WINTER ACCUMULATION AT THE EQUILIBRIUM LINE OF LITTLE ICE AGE GLACIERS, BROOKS RANGE, ALASKA

Glacial geologic records in northern Alaska provide a basis for quantitative reconstruction of paleoclimate, and indicate colder but drier conditions during the Little Ice Age (LIA). A close linear relation was found between ablation season (JJA) temperature and winter accumulation at the equilibrium-line altitude (ELA) of 30 modern glaciers around the Northern Hemisphere. This relation was used to reconstruct the winter accumulation on glaciers of the LIA by assuming JJA temperature, which was based on environmental lapse rates applied to modern instrumental data combined with LIA summer temperature estimates from existing tree-ring data. To determine ELAs, LIA moraines were mapped from aerial photographs for 120 glaciers across the Brooks Range, of which five were field checked. Lichenometric age estimates of Neoglacial moraines for these five glaciers suggest glaciers reached their maximum LIA extent between 600 and 200 yr BP (BP=before 1950 AD). ELAs were reconstructed for both modern and LIA glaciers using an accumulation-area ratio of 0.58 and GIS software. The average ELA was 25 +/- 17 m lower during the LIA than the modern (1970 AD). Applying reasonable environmental lapse rates (e.g., 5 – 9 °C km^-1) to the average ELA lowering and holding precipitation constant, suggests that summer temperatures were not more than 0.2 °C cooler in the Brooks Range during the LIA as compared to the modern. Dendroclimatic records for northern Alaska, however, suggest that summer temperatures in the Brooks Range were at least 0.8 °C cooler during the LIA. Therefore, the minimal ELA lowering suggests drier conditions prevailed in the Brooks Range during the LIA than the modern. To estimate the amount it was drier, we used the relation between winter accumulation and summer temperature on modern glaciers. Comparison between winter accumulation estimates at modern and LIA glacier ELAs suggests that winter accumulation was reduced ~16% during the LIA. The sensitivity of this analysis to the assumed lapse rate, LIA cooling, ELA lowering, and slope of the regression was tested. The results indicate higher accumulation would have required LIA summer temperature to be no lower than the modern, which seems unreasonable. Decreased winter accumulation might reflect a prolonged southerly extent of the Arctic front during the LIA.