Glacial-Geologic Evidence for Decreased Precipitation During The Little Ice Age in The Brooks Range, Alaska

Abstract

We mapped Little Ice Age (LIA) moraines in the Brooks Range to estimate former equilibrium-line altitudes (ELAs), and combined this information with available proxy temperature estimates to infer precipitation trends because little is known about precipitation changes associated with centennial-scale climate variability in the Arctic during the late Holocene. The Brooks Range, northern Alaska (68°N), hosts hundreds of extant glaciers that exhibit geomorphic evidence for multiple fluctuations in ice extent during the past millennium. Our lichenometric age estimates for LIA moraines in the forefields of five cirque glaciers in the Sagavinerktok River valley and Oolah Valley suggest two intervals of LIA moraine formation centered around a.d. 1250 and 1650. The outermost LIA moraine was mapped on aerial photographs for 114 relatively large (1.2 ± 0.5 km) and geographically simple glaciers along a 700-km-long transect following the range crest. At their maximum extent during the LIA, these glaciers were an average of 0.2 ± 0.1 km longer than the ice margins shown on most recent U.S. Geological Survey topographic maps (1956 and 1972). The ELA was estimated using an accumulation-area-ratio method and GIS analysis. The reconstructed ELAs needed to maintain an equilibrium length for the LIA glaciers were an average of 51 ± 29 m lower than for the smaller glacier sizes of the mid 20th century. This small ELA lowering during the LIA is less than would be expected from available proxy temperature estimates from elsewhere in Alaska that indicate warm-season temperature reductions of about 1 °C. To explain this discrepancy, we suggest that precipitation decreased during the LIA. A prolonged southern displacement of the Arctic front might explain the drier conditions in the Brooks Range during the LIA.