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Ellis, J.M., 1982

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Ellis, J.M., 1982, Holocene glaciation of the central Brooks Range, Alaska: Buffalo, New York, State University of New York, Ph.D. dissertation, 397 p., illust., map, 1 folded map.

Abstract

The central Brooks Range was glacierized in higher, north-facing cirques the past ~5,000 yr. Lichenometric mapping of deposits downslope of >50 receding cirque glaciers, along with five radiocarbon dates directly associated with Neoglacial moraines, defines seven major expansions of similar magnitude during this interval. These advances were accompanied by ELA depressions of 100-200 m below present. Reconstructions of Neoglacial maxima utilized an accumulation-area ratio (AAR) of 0.67 and imply a lowering of ~1 degree C from present summer temperatures. The four oldest expansions are lichenometrically dated (+/- 20% age reliability) at 4,400, 3,500, 2,900, and 1,800 yr B.P. Radiocarbon analysis of dead moss that is emerging undisturbed from beneath a receding glacier toe dates a Neoglacial advance at 1,120 +/- 180 yr B.P. This site suggests that for the past millennium cirque glaciers were continuously in more advanced positions than they are under the present climatic regime. Three expansions dated at 1,150 +/- 200, 800 +/- 150, and 390 +/- 100 yr B.P. (A.D. 1410-1600) occurred during this glaciologically favorable interval. Initial recession from the last major advance was slow as the ice margins remained close to their maximum extents until A.D. 1640-1750. Retreat was most rapid after A.D. 1870 and decelerated after the mid 1900s based on historical photographs and lichenometry. Although glaciers across the central Brooks Range have similar chronologies, dimensions, and shapes, varying environments promote deposition of different types of Neoglacial deposits. Moraines without ice cores occur in cirques with minimal bedrock exposed in cirque cliffs (about 80 m) and extensive direct radiation during the ablation season (about 92%). However, three out of four moraines studied are cored with glacier ice. The most important factor controlling formation of glacier-cored moraines is increased supply of debris as measured by height of bedrock in cirque cliffs (about 170 m). Glaciers at relatively low altitudes are associated with Neoglacial moraines upslope of early Holocene rock glacier tongues. The glaciers of granitic terrain differ respectively from those of sedimentary terrain in direct radiation energy received at the deposit (65 +/- 20% versus 85 +/- 10%), bedrock exposed in cirque cliffs (245 +/- 135 versus 130 +/- 90 m), and amount of ELA depression as measured from present mean glacier altitude to the ELA attained during Neoglacial maxima (130 +/- 60 m in contrast with 70 +/- 35 m).

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