last glaciation, and the magnitudes and rates of climate change necessary to drive this retreat. This goal was met through the application of a geologic dating method that determines the duration of exposure of a glacial deposit or landform. The method, termed cosmogenic 10Be surface exposure dating, combines the known production rate of the isotope 10Be and its measured concentration in surface sediment to determine the exposure age of a glacial feature. This geologic dating method provides precise ages of glacial features, which are critical to understanding the timing of events of the last glaciation. The method is especially useful for determining exposure ages of glacial moraines – ridges of sediment deposited by glaciers that delimit the former margin of a glacier – and of glacially eroded bedrock. The set of 10Be exposure ages of glacial features generated by this study includes new ages of seven moraines, each indicating the time when glaciers began retreating from their maximum extent. Additionally, 10Be exposure ages from two settings indicate the pace of ice retreat. Intellectual Merit. By themselves, the results of glacier modeling experiments do not provide unique estimates of temperature and precipitation at the culmination of the last glaciation, but can be combined with results of the best available computer models of temperature during the last glaciation to solve for precipitation. Together, the combined modeling results indicate strong temperature depression coupled with less-than-modern precipitation at the culmination of the last glaciation in the Northern Rocky Mountains of Montana, moderate temperature depression and less-than-modern precipitation in the Middle and Southern Rocky Mountains in Wyoming and Colorado, and moderate temperature depression coupled with more-than-modern precipitation in the southernmost Rocky Mountains. When combined with geological age constraints on the pace of ice retreat, results of glacier modeling in two settings in Colorado indicate only slow increases in temperature (0.1°C per thousand years) after initial ice retreat during the time interval 21,000-16,000 years ago, but relatively rapid changes in temperature (0.7-1.0°C per thousand years) during the interval 16,000-13,500 years ago. Although geologic ages of glacial features in these two settings are being refined, the latter finding represents an important step toward quantifying the timing and magnitude of regional climate change during the transition from the last glaciation to the current warm period. Broader Impacts. In addition to the intellectual merits described above, this study provided valuable scientific training of undergraduates at Colorado College and SUNY Geneseo. Eighteen students were trained in field and laboratory research methods used in this study and in scientific writing and professional presentation. Nine students who participated in this research are currently enrolled in or have completed a graduate degree in geology, thereby preparing to become important contributors to the advancement of geological science in the 21st century. Furthermore, the glacier modeling methods used in this study have been refined to facilitate its application to settings beyond the Rocky Mountains and to improve the overall accuracy of model output. These improvements will facilitate new research on the last glaciation within and beyond the U.S. Rocky Mountains, thereby contributing to a better understanding of climate changes during this event.
