pre-late-wisconsin glacial history, coastal ahklun ... 2006 readings... · numericalages because...

*Corresponding author. Tel.: 1-520-523-7192; fax: 1-520-523-9220.E-mail addresses: [email protected] (D.S. Kaufman), will-

[email protected] (W.F. Manley), [email protected] (S.L. Forman),[email protected] (P.W. Layer).

Quaternary Science Reviews 20 (2001) 337}352

Pre-Late-Wisconsin glacial history, coastal Ahklun Mountains,southwestern Alaska } new amino acid, thermoluminescence,

and 40Ar/39Ar results

Darrell S. Kaufman!,*, William F. Manley", Steve L. Forman#, Paul W. Layer$!Departments of Geology and Environmental Sciences, Northern Arizona University, Flagstaw, AZ 86011-4099, USA

"Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, USA#Department of Earth and Environmental Sciences, University of Illinois, Chicago, IL 60607-7059, USA

$Department of Geology and Geophysics, University of Alaska, Fairbanks, AK 99775-0780, USA

Abstract

New stratigraphic and geochronologic data from the Togiak Bay area of southwestern Alaska indicate that glaciers advanced fromthe southern Ahklun Mountains at least three and as many as six times prior to the late Wisconsin. The oldest glaciations arerepresented by glacial}marine sediment in coastal exposures on Hagemeister Island. The extent of amino acid (isoleucine) epimeriz-ation in fossil molluscs indicates that at least one, and possibly four, older middle Pleistocene glacial intervals are represented, withage estimates spanning &500}280ka and averaging &400$100ka. The youngest glacial-marine drift on Hagemeister Island maycorrelate with the eruption of the Togiak tuya. A new 40Ar/39Ar age on basalt that overlies pillow lava indicates that the volcanoerupted through glacial ice at least 300m thick 263$22 ka. The youngest drift in the region overlies the Old Crow tephra(140$10 ka) and a 70$10 ka basaltic lava #ow dated by thermoluminescence analysis of underlying baked sediment. The driftdelimits #at piedmont lobes that spread out onto the continental shelf and terminated '100 km from their source areas during theearly Wisconsin (sensu lato). The glacial-geologic evidence suggests that major expansions of glaciers were out of phase with global icevolume. ( 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Although no continental ice sheet ever covered westernAlaska, pre-late-Wisconsin mountain glaciers there weremuch more extensive than during the most recent globalglacial maximum (e.g., Hamilton, 1994). The advance oflate Wisconsin ice was restricted to mountainous areas,thereby preserving an extensive record of older gla-ciations. The few coastal regions of the state that exposeinterstrati"ed glacial and marine deposits are parti-cularly important; they a!ord an opportunity to inte-grate the local record of glacial #uctuations with theglobal record of eustatic sea-level changes. The AhklunMountains (Fig. 1) are the only glaciated terrain in west-ern Alaska between the Alaska and Seward peninsulas.They are truncated on the south by the Bering Sea where

high coastal blu!s expose thick sequences of Quaternarydeposits. The deposits have a high potential for obtainingnumerical ages because they contain fossiliferous marinedeposits and tephras erupted from the Aleutian vol-canoes, and because they are interstrati"ed with basalticlava from local sources.

The purpose of this study is to apply a suite ofgeochronological methods, including amino acid, thermo-luminescence, tephrochronology, and 40Ar/39Ar tech-niques, to determine the timing of pre-late-Wisconsinglacier advances in the southwestern Ahklun Mountains,particularly the lower Togiak River valley and islandso!shore (Fig. 2). The geochronological data are necessaryto correlate the local stratigraphic record of paleoen-vironmental changes with the Pleistocene chronostrati-graphic framework developing across Beringia (e.g.,Kaufman and Brigham-Grette, 1993; Brigham-Gretteand Hopkins, 1995) and with other regional and globalrecords of climate change. These comparisons will facilit-ate an improved understanding of the paleoclimatologi-cal controls on glaciation in western Alaska and theirrelation to the global climate system.

0277-3791/01/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 1 2 - 8

Fig. 1. Southern Ahklun Mountains region showing the maximum extent of late Pleistocene glaciers (bold line where well-de"ned by terminal moraineor outermost position of hummocky drift; narrow line where inferred). Ice-free areas within glacial limit are not shown. Stars indicate newly discoveredlocalities of the Old Crow tephra. Light shading depicts mountainous area '200m; darker shading are elevations '600m; darkest shading aremodern lakes.

1.1. Study area

The Ahklun Mountains form the highest range inAlaska west of the Alaska Range and north of the AlaskaPeninsula (Fig. 1). They trend about 250 km northeast tosouthwest and are #anked by the Kuskokwim River andNushagak lowlands on the west and east, respectively.The range reaches its highest elevations in the northeast,where summits exceed 1500m and shelter dozens of mod-ern-day glaciers. Repeated glaciations during the Pleis-tocene have excavated an extensive network of valleytroughs, most of which are controlled by underlyingbedrock structure. Along the steep eastern range front,the troughs are "lled by an interconnected system ofelongate, glacially over-deepened lakes dammed by ter-minal moraines. The southern (coastal) and western#anks are dissected by broad, fault-bounded valleys,forming a more di!use mountain front of rolling uplandspunctuated by small, rugged massifs. Beyond the moun-tains, at the limits of glacier advances, moraines andice-thrust ridges form the principal topographic featuresin the lowlands. Erosion by storm waves has exposed

many kilometers of unconsolidated Quaternary sedi-ment.

The "rst descriptions of Quaternary features along thecoastal segment of the Ahklun Mountains were madeduring reconnaissance studies beginning in 1898 (Spurr,1900; Hamilton, 1921; Mertie, 1938). These workersinterpreted much of the sur"cial deposits between thevillages of Quinagak and Togiak as glacial and glacial-marine drift deposited by glaciers that originated in theAhklun Mountains and spread out along the coast aspiedmont lobes. Hoare and Coonrad (1961a, b) delin-eated undi!erentiated drift units in their reconnaissancebedrock mapping of the Hagemeister Island and Good-news Bay 1 : 250,000-scale quadrangles. Their maps, to-gether with the contributions of Muller (1953), were laterincorporated into the state-wide compilation of glacialdeposits in Alaska (Coulter et al., 1965).

More recent work focused on the record of pre-late-Wisconsin glaciations. Porter (1967) conducted a de-tailed glacial-geologic study of the placer-rich area southof Goodnews Bay. He described evidence for glaciers thatrepeatedly advanced from the Ahklun Mountains and

338 D.S. Kaufman et al. / Quaternary Science Reviews 20 (2001) 337}352

Fig. 2. Location of geochronological sample sites discussed in text. (a) Togiak Bay region; map location shown in Fig. 1. Light shading depictsmountainous area '100m; darker shading are elevations '250m. (b) Shaded relief map of Togiak tuya. Contour interval"40m with lowestcontour at 40 m above sea level.

spread westward as piedmont lobes over the coastallowlands of Chagvan Bay. Lea (1990) described the inter-nal stratigraphy of a laterally continuous arcuate ridgeexposed in cross section in coastal blu!s at Ekuk. Thethrust (composite) ridge was formed by piedmont glaciersfed from the southeastern #ank of the range. It is com-posed of glacial and glacial}estuarine drift deposited byglaciers that entered a macrotidal estuary similar to thepresent bay: the Nushagak Formation. Its age is between75 and 90ka, based on a variety of geochronologicalevidence including thermo- and optical-luminescenceages on associated tide-#at mud (Kaufman et al., 1996).Lea (1989) also described evidence for an older advancerecorded by subsurface drift (Nichols Hill drift) on theNushagak Peninsula beyond the limits of Wisconsin gla-ciation.

The results reported in this study are based mainly onexposures in the lower Togiak River valley, aroundTogiak Bay, and Hagemeister Island located &20 kmo!shore (Fig. 2). The area was studied previously inreconnaissance geologic mapping by Hoare and Coon-rad (1961a, b; 1978a). They interpret the Togiak Rivervalley as a graben formed in highly deformed rocks ofMesozoic age and bounded by northeast-trending faults

comprising the southwestern extension of the Denalifault system. They mapped the Quaternary lava that wedated in this study; they described the Togiak tuya(Hoare and Coonrad, 1978b), and reported late Cenozoicmarine molluscs on the east coast of Hagemeister Island(Hoare and Coonrad, 1978a).

2. Methods

2.1. Amino acid geochronology

The utility of amino acids as a geochronological toolfor Quaternary biominerals is now well established (seerecent review of principles and applications by Wehmil-ler, 1993; Rutter and Blackwell, 1995). Proteins and theirconstituent amino acids bound within the carbonatematrix of fossil molluscan shells are degraded in propor-tion to their age and temperature history. For geo-chronological purposes, the most reliable of the complexnetwork of reactions comprising protein diagenesis is theracemization reaction (or epimerization, in the case of theamino acid isoleucine). This reaction involves the inver-sion of amino acids from their protein L-con"guration to

D.S. Kaufman et al. / Quaternary Science Reviews 20 (2001) 337}352 339

their nonprotein D-con"guration. The ratio of the aminoacid D-alloisoleucine to its diasteriomer L-isoleucine(aIle/Ile) measures the extent of epimerization in theamino acid isoleucine and has been used previously forgeochronological purposes in the Arctic, and elsewhere.

Samples were processed by conventional methods atthe Amino Acid Geochronology Laboratory, Utah StateUniversity. They were cleaned by ultra sonication and byacid leaching in dilute HCl to remove 20}30% by weight,followed by vigorous rinsing in puri"ed H

2O and drying

under laminar #ow. Samples were dissolved in 7M HClusing a ratio of 0.2ml mg~1 shell and hydrolyzed bysealing under N

2and heating for 22 h at 1103C, then

dried in a vacuum, rehydrated, and loaded onto a high-performance liquid chromatograph (HPLC). The HPLCemploys step-wise addition of sodium-citrate bu!ersof increasing pH and post-column derivitization byo-phthalaldehyde with electronic integration of #uores-cence detection (Hare et al., 1985). The extent of epimeriz-ation was calculated using the average electronicallycomputed peak height aIle/Ile ratio. Each sample solu-tion was analyzed two or three times. The analyticaluncertainty (internal reproducibility) in aIle/Ile measuredby the coe$cient of variation (p/XM ) was typically 2}5%for fossil samples and 3% (n"50) for the laboratorystandard, which was analyzed regularly to monitor ma-chine performance. In addition, the standards of Wehmil-ler (1984) were analyzed for inter-laboratory calibration.

2.2. Thermoluminescence

Luminescence geochronology is based on the time-dependent dosimetric properties of quartz and feldsparminerals. The technique is especially e!ective for sedi-ments that have been baked. Heating of sediment by theemplacement of an overlying lava #ow eliminates most ofthe stored luminescence signal from mineral grains. Afterthe sediment has been heated, ionizing radiation from thedecay of naturally occurring radioisotopes within thesurrounding sediment results in the trapping of electronsthat accumulate in silicate mineral-grains. Excitation ofsediments by heat or light in the laboratory releases thetime-stored electrons as luminescence emissions. Theintensity of the luminescence can be used as a measure ofsample age by dividing the laboratory-determinedpaleodose (equivalent dose, ED) by an estimate of theradioactivity that the sample received during burial (doserate).

The ED was determined on the polymineral, "ne-grained (4}11lm) fraction by the total-bleach method, asdiscussed by Forman (1989). The residual level, the start-ing point for the accumulation of the environmentalradiation dose is the background signal remaining afterheating to 4503C. This closely approximates the originalresidual level attained after heating by an overlying lava#ow (Forman et al., 1994).

Prior to measurement of the natural and additive doseTL signals, the sediment was preheated at 1243C for 2}3days to obviate potential short-term instability in thelaboratory applied dose, called anomalous fading, whichcan lead to age underestimates (Wintle, 1973). After pre-heating, samples yielded peak TL emissions between 280and 3003C and were shifted (103C to compensate forthermal lag e!ects. The samples exhibited an insigni"cantamount ((10%) of potential fading after an additional30}35 days storage and thus we consider TL ages to bevalid "nite estimates.

Additional beta doses were applied to the natural TLsignal by a series of extended radiations with a calibrated90Sr/90Y source to evaluate the rate of TL ingrowth withirradiation. To accurately interpolate an ED, radiationdoses of at least "ve times the calculated ED were addedto the natural TL signal. The natural- and additive-dosedata were "tted by an exponential function to calculatethe ED (Huntley et al., 1988). ED values were calculatedover a range of temperatures, usually between 250 and4003C, which includes at least 90% of the measured TLsignal and is also the temperature region that exhibitsa pronounced plateau in ED values.

Another critical determination in TL dating is the doserate, which is an estimate of radioactivity of sedimentsurrounding the sample during the burial period. Themajority of radiation is from the decay of varyingamounts of isotopes in U and Th series and 40K contentof mineral grains. The 40K level is calculated from theK content assayed by #ame photometry. The U and Thcontents were determined by alpha counting, which as-sumes secular equilibrium in the decay series. Sur"cialsediments in the western US commonly have U/Th dis-equilibrium levels of 0.5}20%, however (Szabo andRosholt, 1982). This uncertainty in equilibrium status ofU and Th series may increase the error in the TL ageestimate by 10%.

2.3. 40Ar/39Ar

The K}Ar dating method uses the natural decay of40K to 40Ar, which has a half life of 1.25Ga. For thismethod, the K content is usually measured by a tech-nique such as #ame-photometry, while the Ar isotopesare measured on a mass spectrometer. This requires twoseparates of the same sample. The 40Ar/39Ar method isa variant of the K}Ar technique in which the samples areirradiated in a nuclear reactor to produce 39Ar, which isused to determine the K content of the sample. Samplesare fused to release the Ar using a laser heating system(York et al., 1981; Layer et al., 1987). In this technique,only mass spectrometry is required and this allows forthe dating of small samples.

For 40Ar/39Ar analysis, samples were crushed andwashed in deionized water, then sieved and &1mmdiameter, whole-rock chips were selected for irradiation.


The samples were wrapped in Al foil and arranged in twolevels, labeled top and bottom, within Al cans of 2.5 cmdiameter and 4.5 cm height. Three samples of biotiteBern4B with an age of 17.25Ma (C. Hall, pers. commun.,1991) were included on each level with each set of un-knowns to monitor the neutron #ux. The samples wereirradiated for 4MWh in position 5c of the U-enrichedresearch reactor of McMaster University in Hamilton,Ontario.

Following irradiation, the samples and monitorswere loaded into 2mm diameter holes in a Cu trayand placed in a ultra-high vacuum extraction line.The monitors and samples were fused using a 6 W Ar-ion laser. Ar puri"cation was achieved using a liquidN cold trap and a SAES Zr-Al getter at 4003C. Thesamples were analyzed in a VG-3600 mass spectrometerat the Geophysical Institute, University of Alaska Fair-banks. The Ar isotopes measured were corrected forsystem blank, mass discrimination, as well as Ca, K, andCl interference reactions following procedures outlinedin McDougall and Harrison (1988). The weighted meanof the results obtained on the monitor samples was usedon the ensuing calculations for their correspondingsample set.

3. Results and discussion

The glacial sequence of the southern Ahklun Moun-tains features at least four and possibly as many as sevenmiddle and late Pleistocene glacial intervals: (1) at leastone, possibly four, older middle Pleistocene intervalsrepresented by glacial}marine sediment on HagemeisterIsland; (2) a younger middle Pleistocene advance thatcoincided with the eruption of the Togiak tuya (andpossibly with the youngest glacial}marine sediment onHagemeister Island); (3) a series of early Wisconsin (sensulato; used here to include marine oxygen-isotope(sub)stages 5d}4, &115}60ka) advances represented byan extensive drift sheet that includes the glaciallyin#uenced intertidal sediments at Ekuk blu!s (Lea,1990) and terrestrial-based glacier deposits elsewhere;and (4) multiple #uctuations of much more restrictedglaciers during the late Wisconsin. Relative-age and14C data on the late Pleistocene glacial deposits arediscussed elsewhere in this volume (Manley et al., 2001).Additional glacial-geologic mapping of late Pleistocenedeposits, combined with cosmogenic-isotope datingand lake coring is the focus of Master's thesis re-search (e.g., Axford et al., 1998; Briner and Kaufman,2000). Our research on the paleoenvironmental condi-tions recorded by the Pleistocene sedimentary sequencesis ongoing. Here we present brief descriptions of the keystratigraphic sections from which we derive our geo-chronological materials and interpretations of depos-itional environments.

3.1. Older Middle Pleistocene and earlier advances

3.1.1. Stratigraphic evidenceThe oldest depositional record of glaciation in the

Togiak Bay area is on Hagemeister Island. Wave-cutcli!s on the eastern and southern coasts of the islandexpose glacigenic and non-glacial deposits that have beendeformed by glacier ice that advanced onto the continen-tal shelf from the Togiak River valley and from theuplands that surround Togiak Bay. The stratigraphicsection on the east coast was measured on the north sideof a steep ravine cut into the highest segment ('50m) ofthe coastal blu! at 58346.56' N latitude, 160348.36'W longitude (WM97-67-1). It is subdivided into sixstratigraphic units, including three distinctive diamictonunits (Fig. 3a):

(1) A lower, undeformed, clast-rich diamicton rests onbedrock and extends up to 18 m above sea level. Thematrix is sandy and the clasts are dominated by sub-angular to angular boulders of local origin, althoughsub-rounded boulders of exotic lithologies (including ves-icular basalt and granite) are also present. The diamictonis crudely bedded with some beds exhibiting reversegrading. We interpret the diamicton as colluvium depos-ited along a beach cli!. The erratic lithologies may indi-cate reworking from an otherwise undocumented andundated, earlier drift sheet. At 6}8m, the unit is interrup-ted by &3 m of silt and "ne sand forming thin beds thatdip seaward. These beds probably record submergence ina shallow-marine setting, thereby representing the oldestrecorded marine transgression.

(2) The lower diamicton unit is abruptly overlain bya &9 m thick zone of severely folded and faulted sedi-ment dominated by compact, clast-poor, very "ne sandysilt with abundant marine molluscs (Fig. 4a). The clastsare mainly subrounded pebbles to small boulders ofexotic lithologies. In places, their distribution is patchy,with enriched concentrations over distances of deci-meters. A &50 cm thick, contorted bed of sorted, open-work, iron-cemented, rounded granules and pebbles isdeformed into the diamicton at &23 m elevation. Thegranule bed is found near or directly above silty peat andorganic-rich mud that contains compressed, coniferouswood up to 17 cm in cross-section diameter, and a tephrabed up to 2 cm thick. The diamicton is less stoney abovethe granule bed than below. The mollusc fauna includesa variety of taxa (Table 1) dominated by the small tax-odont, Nuculana (N. pernula and N. minuta) and by thelarge bivalve genus, Serripes. Some fossils are well preser-ved and paired, others form casts or molds. We interpretthe shelly, erratic-bearing diamicton as glacial}marinedrift, and the intervening gravelly, peaty beds as inter-glacial deposits of a river/#oodplain or beach/lagoonthat were deformed into the marine sediment. In additionto glacial}marine and inter-glacial terrestrial deposits,


Fig. 3. Measured stratigraphic sections from the (a) east and (b) south coasts of Hagemeister, southwestern Alaska. Site locations shown in Fig. 2.

the presence of nonglacially in#uenced marine depositsin this highly deformed unit is indicated by Natica jan-thostoma (identi"ed by L. Marincovich, California Acad-emy of Sciences). This gastropod presently ranges fromJapan to Kamchatka and is thought to have expandedthroughout the Bering Sea during the warmest marinetransgressions of the middle and late Pleistocene (An-vilian and Pelukian marine transgressions; Hopkins,1967). Because the convoluted sediments lack strati-graphic integrity, we cannot ascertain whether thejuxtaposition of glacial}marine drift with non-glaciallyin#uenced marine sediment resulted from the deforma-tion or whether the two units are conformable.

(3) The main deformed stony mud unit is overlain bymud with multiple tephra stringers, casts of marine mol-luscs, and rare granules and pebbles. The unit extends upto '33m with a covered upper contact. The mud isdominantly massive, but interbeds of "ne sand are exhib-ited locally. The mollusc casts extend up to &28 m andare of an indeterminate bivalve genus. We interpret thisunit as shallow marine deposit and the paucity of stonesas indicating the absence of glacier-ice rafting.

(4) A &2 m thick unit of deformed, bedded, pebble tocobble gravel, and well-sorted, "ne sand with tephraoverlies the covered interval. Clasts are rounded to sub-angular and include a variety of lithologies. The exposureis poor, and the beds too deformed to identify sedimen-tary structures. We tentatively interpret this unit as a #u-vial deposit, perhaps outwash of the advancing glacierthat deposited the overlying till.

(5) The upper diamicton unit is &7m thick and restson a sharp basal contact. The matrix is silty and compact,with clasts dominated by subrounded pebbles to smallboulders of exotic lithologies. We interpret this unit as tillof a terrestrially based glacier that most recently en-croached onto eastern Hagemeister Island. The compact-ness and sharp basal contact indicate lodgment till,although the presence of laterally discontinuous, sorted,"ne-grained beds, and the kettled surface morphology,suggest that the till may have partly been deposited bymelt-out processes.

(6) The section is capped by 2}3m of massive, well-sorted silt, with multiple tephra beds. This unit is composedof loess. It is topped by a mat of peat and surface tundra.


Table 1Marine mollusc faunas from Hagemeister Island!

Field no. (WM97) (Fig. 2)

Genus 67-2B" 67-1C 67-1D 70A 70C 70D 71A 71B

Elevation (m) 19.5 26.0 22.1 17.6 20.0 21.0 16.8 20.4

BivalvesNuculana x x x x x x xSerripes x(?) x x xMya x x xMytilus xMacoma x xCyclocardia xAstarte x

GastropodsNeptunia x x x x xNatica x x# x x xPolinices xTachyrhynchus x

!Identi"cations by L. Marincovich (California Academy of Sciences)."Section WM97-67-2 is located &50m south of section WM97-67-1.#N. janthostoma; extralimital; presently ranges from Japan to Kamchatka.

Fig. 4. Glacial}marine drift exposed on south coast of HagemeisterIsland (WM97-71; location shown in Fig. 2). (a) Close up of massivestoney mud with fragmented and articulated molluscan shells. (b)Prominent white tephra bed traces the tight isoclinal folds in severelydeformed section. Shovel is 50 cm long.

The stratigraphic section on the southern coast ofHagemeister Island is lower (&30 m high), but betterexposed and more continuous laterally ('2 km) thanthe east-coast exposure. We measured several strati-graphic sections. The one described below is located at58333.55@N latitude, 161302.29@W longitude (WM97-71).It is subdivided into four main units (Fig. 3b):

(1) A compact, silty, diamicton is exposed above thecover at the foot of the blu! and extends up to &9 mabove sea level. Clasts are abundant, subrounded tosubangular pebbles to large cobbles of variable andexotic lithologies. The diamicton is probably till of anearly, undated advance.

(2) The lower diamicton is overlain by sorted, horizon-tally bedded, undeformed, sandy pebble and cobblegravel. The upper contact with the overlying deformedunit is sharp, cuts bedding within the gravel, and exhibitslittle relief. We interpret this contact as the basal thrustalong which the overlying, deformed unit was moved.This unit is probably outwash.

(3) The principal unit outcropping along the southcoast is a severely deformed, fossiliferous, stone-poordiamicton with at least one tephra bed. The matrix iscompact, "ne sandy silt. The clasts are rounded to suban-gular pebbles and cobbles of exotic lithologies. Locally,the diamicton contains zones of deformed gravel beds.The molluscs are dominated by the taxodont bivalvegenus Nuculana. The tephra bed is 5}30 cm thick, white,and forms tight isoclinal folds at the measured section(Fig. 4b); 0.6 km west of the section, the same tephra bedthickens to '1m and is laterally continuous across'100 m. Where the tephra thickens, it exhibits cross


Fig. 5. Composite vertical aerial photograph showing outer limit ofhummocky early Wisconsin (sl) drift and interglacial shoreline cut ontopre-glacial colluvium on the northwest side of Hagemeister Strait andshoreline near Cape Pierce (location shown in Fig. 1).

lamina. In both sections, the tephra bed overlies (?)a &5 cm thick bed of shell- and pebble-rich mud, which,in turn, overlies several centimeters of thinly bedded siltand sand. Most commonly, mollusc shells in the pebblymud are fragmented and decalci"ed so that only theircasts are preserved. Locally, they form a shell hash;elsewhere, they are whole or paired. The contact betweenthe tephra and the distinctive pebbly, shelly mud is con-formable as evidenced by stones that protrude into or aredraped by the tephra, depending on whether or not thebeds are overturned. We interpret the bed of pebbly,shelly mud as an interval of glacial}marine drift rewor-ked by currents, or perhaps by a subaqueous debris #ow,in a shallow-marine setting. The eruption of the tephrawas closely associated in time. We interpret the diamic-ton that comprises the bulk of the unit as glacial}marinedrift.

(4) The surface of the glacial}marine unit exhibits reliefof '5 m over distances of hundreds of meters, presum-ably representing primary deformational relief. Thepaleo-swales, like the one at the measured section,contain a sequence of basin-"lling deposits that in-cludes a basal peaty diamicton overlain by thinly beddedpeaty silt and "ne sand, grading into inorganic silt, andcapped by numerous tephra beds, peat, and moderntundra. The initial deposition was by hillslope colluvium,followed by lacustrine sedimentation in a shallow pond,giving way to eolian deposition on a relatively dry land-scape.

Evidence for more than one glacier advance ontoHagemeister Island is equivocal. Without an intact strati-graphic section that includes conformable, intervening,inter-glacial deposits, we cannot rule out the possibilitythat the erratics in the lower diamicton on the east coast,the overlying deformed unit that contains glacial}marinedrift, and the upper till, were all deposited during a singleglacial advance/retreat associated with one marine trans-gression/regression cycle. The strongest evidence for anolder advance is the glacial diamicton with overlyingundeformed outwash at the base of the south-coast expo-sure. These deposits indicated that a glacier advancedover the site, then retreated to deposit the overlyingoutwash, prior to the advance that deformed the overly-ing glacial}marine drift. The glacial}marine drift mayhave been deposited by the same glacier that sub-sequently deformed it; or, the deformation may be asso-ciated with a younger advance. There is no overlying tillto record such an advance on the south coast, however.Our glacial-geologic mapping indicates that, during theearly Wisconsin (sl ), ice extended down HagemeisterStrait and terminated before reaching Pyrite Point(Figs. 1 and 5). The thin ice deposited hummocky drift onthe west coast of Hagemeister Island, but did not ad-vance over the south coast of the island. In contrast, icefrom Togiak Bay did encroached onto the east coastwhere it deposited the upper till (Fig. 1).

The aIle/Ile ratios (discussed below) indicate that partof the deformed unit exposed on the south coast has beenoverturned. AIle/Ile ratios increase upward throughthree levels of the deformed unit below the tephra at siteWM97-70, whereas they decrease upward through twolevels below the tephra at WM97-71. Although we didnot yet attempt to map speci"c structures, we assumethat the spatial distribution of aIle/Ile ratios can beexplained by the well-expressed, severe deformation thatthe sediments have experienced.

3.1.2. GeochronologyThe relative and absolute ages of the middle Pleis-

tocene glacial}marine drift exposed on the eastern andsouthern coasts of Hagemeister Island (Fig. 2) wereevaluated using amino acid geochronology. AIle/Ile ra-tios were measured in 65 individual Nuculana shells col-lected from seven zones (each 1}4 m2 of exposure surface)within the main deformed unit. On the east coast, 19individual shells were analyzed from two zones on oppo-site sides of the prominent ravine that bisects the expo-sure (WM97-67-1 and 67-2; Fig. 2). On the south coast,46 shells were analyzed from "ve zones within the de-formed unit; three collections were made at one section(WM97-70) and two at another 0.6 km to the east(WM97-71). The mean aIle/Ile ratios from these collec-tions range from 0.102$0.016 to 0.175$0.021(Table 2).

The aIle/Ile data can be used to correlate and di!eren-tiate the relative ages of Nuculana shells. The aIle/Ileratios from the "ve south-coast collections cluster intofour distinct groups (aminozones), separated by gaps inthe distribution (Table 3; Fig. 6). The mean aIle/Ile ratiosin the two collections from the east coast correlatedirectly with two aminozones from the south coast.


Table 2Amino acid (isoleucine) epimerization ratios (aIle/Ile) in Nuculana from Hagemeister Island

Lab no. Field no. aIle/Ile! Aminozone"

(UAL) (WM97) XM p n1

2212 67-2B 0.128 0.016 10 II2213 67-1C 0.103 0.004 9 I2214/27 70A 0.102 0.016 10 I2215 70C 0.124 0.013 10 II2216 70D 0.144 0.013 9 III2217 71A 0.175 0.021 8 IV2219 71B 0.144 0.014 9 III

!Mean and standard deviation of `na shells (intershell variation); aIle/Ile values for the Inter-Laboratory Comparison Standards ILC-A, ILC-B, andILC-C analyzed at the Utah Amino Acid Laboratory (UAL) average 0.159$0.002, 0.551$0.004 , and 1.154$0.001, respectively; these are wellwithin the range measured for the same samples by other laboratories (Wehmiller, 1984)."Aminozones identi"ed on the basis of clustering of aIle/Ile results (see text and Table 3).

Table 3Amino acid (isoleucine) epimerization ratios (aIle/Ile) grouped byaminozone and used to estimate the age of Nuculana from HagemeisterIsland

Aminozone aIle/Ile! EstimatedXM p n age (ka)"

I 0.102 0.011 19 280$30II 0.126 0.014 20 350$40III 0.144 0.013 18 410$40IV 0.175 0.021 8 500$60

!Mean and standard deviation of `na shells (intershell variation)."Age estimate based on Eq. (1) (from Miller, 1985), using the mean

aIle/Ile for aminozone. Age uncertainty is based on $1p error of meanaIle/Ile for each aminozone. Both values are rounded to nearest 10 ka.

Fig. 6. Frequency histogram of amino acid (isoleucine) epimerizationratios (aIle/Ile) measured in shells from Hagemeister Island, south-western Alaska. Symbols and error bars represent the mean aIle/Ile$1p and are plotted for each shell collection; circles and trianglesrepresent south- and east-coast exposures, respectively (data listed inTable 2).

Clustering of aIle/Ile ratios from exposures separated byup to 20 km into similar groups indicates that the datare#ect the primary temporal distribution of episodicevents. We interpret each of the four aminozones as aninterval of marine inundation and each gap in the distri-bution of aIle/Ile ratios as an interval of emergence.

Aminozone I comprises samples WM97-67-1C(0.103$0.004) from the east coast, and WM97-70A(0.102$0.016) from the south coast. Their correlation issupported by the molluscan faunas of these two collec-tions that include the most common occurrence ofSerripes in the two exposures. Aminozone II includessamples WM97-67-2B (0.128$0.016) from the east coastand WM97-70C (0.124$0.013) from the south.Aminozone III is distinguished by its stratigraphic asso-ciation with the prominent white tephra bed and thepebbly, shelly mud layer. Shells collected from within0.5m below the prominent tephra at sites separated by0.6 km have remarkably similar aIle/Ile ratios (WM97-71B"0.144$0.014 and WM97-70D"0.144$0.013),lending con"dence to the geochronological integrity ofthe data. Aminozone IV is based only on sample WM97-71A (0.175$0.021) from the south coast, and does notappear to have a correlative in the east-coast exposure.The aIle/Ile ratios are signi"cantly higher in this collec-tion, suggesting that a considerable length of time separ-ates the deposition of these shells from shells with loweraIle/Ile ratios. Each aminozone is represented by erratic-bearing, massive sandy silt that we interpret as gla-cial}marine sediment. One of the shell collectionscontains de"nitive taxonomic evidence for non-glaciallyin#uenced waters (Natica janthostoma), but did not in-clude Nuculana; we are unable to correlate this collectionwith the aminostratigraphic results from other collec-tions.

To estimate the numerical age of the shells fromHagemeister Island based on their aIle/Ile ratios, weassume a reasonable postdepositional temperature and


Fig. 7. Comparison of the extent of amino acid (isoleucine) epimeriz-ation (aIle/Ile) in modern specimens of (a) Nuculana pernula andNuculana minuta, and (b) Nuculana pernula and Mya truncata, heatedsimultaneously in the laboratory at 1103and 1423C. Numbers aboveand to the right of data points are heating times in days. Error bars are$1p of multiple subsamples heated in the same tube. Lines showone-to-one relation.

apply this to an empirically derived equation that relatesaIle/Ile to time and temperature. Miller (1985) used la-boratory-heated and 14C-dated shells and a reversible"rst-order kinetic model to derive an age equation for themolluscan species Mya truncata:

t"Mln[(1#aIle/Ile)/(1!0.77aIle/Ile)]

!0.0194N/1.7710(L16.45-6141/¹), (1)

where ¹ is the e!ective diagenetic temperature (EDT; theweighted mean temperature that represents the integ-rated kinetic e!ect of all temperature #uctuations experi-enced by the sample) in K, and t is time in years.

Because the (apparent) rate of epimerization can bedependent upon taxonomy, and because Eq. (1) is basedon Mya, we performed high-temperature laboratory ex-periments to test whether isoleucine in Nuculana epimer-izes at the same rate as in Mya. Modern specimens of thetwo species of Nuculana (N. pernula and N. minuta) mostcommon in the Hagemeister Island sections were buriedin moist, sterilized sand, sealed in glass tubes, and heatedsimultaneously in a temperature-stable oven at 140and 1103C for 4 and 3 time steps, long enough to induceepimerization of aIle/Ile"0.20 and 0.11, respectively.AIle/Ile ratios at both temperatures, plus the initialaIle/Ile in unheated specimens, show no signi"cant di!er-ence between the two species of Nuculana (Fig. 7a) norbetween Mya and Nuculana (Fig. 7b). On the basis ofthese results, we conclude that Eq. (1) can be applied toNuculana with aIle/Ile (&0.2. A similar conclusion wasreached previously for the mollusc taxodont genus Por-tlandia (Kaufman et al., 1996).

As an approximation of the EDT experienced by theshells on Hagemeister Island, we use the value deter-mined for presumed last interglacial (Pelukian) shellscollected from eastern Bristol Bay (Kaufman et al., 1996;t"125ka; aIle/Ile"0.052; EDT"!5.43C). This EDTcharacterizes a full interglacial}glacial cycle, which weassume is a reasonable approximation for the last&1 Ma. Applying the EDT of !5.43C to the meanaIle/Ile ratios of the four aminozones yields age estimatesthat range from 500$60 to 280$30 ka for the oldestand youngest aminozones, respectively (Table 3). Therange of ages suggests that oxygen-isotope stages 8}13are represented on Hagemeister Island. The mean ages ofthree of the four aminozones coincide with odd-num-bered marine oxgyen-isotope stages (13, 11, 9), althoughother estimates are within the range of errors. The ageuncertainties were calculated using the $1p error aboutthe mean aIle/Ile for each aminozone; they do not takeinto account errors in the EDT estimate used to calibratethe age equation or in the age equation itself, and there-fore underestimate the true uncertainty. In the absence ofde"nitive lithostratigraphic evidence for multiple trans-gressions, we cannot exclude the possibility that onlya single highstand of sea level is represented. If so, then

the overall age estimate based on the entire range ofaIle/Ile ratios is roughly 400$100 ka.

Regardless of the these uncertainties, the aIle/Ile ratiosindependently of the age equation indicate that at leastsome of the marine deposits exposed on HagemeisterIsland correlate with the middle Pleistocene Anvilianmarine transgression. Deposits of this transgression at itstype locality near Nome on Seward Peninsula were pre-viously dated at between 580 and 280 ka, and were corre-lated with marine oxygen-isotope stage 11 (410 ka;Kaufman et al., 1991). AIle/Ile ratios measured in Myafrom the deposits in western and northern Alaska pre-viously ascribed to the Anvilian marine transgressionincrease in proportion with the current mean annualtemperature at each site (Fig. 8). These data suggest thatcorrelative deposits in the Bristol Bay region should haveaIle/Ile ratios of &0.14. Although we cannot be certainwhich aminozone on Hagemeister Island correlates withthe Anvilian transgression at Nome, it appears that de-posits of oxygen-isotope stage 11 are present.

The Quaternary deposits on Hagemeister Island in-clude multiple tephra beds. The thickest and mostlaterally continuous bed is exposed along the south coast


Fig. 8. Relation between current mean annual temperature and theextent of amino acid (isoleucine) epimerization (aIle/Ile) in molluscs atsites where middle Pleistocene marine deposits have been studied inwestern and northern Alaska. Trend line suggests that one of theaminozones on Hagemeister Island correlates with the Karmuk Mem-ber of the Gubik Formation at Barrow (Mya; Brigham-Grette andCarter, 1992), the Cape Blossom or lower Hotham Inlet Formation atKotzebue (Astarte converted to Hiatella based on transformation givenin Kaufman (1992) and Huston et al. (1990), and the type locality of theAnvilian marine transgression at Nome (Hiatella plus Mya; Kaufman,1992).

Fig. 9. Togiak tuya in the lower Togiak River valley (location shown inFig. 2). (a) View to the southwest. (b) Pillow lava at DK95-17 from&25 m below the west-central rim of the tuya (outcrop location(shown in Fig. 2b) is not visible in view). Note glassy rim and inter-pillow basaltic breccia. Hammer is 35 cm long.

where it forms a prominent white marker. S. Preece andJ. Westgate (University of Toronto) analyzed the major-element geochemistry of glass shards from this tephra,but did not "nd a correlative in University of Toronto'sdatabase of Late Cenozoic tephras from Alaska andwestern Canada. Tephrochronological analyses, includ-ing trace-element and "ssion track studies of the promin-ent white tephra, are continuing.

3.2. Younger Middle Pleistocene advance

3.2.1. Stratigraphic evidenceThe next, more recent advance is evidenced by pillow

lava of the Togiak tuya (Hoare and Coonrad, 1978b),a glacially streamlined volcano composed of basaltic tu!and lava. The tuya forms a prominent mountain in thelower Togiak River valley, &15 km northeast of thevillage of Togiak (Fig. 2 and 9). Its top is nearly #at, witha maximum elevation of 353m above sea level. It is 6 kmlong and 2.5 km wide, with its long axis parallel to theregional strike, the orientation of the valley-boundingfaults, and to the #ow of former glacier ice. The #atsummit is cut by two valleys that cross the tuya axis andprobably were formed by meltwater incision associatedwith glaciers postdating the eruption. The surface of thetuya is underlain by a series of olivine basalt #ows thaterupted subaerially. At the northern end of the volcano,the subaerial #ows overlie palagonitized glassy tu!s thatapparently record an explosive eruption phase involvingthe introduction of water into the volcanic vent (Hoare

and Coonrad, 1978b). Farther south, the subaerial cap-ping #ows overlie basalts exhibiting well-formed pillowstructure. A 5m thick bed of pillows is exposed alonga steep slope on the west side of the tuya &25m belowthe summit. The pillows are &1 m across and are rim-med by glassy palagonite; inter-pillow areas are "lledwith palagonitized basaltic breccia (Fig. 9b). These fea-tures strongly indicate that they formed subaqueously.To con"ne a water body at the mouth of the TogiakRiver valley that opens onto the continental shelf re-quires a glacier-ice dam. We argue, as did Hoare andCoonrad (1978b), that the pillow lava formed as the tuyaerupted through glacier ice and thawed an intraglaciallake into which the lava #owed. The elevation of thepillows indicates that the lake surface was 300m abovethe #oor of the lower Togiak River valley. Glacier icesurrounding the lake was therefore at least 300m thick.Because we have not identi"ed the drift associated withthis glaciation, the maximum extent of this advance isunclear. The tuya was overrun by glacier ice followingthe eruption, either during the same glaciation or duringa more recent advance, or both. The ice truncated thevolcano #anks, streamlined the southern end, and depos-ited a thin cover of drift and erratic boulders on itssummit.


Table 440Ar/39Ar laser-fusion data for the Togiak tuya and the Togiak Bay basalts

Sub-sample Cumulative 39Ar 40Ar/39Ar" 37Ar/39Ar 36Ar/39Ar Atmosph40

Ar (%)37Ca/39K 40Ar*/39Ar

KAge (ka)!

XM p XM p XM p

Sample UAF059-02; DK95-17; Togiak tuya (Fig. 2)1 0.14 2.370 5.023 0.008 85.7 9.25 0.12 0.335 0.072 296 642 0.28 1.510 5.113 0.005 80.5 9.41 0.06 0.290 0.081 256 723 0.43 1.959 5.056 0.007 85.7 9.31 0.08 0.278 0.070 246 624 0.59 1.399 5.267 0.005 82.3 9.70 0.08 0.243 0.073 215 645 0.84 2.263 5.123 0.008 86.8 9.43 0.06 0.297 0.053 262 476 1.00 2.526 5.623 0.009 87.6 10.35 0.05 0.312 0.041 276 36

Weighted average 263 22Sample UAF059-03; DK95-22; Togiak Bay basalt (Fig. 2)1 0.17 11.631 6.856 0.041 101.1 12.64 0.08 !0.132 0.176 !117 1562 0.38 13.133 5.213 0.046 100.5 9.60 0.03 !0.061 0.122 !54 1083 0.56 12.207 5.915 0.043 100.2 10.89 0.06 !0.028 0.097 !25 864 0.69 22.427 6.996 0.076 97.9 12.90 0.06 0.463 0.200 409 1775 0.86 11.576 5.616 0.040 99.3 10.34 0.07 0.082 0.105 72 936 1.00 12.222 6.187 0.043 99.2 11.40 0.11 0.093 0.205 82 181

Weighted average (excluding subsample 4) !5 49

!Ages calculated using the constants of Steiger and Jaeger (1977); weighted average of J from standards"0.000490$0.000002."Measured isotopic ratios corrected for blank and decay of 37Ar and 39Ar.

3.2.2. GeochronologyThe Togiak tuya is younger and petrologically di!er-

ent than the valley-#oor basalt that it overlies (Hoare andCoonrad, 1978b). An alkali-olivine basalt #ow on thevalley #oor 20 km north of the tuya is normally magnet-ized and was dated previously by K}Ar at 0.76$0.2Ma(Hoare and Coonrad, 1978b). Its columnar structureand medium-grained, nonporphyritic texture indicatethat it was erupted subaerially. The date provides a max-imum limiting age on the tuya eruption and dates aninterval of ice-free conditions in the lower Togiak Rivervalley.

To more closely date the tuya eruption, we performedsingle-step, laser-fusion 40Ar/39Ar analyses on a cappingbasalt #ow. The dated sample (DK95-17; Fig. 2) wascollected from near the base of the subaerial #ows, about17m above the well-exposed pillow lava described above.We choose to date basalt that erupted subaerially toavoid potential complications of incomplete degassing ofthe chilled pillows. In selecting subsamples for laserfusion, phenocrysts (another potential source of excessAr) were avoided and groundmass-rich subsamples weredated (cf. McDougall and Harrison, 1988). Six sub-samples yielded a weighted average age of 263$22 ka(Table 4). They averaged 85% atmospheric Ar content,which, together with visual inspection, indicates that thebasalt is not signi"cantly altered. The isotopic composi-tions of the subsamples varied only slightly, indicating anisotopically homogeneous sample, but precluding preciseisochron determination of the age. Because we found noevidence for deposition or erosion separating the pillowlava from the capping subaerial #ow, we believe that

40Ar/39Ar age closely limits the age of the glacial lake.The age also overlaps with the amino acid age estimate ofthe youngest aminozone on Hagemeister Island, indicat-ing that the eruption of the tuya may have partiallyoverlapped with the deposition of glacial}marine sedi-ment in Togiak Bay.

3.3. Early Wisconsin (sl) advance

3.3.1. Stratigraphic evidenceThe broad valleys of the Ahklun Mountains, and the

lowlands and continental shelf to the south, are underlainby a regionally extensive drift sheet that retains much ofits primary constructional/deformational relief. The driftcomprises a broad range of sediment types, but is domin-ated by deformed pro-glacial and non-glacial sediment ofoutwash streams and shallow marine embayments, andby ice-contact strati"ed drift. Several distinct stillstandsor readvances seem to be represented by the sequence ofice-stagnation moraines and glacially deformed com-posite ridges (cf. Manley et al., 2001). The drift delimitslow, #at piedmont lobes that spread out onto the conti-nental shelf of northern Bristol Bay, terminating morethan 100km from their source areas. The limit of the driftis clearly expressed as the upper extent of kame-kettletopography against the colluvium-mantled rolling foot-hills of the southern Ahklun Mountains and o!shoreislands (e.g., Fig. 5). The surface is mantled by a nearlycontinuous cover of loess that thickens and is underlainby a conformable sequence of pond deposits "llingpaleo-swales and basins (e.g., the upper 5 m at WM97-71,southern Hagemeister Island, Fig. 4b).


Table 5Dose-rate data for thermoluminescence-dated, lava-baked sedimentfrom Togiak Bay

Field no. (Fig. 2) DK95-22 DK96-48 DK96-33Lab no. (OTL) 575 630 672

a count (ks cm!2)! 0.31$0.02 0.40$0.02 0.31$0.02Th (ppm)! 2.8$0.5 2.9$0.5 3.0$0.5U (ppm)! 1.7$0.2 2.3$0.2 1.6$0.3Unsealed/sealed" 1.00 1.02 1.04K

2O (%)# 1.52$0.02 1.80$0.02 1.61$0.02

Moisture content (%) 15$5 15$5 15$5a-value$ 0.05$0.01 0.05$0.01 0.04$0.01Dose rate (Gy ka~1)% 2.07$0.14 2.40$0.16 2.16$0.15ED temperature range (3C) 250}350 250}350 250}400ED (Gy) 121.6$2.0 193.8$6.2 169.0$10.5Age estimate (ka) 59$5 74$7 78$8

!U and Th ppm values calculated from a count rate, assuming secularequilibrium."Ratio of bulk a count rate under unsealed and sealed counting

conditions; a ratio of '0.94 indicates little or no Ra loss.#K% determined by Activation Laboratory Ltd., Ontario.$Measured a e$ciency factor as de"ned by Aitken and Bowman

(1975).%Dose rate for OTL-575 was calculated by integrating the contribu-

tion of the overlying basalt and the underlying sediment; dose rates forboth samples include a 0.10$0.2 Gy ka~1 contribution from cosmicradiation (Prescott and Hutton, 1988).

We correlate this drift in the Togiak Bay region withthe Nushagak Formation in the eastern Bristol Bay areawhere Lea (1990) reported sedimentological evidencefrom Ekuk blu!, a composite ridge formed by the ad-vance of a piedmont lobe onto the Nushagak lowlandduring a time when relative sea level was close to itspresent-day level. Unlike the drift in the Nushagak Bayarea, however, we have not yet found de"nitive evidencefor glacial}marine sedimentation associated with thedrift in or around Togiak Bay. Therefore, the glacieradvances in the two areas may not be strictly syn-chronous.

3.3.2. GeochronologyThe sur"cial drift of the Togiak Bay region is younger

than the Old Crow tephra (140$10 ka; Westgate et al.,1990). We discovered the tephra for the "rst time in theBristol Bay region at three sites (Figs. 1 and 2): (1) on thenorth coast of Goodnews Bay, where reworked,pumacious lapilli of Old Crow tephra forms small-scalecross sets in well-sorted, "ne sand of a composite ridgethat is part of the drift sheet; (2) on the northwest coast ofHagemeister Strait, where a faulted and folded, &40 cmthick bed of Old Crow tephra underlies the drift; and (3)across the strait on the northwest coast of HagemeisterIsland, where a glaciotectonized, &15 cm thick bed ofthe Old Crow tephra similarly underlies drift. These sitesare located closer to the presumed source of the OldCrow tephra than any previously discovered. A "ssion-track age is currently pending and will be reported ina subsequent paper, along with the results of major-element analysis of the glass that were used to correlatethe tephra with the Old Crow (analyses by S. Preece andJ. Westgate, University of Toronto; and J. Riehle, USGeological Survey).

The drift is also younger than the last interglaciation((125ka), as indicated by (1) the absence of emergentshorelines or beach deposits atop the drift, despite thepresence of a presumed last-interglacial shoreline 2.5 kmbeyond the drift near Pyrite Point (Fig. 5); and (2) thesuperposition of the drift in southeastern Togiak Bayover emerged thinly bedded silt and "ne sand containingmarine diatoms (J. Smol, pers. commun.) and pollen ofinterglacial character (M. Edwards, pers. commun.),which we have tentatively correlated with oxygen-iso-tope substage 5e (Kaufman and Manley, 1996).

A closer maximum-limiting age is provided by newgeochronological data on a lava #ow that separates un-derlying non-glacial sediment from overlying drift. Thesharp and planar base of the 3}5 m thick basalt #ow iswell exposed in two sections separated by &1km alongthe coast of southeastern Togiak Bay (Fig. 2). At bothsites, the lava has baked the underlying sediment, whichis dominated by massive mud to thinly bedded silt, sand,"brous peat, and tephra. The basalt is vesicular andshows no evidence for phreatic conditions. The overlying

drift is bouldery with a sandy matrix, typical of theice-contact strati"ed drift that comprises the kames andeskers of the surrounding landscape. At site DK95-22, wedated the basalt by 40Ar/39Ar laser fusion technique andthe sediment directly adjacent to the lower contact of thebasalt using TL.

The results of Ar isotope analysis on the basalt, whichshowed visible evidence of alteration, were mixed. Five ofsix subsamples yielded ages that are not statisticallydi!erent from zero age with atmospheric Ar contents of'99% (Table 4). From the associated analytical error,the Ar data suggest a weighted average age of (50 ka,indicating that it is younger than the last interglaciation.A single subsample had an age of 409$177 ka, signi"-cantly older than the other subsamples. We suspect thatthis anonymously old apparent age re#ects incorpora-tion of excess Ar.

The inference that the basalt is younger than 400 ka issupported by the results of three new TL analyses. Bakedsediment adjacent to the lower basalt contact yieldeda TL age estimate of 59$5 ka (OTL-575; Table 5). Thisvalue is based on a dose-rate estimate that accounts forseparate radiogenic contributions from the overlyingbasalt and the underlying "ne-grained sediment, bothcon"gured in a hemispheric geometry. To avoid thiscomplication in the dose-rate calculation, we sub-sequently dated two other samples of baked sedimentfarther below the base of the basalt: OTL-672 was from&10 below OTL-575, where the radiogenic contributionfrom the basalt is negligible; it yielded an age of


Fig. 10. Additive-dose build-up curves for thermoluminescence analy-sis on lava-baked sediment sample OTA-630. Inset "gure shows equiva-lent dose (ED) for range of temperatures. A Corning 5-58 "lter was usedin front of the photomultiplier tube (Throne EMI 9635Q).

Fig. 11. Summary diagram showing geochronological data, extent ofglacier ice down the Togiak River valley (black shapes), and the marineoxygen-isotope record (Imbrie et al., 1984; numbers are oxygen-isotopestages). The marine d18O record is used as a proxy for eustatic sea level,which is probably above its average Quaternary position (-55m) duringintervals highlighted in light gray (cf. Porter, 1989). Ages and ageuncertainties on volcanics are represented by the hatched bars (Togiaktuya: 40Ar/39Ar data in Table 4; Old Crow tephra: "ssion track data inWestgate et al., 1990; Togiak Bay basalt: TL data in Table 5). Aminoacid age estimates and uncertainties indicated by circles with verticallines (data in Table 3); associated glacier advances are indicated by thedark gray polygons. The amino acid data suggest that the youngestglacial}marine drift may have been deposited at the same time as theeruption of the Togiak tuya. Extent of ice prior to 100 ka is uncertain.

78$8 ka. The second sample (OTL-630) was from 85 cmbelow the basalt #ow at an exposure (DK96-48) located&2 km southwest of the "rst site. This depth is wellwithin the zone of the 5003C isotherm below a 5m thickbasalt #ow as modeled by Forman et al. (1994). Theresulting age for this sample is 74$7 ka (Table 5;Fig. 10). We calculate the age and age uncertainty for thelava-baked sediment as the mean and standard deviationof the three ages, which overlap at $2p. The resultingage estimate is 70$10 ka and the overlying drift isyounger by some length of time.

The TL age estimate for the basalt below the drift inTogiak Bay overlaps with the recently published age forthe Nushagak Formation (Kaufman et al., 1996). In theNushagak Bay area, luminescence and amino acid geo-chronology converge on an age of 75}90 ka for the gla-cial}marine drift. In both the Nushagak and Togiak Bayareas, the age of the drift is beyond the limit of 14C dating('40 ka), as is shown by numerous analyses on woodwithin the organic-rich, swale-"lling deposits (Kaufmanet al., 1996; Manley et al., 2001). Because we do not knowhow much time separates the eruption of the basalt andthe deposition of the overlying drift at Togiak Bay, how-ever, we cannot exclude the possibility that the drift atTogiak Bay is somewhat younger than the drift inNushagak Bay. In either case, we assign this drift to theearly Wisconsin (sl), and recognize that it may includedeposits of multiple late Pleistocene advances that pred-ate the late Wisconsin (i.e., marine oxygen-isotope(sub)stages 5d}4).

4. Summary and conclusions

Our recent geochronologic and stratigraphic investi-gations of the southern Ahklun Mountains provide new

information on the timing and extent of Pleistocene gla-cier advances. We have identi"ed and estimated the agesof at least three, and as many as six, pre-late-Wisconsinglacial intervals (Fig. 11). The oldest are represented by


glacial}marine sediment on Hagemeister Island, whereamino acid data provide the basis for subdividing thecomplexly convoluted marine sediment into fouraminozones. Based on the age equation derived by Miller(1985), together with assumptions about the averagepostdepositional temperature, the mean age estimates ofthe four aminozones range from &500 to 280ka. Themean age estimates for three of the four aminozones arecentered over odd-numbered marine oxygen-isotopestages (13, 11, and 9), suggesting that they are associatedwith highstands of eustatic sea level. The aminozones areseparated by &75 ka, about the same length of time thatseparates interglacial stages 13}7 in the marine oxygen-isotope record (Imbrie et al., 1984). Given the large uncer-tainties associated with these age estimates, however,other correlations are possible. Furthermore, because thedeformed stratigraphic sequences lack unequivocal evid-ence for multiple transgressive cycles, we cannot excludethe possibility that fewer high-sea-level intervals are rep-resented. In the most conservative interpretation, theamino acid data suggest an overall average age of&400$100 ka for the glacial}marine drift onHagemeister Island.

The correlation of the aminozones with intervals ofhigh eustatic sea level is supported by our lack of evid-ence for signi"cance glacial-isostatic depression. Becausewe have not found erratics or other evidence of glaciationabove &180m on Hagemeister Island, we infer that theice that deposited the glacial}marine drift was relativelythin. This suggests that isostatic depression was limited(less than a few tens of meters) and, therefore, that theemergent marine deposits record a period of high eustaticsea level. The advances seem to have occurred whenglobal sea level was high enough to submerge the BeringSea shelf, perhaps during the intial states of continentalice sheet build up. The glacial-geologic evidence fromHagemeister Island supports similar evidence (e.g., Kauf-man et al., 1996; Brigham-Grette et al., 2001) from otherparts of central Beringia that the expansion of glacierswas out of phase with global ice volume.

Late in the middle Pleistocene, glaciers advanced to "llthe lower Togiak River valley to at least 300m above thevalley #oor. This advance coincided with the eruption ofthe Togiak tuya, dated at 263$22 ka by laser-fusion40Ar/39Ar, and possibly with the deposition of theyoungest glacial}marine deposits on Hagemeister Island.We presently lack evidence from anywhere in the BristolBay region for a glacier advance during marine oxygen-isotope stage 6. This absence suggests that glaciers in theAhklun Mountains remained relatively small duringstage 6. Stage 6 glacial deposits are apparently absent inthe Yukon Cordillera as well (Schweger, 1997), sugges-ting that they have been obliterated by subsequent ad-vances of equal or greater extent. In contrast, early in thelast glacial cycle, during the early Wisconsin (sl ), glaciersreadvanced down the Togiak River valley and overran

a basaltic lava #ow dated by TL at 70$10 ka. This isapproximately the same age, or slightly younger, than theNushagak Formation of eastern Bristol Bay (Kaufmanet al., 1996). Unlike the Nushagak Formation of easternBristol Bay, we have not yet found de"nitive evidence forglacial}marine sediments associated with this advance inwestern Bristol Bay. Glacier advances during the lateWisconsin were limited to the highest mountain valleyswhere glacier termini lay '40 km inland from the pres-ent-day coast (Manley et al., 2001).

The multiplicity of the glacial advances in the TogiakBay region is supported by glacial geologic mapping ofPorter (1967) who described four distinct drift sheets inthe Chagvan Bay area. However, we are unsure of how tocorrelate the glacial episodes discussed in this study withthose de"ned previously by Porter (1967) or with theNichols Hill drift of Lea (1989).

While our understanding of the glacial history of theAhklun Mountains is coming into focus, the interpreta-tions presented here will no doubt be updated andre"ned as new information becomes available. Forth-coming cosmogenic-isotope ages on moraine surfaces,radiometric ages, and geochemical analyses on tephrabeds, will improve the geochronological control. In addi-tion, sediment cores taken from lakes beyond the limit oflate Wisconsin glaciers will enable more detailed recon-structions of environmental changes through the lastglacial maximum.

Acknowledgements

Louie Marincovich (California Academy of Sciences)identi"ed the marine molluscs; Shari Preece and John Wes-tgate (University of Toronto), and James Riehle (US Geo-logical Survey) analyzed the major-element chemistry of thetephras; Warren Coonrad (formerly with US GeologicalSurvey) suggested sites to visit; Matthew Pachell (UtahState University) assisted in the "eld; the Dillinghamo$ce of the Togiak National Wildlife Refuge providedcritical logistical support; Twin Hills Native Corporationallowed access to their land. Thomas Hamilton and Kris-tine Crossen suggested improvements to the manuscript.This research is supported by the National Science Foun-dation, Arctic Natural Sciences (OPP-9529940).

This is PARCS contribution 155.

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