Chlorine-36 and 14C chronology support a limited last glacialmaximum across central Chukotka, northeastern Siberia,
and no Beringian ice sheet
Julie Brigham-Grette,a,* Lyn M. Gualtieri,b Olga Yu. Glushkova,c Thomas D. Hamilton,d
David Mostoller,e and Anatoly Kotovf
a Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USAb Quaternary Research Center, Box 351360, University of Washington, Seattle, WA 98195-1360, USA
c Northeast Interdisciplinary Research Institute, Far Eastern Branch Russian Academy of Sciences, 16 Portovaya St., Magadan 685000 Russiad U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA
e Weston & Sampson Engineers, Inc., 195 Hanover Street, Suite 28, Portsmouth, NH 03801, USAf Chukotka Science Center, Anadyr, Chukotka Region, Russia
Accurate reconstructions of the glacial history of theRussian arctic are essential if we are to further understandthe paleoclimatic history and global context of the circum-arctic. Glacial ice extent across the Russian arctic has beenamong the more controversial topics facing Arctic Quater-nary scientists over the past decade. The idea of a pervasiveEurasian ice sheet (Grosswald 1988; 1998) during the lastglacial maximum (LGM) has been strongly challenged byboth Russian and European workers as part of the EU-
QUEEN program (European Union-Quaternary Environ-ments of the Eurasian North; Svendsen et al., 1999; Thiedeet al., 2001, and references therein). The concept of an EastSiberian Sea ice sheet (Grosswald and Hughes, 1995, 2002)has been partially adopted by Peltier (1994) in the ICE-4Gice-sheet reconstructions and perpetuated in climate modelsas boundary conditions for the LGM (cf., Felzer, 2001).This has happened despite a growing body of field evidenceagainst such an ice sheet (Arkhipov et al. 1986; Vartanyanet al. 1993, Felzer, 2001; Glushkova, 2001; Gualtieri et al.,2000; 2001). The concept of a Beringian ice sheet during theLGM covering all of Chukotka Peninsula and the EastSiberian Sea (Grosswald, 1998) has also been perpetuated inthe literature (Kotilainen and Shackleton, 1995; Clark et al.,
1999) despite longstanding arguments for only valley gla-ciers throughout the region (Glushkova, 1984; 1992).
This paper refutes the notion of a Beringian or EastSiberian Ice Sheet and adds significant new knowledge tothe body of field-based research documenting the restrictedextent of late Quaternary glaciation in western Beringia. Wedescribe the physical evidence for and geochronology ofQuaternary glaciation of the Pekulney Mountains andTanyurer River valley, central Chukotka (Figs. 1 and 2). Weselected this region for our study because it was repeatedlyglaciated and satellite images and aerial photographs revealwell-preserved moraines (Glushkova, 2001; Heiser andRoush, 2001); however, the sequence lacks a numericalchronology. Here we describe the character of the morainesand derive their geochronology from cosmogenic isotopeand radiocarbon age estimates, soil properties, and fluvialterrace development. Aside from this study, the only other36Cl-based chronologies of glaciation that exists in Beringiaare that of Gualtieri et al. (2000) for the Koryak Mountainsin Russia and Briner et al. (2001) for the Ahklun Mountainsin Alaska (Fig. 1). The results from our study are placedwithin the context of the glacial history of northwest Alaska(Kaufman and Hopkins, 1986; Hamilton, 1994) and theBering Strait region of central Beringia (Brigham-Grette,2001; Brigham-Grette et al., 2001).
Physiography and previous work
The Pekulney Mountains are located �200 km northwestof Anadyr, the capital of Chukotka, located on the northwestedge of the Bering Sea. The mountain range reaches analtitude of 1359 m creating a drainage divide between theAnadyr River to the west and the Tanyurer River to the east.It consists of late Jurassic to early Cretaceous island arcvolcanic and plutonic rocks. The Tanyurer River drains anarea of 9750 km2 north of the Anadyr River that is boundedby the Pekulney Mountains to the west and low hills of theSvetney Mountains to the north and east (Fig. 2). Glushkovaand Sedov (1984) mapped the surficial geology of the re-gion at a scale of 1:780,000 using satellite photos andtraditional aerial photos at a scale of 1:50,000. These mapscontributed to regional Quaternary syntheses for northeastRussia first published in English by Velichko et al. (1984)and Arkhipov et al. (1986). Glushkova (1992) later mappedand assigned relative ages to two sets of moraines in theTanyurer Valley. Relative ages were assigned based uponoverall moraine morphology, relative morphostratigraphicposition and the number and height of fluvial terraces.Moraines close to the eastern mountain front were assigneda Sartan age and large end moraines at the southern end ofthe Tanyurer River, abutting the Anadyr River, were as-signed a Zyryan age. Heiser and Roush (2001) demonstrated
the reproducibility of this work by mapping moraine com-plexes across most of remote Chukotka using syntheticaperture radar imagery (SAR). Their work supports theearlier Russian work by identifying three sets of morainesrepresenting glaciation of variable extent. Using brightnesscontrasts of the imagery, they estimated ice-proximal andice-distal slope angles of moraines and, by comparison withSAR images of well-studied moraine sequences in Alaska,suggested ages of middle Pleistocene, Zyryan (early Wis-consinan), and Sartan (late Wisconsinan/LGM) for nestedsequences found regionally across Chukotka.
Our work builds on the mapping of these previous stud-ies by placing a numerical chronology on the deposits.Lacking such a chronology, Grosswald (1998; Grosswaldand Hughes, 1995, 2002) and Hughes and Hughes (1994)have arbitrarily and repeatedly assigned an LGM age to themost extensive moraines in this region to support theirhypothesis of a Beringian Ice Sheet (variously named EastSiberian Ice Sheet, or just part of their pervasive RussianArctic Ice Sheet).
Methodology
We field checked the nested moraines on Glushkova’s(1992) map during a foot traverse of the Kuveveem Rivervalley followed by rafting 125 km down the Tanyurer Riverfrom the mouth of the Kuveveem River to the Anadyr River(Fig. 2). Within a few kilometers of either side of both riverswe conducted morphometric studies on the moraines fol-lowing the recommendations of Kaufman and Calkin (1988)and Peck et al. (1990) with an emphasis on slope angles,moraine crest width and axial relief. Erratics on morainecrests and glacially scoured bedrock were sampled for ex-posure dating using cosmogenic 36Cl.
In general, we collected cosmogenic samples from theupper 5–10 cm of the highest, flattest and most stableerratics, selecting sample sites as far away from the edges ofthe rock as possible so as to minimize the loss of 36Clproduced by thermal neutron activation. Samples werechemically prepared and AMS (atomic mass spectrometer)measurements were performed at PRIME Lab/Purdue Uni-versity. Ages were calculated using RICH (rock in situproduced cosmogenic-nuclide history; Dunne et al., 1996),a program that uses a calcium production rate of 73.4 � 5atoms/gram/year, a potassium production rate of 154 � 10.2atoms/gram/year, and a surface thermal neutron flux (scaledfor elevation and latitude) of 307 � 24 neutrons/g-rock/yr inorder to determine the production rate of 36Cl from neutronabsorption by 35Cl. Based on historical snow data forAnadyr and Markova, corrections were made for 91 days of50-cm thick snow cover/year in all samples. Dip and hori-
Fig. 1. Central Beringia, showing place names mentioned in text. Kg � Kigluaik Mtns., Bd � Bendeleben Mtns. PM � Pekulney Mtns. Dashed gray lineindicates the extent of hypothesized Beringian Ice Sheet (Grosswald and Hughes, 2002).
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zon-shielding corrections were minimal in all cases, butwere also accounted for in final age calculations. For detailsregarding the use of cosmogenic isotopes as a surface ex-posure dating technique see papers by Phillips et al. (1986,1996); Lal (1991); Zreda and Phillips (1994, 1995); Stone etal., (1996, 1998); Gosse and Phillips (2001); Swanson andCaffee (2001).
We dug test pits down to permafrost (about 75 cm depth)on all moraines and examined soil profiles for rubification,texture, dry consistency, and horizon thickness as an indi-cation of relative-age. In general, these pedogenic propertiesbecome more strongly developed on older landscapes. Theirtrends are best quantified using the profile developmentindex (PDI) of Harden (1982) based on the reasonableassumption that all of the land surfaces in the Tanyurerregion have experienced the same climate in similar topo-graphic settings. Loess was the most common parent mate-rial for soils examined on moraine crests with small differ-ences in vegetation cover. In general and in a relative sense,higher PDI values are characteristic of older land surfaces.We also logged the stratigraphy of fluvial and glaciofluvialterraces in accordance with standard stratigraphic tech-niques and sampled organic horizons for radiocarbon ageestimates.
The regional glacial stratigraphy
On the basis of their relative position and availablegeochronology, the moraines of the Tanyurer Valley aresubdivided here into three sets consistent with the regionalcompilation of Glushkova (2001). The following discussionis organized by reviewing the glacial stratigraphy, morphol-ogy, and geochronology from north to south, from theyoungest to oldest moraines, along the Kuveveem andTanyurer rivers (Fig. 2).
Kuveveem valley moraines
The youngest glacial sequence in the region is welldefined by a set of three nested, concentric moraines foundextending down the valley of the Kuveveem River but wellwithin the Pekulney Mountain front (A on Fig. 2, Fig. 3a).A fourth subdued recessional moraine occurs about 4 km upvalley from the terminal moraine complex. This moraineforms the head of outwash for paired terraces 8–10 m arl(above river level) that cut through the three nested mo-raines and flank the modern Kuveveem River. The nestedmoraines are sharp in relief with maximum proximal anddistal slope angles of 22° and a mean slope angle of 15°.Axial relief along the crest is high and crest width variesfrom 9–11 m. Ancient meltwater channels that traverse themoraines are floored by fresh well-rounded boulders andcobbles (Fig. 3b). Left- and right-lateral moraines consistingof till and ice marginal colluvium can be easily traced for 7km up-valley from the outermost terminal moraine (Fig.
3a). Soils examined on the crest of these moraines in tillshowed little reddening and horizon development resultingin a PDI average of 3.5.
Large stable basalt erratics on moraine crests were sam-pled for 36Cl cosmogenic isotope dating. Four glacial errat-ics from a lateral and terminal moraine yield ages rangingfrom 8500–23,600 yr (Fig. 4). The youngest age of 8500 yr(95S15) is spurious and probably represents a recently ex-humed boulder due to frost action or one fallen onto thelateral moraine from the adjacent high slopes. The remain-ing three ages, however, are from stable sites on the termi-nal moraines. These data cannot be used to differentiatebetween the age of glacial maximum and the initiation ofdeglaciation, since these events could differ by a few thou-sands of years. Rather, we interpret the three ages from16,200 yr to 23,600 yr (95S14, 95S16, and 95S18) to reflectthe time of stabilization of the moraines. The oldest age of23,600 yr is slightly older than the other two ages, but wehave no geological or analytical reason to eliminate thissample. It is entirely possible that either the oldest samplecarries some minor inheritance or that the younger bouldersare slightly younger due to subtle moraine erosion, howeverthis does not change our interpretation.
14C dating of youngest terraces
A prominent outwash terrace extends laterally from theinnermost recessional moraine and through the terminalmoraines at �10 m above the modern river level (m arl).This fill terrace can be traced �20 km down the KuveveemRiver to the Tanyurer River and is geomorphically thoughtto be only slightly younger than the outermost moraine butcoincident in age with the inner moraine (head of outwash).Radiocarbon age estimates were made from several samplesof plant detritus excavated from exposures of slackwater siltand organic pods preserved in coarse gravels on the northside of the river (Fig. 5 and Table 2 ). Near the outermostmoraine front, millimeter-length wood fragments in glacio-tectonically deformed gravels yielded an AMS age of 18,130 � 440/� 430 cal yr B.P. (GX-21531). About 1.1 kmdown river from the outer terminal moraine (Fig. 2, site 1)a thin 2–3 cm organic-rich silt containing plant macrofossilswas excavated from a zone at 6.5 m above the river. A bulksample from the silt consisting of floodplain bryophytic peatyielded an age of 21,500 �2750/�2050 14C yr B.P. (GX-21,525). Subsequently, two AMS ages on angiosperm twigsfrom the same sample yielded ages of 19,290 � 360 cal yrB.P. (AA-25666) and 18,590 � 420/-410 cal yr B.P. (AA-25,667). Detrital organics (wood and plant fragments) col-lected along the exposure at the same level from a fine-grained pocket of sand in coarse gravels yielded a bulk ageof 18,290 � 520 cal yr B.P. (MAG 1502). At a second sitelocated 90 km down river from the intersection of theKuveveem River with the Tanyurer River (Fig. 2, site 2), a10-m arl terrace consisting of well-sorted gravel overlain bymassive fine silt and sand lenses enclosed detrital organic
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pods yielding an age of 23,860 � 1410 14C yr B.P. (GX-21,530). The coarser sandy soils on this prominent terrace inseveral locations show reddening of an 18- to 33-cm-thick Bhorizon. These data contribute to PDI values from 5 to 14.
Below the 10-m arl terrace a lower fluvial terrace is
present at �6 m arl along the length of the Tanyurer River(Fig. 5). This is a fill terrace consisting largely of well-stratified sand and silt; macrofossils are common. Radiocar-bon ages on individual pieces of wood and detrital organicsat three sections yielded ages between 6930 and 13,370 cal
Fig. 3. a. Two of the three moraine ridges forming the terminal moraine complex in the Kuveveem River valley. The left lateral moraine can be seen extendingup valley. Chlorine-36 ages of 8500 yr (95S15) and 16200 yr (95S14) were determined from erratics on matching moraines from the opposite side of thevalley. b. Broad meltwater channel dissecting the moraine complex. Chlorine-36 ages of 23,600 yr (95S16) and 16,700 yr (95S18) were determined fromerratics on matching moraines from the opposite side of the valley.
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yr B.P. (Table 2; GX-21532, GX-21526, GX-21533, GX-21534). The stratigraphy and palynology of these exposureswill be described in a paper elsewhere; however, the age ofthis terrace helps to constrain the age of the 10 m arl terrace.
Tanyurer valley moraines
East of the Kuveveem River, the broad valley of theTanyurer River contains the morphologic and stratigraphicevidence of at least two glacial advances that were morespatially extensive than the glacial landforms describedabove (Fig. 2). The first set of moraine ridges is locatedroughly halfway down the valley (south end of terrainmarked as B on Fig. 2). North of these moraines the land-scape is dominated by dead-ice topography characterized byirregular hills, hummocks, and numerous interveningkettled and thermokarst lakes. These moraines and land-scape to the north constitute what we define as theChumyveem River moraine complex.
The second set of extensive nested moraines is located atthe southern end of the Tanyurer Valley. Here along thenorth side of the Anadyr River broad concentric morainesmark the terminal position of several lobes of ice that oncefilled the entire Tanyurer Valley. The glacier once abuttedthe Rarytkin Mountains to the south blocking drainage ofthe Anadyr valley creating a large proglacial lake over thearea surrounding Lake Krasnoye (Glushkova, O. Yu., per-sonal communication, 1995). These moraines and landscapeconstitute what we define as the Anadyr River moraines.
Chumyveem river moraine complex
The landforms and stratigraphy of the central TanyurerValley were investigated along the Tanyurer River with afocus on fluvial terrace exposures as well as moraines dis-sected by the river. In a few places it was also possible to
study exposures of moraine cross-sections and drumlinizedbedrock uplands within a few kilometers of the river.
The large concentric moraines in this part of the valleycan be traced for tens of kilometers; they outline two ormore former ice lobes which emanated from the PekulneyMountains and the southern flank of the Svetney Mountains(B on Fig. 2). Numerous morphologic features indicate thatthese moraines are much older than those investigated inKuveveem River valley. Moraine crests are 130–150 mwide and are smoothly graded, with no perceptible reliefalong their crests or down their gentle (1° to 3°) flankingslopes that merge with broad solifluction aprons at theirbases. No surface erratics protrude through the thick loesscover, which extends below the permafrost table at 75 cmdepth. Soils show deep reddening to 36 cm depth, and havePDIs in the range of 17 to 36.
Other landscape features that have been overriden by icein the central axis of the valley also suggest significant age.Tors and altiplanation terraces have been eroded out byperiglacial activity from ice-molded upland surfaces, and aridge streamlined in the direction of ice flow is cut bytransverse notches as much as 2.8 m deep and 4.2 m widethat have weathered out along joint planes (Fig. 6). Lateralcutting by the Tanyurer River has eroded the drift sheet toa width of several tens of kilometers in places, and as manyas three major fluvial terraces at heights up to 52 m aboveriver level are present along this sector of the valley.
Because erratic boulders were absent from moraine sur-faces, we sampled an erratic granite boulder resting on avesicular basalt outcrop from an ice-molded bedrock sur-face about 85 m above the valley floor. The granite yieldeda 36Cl age of 47,800 yr (95T28) and the basalt an age of40,000 yr (95T29). These may be minimum ages because ofthe apparent greater relative age of this drift sheet andbecause greater minimum ages have been obtained on cor-relative drift from nearby localities (see below).
Roughly 30 km downstream at site 4 (Fig. 2), sampleswere collected from an ice-moulded upland (120–126 melevation) on the west side of the river. The granitic erraticat this location yielded a 36Cl age of 41,500 yr (95T36). Theerratic is well rounded, and in a geomorphologically stableposition. However, it was difficult to obtain a sample fromthe center of this rock and as a result, an edge sample wastaken. Due to its size (14 cm in height, 41 cm in width and75 cm in length) the rock has probably experienced pro-longed periods of soil and/or snow cover, partially shieldingit from cosmic-ray bombardment. Due to these factors weinterpret this age to be a minimum age estimate.
Three other samples from this upland (site 4) were takenon glacially moulded bedrock (Fig. 6). The samples yieldages ranging from 55,500 to 69,470 yr (Fig. 4; Table 1). Weinterpret these ages to be minimum estimates on deglacia-tion, since the bedrock had to have been fully exposed(uncovered by ice) before cosmogenic isotopes could accu-mulate.
At site 5 (Fig. 2), Sample 95T41 (36,600 yr: Fig. 4 and
Fig. 4. Chlorine-36 ages from the Kuveveem and Tanyurer river valleys.Error bars as given in Table 1 are shown. All samples were corrected fordip, horizon, and snow shielding.
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Table 1) was taken from one of the largest erratics found inthe valley (15 cm in height, 16 cm in width, 35 cm in length)located on a moraine with a narrow, flattened, over-steep-ened crest and oversteepened flanking slopes of 10° com-pared to slope angles of 1–2° for other moraines in thecentral Tanyurer River valley (Mostoller, 1997; Heiser andRoush, 2001). Field observations of the moraine morphol-ogy at this site indicated that the crest of this moraine hadlikely been reworked and the slopes over-steepened byfluvial processes: hence the age confirms our field observa-
tions and is much younger than that of the moraine at thetime of its deposition.
The 6-m and 10-m fluvial and glaciofluvial terraces de-scribed in the previous section are stratigraphically insetinto two higher terrace remnants at 16 m arl and 23 m arlalong the central Tanyurer River (Fig. 5). The highest ter-race is preserved in only a few places at 23 m arl and nostratigraphic exposures could be located along its erodedslopes. The second terrace at 16 m arl is poorly preservedalong the length of the river. However, it is inset against one
Fig. 5. Schematic terrace morphostratigraphy in the Tanyurer Valley. The low Holocene terrace is found along the length of the valley; however, the higherolder terraces are more fragmented with increasing age. The oldest (highest) terrace at 23 m was identified along the river at only one location withoutadequate exposures. Topographic maps at 1:100,000 suggest that other surfaces at approximately this level exist away from the river. Ages are given in calyr B.P. years and are found in Table 2.
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of the older moraines dissected by the river system. Theterrace at site 5 (Fig. 2) consists of sandy stratified alluviumenclosing a well-preserved 30-cm-diameter log of cotton-wood (Populus suaveolens) 14C dated to �42,380 14C yrB.P. (GX-21527, Table 2). Peaty organic lenses from thesurrounding sand gave ages of �36120 14C yr B.P. (GX-21,528) and �36,000 14C yr B.P. (MAG 1504). The pollenassemblage from these same peaty organic lenses is inter-glacial in character, but we cannot exclude the possibility
that the terrace dates from the Karginsky interstade (middleWisconsinan) (Lozhkin, A.V., personal communication,1996).
Fossil wood was also found at the base of an undisturbedterrace located 3.5 km up-river from the confluence of theChumyveem River with the Tanyurer River (Fig. 2, site 6)and yielded an age of �42,980 14C yr B.P. (GX-21529).Because the Chumyveem River cuts across the kettled andhummocky terrain north of the moraines in the central
Fig. 6. Ice-moulded bedrock with postglacial joint etching, central Tanyurer River valley (site 4, Fig. 2). Samples 95T32, 95T33, 95T34 and 95T35 were takenfrom bedrock at this site and sample 95T36 was taken from a granite erratic at this locality (see Table 1).
Table 1NR/S ratios and 36Cl ages for the Kuveveem and Tanyurer River valleysa
a Based on a calcium production rate of 73.4 � 5 atoms/gram/year and a potassium production rate of 154 � 10.2 atoms/gram/year (Zreda et al. 1991).b Indicates sample taken from basalt bedrock.c 35Cl/37Cl ratio for chloride in the AMS sample.d Mass of chloride added by carrier.e 35Cl/37 ratio from AMS of chemical processing blank for chloride in AMS sample.f Mass of chloride in carrier in chemical processing blank.g Post-leach calcium concentration.h Post-leach potassium concentration.
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Tanyurer valley, our radiocarbon age provides additionalevidence for the antiquity of this glaciated landscape be-yond the range of radiocarbon dating.
Anadyr river moraines
The most extensive glacial advances preserved in theTanyurer River valley are defined by subtle lobate morainesbordering the north side of the Anadyr River at the southernend of the valley (C on Fig. 2). The moraines, which stand60–80 m arl, have proximal and distal slopes of �2°, asestimated by Heiser and Roush (2001) and later confirmedby us in the field. The moraines are mantled with thickloess, and surface erratics are absent. Their surfaces arecovered with thickets of stone pine (Pinus pumila) and a soilpit at one site in a clearing revealed a well-developedB-horizon used to define a PDI of 25.
Like the moraines, the landscape north of them is sub-dued with fewer kettle lakes than found on the glaciatedlandscape north of the intermediate age moraines (Fig. 2).The broad moraines across this area can be clearly traced onair photos and topographic maps eastward for 30 km to thenorth of Anadyr Bay. At its maximum extent, the ice form-ing these moraines abutted the north flank of the RarytkinMountains and impounded a large glacier-dammed lake inthe Anadyr River valley, of which the present-day LakeKrasnoye may be a small remnant. Study of the sedimentsin this lake could significantly add to our understanding ofthe paleoenvironment and age of these moraines.
The age of these extensive moraines has been controver-sial. Glushkova (1984) suggested a Zyryan age based on the
limited number of nested terraces found north of the mo-raines. Heiser and Roush (2001) suggested a middle Pleis-tocene age based on a comparison of brightness featureswith moraines of similar relative age in Alaska. In contrast,Grosswald (1998) and Grosswald and Hughes (1995) referto these same extensive moraines as LGM and used them asevidence for East Siberian Sea ice sheet based on theirhypothetical ice sheet modeling. We have no direct agecontrol on these extensive moraines except by relative po-sition to numerically-dated sites to the north (36Cl and 14Cages). Therefore, we believe based on morphometric crite-ria, the moraines are likely middle Pleistocene in age orolder.
Discussion
The results of this work demonstrate that the LGM in thePekulney Mountains was characterized by the developmentof valley and alpine glaciers-not a pervasive ice sheet.Cosmogenic isotopic and radiocarbon ages show that theKuveveem Valley moraines dates to �20,000 yr ago andtherefore was deposited during Sartan (LGM) time (A onFig. 2). We interpret cosmogenic isotope ages � 30,000 yras well as non-finite radiocarbon dates to imply an age noyounger than Zyryan (early Wisconsinan or older) for theChumyveem River moraine complex found in the centralTanyurer Valley (B on Fig. 2). Morphometric analyses,landscape character, and relative loess cover are all consis-tent with this interpretation. The most extensive AnadyrRiver moraines (C on Fig. 2) at the south end of the
Table 2Radiocarbon age estimates from the Pekulney Mountains and Tanyurer River valley
Site Lab IDa Field ID Material dated 14C age(14C yr B.P.)
a Laboratories: AA, NSF-Arizona AMS Facility; GX, Krueger Enterprises, Inc./Geochron; MAG, NEISRI-Magadan, Russia Laboratory.b Macrofossil identifications by John McAndrews, Royal Ontario Museum, Canada.c Calibrated using CALIB. 4.3 (Stuiver and Reimer, 1993)
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Tanyurer valley are even more subdued implying an evenolder, perhaps early middle Pleistocene age; however, thisage estimate is less constrained.
The relative-age and numerical age estimates providedhere for the glacial and fluvial stratigraphy of the TanyurerRiver are consistent with the Pleistocene history knownover most of Chukotka as well as western and northern partsof Alaska. In Chukotka, Glushkova (2001) has also demon-strated that the most extensive glacial advances marked byclear moraines were middle Pleistocene in age and, they lietens of kilometers beyond well-dated moraines of Sartanage. On outer Chukotka Peninsula, Brigham-Grette et al.(2001) have demonstrated that two ice advances, both olderthan the LGM but younger than marine oxygen isotopestage 11, extended beyond the modern Russian coast. Gual-tieri et al. (2000) showed that the most extensive well-preserved moraines in the northern Koryak Mountains southof Anadyr are at least Zyryan in age. Across Chukotka,Sartan ice was limited to cirque and small valley glaciers inlocal mountain systems (Ivanov, 1986; Glushkova, 2001,and unpublished maps).
In the Ahklun Mountains (Briner et al., 2001) the Ben-deleben and Kigluaik mountains of Seward Peninsula(Kaufman and Hopkins, 1986), and the western BrooksRange (Roof, 1995; Huston et al., 1990; and Hamilton,1994) the most extensive well-preserved glacial depositswere not formed during the LGM but during the MiddlePleistocene. Especially on Seward Peninsula, valley glaciersextended some 10–30 km beyond the early WisconsinanSalmon Lake moraines and some 15–40 km beyond therestricted Mt. Osborn (LGM) ice limits (Kaufman and Hop-kins, 1986). The late Pliocene and early Pleistocene glaci-ations were even more extensive than the middle Pleisto-cene in many areas. In the Alaskan Brooks Range, Hamilton(1994, and references therein) demonstrated this point withabundant evidence for scattered erratics, drift patches, andU-shaped valleys with late Tertiary fills. He hypothesizedthat the moisture source for these oldest large valley glaci-ations may have been a seasonally ice-free Arctic Ocean.
Data presented here demonstrate that Grosswald’s hy-pothesis (1988, 1998, Grosswald and Hughes, 1995, 2002)for a Beringian Ice sheet during the LGM is invalid. Instead,we prove in this study that the extensive moraines he usedfor suggesting Beringian Ice sheet limits are in fact, ofmiddle Pleistocene age or older. Our study confirms thatLGM ice was limited to local mountain areas (cf., Heiserand Roush, 2001).
Limited LGM glacial ice extent across Chukotka andnorthwest Alaska is consistent with paleoecological andgeomorphological evidence for aridity over most of centraland western Beringia (Carter, 1981; Hopkins, 1982; Lozh-kin et al., 1993; Anderson and Brubaker, 1994; Edwards andBarker, 1994; Mock et al., 1998; Bartlein et al., 1998;Felzer, 2001). Sancetta et al., (1985) suggested that sea icepersisted 9 months of the year over the deep Bering Sea.Modeling by Seigert et al., (2001) suggested that the height
and distribution of the Scandinavian and Barents/Kara icesheets during the LGM deprived northern arctic Russia andwestern Beringia of moisture to support large ice sheets. Inthe model, westerly winds lacked adequate moisture overthe emergent subcontinent of Beringia to grow an ice sheet.Increased continentality owing to emergence of the Bering-Chukchi shelves and long-lasting seasonal sea-ice coverover the deep Bering Sea limited the advection of moisturefrom the south. Despite lower regional temperatures assuggested by regional pollen evidence (Bartlein et al.,1998), effective moisture was limited and the climate wastoo dry.
Why were ice sheets across Chukotka and central Ber-ingia more extensive during earlier glaciations, especiallyduring parts of the early and middle Pleistocene? Thislargely remains an unanswered question. However, it mustindicate a significant change in available moisture from thewesterlies, the North Pacific and perhaps the Arctic Ocean.Such a major expansion of ice requires a more dramaticchange in the oceans and the atmosphere, a system we stilldo not fully understand.
Conclusions
The Pekulney Mountains and Tanyurer River valley arekey regions for examining the nature of glaciation acrossmuch of northeast Russia. Based upon radiocarbon and 36Clage estimates we have shown that alpine-style glaciationwas extremely limited during the Sartan Glaciation (ca.20000 yr ago) across Chukotka, as demonstrated in theKuveveem River. During a preceding ice advance that ourcosmogenic and 14C data indicate was at least Zyryan (earlyWisconsinan) or mid-Pleistocene in age, glacial ice ex-tended 140 km down valley of the Sartan limit. Valleyglaciers emanating from regional mountain systems weremost extensive during an earlier glaciation that we infer tobe of at least middle Pleistocene age. This research providesdirect field evidence against Grosswald’s (1998; Grosswaldand Hughes, 2002) conjecture that there was a Beringian IceSheet during the LGM. It is our opinion that this hypothesisshould finally be abandoned.
Acknowledgments
This research was funded by National Science Founda-tion-Office of Polar Programs Grant 9423730 and BeringianHeritage Funds from the National Park Service to Brigham-Grette. We thank our Russian host institutions, namely theNortheast Interdisciplinary Research Institute, Magadan,and the Chukotka Science Center, Anadyr, for their in-kindsupport of our research, especially arranging permits andaccess to Chukotka. Patricia Heiser and the Alaska SARFacility provided SAR images. The University of ArizonaAMS Facility, Krueger Enterprises. Inc./Geochron, and the
396 J. Brigham-Grette et al. / Quaternary Research 59 (2003) 386–398
NEISRI-Magadan Laboratory determined the radiocarbonage estimates. Chlorine-36 cosmogenic isotope chemicalpreparations and AMS analyses were performed by PRIMELab/Purdue University. John McAndrews, Royal OntarioMuseum, Canada, kindly did macrofossil identifications forthis project. We thank Robert Ackert and John Gosse fortheir constructive review and suggestions for improve-ments.
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