Chronology of Holocene Glaciation, Central Brooks Range, Alaska

Chronology of Holocene glaciation, central Brooks Range, Alaska

JAMES M. ELLIS* Department of Geological Sciences, State University of New York at Buffalo, Buffalo, New York 14226 PARKER E. CALKIN

ABSTRACT

The central Brooks Range was glacierized pressions of 100 to 200 m below levels and also because of a unique photographic rec- in the highest, north-facing cirques during maintained in the late 1970s. Environmental ord dating from 1911 (Ellis and others, 1981). late-middle to late Holocene (Neoglacial) lapse rate estimates and 1977-1981 glacio- Details of late Quaternary valley glaciation in time. This Neoglaciation involved at least 5 logic-meteorologic measurements suggest the central Brooks Range are reviewed by Ham- major cirque-glacier expansions of similar that summer air temperatures accompanying ilton and Porter (1975) and Hamilton (1982). magnitude, as based on lichenometric map- Neoglacial maxima were, respectively, 1 °C The last Pleistocene glacial advance to reach the ping of more than 50 glaciers and radiocar- or 3 to 4 °C cooler than those of the late north flank of the Brooks Range, termed the late bon dates directly associated with 5 moraines. 1970s. Across the central Brooks Range, the Itkillik (Hamilton, 1978, 1979a), is represented Initial stabilization of debris-covered glaciers Neoglacial maxima ELAs rise northeastward by arcuate end moraines ~45 km north of took place by early Holocene time, but evi- and northward from 2 to 5 m km'1. Atigun and Anaktuvuk Passes (Figs. 2 and 3). dently no moraines formed during this This advance culminated about 13,000 to interval. INTRODUCTION 12,500 yr B.P.; a recessional stand confined to Few morainal ridges are preserved that some tributary valleys has been mapped as late date from the older expansions, but they have The bouldery deposits associated with cirque Pleistocene Alapah Mountain drift (Hamilton been lichenometrically dated (± 20% age reli- glaciers in the central Brooks Range provide a and Porter, 1975). Most valleys of the central ability) to 3 separate intervals: 4400, 3500, detailed record of glacial fluctuations through Brooks Range were free of valley glaciers by and 2900 yr B.P. Twelve morainal complexes late-middle to late Holocene (Neoglacial) time. 11,000 to 10,000 yr B.P. have ridges indicative of glacial advance that Resolution of glacier responses in the Brooks Relatively undissected and unvegetated gla- lichenometrically date at 1800 ± 500 yr B.P. Range is particularly significant, because the cial deposits at thresholds of cirques across the Relict lichens that are now emerging undis- combined effects of high latitude and remoteness Brooks Range were assigned by Porter and Den- turbed from beneath a receding glacier toe from the main moisture source, the Pacific ton (1967) to the Fan Mountain glaciation of imply that this time was one of prolonged Ocean, cause these glaciers to be the least active Detterman and others (1958) on the basis of recession in at least some parts of the Brooks in the United States (Meier and others, 1971). morphologic similarities with cirque deposits in Range, however. Radiocarbon analysis of Development of this new and detailed glacial the Fan and Alapah Mountain areas (-65 km dead moss at this same site dates a subsequent history has been made possible by the applica- west of Atigun Pass) (Fig. 1). Near Anaktuvuk Neoglacial advance across the cirque floor at tion of lichenometry as an absolute dating tool Pass, Porter (1966) recognized two end mo- 1120 ± 180 yr B.P. Our data suggest that for (Calkin and Ellis, 1980, and in press). This has raines: an inner set within the cirques (Fan the past -1,100 yr, cirque glaciers have been been supplemented by radiocarbon dating, se- Mountain II) and an outer set (Fan Mountain I) continuously in more extended positions than quential glacier photographs (Hamilton, 1965), lying within 3 km of cirque thresholds. Hamil- they are today. During this glaciologically fa- and relative dating methods associated with ton (1965) also suggested a two-stage Fan vorable interval, the last two major advances weathering and soil formation (Ellis, 1982; Ellis Mountain record for the Arrigetch Peaks and occurred at 800 ± 150 and 390 ± 90 yr B.P. and Calkin, 1983). extended this division to other areas of the cen (A.D. 1410-1600). Glaciers across the central In the 3 field areas across 225 km of the central Brooks Range (Hamilton, 1978, 1979a, Brooks Range stayed close to their maxima tral Brooks Range (Fig. 1), 51 cirque glaciers 1981) for small-scale reconnaissance mapping. until A.D. 1640-1750. Historical photographs and their downslope deposits were studied and Porter and Denton (1967) considered these and lichenometry show that retreat was most reconstructed to maximum Neoglacial dimen- cirque deposits to be post-Hypsithermal (Neo- rapid after A.D. 1870 and decelerated after sions (Ellis, 1982). The main study area was the glacial) in age, but dating only to the later part of the mid-1900s. Recession from this most re- very accessible Atigun Pass region, where depos- this interval. Similarly, the original Fan Moun- cent Neoglacial maximum has amounted to its downslope of 41 cirque glaciers were mapped tain glaciation was considered by Detterman 150-700 m and continues at present. (Fig. 2). The centrally located Anaktuvuk Pass and others (1958) to represent an extremely re- The cirque glacier advances were accom- (Fig. 3) was chosen because of previous recon- cent and short-lived advance that left numerous panied by equilibrium-line altitude (ELA) de- naissance work on cirque deposits (Detterman ice-free cirques with Fan Mountain moraines. and others, 1958; Porter, 1966). Glaciers in the Hamilton (1979b, 1981) reassigned Fan Moun- *Present address: Gulf Oil Exploration and Produc- tain to the entire span of late Holocene Neogla- tion Company, P.O. Box 36506, Houston, Texas Arrigetch Peaks (Fig. 4) are significant because 77236. of their different climatic and lithologic settings ciation. In this paper, we present lichenometric

Geological Society of America Bulletin, v. 95, p. 897-912, 19 figs., 2 tables, August 1984.

897

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Point Barrow

Arctic Ocean

— 70 N

Yukon Arctic Circle »Territory, \Canada

66 N V Yukon R. -Trans-Alaska ! Oil Pipeline 164 W 160 W 156 W 152W 148 W 144 W

Figure 1. Location map of central Brooks Range, northern Alaska, showing Arrigetch Peaks, Anaktuvuk Pass, and Atigun Pass study areas.

and radiocarbon evidence obtained during five exposure in cirque walls (potential supraglacial PHYSIOGRAPHIC SETTING field seasons (1977-1981) that demonstrates debris), topographic screening of direct solar ra- that the Neoglacial record of the central Brooks diation, mass-wasting history of the cirque dur- The three study areas are all above the limit of Range is probably older and certainly more ing late Pleistocene déglaciation, and altitude, boreal spruce forest and within the zone of con- complicated. determine the type of Neoglacial moraine depos- tinuous alpine permafrost (Ferrians, 1965). ited (Ellis and Calkin, 1983). Most cirque glaci- Cirque glacier moraines are generally unvege- CIRQUE GLACIERS AND ers, particularly those in the Arrigetch Peaks, are tated, except for lichen growth; however, a sub- THEIR DEPOSITS associated with moraines that are cored with stantial cover of lichens, algae, mosses, sedges, glacial ice. These moraines have marked relief and grasses may occur on the older Neoglacial Glaciers of the central Brooks Range are and sharp-crested fronts in the downvalley direc- moraines and on rock glacier tongues. small, and those studied displayed relatively tion, and they may grade into zones without ice debris-free areas of 0.01 to 2.0 km2, lengths of cores. Moraines that lack cores of ice have subtle Atigun Pass Area 50 to 2,500 m, and slopes of 8° to 23°. They relief and are exceptionally stable. They also have orientations that are markedly concen- have the clearest lichenometric record and pro- The Atigun Pass area is underlain by thrust- trated along 011°, thereby minimizing insolation vide the best data on déglaciation rates. faulted and steeply folded marine and nonma- (Ellis and Calkin, 1979; Ellis and others, 1981). Occasionally, morainal ridges downslope of rine sedimentary rocks (Brosgé and others, This predominantly north-facing pattern reflects glacial ice are superimposed upon the heads of 1979). Most of the peaks that reach above 2,000 marginal climatic conditions and significant con- tongue-shaped rock glaciers. These moraines are m, the glacierized cirques, and the Neoglacial trol exerted by solar radiation on glacierization glacier-cored and form a transition zone (Foster moraines in this region are composed of sili- during Holocene time. Glaciologic studies in the and Holmes, 1965) between the^xposed core of ceous conglomerates and sandstones or quartz- eastern Brooks Range showed that radiation ac- glacial ice and the downvalley rock glacier. The ites of the resistant Devonian Kanayut Con- counts for -60% of ice and snow ablation lichenometric record preserved on debris in the glomerate. Almost all cirque glaciers occur (Wendler and Weller, 1974). Under the pres- transition zone is used to help to construct the north of the Continental ¡Divide and rise north- ent climate, cirque glaciers across the Brooks Neoglacial history of the cirque glaciers. In our ward from minimum altitudes of -1,500 m Range are wasting away. interpretation of cirque-glacier fluctuations (see near the Divide to maximum altitudes of Cirque glaciers are fronted by different types Haeberli and others, 1979), however, the down- —2,000 m near the northern front of the range of deposits in the central Brooks Range. Factors valley rock glaciers that show clear evidence of (Ellis and Calkin, 1979). The Continental in the cirque environment, including bedrock movement (White, 1981) have not been used. Divide is the Pacific-Arctic drainage divide and

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correlates with the transition zone between the extreme continental climate of Alaska's interior and the generally cooler Arctic regime of the North Slope (Brown, 1980). At Atigun Pass (1,440 m), estimated mean annual temperature is about -14 °C, and annual precipitation ranges between 0.4 and 0.7 m, of which about 50% is snow.

Anaktuvuk Pass Area

Field work was undertaken in cirques incised within the Kanayut Conglomerate at the heads of Inukpasagruk and Itikmalakpak Creeks (Fig. 3). Glacierized cirques are more sparse here than farther east around the higher Atigun Pass (Hamilton, 1979a), and only one of the mapped cirques contained exposed glacier ice (48 in Fig. 3). This ice mass was fronted by a glacier-cored moraine; its Neoglacial record was combined with the extensive Atigun Pass data because of similarities in bedrock, latitude, and altitude.

The Arrigetch Peaks

Granitic rocks (Nelson and Grybeck, 1980) support summits to 2,150 m and deeply incised cirques with nearly vertical walls in the Arri- getch Peaks, 225 km southwest of Atigun Pass. The moisture regime of the Arrigetch area appears to be transitional between the wetter mari- time conditions of the western Alaska coast and the relatively dry continental climate of interior Alaska (Hamilton, 1981). Sparse climatic data suggest that the Arrigetch Peaks are slightly warmer and wetter than the more easterly Atigun Pass area. Mean glacier altitudes range from 1,300 to 1,750 m. Of 11 glaciers that were reconstructed to their Neoglacial maxima, 9 were lichenometrically mapped in the field. The earliest set of glacier photographs in the central Brooks Range was made here in 1911 by Figure 2. Location map of Atigun Pass area. Glaciers 1-12 have Neoglacial Philip S. Smith (1913). His photography was moraines largely without glacier cores, glaciers 13-34 have moraines with gla- repeated in 1961 by T. D. Hamilton (1965) and cier cores, and those numbered 35-41 have Neoglacial moraines superimposed in 1979 by Hamilton and Ellis (Ellis and others, upslope of a rock glacier tongue. Letters a-i refer to radiocarbon-dated sites 1981, Figs. 6 and 7). This has allowed us to listed in Table 1. monitor stability, lichen colonization, and deglaciation rates for the 68-yr period. spread here. In particular, the species Rhizo- R. geographicum thallus diameters as much as DATING OF MORAINES AND carpon geographicum s.l., with a life span that 150 mm are tied to an absolute growth curve ROCK GLACIERS may reach to 8,000 or 9,000 yr in the Arctic that is extrapolated to 5000 yr B.P. and as- Field Methods and of world-wide use in lichenometry (Locke signed a subjective ± 20% age reliability and others, 1979), has provided consistent (Fig. 5). Lichenometry is the major tool for relative growth patterns on the bouldery cirque glaci- The various geomorphic units were mapped and absolute dating of Holocene rock sub- ers. In this study, six lichen species are used; by recording thallus diameters of the six lichen strates above tree line in the Brooks Range however, all thallus diameters are converted species while traversing as much area as possi- (Calkin and Ellis, 1980). Several lichen taxa to appropriate R. geographicum dimensions ble. Only the maximum diameter of the largest with predictable growth patterns are wide- by use of measured interspecific growth ratios. lichen thallus is used as an indicator of sub-

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+ 152° 00' W 151°00'W 68o30'N

Latest Itkillik ice margin with approx-. imate flow directions (Hamilton, 1979)

^ lake

.:.v.-.. cascading glacial CiT'i' deposit

'f'•-••••••• , v Figure 4. Location map of the Arrigetch Peaks. Glaciers 49-51 are Continental Divide 68°00Nt- fronted by moraines without ice cores; glaciers 52-58 have glacier- Figure 3. Location map of Anaktuvuk Pass area. Empty cirques of cored moraines, and a tongue-shaped rock glacier with an exposed Pleistocene age are noted as 42, 43, and 45; a tongue-shaped rock glacier core and Neoglacial moraines upslope is numbered 59. These glacier with no visible glacier core is in cirque 44; lobate rock glaciers 11 glacier lobes were planimetrically reconstructed to their maximum are located in cirques 46 and 47; and a cirque glacier fronted by a Neoglacial dimensions; however, 49 and 57 were not lichenometri- Neoglacial moraine with an ice core is in cirque 48. cally mapped in the field.

strate age, because it is assumed to be the old- assembled to help to differentiate various ages 160 est and to possess the optimum growth rate for of moraines and rock glaciers (Ellis, 1982). 140 Error Bars the site being studied (Beschel, 1961). This On moraines and rock glaciers that date from i 20% technique yields minimum ages (Karlen, 0 to -12,500 yr B.P., 22 soils were examined. ï 120 Qualitative 1979). In addition, the ages that are derived The major data collected include dry Munsell Age Reliability date stabilization of debris ridges and the be- color, horizon thickness, and colormetric pH. 100 ginning of glacial retreat from advanced ice Soil pits were located on crests of deposits positions, not times of maximum glacier where drainage was unimpeded during the 80 expansions. thaw season. Preliminary attempts were made Increased precipitation and temperature, to use stone weathering as a relative-dating 60 together with longer growing seasons, favor tool in the vicinity of Atigun Pass. The most more rapid lichen growth (Beschel, 1961); successful method is termed pebble relief, and 40 therefore, lichens may grow slightly faster on it involved the measurement of relief of highly the moraines of the more westerly Arrigetch resistant chert pebbles and quartz veins above 20 Peaks than those of the Atigun Pass area. The the general rock surface. -I- granite and the siliceous Kanayut Conglomer- 0 1000 2000 ;sooo 4000 !>000 ate provide comparable lithologies for lichen Radiocarbon Ages Radiocarbon Years Since Substrate Stabilization growth across the central Brooks Range. On massive bedrock and boulders located just We obtained nine radiocarbon dates on Figure 5. Rhizocarpon geographicum s.l. outside Neoglacial deposits in both terrains, buried organic material in the Atigun Pass growth curve, central Brooks Range. Shaded R. geographicum s.l. attain thallus diameters area that are applicable to our Neoglacial zone indicates subjective ±20% age reliability in excess of 250 mm. chronology (Table 1; Fig. 2). Of these, 5 were for the growth curve. Colonization is as- A preliminary chronosequence of soil de- directly associated with Neoglacial cirque sumed to occur 30 yr after substrate stabiliza- velopment in the Atigun Pass area has been deposits; the others were useful for absolute tion (see Calkin and Ellis, 1980).

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calibration of lichen growth curves and epi- TABLE 1. RADIOCARBON DATES sodes of stream alluviation. Conversions of Ub Age Location Description radiocarbon year B.P. to calendar year B.P. sample (>r) (Fig. 2) follow the detailed correction curve of no.

Oeschger (1975, Fig. 6) to A.D. 1200. BGS 547 210 • 90 a Peat layer at 0.65-m depth in alluvial fan at 900-m altitude in TAPS corridor (Alyeska Material Site A major expansion of Buffalo Glacier MS'112-2). Associated with R. geographicum on fan's aggradational upper surface • 12 mm in diameter. (Fig. 6) is dated at 320 ± 100 yr B.P., or A.D. BGS 522 320 . 100 b Woody vegetation preserved in bedrock joints underneath lateral margin of Buffalo Glacier (1 in 1450 to 1650 (b in Table 1). This age is espe- Figs. 2, 6) outermost, non-glacier-cored moraine (Calkin and Ellis, 1980, Fig. 6). Located at cially significant because of the stability of the 1,615-m altitude: associated with R. geographicum • 20 mm and Alecioria minuscula 140 mm in diameter growing on upper surface of the overriding moraine. ice-free moraine. It records deposition of the BGS 671 380 • 90 c Peal layer at 1.5-m depth in alluvial fan at 1.070-m altitude in TAPS corridor. Dales beginning of outermost lateral moraine on a near-hori- aggradation on fan surface. zontal bedrock surface (Calkin and Ellis, BGS 549 480 - 140 d Plant fragments from interface between older, downslope rock glacier lobe and overriding ice-cored, debris lobe 30 m thick. Sample may date advance of Wolverine Glacier (41 in Fig. 2) upper 1980, Fig.6). Terminal moraines with substan- transition zone over stabilized rock glacier.

tial ice cores at Ram and Kid Glaciers (16 and BGS 613 540 • 180 e Grassy plant fragments from interface between cirque threshold and Ram Glacier (16 in Fig, 2) 18 in Fig. 2) apparently were deposited on terminal, ice-cored debris lobe. Sample dates advance of lobe or mass-wasting over site. their bedrock cirque thresholds by 540 ± 180 BGS 548 800 • 90 r Peat layer 0.95-m depth in alluvial fan at 1,220-m altitude near Alyeska Material Site MS-I10.2. Maximum R. geographicum diameters of 33 mm measured on boulders at fan surface in vicinity and 1500 ± 150 yr B.P., respectively (e and i of dated material.

in Table 1). At the glacier-cored rock glacier BGS 614 1.120 • 180 g Emergent mosses collected in situ from around boulders being exposed by receding Golden Eagle Glacier (7 in Figs. 2 and 7). Dates glacier's last Neoglacial advance over site (see Calkin and Ellis, named Wolverine (41 in Fig. 2), organic 1981, Fig. 3).

material was excavated from between the toe BGS 670 1.300 • 100 h Peat layer at 0.85-m depth in alluvial fan at 1.130-m altitude in TAPS corridor. Maximum R geo- of the ice-cored Neoglacial moraine complex graphicum diameters of 50 mm measured on boulders at fan surface in vicinity of dated material (Calkin and Ellis, in press). and the overridden rock glacier tongue; it pro- BGS 615 1.500 • 150 Plant fragments from interface between bedrock cirque threshold and bouldery, ice-cored terminal vides an age of 480 ± 140 yr B.P. (d in Table moraine of Kid Glacier (18 in Fig. 2). Sample may dale advance of lobe or mass-wasting event 1). Radiocarbon evidence obtained at the toes over site. of the three steep-fronted, glacier-cored moraines must represent minimum ages of glacial At the gently sloping toe of Golden Eagle 180 yr B.P. was obtained for the dead mosses advance. This is because they may relate more Glacier (Fig. 7), glacial retreat has exposed an (g in Table 1). This dates a Neoglacial ad- to a mass movement event or downslope creep 2 area of -800 m bearing undisturbed, non- vance across the now deglaciated site and in- of the debris lobe. Taken together with the sorted patterned ground and in situ patches of dicates that for the past 1,100 yr the cirque Buffalo Glacier date, the 4 radiocarbon ages unidentified dead mosses partially enveloping glacier was continuously in a more advanced indicate a widespread phase of cirque glacier hundreds of lichen-covered boulders and cob- position than it is under the present climate. expansion from 720 to 220 yr B.P. and per- bles ranging to 1.5 m in diameter (Calkin and The mosses and lichens disintegrate and dis- haps an advance ~ 1,000 yr earlier. Ellis, 1981). A radiocarbon date of 1120 ± appear within a few years after emergence.

RADIOCARBON SITE

Figure 6. View south toward Buffalo Gla- cier (1 in Fig. 2), showing location of 320 ± 100 yr B.P. radiocarbon date at edge of Neo- Figure 7. View east toward Golden Eagle Glacier (7 in Fig. 2), showing location of 1120 ± glacial moraine. 180 yr B.P. radiocarbon date obtained on emergent moss at toe of receding glacier.

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0 20 40 60 80 100 120 140 I I * Figure 8. Maximum thallus diameters of R. geographicum mapped 2 I I on moraines extending from the margins of cirque glaciers in the I I 4 I I Atigun Pass, Anaktuvuk Pass, and Arrigetch Peaks areas. Those I I I diameters that were measured on thin drift directly over bedrock 6 I I * (designated *) may be the most reliable as age criteria. Symbols at 8 right indicate glaciers with moraines largely free of glacier cores (M), I 10 moraines with glacier cores (MG), and moraines superimposed up- 12 slope of rock glacier tongues (TRGC). 14 I I 16 I I ? i I I I The length of the preceding, ice-free episode 18 i I I I I I at this site is suggested from measurements of 20 M I ? relict R. geographicum s.l. found on the I I S3 22 I emergent and undisturbed boulders. Here, the I I 24 largest, well-defined thalli had maximum di- z MG 26 ameters ranging from 62 to 72 mm. This sug- 0 gests a minimum duration of 1,500 to 2,500 yr 1> 28 for an ice-free interval prior to the 1120 yr I I I 30 I I I B.P. advance. Lichenometric mapping of live o> g 32 lichens populating the deglaciated surface e> downslope of the relict 14C site indicates that 34 • I ? I ? I I III recent deglaciation from the most advanced 36 position was not uniform, but most rapid after 38 • TRGC A.D. 1750. 40 I Alluvial-fan aggradation may be related to I I I I 48 I I - MG * * * cirque glacier expansions at valley heads in the * M 50 - * * * same way that terrace alluviation appears to I be contemporaneous with increased cirque 52 I ? MG glacier activity (Hamilton, 1980). Layers of 54 • *I I I I peat were located immediately beneath four 56 - *l 598 -TKGC aggradation fan surfaces. They provided radi- _J I I L_ ocarbon ages of 210 ± 90, 380 ± 90, 800 ± 90, 0 20 40 60 80 100 120 140 and 1300 ± 100 yr B.P. (a, c, f, and h in Table Rhizocarpon geographicum maximum thallus diameters (mm) 1 and Fig. 2). yr (Figs. 8 and 10) (Bruen, 1980a). Soils of zone of moraines characterized by stabiliza- Lichenometric, Soil, and freshly deposited tills that have not yet been tion ages of (L) 1800 ± 400, 1400 ± 300, Weathering Evidence oxidized demonstrate pH values averaging 1200 ± 250, and 480 ± 100 yr B.P. (Calkin 7.7. Soils on outer ridges that are lichenomet- and Ellis, in press, Fig. 5). Pika Rock Glacier Moraines downslope of 51 cirque glacier rically dated at -2000 yr B.P. are distinctly shows a marked trend in soil development tongues were lichenometrically mapped (Fig. 8). better developed (Fig. 10). across the Neoglacial transition zone and To clearly delineate an age as lichenometrically Morainal lobes (22) are extensively cored downvalley onto the rock glacier (Fig. 11). determined, (L) is placed before the derived age, with glacial ice (13-34 in Fig. 2). Within this Weathering of boulders and cobbles with followed by the ± 20% age reliability (Fig. 5). group, closely spaced cirque glaciers show time in the sedimentary terrain of Atigun Pass All thallus measurements in this paper refer to very similar lichenometric records (15-19 in is shown in Figure 12 (A-H). Distinctive red R. geographicum. Fig. 8). Major ridge-building events termi- and green shale boulders from the middle Atigun Pass Area. Morainal complexes nated about (L) 1900 ± 400, 800 ± 200, and member of the Kanayut Conglomerate (Fig. (12) were deposited with either no cores or 350 ± 80 yr B.P. A recessional glacier-cored 12A) disintegrate in <500 lichenometric years. small cores of ice (1-12 in Fig. 2). The largest ridge system dated at (L) 100 ± 20 yr B.P. is Percentage of lichen cover rapidly increases R. geographicum lichens found on a Neogla- also evident. Lichen thalli indicative of stabili- from 0 to 400 lichenometric years (Figs. cial moraine in the central Brooks Range were zation beyond 2000 yr B.P. are rare on the 12A-12C); however, from 400 back to - 5,000 at the stable debris lobe of Triple East Glacier ice-cored moraines studied; however, some yr, the percentage of lichen cover varies over a (Fig. 9). Here, the ridges were characterized dating to (L) 3500 ± 700 and 2900 ± 600 yr wide range (Figs. 12D-12E). The percentage of by maximum lichens of 145, 97(?), 70, 35, B.P. are preserved along outer ridges of debris lichen cover is nearly constant or reflects and 24 mm. These in turn represent morainal lobes. boulder disintegration for surfaces that stabil- stabilization events of (L) 4500, 3500, 3000, Glacier-cored, tongue-shaped rock glaciers ized prior to middle Holocene time (Figs. 2000, and 450 yr B.P. (+20% age reliability). (seven) were mapped in the Atigun Pass area 12F-12H). Figure 12 shows the marked de- Grizzly Glacier (3 in Fig. 2) also preserves (35-41 in Fig. 2). Harlequin Duck Rock Gla- crease in boulder exposure with time. an extensive Neoglacial record, showing 4 cier (Ellis and Calkin, 1979, Fig. 4D) dis- Lichenometry, soils, and weathering data major stabilization events in the past —2,000 plays a prominent glacier-cored transition indicate that rock glacier lobes were initiated

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Figure 9. A. View west toward Triple East Glacier complex. Arrow shows position oi morainal lobe of Triple East Glacier (12 in Fig. 2). B. Lichenometric map of Neoglacial moraine complex downslope of Triple East Glacier.

by early Holocene time, after valley déglacia- tion and before construction of Neoglacial moraines. Soils show a progressive decrease in pH values from 7.5 to 8.0 for newly deposited till in glacierized cirques to 4.7 to 5.4 for late Pleistocene moraines downvalley. Rock glaciers have intermediate soil pH values of 4.8 to 6.4 in the upper solum (Fig. 11). Pebble relief on late Holocene drift in cirques is ~1 mm, whereas for valley moraines it averages ~6 mm; rock glacier values are intermediate at ~4 mm (Ellis, 1982). ©65 Rhizocorpon geographicum maximum digmeter (mm) Anaktuvuk Pass Area. Lichenometric mapping in two tributaries of Inukpasagruk Creek © 67ei R. eupetraeoides/inarense mgximum digmeter (mm) (Figs. 3 and 13) demonstrates that the Neogla- ^20— R. qeogrgphicum isophyse (line of equal growth,mm) cial or Fan Mountain glaciation was not as extensive as previously described by Porter (1966, Fig. 18). The two tributaries hang over Inukpasagruk Creek with subtle late Pleisto- cene (Alapah Mountain) moraines at their mouths (Porter, 1966, PI. 11; Hamilton, 1979). R. geographicum in excess of 250 mm, boulders with rounded corners, hollows weathered to depths of 4 cm on Kanayut stones, and lichen cover to 95% along the length of both branches of the west tributary (Fig. 13B) indicate that the 2 empty cirques there date to late Pleistocene time. A deposit in the west cirque of Mount Ah- gook (44 in Figs. 3 and 13B), previously interpreted as superimposed moraines of Fan Mountain age (Porter, 1966, PI. 13), is re- classified as a tongue-shaped rock glacier (Fig. 14). Its altitude of «1,350 m is well below the lowest glacier-cored rock glacier with a transition zone near Atigun Pass. This altitude is similar, however, to other inactive, tongue-shaped rock glaciers without exposed cores of ice (Ellis and Calkin, 1979). An ice mass at 1,700-m altitude in the Itikmalakpak drainage (48 in Fig. 3) built morainal ridges during the Neoglacial (Fig. 8). The Arrigetch Peaks. Distinct morainal lobes (nine) were mapped in the Arrigetch Peaks. Here, the destabilizing effects of melt- ing glacier-ice cores are marked and cause lichens to be more sparse and inconsistent in size than in the sedimentary Atigun and Anaktuvuk Pass areas. In addition, the algae 0 200 m Trentepohlia iolithus is ubiquitous on Neogla- B

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individual morainal ridges and map units on cirque glacier moraines in the three field areas are very similar at and beyond the 20-mm thalli diameter (Fig. 8). This similarity suggesls that (1) the ice masses reacted synchronously to climatic changes, (2) lichen colonization times and growth rates do not vary significantly with lithology or substrate instability, and/or (3) the lichenometric method is too imprecise to distin- guish colonization differences or slight changes in climate across 225 km of the central Brooks Range. In the more easterly areas, there if. some evidence for ridge construction and stabilization since 20-mm lichen time as numerous ice-cored moraines host 18, 15, and 11-12 mm R. geo- à graphicum. This lichenometric pattern is absent, however, in the more westerly Arrigetcn area (Fig. 8). The lichenometric age groupings of Neogla- 200 m cial moraines found on the 51 debris lobe;, in the central Brooks Range are derived by replotting the maximum thallus diameters in Figure 8 as a frequency histogram. This histogram is correlated with the growth curve for R. geographicum in Figure 16. Maximum lichen diameters appear to cluster into seven groups representing five or more major periods of moraine stabilization (Table 2). The number of ridges that comprise: these seven groups progressively decreases wi:h age. pH=6.0 In addition, the ridges of various ages nest 2.5Y6/4 closely along the perimeter of Neoglacial lobes pH=7.5 in a belt =650 m wide. These factors and other 5Y5/3 field evidence indicate that younger glacial expansions destabilized significant portions of older ridges, yet there is little evidence of these younger advances breaking through older deposits. Apparently, the major expansions B during the Neoglacial have each been of similar magnitude. The timing of these advances is less reliable with increasing age because of the qual- Figure 10. A. View southeast from Continental Divide toward Grizzly Glacier (3 in Fig. 2; itative ±20% age reliability assigned to licheno- photograph by M. P. Bruen, 1978). B. Lichenometric map of Neoglacial deposit downslope of metric ages (Fig. 16) and the reduced number of Grizzly Glacier (modified from Bruen, 1980a), with soil profiles shown. preserved morainal surfaces. In addition, this reliability range is increased beyond the 20% of the cial moraines of the Arrigetch Peaks and along the perimeter of 2 debris lobes, suggest- mean thallus size, because each of the major further inhibits lichen development. ing stabilization at (L) 1800 ± 400 and 1120 ± stabilization events incorporates a range of thal- Some of the best recessional chronologies 300 yr B.P. (Fig. 15). There is evidence for lus diameters (Table 2). were obtained from stable, thin drift sheets major activity of cirque glaciers prior to 2000 The oldest cirque glacier deposits that may be that were deposited directly on bedrock be- yr B.P. (Ellis and others, 1981, Fig. 9); how- associated with the initial expansion or re uvena- hind terminal moraines dated at (L) 390 ± 90 ever, interpretation and dating are difficult. tion of cirque glaciers in the central Brooks yr B.P. in the Arrigetch area (Fig. 8). The de- Range have been lichenometrically dated at glaciated areas upslope are characterized by HOLOCENE GLACIAL 4400 ± 900 yr B.P. (Fig. 17). Lichenometric lichens 16 mm in diameter, suggesting that CHRONOLOGY measurements on 8 other ridges in the central the ice margins remained close to their maxi- Brooks Range suggest moraines also stabilized at mum extent until (L) 170 ± 40 yr B.P. At Cirque Glaciers and Their Moraines (L) 3500 ± 700 and 2900 ± 600 yr B P. The Arr-10 West (Fig. 15), the ice margin has re- older ages have been substantiated by our sub- treated 200 to 300 m since that time. Nested The maximum thallus diameters of Rhizo- sequent work in the western and northeastern ridges without cores of ice were preserved carpon geographicum s.l. that characterize Brooks Range. In these areas, mapping has

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demonstrated 3 and 2 more moraines that date, respectively, within the 3500 and 4400 yr B.P. time intervals. Live lichens on 12 moraines indicate a glacial GLACIER CORE advance and subsequent morainal stabilization phase at (L) 1800 ± 400 yr B.P. At Golden Eagle Glacier (Fig. 7), however, lichenometric measurements on emergent but relict lichens suggest a period of glaciologically unfavorable conditions during this time. Our chronology (Fig. 17) tentatively includes the advance indicated by live lichens. From 1200 yr B.P. to the present, the lichenometric chronology is enhanced by combining it with radiocarbon dates (Table 1) and sequential glacier photographs. A major glacial advance, radiocarbon dated at 1120 ± 180 yr B.P. and lichenometrically dated at 1150 ± 300 yr B.P., is well established for the central Brooks Range. There also appears to have been a subsequent readvance (L) 800 ± 200 yr B.P. A period of alluvial-fan activity may have accompanied this expansion, if one aggradational event, radiocarbon dated at 800 ± 90 yr B.P., is representa- tive. The last major Neoglacial expansion is lichenometrically dated at (L) 390 ± 90 yr B.P. (A.D. 1410-1600); radiocarbon dates directly associated with cirque glacier deposits range from 320 ± 100 to 540 ± 180 yr B.P. (A.D. 1200-1650). The well-documented and widespread lichenometric range is chosen to represent the culmination of the last major expansion in the central Brooks Range. Ice margins remained close to this advanced position until about A.D. 1640-1750, when marked recession commenced. In the Arrigetch Peaks at cirque glacier Arr-4 (56 in Fig. 4), lichenometric mapping and sequential photographs suggest deglaciation was most rapid after about A.D. 1870 (Ellis and others, 1981). Hamilton (1965) estimated that both arms of Arr-4 retreated 200 m upvalley to 100-m higher altitude during the period A.D. 1911 to 1962. No lichens were found within 15 m of the ice margin; this distance may represent deglaciation during the ~30 yr assumed necessary for lichen colonization. The 15-m distance indicates the recession rate has slowed since the 1950s. Evi- dence at Golden Eagle Glacier (Fig. 7) suggests that glaciers are at present receding farther into their cirques than they have during the past Figure 11. A. View southwest from Continental Divide toward Pika Rock Glacier (37 in 1,100 yr. Fig. 2; photograph by M. P. Bruen, 1978), showing downslope limit of transition zone contain- ing Neoglacial moraines. B. Lichenometric map of Pika Rock Glacier (modified from Bruen, 1980a), showing soil profiles. Rock Glaciers remnants of valley glaciers and/or incipient (1) stabilization and weathering of upper sur- Rock glaciers now present in the cirques were cirque glaciers (Beget, 1983), creating glacier- faces of rock glaciers during this period (Fig. probably initiated by increased mass wasting cored rock glaciers. A climatic optimum (Hypsi- 12), (2) a lack of morainal deposits of this age in during late Pleistocene deglaciation. The debris, thermal Interval) is inferred for the early to the cirques, and (3) the common occurrence of shed from oversteepened valley walls, covered early-middle Holocene period, on the basis of empty cirques with altitudes and orientations

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/8/897/3434705/i0016-7606-95-8-897.pdf by guest on 25 September 2021 Figure 12. Development of lichens and weathering with time on moraines and rock F. Early Holocene, tongue-shaped rock glacier surface. G. Early Holocene, inactive, glaciers of sedimentary rocks, Atigun Pass. A. 10 yr; note shale boulder in foreground lobate rock glacier surface. H. Late Pleistocene end moraine (—12.500 vr B.P.. at north starting to disintegrate. B. ~80 lichenometric years of exposure. C. -400 lichenometric end of Atigun Valley, Fig. 2). years. D. -1,200 lichenometric years. E. -4,500 lichenometric years (see Fig. 9B).

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to Neoglaciation; their moraines have eliminated any evidence of a previous rock glacier lobe emanating from the cirque (Figs. 9 and 10).

PRESENT AND PAST EQUILIBRIUM-LINE ALTITUDES

Determination of past equilibrium-line altitudes (ELAs) for glaciers reconstructed to their Neoglacial maxima was accomplished planimetrically by application of an assumed accumulation area to ablation area ratio of 2:1 or a ratio of accumulation area to total glacier area (AAR) of 0.67 (Meier and Post, 1962; Porter, 1970, A.Porter (1966, Fig. 18) 1975; Gross and others, 1976). The results are applicable to all of the seven Neoglacial maxima (Table 2), because, with few exceptions, their terminal and lateral moraines are closely nested. The shape of reconstructed cirque glaciers in the central Brooks Range demonstrates high sensitivity to climatic change. The area-altitude distribution or shape of Yellowjacket Glacier (22 in Fig. 2), shown in Figure 18, is typical for cirque glaciers in the central Brooks Range. A substantial portion of the surface area is concentrated around the steady-state ELA attained during Neoglacial maxima. Marked changes in the mass balance occur with only minor fluctuations (-100 m) of the ELA. Little data are available B. This study with iichenometric techniques applied to mapping on the response time of these glaciers to changes in climate. Climatic sensitivity is indicated, Fan Mt. H Moraines ^ Alapah Mt./Latest Itkillik however, by their shape, relatively high altitude ^ Drift (late Pleistocene age) Fan Mt. X Moraines within the range, and northerly orientation. 0 Lake Equilibrium-line altitudes reconstructed for * R qeoarqphicum thalli the Neoglacial maxima of glaciers fronted only 2Ô0-270 mm in Drainage divide by moraines range between 1,300 and 1,380 m diameter in the Arrigetch Peaks and rise eastward to an 43) Inventory number average of 1,755 ± 70 m in the Atigun Pass area. ( Fig. 3) Average altitudes of ELAs for glaciers leading into transition zones and rock glaciers in the Figure 13. Four cirques draining into Inukpasagruk Creek, Anaktuvuk Pass area. Compare Atigun Pass area are 1,640 ± 80 m. The lower (A) Porter's (1966, Fig. 18) interpretation of Neoglacial events with (B) that of this study. altitude for glaciers associated with rock glacier tongues may be due in part to their concentra- tion along the southern part of the range (35-41 similar to those of nearby cirques that are now altitude. These ice cores built Neoglacial mo- in Fig. 2). Neoglacial maxima ELAs for all occupied by glacier-cored rock glaciers. The raines, overriding and/or incorporating the cirque glaciers in the Atigun Pass area rise 1 empty cirques demonstrate that the early to downslope rock glacier tongue to varying de- northward at -5 m km' (Fig. 19) and between early-middle Holocene climate was such that grees (Fig. 11). These Neoglacial deteriorations the Arrigetch Peaks and the Atigun Pass area by 1 glacial ice unprotected by debris disappeared. were not severe enough, however, to initiate gla- —2 m km" . Both of these gradients reflect pre- Debris-protected ice in similar cirques survived cierization of empty cirques that are at similar dominantly southerly to southwesterly sources the climatic optimum, however, evolving into altitudes and orientations. of moisture for the central Brooks Range. glacier-cored rock glaciers by early Holocene At higher altitudes, north-facing cirques either Thicknesses of glaciers averaged 65 m or time. were empty and subsequently glacierized, or greater during Neoglacial maxima, as based Subsequent climatic deteriorations in late- their debris-protected glacial ice was activated on field measurements of remnant glacier cores middle to late Holocene time were severe by snow-line (equilibrium-line altitude) depres- at moraine snouts. Theoretical reconstructions enough to activate the headward portions of sion as early as ~(L) 4400 ± 900 yr B.P. These based on Nye (1965) and Weertman (1971) glacier-cored rock glaciers that were of sufficient higher-altitude glaciers responded most strongly indicate maximum thickness ranges of 43 to 115

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Neoglacial Disturbed Zone Early Holocene

Figure 14. A. View west toward inactive rock glacier (44 in Fig. 3) in tributary cirque west of Mount Ahgook; formerly mapped as Fan Mountain I and II lobes (Porter, 1966, Pi. 13). B. Lichenometric measurements of R. geographicum thalli along a longitudinal traverse of rock glacier showing early Holo- cene-late Pleistocene age, low altitude, and snow-covered disturbed zone near head.

Previous Previojs Fan Mt. E Fan Mt. I

and 75 to 200 m, respectively. Ablation since ciated with an ELA depression of — 200 m. This land (Burrows, 1975). These all show five or the last Neoglacial maximum is estimated at 30 3 to 4 °C change may better estimate the range more pulsations from middle to late Holocene m (range 27-39 m), on the basis of the relief of of summer temperatures that accompanied time that are noncyclic. lateral moraines above present glacier surfaces. Neoglacial ELA fluctuations in the central The magnitudes of each of the major advan- We suggest that only -35 m of ice remains in Brooks Range. ces in the central Brooks Range were similar many glacierized cirques. (Fig. 17). This situation was also found in de- Equilibrium-line altitudes for 1977-1981 COMPARISON WITH OTHER tailed studies of the Alps (as summarized in have been determined glaciologically at 3 repre- GLACIAL STUDIES Grove, 1979), southern Alaska (Demon and sentative glaciers near Atigun Pass (Bruen, Karlen, 1973), and Scandinavia (Karlen, 1973). 1980b; Ellis and Calkin, 1982). They were at or The last major Neoglacial expansion in the Miller (1973), however, recorded the most re- above the heads of the glaciers during the late central Brooks Range is dated at A.D. cent Neoglacial advance as represeniing the 1970s. ELAs reconstructed for the Neoglacial 1410-1600. Extensive compilation of Holocene maximum in Baffin Island. Benedict (1973) sug- maxima were depressed 100 to 200 m below the glacial records by Grove (1979) and global gested a middle Holocene advance in the central glaciologic ELAs of the 1970s. A lowering of Neoglacial moraines by Davis (1980, p. 73-74) Rockies as the largest of the Neoglacial there. summer air temperatures by only -1 °C from suggests the near synchrony of this last major Mercer (1976) indicated the first Neoglacial ad- values attained in the late 1970s could account Neoglacial expansion with others throughout vance (4200 to 4600 yr B.P.) in Patagonia as the for ELA depressions of 100 to 200 m and max- the world. The evidence available at present most extensive; he reported that the most recent imum Neoglacial expansions, if 2 assumptions suggests, however, a lack of synchroneity or a (<300 yr B.P.) advance was the smallest. Neo- are made. (1) ELA depressions are strongly de- failure of dating methods to delineate synchro- glacial moraines in the northeastern Brooks pendent upon reduced summer temperatures, nous events beyond 500 yr in age (Grove, Range that are older than -100 yr demonstrate and (2) the environmental lapse rate is ~0.6 1979). The central Brooks Range sequence is a lichenometric history similar to the central °C/100 m (see Porter, 1966). Air temperature similar in form to sequences from most other Brooks Range chronology (Calkin arid Ellis, records maintained near the 3 glaciers from regions (Davis, 1980), including Baffin Island 1982). 1977 to 1981 showed, however, that a lowering (Miller, 1973), the eastern Alps (Heuberger, The formation of glacier-cored rock glaciers of summer temperatures by 3 to 4 °C was asso- 1974), Patagonia (Mercer, 1976), and New 5'ea- and stabilization of their u pper surfaces were the

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Figure 15. A. View south toward Arrigetch Peaks glacier Arr-10 (51 and 53 in Fig. 4), with glaicer-cored moraines at the far left and far right. A thin, stable moraine without an ice core extends downslope from the center ice tongue. S. A. Walti stands in the right fore- "» \ I — s rî'/V-Mv , 0/ ground at the limit of the Neoglacial maxima. B. Lichenometric map of Arr-10. Note thin drift on bedrock where déglaciation rates can be most accurately determined with lichen- Ice-cored ometry. Moraine

Ice-cored Moraine main glacial events during early Holocene time in the central Brooks Range. Rock glaciers were also formed during this interval in the Canadian Rockies (Luckman and Crockett, 1978; John- son, 1980), the Colorado San Juan Mountains (Carrara and Andrews, 1976), and the Swiss Alps (Haeberli and others, 1979). A comparison of the central Brooks Range ELA depression (calculated from ELAs averaged in the late 1970s to reconstructed ELAs for Neoglacial maxima) with other glacierized alpine regions is as follows (see Porter, 1975, Fig. 9, Table 2);

(a) 140 m Southern Alps of New Zealand (e) 100-200 m central Brooks Range Brooks Range, suggests a world-wide climatic (b) 200-250 m Cascades (Scott, 1977) (this study). deterioration of similar magnitude. In the central (c) 150 m Colombian Andes (Herd, Brooks Range this climatic deterioration has oc- 1974) The similarity of Neoglacial ELA depressions curred five or more times during the latter half (d) 100-200 m Alps (Heuberger, 1968) on a world-wide scale, including the central of the Holocene.

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160 Figure 16. Frequency histogram of R. Geographicum maximum thallus diameters for cirque glacier deposits of 140 central Brooks Range correlated with R. geographicum Error Bors growth curve (Fig. 5). Seven clusters of similar-sized Qualitative 120 thalli are shown on the growth curve as rectangles; the size Age Accuracy of the rectangles indicates the age of each cluster and thalli 100 diameter ranges. Estimated plus or minus age reliability drawn as error bars. Arrows along growth curve's abscissa 80 point to mean age of R. geographicum size concentrations. Curve is not fixed to any given year. To convert radiocar- 60 SI bon years given on the abscissa to radiocarbon yr B.P. i (before A.D. 1950), subtract A.D. 1950 from the current -40 A.D. date and deduct this value from the graph's radiocarbon age. 20

• + + I _i I I I II. 0 0 1000 2000 3000 4000 5000 18 15 12 9 6 3 0 Radiocarbon Years Since Substrate Stabilization Number of Moraines

Figure 17. Holocene glacier chronology for the central Brooks Range, showing periods of cirque glacier expan- TABLE 2. L1CHENOMETRIC TABULATION OF NEOGLACIAL sions followed by moraine stabilization. The 4 vertical Present EVIDENCE PRESERVED IN THE CENTRAL BROOKS RANGE error bars parallel to the time line on the left (5500 to 1500 Retreat Adv ince yr B.P.) are the age reliability ranges of ± 20% about the No. of R. geographicum Lichenometric age morainul maximum thallus (±209f) mean lichen ages. Radiocarbon ages (Table 1) are depicted ridges diametcis (mm) by asterisks; all were interpreted as being related to glacier

48 20 25 (L) 390 90 yr B.P.. A.D. 1410- 1600 advances rather than retreats. Two radiocarbon ages (320 24 32 35 800 200 ± 100 and 1120 ± 180 yr B.P.) are exceptionally reliable as 15 42 45 1150 300 12 60 66 1800 400 indicators of glacial expansions. 5 95 - 97 2900 600 3 113- 115 3500 700 2 140- 145 4400 900

TD Bai ao .9? 2000- o o PROPOSAL FOR PRINCIPAL <25 « 4000- AND SUPPLEMENTARY to o REFERENCE SECTIONS Q. 6000- a> ? Climatic O x- 8000- I Optimum The term "Fan Mountain" (Detterman and .9? o \ Rock glocier others, 1958) is retained to describe the cirque 10000- formation glacier advances and their deposits of late- CD N JXL 12000 middle to late Holocene (Neoglacial) age, in O \Valley deglaciatkn order to maintain continuity with the literature. X) S. o The Fan Mountain glaciation as redefined in this tn Retreat Advance paper, however, is composed of more than two stages, and its deposits do not occur in ice-free cirques, as suggested in earlier literature. Coa- age was not designated in the Fan Mountain 12, and 35 in Fig. 2). Buffalo Glacier (Fig. 6) is lescing lobate rock glacier lobes in ice-free area, however. In addition, the Fan Mountain formally proposed as the principal reference sec- cirques (probably early Holocene in age) may area is relatively inaccessible, has extensive out- tion of the Fan Mountain, glaciation in the cen- have been misinterpreted as Fan Mountain mo- crops of limestones that are unfavorable for lich- tral Brooks Range (see Calkin and Ellis, 1980, raines during previous reconnaissance studies enometric dating techniques, and the Holocene Fig. 8). The moraines of the other three glaciers near Fan Mountain (Fig. 1) (Detterman and glacial deposits there have not been mapped are designated as supplementary reference sec- others, 1958, Fig. 10). beyond the 1:250,000 scale (Hamilton, 1979a). tions (Fig. 9) (Calkin and Ellis, 1981, Fig. 1, and Fan Mountain continues as the type locality We propose to establish principal and supple- in press, Fig. 5). These cirque glacier deposits for Neoglaciation, because it is the specific geo- mentary reference sections in the Atigun Pass span the Neoglacial time interval, two have crit- graphic locality where the cirque-glacier depos- area, where large-scale, surficial geologic-lich- ical radiocarbon ages, and all are relatively ac- its were originally defined and named. A type enometric maps have been published for four cessible due to proximity to the trans-Alaska oil section for cirque glacier deposits of Neoglacial cirque glaciers and their deposits (glaciers I, 7, pipeline haul road.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/8/897/3434705/i0016-7606-95-8-897.pdf by guest on 25 September 2021 Plan View of Yellow Jacket Glacier

Present Ice Reconstructed Surface _ . . Ice Surface 1800 m for Neoglacial Maxima

\ -7—2-AAR = 0.33 "®O 1400 ZD |= 1300 Y^-^SOm ] ] AAR = 0.67 Vs. ~J (Neoglacial \\. Maxima V^ J ELA) Present L>/ Moraine

1525 m Neoglacial 1525 m i 1 Ice Surface 0 500m

68° 10' N 68° 20 N Figure 18. Present-day and reconstructed plan views and Latitude area-altitude distribution of Yel- lowjacket Glacier (22 in Fig. 2), Figure 19. North-south profile through Atigun Pass, with (1) upper trend a typical cirque glacier in the 6000 present ice surface 1800 surface representing equilibrium-line altitudes (ELAs) for glaciers fronted by central Brooks Range. Both dia- 5800 -o- Neoglacial moraines without ice cores (M) and those with glacier-cored deposits (MG), and grams show a Neoglacial maxi- maxima surface AAR = 0.33 (2) lower trend surface representing ELAs of cirque glaciers fronted by Neogla- ma EL A of 1,630 m obtained by — 5600 1700 -5 03 ELA= 1720 cial transition zones of rock glacier tongues (TRGC). The trend surfaces rise using an accumulation ratio ? 5400 TD northward at ~5 m km"1. (AAR) of 0.67. By raising this -AAR = 0.67 3 _.„ - ELA= 1630m 1600 £ ELA only 90 m, to an altitude of < 5200 Difference (Neoglacial < 1,720 m, the AAR reverses to Neogiaciat surface ^ima) 5000 0.33, significantly reducing the 1500 size of the accumulation zone. 10 20 30 ELA depressions to the maxima Area (%) level of 1,630 m rapidly enlarge Yellow Jacket Glacier Area - the accumulation zone. This Altitude Distribution characteristic shape makes these cirque glaciers sensitive to climatic changes.

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CONCLUSIONS REFERENCES CITED 1980, Episodic Holocene alluviation in the central Brooks Range Chronology, correlations, and climatic implications, in Alber.. N.R.D., Beget. J. E.. 1983, Radiocarbon-dated evidence of worldwide early Holocene and Hudson. T„ eds.. The U.S. Geological Survey in Alaska climate change: Geology, v. H. p. 389 393. Accomplishments during 1979: U.S. Geological Survey Circu ar 823-B. 1. Lichenometry has proved to be the most Benedict. J. B., 1973, Chronology of cirque glaciaiion. Colorado Front Range: p. B21 • B24. useful tool for absolute and relative dating of Quaternary Research, v. 3, p. 584 599. 1981, Surficial geologic map of the survey Pass quadrangh. Alaska: Beschel, R. E., 1961, Dating rock surfaces by lichen growth and its application U.S. Geological Survey Miscellaneous Field Studies Map V1F-1320, Holocene glacial deposits in the arctic and alpine to glaciology and physiography (lichenometry), in Raasch, G. O.. ed„ scale 1:250,000. Geology of the Arctic. Volume II: Toronto. University of Toronto Press, 1982, A late Pleistocene glacial chronology for the southe n Brooks central Brooks Range. p. 1044 1062. Range: Stratigraphic record and regional significance: Geobgical So- 2. Lichenometric patterns and maximum li- Brosge. W. P.. Reiser. H. N„ Dutro, J. T„ Jr.. and Detterman. R. L„ 1979, ciety of America Bulletin, v. 93, p. 700 716. Bedrock geologic map of the Philip Smith Mountains quadrangle. Hamilton, T. D., and Porter. S. C.. 1975, Itkillik glaciation in the Brooks chen diameters mapped on the various Neogla- Alaska: L.S. Geological Survey Miscellaneous Field Studies Map V.F- Range, northern Alaska: Quaternary Research, v. 5, p. 471 '(97. 879-B, 2 sheets, scale 1:250,000. Herd, D. G., 1974, Glacial and volcanic geology of the Ruiz-Tolimi volcanic cial deposits located across the central Brooks Brown. J., 1980, The road and its environment, in Brown. J., and Berg, R. L„ complex, Cordillera Central, Colombia [Ph.D. dissert. : Seattle. Range are remarkably similar, suggesting (a) the eds.. Environmental engineering and ecological baseline investigations Washington, University of Washington. along the Yukon River Prudhoe Bay haul road: Hanover, New Hamp- Heuberger, H„ 1968. Die Alpengletscher im Spät- und Postglazial (Theglaciers ice masses reacted synchronously to climatic shire. U.S. Cold Regions Research and Engineering Laboratory Report of the Alps in late- and post-glacial time): Eiszeitalter und Gegenwart, 80-19, p. 3 52. v. 19, p. 270-275. changes, (b) lichen colonization times do not Bruen. M. P., 1980a. Morphology and process of a cirque glacier and rock 1974, Alpine Quaternary environmerts. in Ives, J. D„ and Barry, R. G., vary significantly with lithology or substrate sta- glaciers at Atigun Pass, Brooks Range. Alaska [M.A. thesis]: Buffalo. eds., Arctic and Alpine environments: London, Methuen, p. 318 338. New York, State University of New York at Buffalo, 95 p. Johnson. P. G.. 1980, Glacier-rock glacier t ansition in the souihw;st Yukon bility, and/or (c) the lichenometric method is 1980b, Past and present climatic regimes of a cirque glacier and rock Territory, Canada: Arctic and Alpine Research, v. 12. p. 195 204. glaciers. Atigun Pass, Alaska: Geotogical Society of America Abstracts Karlen. W„ 1973, Holocene glacier and climatic variations. Kebnekaise Moun- too imprecise to detect differences in coloniza- with Programs, v. 12, no. 2. p. 27. tains, Swedish Lapland: Geografiska Annater, v. 55A, p. 29 63. tion and growth across 225 km of the central Burrows, C. J.. 1975, Late Pleistocene and Holocene moraines of the Camcion 1979, Glacier variations in the Svartisen area, northern Norway: Geo- Valley. Arrowsmith Range, Canterbury, New Zealand: Arctic and Al- grafiska Annaler. v. 61 A. p. 11 28. Brooks Range. pine Research, v. 7. p. 125 140. Locke, W. W.. Andrews. J. T„ and Webber. P. J.. 1979. A manual for Calkin, P. E.. and Ellis, J. M., 1980. A lichenometric dating curve and its lichenometry: British Geomorpholojiical Research Group Technical 3. Preliminary use of weathering and soil de- application to Holocene glacier studies in the central Brooks Range. Bulletin no. 26, 47 p. Alaska: Arctic and Alpine Research, v. 12, p. 245 264. Luckman, B. H., and Crockett. K. J.. 1978, Distribution and charai teristics of velopment, including soil pH, complements 1981, A cirque glacier chronology based on emergent lichens and rock glaciers in the southern part of Jasper National Pari., Alberta: lichenometry and helps to differentiate cirque- mosses: Journal of Glaciology, v. 27. p. 511 515. Canadian Journal of Earth Sciences. 15, p. 540 550. —— 1982. Holocene glacial chronology of the Brooks Range, northern Meier. M. F., and Post. A. S.. 1962, Recent variations in mass net oudgets of glacier moraines from rock glaciers. Alaska, in Karlen. W.. ed., Holocene glaciers: Striae, v. 18, p. 3 8. glaciers in western North America: International Association of Scien- in press, Development and application of a lichenometric dating cuive. tific Hydrology, Commission of Sncw and Ice Symposiun of Ober- 4. Significant climatic sensitivity is demon- Brooks Range. Alaska, in Mahaney, W. C., ed., Quaternary dating gurgl, Publication No. 58. p. 63 77. methods: Amsterdam, Elsevier. Meier, M. F., Tangborn, W. V., Mayo, L. R. and Post, A. S.. 1971, Combined strated by cirque glaciers in the central Brooks Carrara, P. E., and Andrews. J. T., 1976. Holocene glacial/penglacial record; ice and water balances of Gulkana and Wolverine glaciers. A laska. and Range, as their area-altitude distributions result northern San Juan Mountains, southwestern Colorado: Zeitschrift für South Cascade glacier, Washington, 1965 and 1967 hydrobgic years: Gletscherkunde and Glazialgeologie, v. 11, p. 155 174. U.S. Geological Survey Professional Paper 715-A, 23 p. in much of their surface area being concentrated Davis. P. T., 1980, Late Holocene glacial, vegetational, and climatic history of Mercer, J. H.. 1976, Glacial history of soutfernmost South America: Quater- Pangirtunk and Kingnait fiord area, Baffin Island, N.W.T., Canada nary Research, v. 6. p. 125 166. around the reconstructed equilibrium-line alti- [Ph.D. dissert.): Boulder, Colorado, University of Colorado, p. 73 74. Miller. G. H., 1973, Late Quaternary glacial and climatic history cf northern tude (ELA) of Neoglacial maxima. Denton. G. H„ and Karlen, W„ 1973, Lichenometry: Its application to Holo- Cumberland Peninsula, Baffin Island. N.W.T.. Canada: Quaternary Re- cene moraine studies in southern Alaska and Swedish Lapland: Arctic search, v. 3, p. 561 583. 5. During the Neoglacial maxima, expanded and Alpine Research, v. 5, p. 347 372. Nelson, S. W„ and Grybeck. D., 1980. Geologic map of the Survey Pass 2 Detterman, R. L. Bowsher, A. L., and Dutro. J. T., Jr., 1958, Glaciation on the quadrangle. Alaska: U.S. Geological Survey Miscellaneous Field Stu- cirque glaciers averaged only 0.65 km in area, Arctic Slope of the Brooks Range, Alaska: Arctic, v. 11, p. 43 61. dies Map MF-I176-A, scale 1:250.000. with steady-state thicknesses of ~65 to 100 m. Ellis. J. M., 1982, Holocene glaciation of the central Brooks Range. Alaska Nye, J. F., 1965. The flow of a glacier in a channel of rectangular, e lipticai. or [Ph.D. dissert.]: Buffalo. New York, State University of New York at parabolic cross-section: Journal of Glaciology, v. 5, p. 661 (.90. In many glacierized cirques, there may be only Buffalo. 396 p. Oeschger, H„ 1975, Die CI4-Daterierung in Gletschervorfeld ('V dating in Ellis. J. M., and Calkin, P. E., 1979, Nature and distribution of glaciers, Neo- the glacier foreland), in Messerli. B„ : nd others. Die Schwan jungen des —35 m of ice remaining as the glaciers continue glacial moraines, and rock glaciers, east-central Brooks Range. Alaska: Unteren Grindelwaldgletschers seil sem Mittelalter: Zeitschrift für Glet- to shrink under the present climatic regime. Arctic and Alpine Research, v. 11, p. 403 420. scherkunde und Glazialgeologie, v. 1 I. p. 61. 1982, 1977 1981: An interval of descending ELA's and coo'ing Porter, S. C„ 1966, Pleistocene geology of Anaktuvuk Pass, central Brooks 6. Within the Atigun Pass area, four cirque temperatures, central Brooks. Range. Alaska: Geological Society of Range. Alaska: Arctic Institute of North America Techniea Paper 18. America Abstracts with Programs, v. 14, no. 7. p. 483. 100 p. glaciers and their Neoglacial deposits are specifi- 1983, Environments and soils of Holocene moraines and rock glaciers. — - 1970. Quaternary glacial record in Swat Kohistan. west Pak stan: Geo- central Brooks Range. Alaska, in Evenson, E.. Schliichter, Ch., .ind logical Society of America Bulletin, 81. p. 1421 1446. cally designated as principal and supplementary Rabassa. J., eds.. Tills and related deposits: Rotterdam, A. A. Balke na, 1975, Equilibrium-line altitudes of late Quaternary glacers in the reference sections. Fan Mountain remains the p. 315 328. Southern Alps, New Zealand: Quaternary Research, v. 5, p. 27 47. Ellis. J. M., Hamilton. T. D.. and Calkin, P. E„ 1981, Holocene glaciation of Porter. S. C., and Denton, G. H., 1967, Chronology of Neoglaciation in the type locality for Neoglaciation of the central the Arrigetch Peaks. Brooks Range. Alaska: Arctic, v. 34, no. 2. North American Cordillera: American Journal of Sciente, v. 265, p. 158 168. p. 177 210. Brooks Range. Ferrians. O. J., Jr., 1965, Permafrost map of Alaska: U.S. Geological Survey Scott, W. E.. 1977, Quaternary glaciation arid volcanism, Metolius River area. Miscellaneous Field Studies Map 1-445. scale 1:2.500,000. Oregon: Geological Society of Amerca Bulletin, v. 88, p. 113 124. Foster. H. L„ and Holmes. G. W„ 1965, A large transitional rock glacier in the Smith, P. S„ 1913. The Noatak-Kobuk regien, Alaska: U.S. Geolog cai Survey ACKNOWLEDGMENTS Johnson River area. Alaska Range: U.S. Geological Survey Professional Bulletin 536, 160 p. Paper 525-B. p. BII2 BI16. Weertman, J., 1971. Shear stress at the base of a rigidly rotating cirque glacier: Gross. G.. Kerschner. H., and Patzelt, G.. 1976, Methodische Untersuchungen Journal of Glaciology, v. 10. p. 31 38. This research was supported by National über die Schneegrense in Alpinen Gletschergebieten (Methodical inves- Wendlcr, G.. and Weller, G.. 1974. A heat-balance study of McCall Glacier, tigations on the snowline in glacierized areas of the Alps): Zeitschrift für Brooks Range, Alaska: A contribuì on to International H ^drological Science Foundation Grants DPP 7619575 and Gletscherkunde und Glazialgeologie. v. 12, p. 223 251. Decade: Journal of Glaciology, v. 13, no. 67. p. 13 25. Grove. J. M., 1979. The glacial history of the Holocene: Progress in Physical White, S. E., 1981. Alpine mass movement forms (noncatastrophic) Classifica- DPP 7819982 through the Research Founda- Geography, v. 3, p. I 54. tion, description, and significance: Arctic and Alpine Research, v. 13, tion of the State University of New York. We Haeberli, W., King, L„ and Flotron. W., 1979, Surface movement and lichen- p. 127 137. cover studies at the active rock glacier near the Grubengletscher, Willis. are grateful to T. D. Hamilton for stimulating Swiss Alps: Arctic and Alpine Research, v. 11, p. 421 441. Hamilton, T. D., 1965, Comparative photographs from northern Alaska: Jour- our initial interest in this study and to L. J. nal of Glaciology. v. 5. p. 479 487. Onesti, M. P. Bruen, S. A. Walti, T. V. Lowell, — - 1978, Surficial geology of the Philip Smith Mountains quadrangle. Alaska: U.S. Geological Survey Miscellaneous Field Investigations Map and L. A. Haworth for help with various aspects MF-879-A, scale 1:250.000. 1979a, Surficial geologic map of the Chandler Lake quadrargle, of the field work and data analysis. W. Karlen Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF- improved our understanding of lichenometric 1121. scale 1:250,000. 1979b. Late Cenozoic glaciations and erosion intervals, north-ccniral mapping techniques. Rice University provided Brooks Range, in Johnson. K. M.. ed.. The United States Geological Manuscript Rfi fivhj by rm Sex im Drt i mhí k 30. 1982 Survey in Alaska Accomplishments during 1978: U.S. Geological RFVISFnMANUSCKIKrRH HVFnSlPTFMBFK ,6, 1983 space to Ellis during the writing stage. Survey Circular 804-B, p. B27 B29. Manum RII't Ac« htf o Si IMI MHI k 16, 1983

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