EQUILIBRIUM-LINE ALTITUDES DURING THE LAST GLACIAL MAXIMUM ACROSS THE BROOKS RANGE, ALASKA

Equilibrium-line altitudes (ELAs) were derived for 383 reconstructed glaciers across the Brooks Range to investigate the regional pattern of climatic forcing, primarily winter precipitation, on glacier extent during the last glacial maximum (LGM).  Glacier outlines were delimited based on new and published field mapping, combined with original interpretations of aerial photographs.  Glacier margins were digitized from 1:63,360-scale maps into a geographic information system (GIS) with a DEM on a 60 m grid.  Several geometric characteristics of each glacier were calculated, along with the ELA using the accumulation area ratio method (AAR=0.58).  We restricted our analysis to relatively simple cirque and valley glaciers that deposited clearly identifiable LGM moraines, and that did not merge with the complex transection glacier ice that filled most troughs of the range.  The glaciers used in this analysis had areas ranging from 0.14 – 120 km², compactness from 0.03 – 0.71, slopes from 4 – 30°, and volumes from 0.002 – 17 km³.  Their ELAs rose from about 470 m in the western Brooks Range to 1860 m in the east, over a distance of 1000 km, and were about 200 - 400 m lower than modern glaciers.  A third-order surface was generated to approximate the ELA trend across the range, and to investigate the source of local-scale variations.  The trend surface lowers toward the west and south, similar to previously derived trends based on glaciation thresholds.  In addition, ELAs in the northeastern part of the range lower northward toward the Beaufort Sea, a trend not seen as strongly in other studies.  Glacier size is significantly correlated with distance from the trend surface, with smaller glaciers farther below the surface.  The regional ELA pattern points to the primary source of moisture from the North Pacific, with a minor contribution possibly from the Beaufort Sea.  The unexpected trend in the northeast part of the range is supported by recent field mapping, where anomalous ice distribution and ELAs reflect a complicated pattern of atmospheric circulation and late Quaternary tectonism.  Nonetheless, the strong indication of moisture flow from the southwest contradicts general circulation models that simulate southeasterly flow over Alaska in response to the modeled glacial anticyclone.