Search for: All records

The glacial geology community has developed digital reconstructions of Pleistocene glaciers in the western U.S. through decades of regionally focused research and interpretations of geologic maps. These paleoglacier reconstructions afford an opportunity to develop paleoclimate reconstructions for the Late Quaternary in the western U.S., especially when combined with cosmogenic chronologies and other paleoclimate proxies and model output. Here, we present a geospatial database of Late Pleistocene mountain glaciers for the conterminous western U.S. based on compilations of paleoglacier reconstructions spanning glaciated mountains in the region. The database consists of paleoglacier outlines as georeferenced polygons drawn at scales ranging from 1:24,000 to 1:100,000, reflecting differences in available mapping data and degrees of confidence in identifying glacial deposits and landforms used to identify paleoglacier limits. The database is available as a web feature service designed to be easily represented in a geographic information system or web mapping application to enable visualization of the pattern of Late Pleistocene mountain glaciation and analysis of paleoglacier outlines and derivative products, such as equilibrium-line altitudes and boundaries of modeled paleoglaciers. We illustrate potential applications of the database for visualization and data assimilation with an example from mountains neighboring the Lake Bonneville basin, where paleoglacier outlines are based on 1:24,000 scale mapping of glacial deposits and landforms and cosmogenic chronologies of moraines are abundant. For this research, the database enables an analysis of the pattern of glaciation in the region and, through assimilation with chronological data, an assessment of the relative timing of glacier maxima and the time when Lake Bonneville overflowed. While the database can be easily shared among users and represented in a geographic information system, development of the database requires community input to maximize its utility for users across disciplines. A goal of this presentation is to encourage interested users to share ideas for developing an accessible, scalable, and community-supported database of paleoglaciers. 

Important information about past climates can be determined from reconstructed equilibrium line altitudes (ELA) of mountain paleoglaciers, specifically the temperature and precipitation accompanying a glacier in equilibrium. Previous reconstructions of Late Pleistocene ELAs of mountain glaciers across the western United States have been used to infer the pattern of temperature and precipitation change across the region, although most of the work was based on presumed ages and limited mapping of glacial deposits and landforms. Cosmogenic nuclide exposure dating of moraines combined with updated mapping and aerial imagery afford an opportunity to revisit the pattern of regional ELAs during multiple episodes of the last Pleistocene glaciation. The goal of this research is to reconstruct ELAs in the same region of previous reconstructions based on glacial sediments that have been dated using cosmogenic nuclide exposure ages. We focus on the large number of glacial valleys with moraines corresponding to the Last Glacial Maximum (LGM; 26.5-19.0 ka). Paleo-ELAs are estimated using the toe to headwall altitude ratio and the accumulation area ratio determined from published glacier reconstructions and existing glacial mapping. Cosmogenic-exposure ages of moraines are compiled from the informal cosmogenic nuclide exposure age database for alpine glacial features (ICE-D Alpine) and represented in a geographic information system along with ELAs for each glacial valley. A reconstructed ELA surface spanning the conterminous western United States is produced using existing algorithms in ArcGIS. Results show reconstructed ELAs generally lower than initially estimated and a larger range of ELAs across the region. In the Sierra Nevada, ELAs increase southeastward, which is consistent with previous estimates, spanning a range from 1800 to 2800 m asl. ELAs rise eastward across the Basin and Range toward the western shore of the area covered by Lake Bonneville, and then decrease eastward toward the Wasatch Mountains. This pattern is inconsistent with previous estimates and may reflect a west-to-east precipitation gradient that differs from modern climate. We discuss this pattern and broader features of the ELA surface of the LGM and later episodes of the last Pleistocene glaciation.