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APG Atlas: Overview PURPOSE Our goal is a comprehensive and consistent overview of former glacier limits across Alaska. Our hope is to facilitate outreach, education, and interdisciplinary research in the fields of geology, geography, biology, archeology, and natural history. Citation: SUMMARY Three decades after the last Alaska-wide compilations of glacial geology (Karlstrom et al., 1964; Coulter et al., 1965), we have coordinated a broadly collaborative effort to create a digital map of reconstructed Pleistocene glaciers. The Alaska PaleoGlacier Atlas is a geospatial summary of Pleistocene glaciation across Alaska. The layers in the atlas depict: 1) the extent of glaciers during the late Wisconsin glaciation (i.e. Last Glacial Maximum, about 20,000 years ago), and 2) the maximum extent reached during the last ca. 3 million years by the northwestern Cordilleran Ice Sheet, ice caps, and valley glaciers. The atlas is targeted for a scale of 1 to 1,000,000 -- suitable for visualization and regional analyses. Former glacier extents are based on decades of field-based mapping, air-photo interpretation, and a variety of dating methods. In all, the first version combines glacial-geologic information from 26 publications and 42 source maps. Revisions will be made and released as time and resources allow. A companion paper (Kaufman and Manley, 2004; part of an INQUA effort for a global atlas with regional reviews) summarizes the glacial-geologic evidence and highlights recent revisions, remaining uncertainties, and implications for paleoclimate forcing. Pleistocene glaciers once covered >1,200,000 km2, from the continental shelf bordering the North Pacific to the northern foothills of the Brooks Range. Late Wisconsin glaciers occupied 727,800 km2 -- nearly ten times the area of modern glaciers, but only 48% of the state. Alaska's glaciers expanded more than 20 times during the last 3 million years in response to cold and snowy conditions. During the late Wisconsin glaciation, when sea level fell approximately 125 m (approx. 400 ft), the Bering Land Bridge was exposed as a broad tundra plain, and much of the state escaped glaciation due to a cold but dry climate. Deposition and erosion by glaciers in the recent geologic past have greatly influenced Alaska's landscapes and ecosystems. GIS PROCESSING We utilized standard GIS techniques to compile the glacier limits, incorporating information from a variety of sources (see REFERENCES below). Processing was conducted with two statewide coverages, one for each age limit, using North American Datum 1927 and an Alaska Albers projection. A first draft of the digital atlas was created by scanning, georectifying, and digitizing the map of Coulter et al. (1965). Additional maps (from six publications and unpublished mapping by WFM and DSK) were scanned and georectified. Polygons delineating former glaciers were then added and edited in ArcMap (v. 8.1 and 8.2, digitizing onscreen at resolutions better than the source maps, commonly in streaming mode with vertex spacing of 1-5 km). This version was distributed for community review, and subsequently updated to include limits from 19 additional publications, as well as unpublished research by the authors and 15 contributors. In sum, the digital atlas integrates information from 42 source maps. UNCERTAINTIES Finally, we classified the glacier limits according to "certainty", a measure of confidence relating to both age determination and geographic position. For example, well-dated moraines were ascribed a high level of certainty, comparable to a solid line on traditional glacial geologic maps. Intermediate certainty was encoded for limits without well resolved ice-marginal features, or those with little or no direct geochronologic control (comparable to dashed lines for "uncertain" boundaries on traditional maps). A low level of certainty was attributed to arcs bounding polygons where ice limits are inferred for offshore positions, or for areas lacking detailed air-photo interpretation or significant, field-based glacial-geologic study. The measures of certainty are based on available mapping and publications, and are commensurate with the target scale of 1 to 1,000,000. The APG Atlas is only as accurate and complete as the source maps. Version 1 is considered sufficiently well resolved for public release. Relatively small glacier systems were not included in some areas due to lack of information, including St. Lawrence Island, Horn Mountains, Shotgun Hills, Taylor Mountains, parts of the Yukon Tanana Upland, and the Blackburn Hills, as well as fringes of the Ahklun Mountains, central and eastern Brooks Range, and the Cordilleran Ice Sheet. Sufficient glacial-geologic mapping is lacking for many of the Aleutian Islands, and for the continental shelves of the Gulf of Alaska. In many areas of the state, significant uncertainties remain. Research through field-based efforts, remote-sensing techniques, and geochronologic methods are needed to improve the levels of certainty shown here, and at larger scales. Future contributions are encouraged. Glacial geologists with information on former glacier limits in Alaska should contact William Manley for a basemap and to coordinate the editing. As time and resources allow, the atlas will be updated on the website. Feedback of any kind is welcome. REFERENCES AND SOURCES
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