Abstract

The central Brooks Range was glacierized in the highest, north-facing cirques during late-middle to late Holocene (Neoglacial) time. This Neoglaciation involved at least 5 major cirque-glacier expansions of similar magnitude, as based on lichenometric mapping of more than 50 glaciers and radiocarbon dates directly associated with 5 moraines. Initial stabilization of debris-covered glaciers took place by early Holocene time, but evidently no moraines formed during this interval.

Few morainal ridges are preserved that date from the older expansions, but they have been lichenometrically dated (± 20% age reliability) to 3 separate intervals: 4400, 3500, and 2900 yr B.P. Twelve morainal complexes have ridges indicative of glacial advance that lichenometrically date at 1800 ± 500 yr B.P. Relict lichens that are now emerging undisturbed from beneath a receding glacier toe imply that this time was one of prolonged recession in at least some parts of the Brooks Range, however. Radiocarbon analysis of dead moss at this same site dates a subsequent Neoglacial advance across the cirque floor at 1120 ± 180 yr B.P. Our data suggest that for the past ∼1,100 yr, cirque glaciers have been continuously in more extended positions than they are today. During this glaciologically favorable interval, the last two major advances occurred at 800 ± 150 and 390 ± 90 yr B.P. (A.D. 1410–1600). Glaciers across the central Brooks Range stayed close to their maxima until A.D. 1640–1750. Historical photographs and lichenometry show that retreat was most rapid after A.D. 1870 and decelerated after the mid-1900s. Recession from this most recent Neoglacial maximum has amounted to 150–700 m and continues at present.

The cirque glacier advances were accompanied by equilibrium-line altitude (ELA) depressions of 100 to 200 m below levels maintained in the late 1970s. Environmental lapse rate estimates and 1977–1981 glaciologic-meteorologic measurements suggest that summer air temperatures accompanying Neoglacial maxima were, respectively, 1 °C or 3 to 4 °C cooler than those of the late 1970s. Across the central Brooks Range, the Neoglacial maxima ELAs rise northeastward and northward from 2 to 5 m km−1.

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