Equilibrium‐line altitudes during the Last Glacial Maximum across the Brooks Range, Alaska

Abstract

AbstractEquilibrium‐line altitudes (ELAs) were estimated for 383 reconstructed glaciers across the Brooks Range, northern Alaska, to investigate their regional pattern during the Last Glacial Maximum (LGM). Glacier outlines were delimited based on published field mapping and the original interpretations of aerial photographs. Glacier margins were digitised from 1:63 360‐scale maps into a geographic information system (GIS) with a digital elevation model on a 60‐m grid. ELAs were calculated for each reconstructed glacier using the accumulation area ratio method (AAR = 0.58). The analysis was restricted 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 to 120 km2. Their ELAs rose from 470 m a.s.l. in the western Brooks Range to 1860 m a.s.l. in the east, over a distance of 1000 km. The ELAs were fitted with a third‐order polynomial trend surface to model their distribution 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, which has not been reported as strongly in other studies. Modern glacier ELAs also lower toward the southwest. The depression of LGM ELAs from modern glacier ELAs is greatest in the central Brooks Range (a maximum of 700 m), and decreases to the east (200 m). The regional pattern of LGM ELAs points to the primary source of moisture from the North Pacific, as it is today. The unexpected trend of LGM ELAs in the northeast part of the range is supported by recent field mapping, where anomalous ice distribution and ELAs reflect complicated LGM climate patterns and possibly late Quaternary tectonism. Copyright © 2005 John Wiley & Sons, Ltd.