Transgressions: rethinking Beringian glaciation

ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 110 (1994) 275 294

Transgressions: rethinking Beringian glaciation Beverly A. Hughes a, Terence J. Hughes b "Institute for Quaternary Studies, University of Maine, Orono, ME 04469, USA b Institute for Quaternary Studies and Department of Geological Sciences, University of Maine, Orono, ME 04469, USA • Received 7 December 1992; revised and accepted 9 December 1993

Abstract

The purpose of this investigation is to encourage a fresh look at Pleistocene Beringia. Heretofore, flooding of Bering Strait has been cited as the only barrier to migration, with marine sea transgressions being a "sea gate" that closed off migration during glacial interstadials and interglaciations. However, the possibility exists that glacial advances were also barriers, with marine ice transgressions being an "ice gate" that closed off migration during glacial stadials and glacial maxima. This possibility proceeds from the Marine Ice Transgression Hypothesis (MITH), which states that marine ice sheets form on the broad Arctic continental shelf of Northern Hemisphere continents when sea ice thickens, grounds and domes in shallow water, and then transgresses landward as continental ice sheets and seaward as floating ice shelves (Hughes, 1987). Landward transgression is onto coastal lowlands. During Pleistocene glaciations, a marine ice sheet extending from Spitsbergen to Greenland may have transgressed the circumpolar continental landmass at its lowest and narrowest gap, central Beringia, and calved into the Pacific Ocean. Four models of Beringian glaciation are presented, based on the distinction between marine glaciation and highland glaciation. Central Beringia was glaciated only in highlands in the traditional model (Hopkins et al., 1982), was also glaciated by a self-sustaining ice shelf floating over the deep ocean basins of the Bering Sea in the model by Grosswald and Vozovik (1984), was glaciated by a marine ice sheet that covered highlands, the continental shelf, and supplied the ice shelf in a model for maximum Pleistocene glaciation, and was glaciated by a marine ice sheet in the Chukchi Sea that merged with highland glaciers, transgressed the continental shelf of the western Bering Sea, and calved into the southern Bering Sea along the edge of the continental shelf in a model for the last glaciation. Field tests are suggested to assess the viability of these four models. The first model is already established for highland glaciation in Alaska, but less established in Siberia. The last model should be the easiest to evaluate for marine glaciation. The last model limits human migration across the Beringian land bridge to brief intervals between stadials and interstadials of the last glaciation cycle, when both the ice gate and the sea gate were opened to human migration. This model can influence the sea change now underway among Quaternary scientists studying peopling of the Americas, based on the archaeological, linguistic and ethnic diversity among native American populations. I. Introduction

The Beringian land bridge between Eurasia and the Americas is shown in Fig. 1. It is now closed, following marine transgression when sea level rose during the last deglaciation. Marine transgressions traditionally refer to flooding by water. However, the Beringian land bridge m a y also have been 0031-0182/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(93)E0208-B

closed during m u c h of the last glaciation by marine ice transgressions (Hughes, 1987). A characteristic o f the last glaciation is that its m a x i m u m southern extent apparently was produced by marine ice transgressions onto lowlands. Laurentide ice was most extensive beyond the H u d s o n Bay Lowlands south o f H u d s o n Bay (Mayewski et al., 1981). Scandinavian ice was

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276

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Fig. 1. Beringian location map. Bathymetry contours are from the 200 m isobath to present-day sea level (from Ackerman, 1988, fig. 2). Identified by letters are the Gulf of Anadyr (A), Kotzebue Sound (K), Norton Sound (N), Bristol Bay (B), Bristol Trough (BR), Bering Trough (BE), Herald Canyon (H), Saint Lawrence Island (SL), Wrangel Island (/,tl) and the New Siberiari Islands (NSI).

most extensive on the North European Plain south of the Baltic Sea. Although data are scanty, Astakhov and Isayeva (1988) and Grosswald and Goncharov (1991) maintain that Siberian ice was most extensive in the West Siberian Lowland south of the Kara Sea. Of all lands bordering the Arctic during the last glaciation, the lowest was central Beringia, which was then a land bridge between Siberia and Alaska, but today lies beneath the shallow water of the Chukchi and Bering seas. If marine ice transgressions were indeed a dominant feature of the last glaciation, as has been proposed

(Denton and Hughes, 1981; Hughes, 1987), with a continuous marine Arctic Ice Sheet that covered polar continental shelves from Spitsbergen to Greenland and transgressed onto the circumpolar continental landmass where the landscape was lowest, did it breach that landmass at its lowest and narrowest gap, central Beringia, and pour directly into the Pacific Ocean? This question can be addressed in four ways. First, how strong is evidence for a marine Arctic Ice Sheet during the last glaciation? Second, what is the relationship between marine glaciation and

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highland glaciation in Beringia? Third, what evidence supports Beringian glaciation? Fourth, what are the consequences for peopling of the Americas if the Beringian land bridge was closed by an "ice gate" during stadials of the last glaciation and by a "sea gate" during interstadials and during the Holocene interglaciation?

2. Marine ice sheets in the Arctic

The very existence of marine ice sheets in the Northern Hemisphere has been disputed. Holocene raised beaches on islands on Arctic continental shelves provided the first dated evidence; specifically in the Barents Sea of Arctic Europe (Schytt et al., 1968) and on the Queen Elizabeth Islands of Arctic Canada (Blake, 1970). Boulton (1979) challenged the concept of a marine ice sheet in the Barents Sea, based on his view that glaciation of Spitsbergen did not even reach the present shoreline on the west coast. However, Mangerud et al. (1992) have shown that Spitsbergen ice reached the edge of the western continental shelf during the last glacial maximum, in essential agreement with the glacial reconstruction by Isaksson (1992). Moreover, extensive seismic mapping and sample coring of the Barents Sea floor have demonstrated conclusively that a marine ice sheet existed during the last glacial maximum (A. Elverhoi and Solheim, 1983; Solheim and Kristoffersen, 1984; Vorren and Kristoffersen, 1986; Vorren et al., 1988; J. Elverhoi et al., 1990; Weinelt et al., 1991; S~ettem et al., 1992; Gataullin et al., 1993). England (1976, 1987) and England and Bradley (1978) cited weathering characteristics on perched moraines and other glacial geological evidence to argue against a marine Innuitian Ice Sheet over the Queen Elizabeth Islands, especially Ellesmere Island. However, Blake (1977, 1978, 1987, 1992), Weideck (1978), Hudson (1983) and De Freitas (1990) have shown conclusively that thick marine ice connected Ellesmere Island and Greenland, with marine ice streams flowing out both ends of Nares Strait. The only present-day marine ice sheet is grounded on the continental shelf of Antarctica. It is called the West Antarctic Ice Sheet because it

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is grounded in the Western Hemisphere, but it becomes afloat in the embayments of the Ross Sea and the Weddell Sea to become the Ross Ice Shelf and the Filchner-Ronne Ice Shelf, respectively (Van der Veen and Oerlemans, 1987). By analogy, a former marine ice sheet grounded on the Arctic continental shelf should have become afloat over the deep basins of the Arctic Ocean and perhaps of the Norwegian, Greenland and Labrador seas, and of Baffin Bay. If this happened, the marine ice sheets and their ice shelves would have been an Arctic Ice Sheet that behaved as a single dynamic system, just as the Antarctic Ice Sheet does today (Hughes et al., 1977; Denton and Hughes, 1981). Disintegration of an ice shelf in the Norwegian and Greenland seas was proposed by Mercer (1969) to explain Younger Dryas cooling in Europe. Broecker (1975) proposed an ice shelf as much as 2000 m thick over the Arctic Ocean during the last glacial maximum to explain oxygen isotope records. Using their finite-element model of ice-shelf dynamics, Lindstrom and MacAyeal (1986) concluded that an ice shelf in the Norwegian and Greenland seas could have extended as far south as Iceland during the last glaciation. Lindstrom (1990) used a finite-element model to reconstruct the last glaciation cycle in Eurasia, in which a marine ice sheet on the Arctic continental shelf spread southward onto the mainland and spread northward as an ice shelf that thinned from 1500 m thick over the Chukchi foreland to 1000 m thick through Fram Strait. Vogt et al. (1993) have reported iceberg plowmarks from north to south through Fram Strait on the sea floor at depths of 450 m to at least 850 m. However, the distribution of Greenland erratics in sediment cores from Fram Strait (Spielhagen, 1992) and Canadian erratics in sediment cores from the western Arctic Ocean (Bischof and Clark, 1992) is not mapped well enough to allow an ice shelf to be distinguished from icebergs imbedded in sea ice as the means of transport. More coring should reveal a dispersal pattern that is characteristic of either ice-shelf flow or pack-ice drift. However, Jones (1993) has shown that the entire Arctic Ocean, from the Chukchi Foreland to Fram Strait, was abiotic from 33 to 13 ka, with minimal biotic activity from 13 to 8 ka. An ice shelf is

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much more likely to impose an abiotic environment than is sea ice, which always has leads. Marine ice transgressions seem to occur early in the glaciation cycle, before highland ice caps reach the shores of polar seas. Boulton and Clark (1990) maintain that the earliest Laurentide glacial geology was produced by marine ice transgression onto Keewatin from the direction of Foxe Basin in Arctic Canada. Kleman (1992) shows from glacial geology in Sweden that early highland glaciation along the Scandinavian mountains did not reach the Gulf of Bothnia, which was the marine center of the late Weichselian Scandinavian Ice Sheet. Grosswald (1980, 1984a,b, 1988)contends that Russian glacial geology is explained only if marine ice transgressed onto the North Russian Plain from the Barents Sea and onto the West Siberian Lowland from the Kara Sea during the last glaciation cycle, with highland glaciation on the Russian mainland restricted to the northern Ural Mountains, the Taymyr Peninsula, the Putorana Plateau and the mountains of eastern Siberia. However, the Taymyr Peninsula and the Putorana Plateau were later overridden by marine ice from the Kara Sea. Grosswald et al. (1992) describe glacial geology, including dated sediments in the lake of a hill-hole pair produced by glacial tectonics, indicating that marine ice from the Laptev Sea transgressed onto land southeast of the Lena River delta during the last glacial maximum, whereas highland glaciation in the nearby Verkhoyansk Mountains did not extend to such low elevations.

3. Marine glaciation and highland glaciation Beringia lies between two very cold and dry Arctic environments that nonetheless supported marine ice sheets during the last glacial maximum, the Queen Elizabeth Islands to the east and the Laptev Sea to the west. In contrast, mainland Beringian glaciation was restricted to mountain highlands in northeastern Siberia and in Alaska (Hopkins, 1982; Hamilton and Thorson, 1983). Mountain glaciers reached the sea only along the southern coast because they were nourished by moisture bearing winds from the Pacific Ocean, as predicted by the highland origin, windward growth

hypothesis of glaciation proposed by Flint (1971). These contrasts allow the possibility that a marine ice sheet may have existed on the continental shelf of central Beringia, beginning in the Chukchi Sea, passing through Bering Strait into the Bering Sea, and calving into the Pacific Ocean, as proposed by Hughes (1987) in the marine ice transgression hypothesis of glaciation. Marine ice sheets begin when sea ice thickens and grounds on shallow continental shelves. Before grounding, thickening is mainly by bottom freezing. A corehole through the Ronne Ice Shelf in Antarctica shows that the lower 395 m, in ice 465 m thick, was by bottom freezing (Engelhardt and Determan, 1987). After grounding, marine ice thickens by surface accumulation of meteoric ice. Highland glaciation in eastern Beringia is illustrated schematically in Fig. 2, which is a cartoon transect across Alaska along 152°N. Along this transect, the Alaska Range rises from the present-day coastline in the south, the Yukon River crosses a central lowland and the Brooks Range rises in the north, beyond which a coastal lowland continues to the Beaufort Sea. The equilibrium line altitude (ELA) of highland glaciers tends to rise from the south to the north, and did so throughout the last glaciation (Ostrem, 1973; P6w6, 1975; Porter et al., 1983). This phenomenon is explained by the reduction in precipitation as the environment changes from maritime to continental and from temperate to polar toward the north, which is counter to the global trend for the ELA to lower from the equator to the poles (Robin, 1988; Broecker and Denton, 1989; Pelto, 1992). The phenomenon can be quantified by

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Fig. 2. The relationship between the equilibrium-linealtitude (dashed) and snowlines(solid) for Alaskaeglaciers.Snowlines are shown for subpolar maritime(SM), subpolar mixed(SX), polar mixed (PX) and polar continental (PC) climateswhen moving from south to north across the Alaska Range (A), the Yukon River (Y), and the BrooksRange (B).

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postulating stacked snowlines for input into glaciation models (Pelto et al., 1990; Fastook and Hughes, 1991; Hughes, 1992). These snowlines all dip from the equator to the poles and, from lowest to highest, characterize the mass balance of glaciers for temperature maritime (TM), subpolar maritime (SM), subpolar mixed maritime-continental (SX), polar mixed maritime-continental (PX) and polar continental (PC) climatic regimes (Pelto et al., 1990). Fig. 2 depicts stacked snowlines for the SM, SX, PX and PC climatic zones across Alaska, so their intersections with the south (SM) and north (SX) slopes of the Alaska Range and with the south (PX) and north (PC) slopes of the Brooks Range allow the ELA on glaciers moving down these slopes to be higher toward the north. This left central Alaska unglaciated during the last glacial maximum, even though all four snowlines were lower. This situation restricted highland glaciation in both eastern and western Beringia, although larger late Pleistocene snow accumulation rates mapped by Grosswald et al. (1986) in northeastern Siberia allowed more extensive glaciation of western Beringia. Conditions for glaciation could have been quite different in central Beringia, as illustrated schematically in Fig. 3. In eastern and western Beringia, snowlines intersect glaciers moving down steep mountain slopes. A lower snowline during the last glacial maximum allows glaciers to move further

NORTH

SOUTH

Fig. 3. Responses of mountain glaciers (top) and a marine ice cap (bottom) to lowered snowlines. As the snowline lowers from the solid line to the dashed line, mountain glaciers advance reversibly, but the ice cap advances irreversibly toward the pole and, depending on the dip of the snowline, either reversibly or irreversibly toward the equator (see Weertman, 1961; Hughes, 1992).

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downslope until their ablation zone becomes wide enough to restore mass-balance equilibrium with their widened accumulation zone. In central Beringia, on the other hand, glaciation begins when sea ice thickens and grounds on the shallow continental shelf. Since the surface of the grounded ice is virtually at sea level, summer meltwater ponds and freezes during the following winter instead of draining away. Therefore, even though the snowline is above the ice surface, all precipitation is logged as ice accumulation until the grounded ice domes enough to allow meltwater runoff and genuine ablation around its perimeter. Unlike mountain slopes, however, the continental shelf is essentially flat. When the snowline lowers, the marine ice dome advances to the edge of the continental shelf in directions where the snowline dips. The snowline dips to the north in Fig. 3, so the marine ice dome must advance northward to the edge of the continental shelf because its ablation zone narrows to the north. If central Beringia has the stacked snowlines shown in Fig. 2 and postulated for eastern and western Beringia, the marine ice dome advances southward to the edge of the continental shelf because its ablation narrows to the south. Weertman (1961) has computed the span of an ice sheet on a flat bed for various accumulation and ablation rates when the ELA changes during a glaciation cycle. The critical condition for a central Beringian marine ice sheet is grounding of sea ice over the entire Chukchi Sea, with runoff of summer meltwater being much less than ice accumulation so that the marine ice dome becomes high enough to intersect the snowline. If the ELA rises to the south, southward advance of the marine ice sheet will be determined by how fast ice can funnel through Bering Strait before it fans out and melts in the northern Bering Sea. If the ELA rises to the north, southward advance of the marine ice sheet will be halted only by calving in deep water at the edge of the Bering Sea continental shelf. The controlling condition in both cases is a low, broad marine ice sheet that pulls down the ELA in central Beringia and accumulates ice faster than it sheds meltwater. Whether it dips to the north or the south, the ELA in Beringia is determined in part by

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Milankovitch insolation cycles shown in Fig. 4 for the last 300,000 yr. Insolation is controlled primarily by Earth's axial tilt cycle of 41,000 yr in latitudes above 75 ° and primarily by Earth's axial precession cycle of 23,000 yr in latitudes below 50 °. Beringia lies between 75 and 50°N so both tilt and precession regulate insolation. Fig. 4 also shows oxygen isotope variations in marine microfossils, which are primarily a measure of global ice volume and secondarily a measure of sea-surface temperature (Prentice and Matthews, 1988). The southern margin of a central Beringian marine ice sheet would be sensitive to both ice volume and sea-surface temperature. Changing ice volume changes sea level, which regulates the ice calving rate of ice ablation by calving (Hughes, 1992). Sea-surface temperature changes the mass balance of an ice sheet (Fastook and Prentice, 1994). Therefore, advance and retreat of the southern margin of a marine ice sheet in central Beringia can be at least as rapid as the oxygen isotope fluctuations in Fig. 4, which seem to correlate with insolation variations. However, advance and retreat may be even faster and less predictable if they correlate with more frequent and irregular indicators of abrupt sea level and climatic change, ~lgO (°/oo) m

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Fig. 4. A comparisonof insolationat 75 and 50°Nwith oxygen isotope stratigraphyfrom the V19-29 core for the last 300,000 yr (produced by James L. Fastook for Denton and Hughes, 1983.)

such as Dansgaard-Oeschger events in coreholes through the Greenland Ice Sheet (Paterson and Hammer, 1987; Johnsen et al., 1992; Taylor et al., 1993) and Heinrich events in sediment cores from the North Atlantic Ocean (Heinrich, 1988; Broecker et al., 1992; Andrews and Tedesco, 1992). Mrrner (1984a,b,c, 1987) argues that abrupt short-term climatic changes on a scale from 10 to 1000 yr are regional rather than planetary, and are caused primarily by a "planetary beat" of the Sun, Earth, Moon system. This beat causes interactions between Earth's rotation rate and gravitational deformations of Earth's equipotential surfaces in ways that redistribute heat on Earth's surface by changing oceanographic circulation. The primary equipotential surfaces are the ocean surface, the surfaces of phase transformations in Earth's mantle, and the mantle-core interface, all of which are affected by the redistributions of ice and water mass on Earth's surface during a glaciation cycle. He maintains that the climatic record of oxygen isotope stratigraphy in Fig. 4, although it correlates with minor insolation changes caused by cycles of Earth's axial tilt and precession, may be forced by Earth's geophysical response to the planetary beat. That beat causes the tilt and precession cycles, but also causes short-term, more abrupt climatic forcing linked to atmospheric carbon dioxide variations controlled by changing rates of nutrient upwelling, as oceanic circulation responds to crustal isostatic adjustments (Mrrner, 1987). Southern Scandinavia has a classic record of regional climatic changes having amplitudes ranging from 1 to 10°C, but with periods between 50 and 150 yr that point to a common origin in the planetary beat (M6rner, 1984a,b,c). Eastern Beringia also seems to have a record of rapid regional climatic change (Hamilton, 1991). Scandinavia and Beringia occupy the same latitude band. A marine ice sheet occupied the Baltic Sea in the center of Scandinavia. Did one occupy the Chukchi and Bering seas in central Beringia? The Grande Pile bog in central France has a pollen record spanning 140,000 yr. During the last glaciation its floral record was strikingly similar to the Beringian flora (Woillard, 1978; Colinvaux, 1989), and the Scandinavian Ice Sheet lay just to the north. Therefore, Beringian flora need not preclude

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an ice sheet in central Beringia. In addition, a marine ice sheet in central Beringia would have been a precipitation sink for moisture-bearing winds from the North Pacific, thereby enhancing the arid climate of central and northern Alaska that was part of the "mammoth steppe" proposed by Guthrie (1989, 1990).

4. Models for Beringian glaciation The possibility of short-term abrupt regional climatic changes in Beringia is important because dating Alaskan Quaternary history has depended heavily on amino-acid geochronology, which requires long-term temperature stability (Miller and Brigham-Grette, 1989). Extensive glaciation would tend to lower and stabilize regional temperatures in unglaciated parts of Beringia, thereby making amino-acid geochronology more reliable in these regions. Without a marine ice sheet in central Beringia, Beringian climate would be more erratic and amino acid dates more problematic. Amino acid dates from sites under the marine ice sheet would be dependent on the basal ice temperature, which would be comparable to the nearsurface permafrost temperature where the marine ice sheet transgressed onto land. Four glaciation models will be examined. The traditional model of Beringian glaciation is presented in Fig. 5, and is defended by Hopkins (1982), updated for Alaska by Hamilton (1991). During Pleistocene glaciations, Beringia was an arid grassy landscape that has no counterpart today, and which Guthrie (1989, 1990) calls the "mammoth steppe" because it supported large grazing animals that are mostly extinct. Hibbert (1982) has traced the history of this concept, beginning with Russian paleoecologists working in western Beringia. Fig. 5 shows the ice extent in a mammoth steppe environment during the penultimate glaciation and the last glaciation, according to Hopkins (1973). Subsequent work showed that some glacial geology assigned to the penultimate glaciation was earlier, so the maximum ice extent in Fig. 5 is the maximum extent of Quaternary glaciation, but its synchroncity is now in doubt (Hamilton, 1991). Using various

Fig. 5. Beringia during the last glacial maximum according to the generally accepted view. (Modified from Kaufman et al., 1991, fig. 1). The maximum extent of the last glaciation and the maximum extent o f Pleistocene glaciation are shown by solid and dashed hatchured lines, respectively. In Figs. 5-8, hatchured lines enclose glaciated areas, black areas are glacially impounded lakes, white areas are unglaciated land, dotted areas are abyssal ocean basins, heavy lines are present-day shorelines, light lines are shorelines at glacial maxima, icesheet elevations are contoured at 500 m intervals, ice-sheet flowlines are perpendicular to ice-sheet elevation contours, and dashed flowlines are on floating ice shelves.

dating criteria, Huston et al. (1990) assigned an age between 600 and 500 ka to Baldwin Peninsula, a glacial tectonic push moraine at the eastern end of Kotzebue Sound. This moraine was the terminus of glaciers originating in the Brooks Range northeast of Kotzebue Sound. Anderson (1988) published pollen records from lakes showing that the intervening region has been unglaciated for the last 28,000 yr. Late Cenozoic glaciation in the Brooks Range of Alaska advanced northward repeatedly, coming within 30 km of the modern Beaufort Sea coast (Dinter et al., 1990). Glaciation of Seward Peninsula advanced into Norton Sound between 580 and 280 ka, according to radiometric dating by Kaufman et al. (1991). Ice advanced into Bristol Bay from the surrounding mountains

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during the Pleistocene (Detterman, 1986), but not during the last glaciation, when the intervening lowlands were covered by eolian sand sheets and other nonglacial deposits (Lea, 1989, 1990; Lea and Waythomas, 1990; Lea et al., 1991). In the traditional model of Beringian Glaciation, glaciers from the Chukchi Peninsula of easternmost Siberia reached northwestern Saint Lawrence Island in the Bering Sea, but not during the last glacial maximum (Ivanov, 1986; Brigham-Grette et al., 1992; Heiser et al., 1992). Since mammoth teeth on Wrangel Island date from 20 ka, the Chukchi Sea would probably have been unglaciated but the northern lowland of Wrangel Island is covered by marine sediments, so it has rebounded isostatically from an earlier glaciation (Vartanyan et al., 1993). Seasonal sea ice covered the deep ocean basin of the southwest Bering Sea during the last glaciation, according to diatom stratigraphy (Sancetta, 1982; Sancetta and Robinson, 1983; Sancetta et al., 1985). Biological activity apparently has been continuous in this region for at least the last 50,000 yr (Morely and Robinson, 1986). The Quaternary glacial history in western Beringia cannot be correlated readily with the glacial history of central and eastern Beringia, in large part because Russian work has not been widely disseminated (Brigham-Grette and Hopkins, 1992). The most recent attempt at correlation was by Brigham-Grette and Carter (1992). The prevailing view, summarized by Biruykov et al. (1988), is that western Beringia was glaciated only in the highlands during the last glacial maximum, but highland glaciers on Kamchatka Peninsula reached sea level owing to high precipitation on mountain slopes facing the Pacific Ocean. Grosswald and Vozovik (1984) and Grosswald (1988) challenged the conventional view of glaciation in western and central Beringia. Citing the higher snow precipitation rates in western Beringia compared to eastern Beringia during Late Pleistocene glaciations (Grosswald et al., 1986), they proposed substantially more glaciation of western Beringia during the last glacial maximum, and postulated a thick ice shelf floating over the deep ocean basin of the southwestern Bering Sea that was pinned to islands in the Aleutian chain and was grounded against the Bering Sea continen-

tal shelf, as shown in Fig. 6. Grosswald and Vozovik (1984) cited the Ronne Ice Shelf of Antarctica as a present-day analog. It floats in the Weddell Sea between the Antarctic Peninsula and the Antarctic mainland, and is comparable in size with the proposed Bering Sea ice shelf, which would have floated in the Bering Sea between the Alaska Peninsula and the central Beringian mainland. Moreover, much of the Ronne Ice Shelf thickness is by basal freezing of seawater (Crabtree and Doake, 1986; Engelhardt and Determan, 1987). Rutford Ice Stream supplies the Ronne Ice Shelf and occupies a deep trough at the base of the Antarctic Peninsula, which Grosswald and Vozovik (1984) noted to propose a similar glacial origin for the deep submarine troughs north of the Alaska Peninsula. They proposed a fringe of marine ice domes along the southern continental shelf of central Beringia, and mountain glaciers along the Alaska Peninsula and the Kamchatka Peninsula, as the source of ice supplying their Bering Sea ice shelf, along with heavy snow precipitation on the ice-shelf surface. In addition, based

Fig. 6. Beringia during the last glacial maximum according to Grosswald (1988). A glacially impounded lake in the Bering Sea has been added to Grosswald's reconstruction.

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on glacial tectonics in and around the New Siberian Islands, Grosswald (1988) postulated a marine ice sheet in the Laptev, East Siberian and Chukchi seas that transgressed onto the Siberian mainland, and merged with a highland ice cap on Chukchi Peninsula. An ice shelf floating over the deep ocean basin of the southwestern Bering Sea is contrary to a seasonal sea ice cover deduced by Sancetta (1982), Sancetta and Robinson (1983) and Sancetta et al. (1985) from the diatom record. Marine ice domes along the southern continental shelf of central Beringia would expand northward if the ELA for a single snowline dipped northward, as shown in Fig. 3, in which case the ice shelf would probably lie in an ablation zone and would not form (Oerlemans and Van der Veen, 1984). If the ELA for stacked snowlines dipped southward, as shown in Fig. 2 for Alaska, the marine ice domes would have impounded water from Alaskan and Siberian rivers and the northeastern Bering Sea would be a huge lake accumulating lacustrine sediments during much of Quaternary glaciation cycles. There is no widespread record of lacustrine sediments in central Beringia (Ackerman, 1988). Strong evidence for marine ice transgression from the Laptev Sea during the last glacial maximum exists southeast of the Lena River delta (Grosswald et al., 1992). The only evidence against a marine ice sheet in the East Siberian and Chukchi seas at that time is a 20 ka date on a single mammoth tooth on Wrangel Island (Vartanyan et al., 1993). However, glacial erratics are widespread on Wrangel Island and glacial through valleys cross Chukchi Peninsula from north to south (M.G. Grosswald, pers. comm., 1992). Present knowledge allows a marine ice sheet on the Siberian continental shelf of the Laptev, East Siberian and Chukchi seas during Quaternary glaciation cycles, including the last glacial maximum in the Laptev Sea and the western East Siberian Sea. Additional dating on Wrangel Island is necessary to establish conclusively that it was not glaciated at the last glacial maximum. Conclusive evidence for or against a marine ice sheet will require coring into the Pleistocene on the Arctic continental shelf of Siberia. An ice shelf floating over the deep basin of the southwestern Bering

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Sea during the last glacial maximum is incompatible with the microfossil record, but it remains a possibility for earlier glaciation cycles (Morely and Robinson, 1986). Fig. 7 presents a model for possible maximum Pleistocene glaciation of Beringia (Hughes et al., 1991 ). At least six Late Cenozoic marine transgressions have been discovered on the Arctic Coastal Plain of Alaska, and several other transgressions when sea level was lower have been discovered on the continental shelf of the Beaufort Sea (Dinter et al., 1990; Brigham-Grette and Carter, 1992). The oldest two, called the Colvillian and the Bigbendian transgressions, are dated between 2.7 and 2.48 Ma and at about 2.48 Ma, respectively, and were from 40 to 60 m and from 35 to 60 m above present-day sea level. Both transgressions may reflect postglacial isostatic rebound or late Cenozoic tectonic activity associated with the Barrow Arch. Rebound is favored by the presence

Fig 7. Beringia during the greatest Pleistocene glaciation if snowline lowering was controlled by the insolation cycles in Fig. 4 and snow precipitation was augmented by evaporation from glaciallyimpounded lakes. The extent of marine glaciation is produced by the snowline lowering that produces the maximum highland glaciation in Fig. 5, and is adapted from the ice-sheet reconstructionby Hughes et al. (1991).

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of glacial erratics in the marine sediments. Tectonism is favored by the presence of Pacific molluscs, indicating that the regional climate was too warm to allow extensive glaciation. The Fishcreekian transgression was between 2.48 and 2.14 Ma, when Late Cenozoic glaciations had begin, and left marine deposits containing dispersed Canadian erratics at elevations of 25-35 m, with contorted bedding suggestive of glacial tectonics. This is the first post-tectonism evidence that a marine ice sheet may have developed on the continental shelf of the Beaufort and Chukchi seas, and transgressed onto the Arctic Coastal Plain of Alaska far enough to isostatically depress the bed so that marine sediments were deposited before isostatic rebound was complete, with these sediments subsequently contorted by glacial tectonics during the following marine ice transgression. Relatively warm marine water transgressed onto a much colder Arctic Coastal Plain during the Fishcreekian transgression. These conditions favored glaciation of the Brooks Range. The Wainwrightian transgression, between 540 and 158 ka, left marine deposits with erratics at elevations between 20 and 25 m that may represent as many as three separate transgressions over this timespan. This was probably the timespan when the maximum glaciation of Beringia occurred, when glaciers from the Brooks Range produced Baldwin Peninsula in Kotzebue Sound (Huston et al., 1990), glaciation of Seward Peninsula invaded the present-day Bering Sea (Hopkins, 1973; Kaufman et al., 1991), erratics from glaciated mountains surrounding Bristol Bay were deposited along the Aleutian Islands (Black, 1976; Thorson and Hamilton, 1986), and expansion of an ice cap on Chukchi Peninsula in easternmost Siberia reached Saint Lawrence Island in the present-day Bering Sea (Brigham-Grette et al., 1992; Heiser et al., 1992). North-south through valleys in the deLong Mountains of the western Brooks Range indicate that a marine ice sheet in the Chukchi Sea may have transgressed beyond the Arctic Coastal Plain. Fig. 7 depicts a maximum glaciation during which a marine ice sheet in the Chukchi Sea overran Chukchi Peninsula, creating numerous north-south through valleys (M.G. Grosswald, pers. comm., 1992), transgressed onto the Arctic

Coastal Plain of Alaska as a frozen-based ice sheet, and poured through Bering Strait as a marine ice stream that produced a fore-deepened channel and sent marine ice lobes into Kotzebue Sound and Norton Sound, before merging with ice streams originating from mountain glaciers converging on Bristol Bay to the east and on the Gulf of Anadyr to the west. These three ice streams supplied an ice shelf that floated over the deep marine basin in the southwestern Bering Sea and calved into the Pacific Ocean between ice caps on the Aleutian Islands. If this maximum glaciation existed, and was the penultimate pre-Eemian glaciation, delayed isostatic rebound could have left the Eemian marine shorelines described by Dinter et al. (1990), Hamilton and Brigham-Grette ( 1991 ), and Hamilton (1991) that are nearly continuous at elevations of 10-12 m along the western and northern coasts of Alaska and formed from 130 to 120 ka. Glacial tectonics on the northern side of Saint Lawrence Island (Brigham-Grette et al., 1992; Heiser et al., 1992) would have been produced if Saint Lawrence Island penetrated into cold overriding ice. The ice stream from Bristol Bay would have deposited the Aleutian Island erratics reported by Black (1976), as noted by Thorson and Hamilton (1986). Wainwrightian erratics from the Canadian Arctic islands could have been deposited prior to isostatic rebound, after a frozen-based marine ice sheet retreated from the Arctic Coastal Plain and collapsed in the Beaufort Sea. If fast ice persisted after collapse of a marine ice sheet in the Chukchi, Beaufort and Bering seas, successive marine terraces would not form along these shorelines, as they did where Laurentide marine ice collapsed over Hudson Bay and Scandinavian marine ice collapsed over the Gulf of Bothnia. Today, fast ice in the Laptev Sea (Dethleff, 1992) prevents raised beaches from forming, even though a marine ice sheet transgressed onto the adjacent Siberian coast during the last glaciation and apparently remained well into the Holocene (Grosswald et al., 1992). If successive marine terraces did form after collapse of marine ice sheets in the Chukchi, Beaufort and Bering seas, rapid coastal erosion may have removed all but the highest raised beaches. Coastal erosion has been very rapid along these shores

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during the Holocene (Dinter et al., 1990) and is today (J. Brigham-Grette, pers. comm., 1993). A consequence of the maximum glaciation in Fig. 7 would have been a vast ice-dammed lake that inundated the river valleys of central Alaska. This lake would be a moisture source for precipitation over interior Alaskan mountain ranges. Although lacustrine sediments from ice-dammed lakes are widespread over interior Alaska (Hamilton and Brigham-Grette, 1991; Hamilton, 1991), there is no evidence for a lake of the kind shown in Fig. 7. This argues against the maximum glaciation depicted in Fig. 7. On the other hand, drainage of such a hypothetical lake would provide the energy to cut the huge submarine canyons, Bristol Trough and Bering Trough on the continental slope west of Bristol Bay, and Herald Canyon in the Chukchi Sea (Ackerman, 1988). Fig. 8 presents a possible model for the last glaciation of Beringia (Hughes et al., 1991). Early in the glaciation cycle, marine ice grounded in the Chukchi and Beaufort seas may have transgressed onto the frozen Arctic Coastal Plain of Alaska. It isostatically depressed the bed up to 7 m where the Flaxman Formation, which includes Canadian erratics, was deposited during the subsequent Simpsonian transgression between 80 and 70 ka. Without a prior marine ice transgression to isostatically depress the Arctic Coastal Plain, Dinter et al. (1990) were faced with the problem of explaining the Simpsonian transgression at a time when global sea level was lower than at present and there was no tectonic activity. Polar easterly winds, strengthened by katabatic winds flowing off the marine ice sheet, produced linear sand dunes along the Arctic Coastal Plain and loess sheets in the foothills south of the dunes. The regional climate was so cold and d~y that streams from the Brooks Range were too weak to incise the loess and dune landscape (Dinter et al., 1990). This implies that the marine ice sheet advanced over a frozen bed on the Arctic Coastal Plain, so it would have produced little if any glacial or periglacial geology, apart from possible glacial erosion that produced Harrison Bay and Prudhoe Bay during the last glacial maximum subsequent to the Simpsonian transgression. This erosional event may have involved glacial tectonics that left frozen patches of the bed undisturbed and able to

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Fig. 8. Beringia during the last glacial maximum if a marine ice sheet in the East Siberian and Chukchi seas transgressed into the Bering Sea. Glaciation in the Bering Sea is adapted from the ice-sheet reconstructionby Hughes et al. (1991), but constrained by field data from Brigham-Grette et al. (1992) and Heiser et al. (1992). Ice-shelf flowlines are shown transporting Canadian erratics from Coronation Gulf to the glacioisostaticaUy rebounding Arctic Coastal Plain of Alaska (Dinter et al., 1990) after collapse of a marine ice sheet on the Beaufort Sea continental shelf, see Fig. 7. evolve into the offshore barrier islands when postglacial coastal erosion was migrating southward (Hopkins and Hartz, 1978; Rodeick, 1979; Dinter et al., 1990). A marine ice sheet in the Chukchi Sea probably would have covered Wrangel Island, which may have become deglaciated and rebounded isostaticalty during the last glacial maximum if its lowlands became blanketed with marine sediments and were inhabited by mammoths at that time (Vartanyan et al., 1993). At some time during the last glaciation, ice from Chukchi Peninsula advanced across the exposed Bering Sea floor to Saint Lawrence Island, where it produced glacial tectonics, deposited till and erratics, and left a younger recessional morainal ridge that extends northeast from Saint Lawrence Island for some 150 km (Brigham-Grette et al., 1992; Heiser et al.,

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1992). South-facing alpine glaciers on Saint Lawrence Island reached present-day sea level at that time (Heiser et al., 1993), and the snowline rose from 150 m on Chukchi Peninsula near Provideniya and on southwestern Saint Lawrence Island to 300 m in the Kigluaik Mountains on Seward Peninsula (Brigham-Grette et al., 1992). These constraints, together with the heavier Late Pleistocene snow precipitation rates in Siberia compared to Alaska (Grosswald et al., 1986), allow much more extensive highland ice in western Beringia than in eastern Beringia during the last glaciation, with marine ice confined to the Laptev, East Siberian, Chukchi and Beaufort seas, and spilling over into the northwestern Bering Sea. Fig. 8 presents this view. Brigham and Miller (1983) use temperaturedependent diagenetic changes in amino acid epimerization rates to show that the Alaskan coast of the Chukchi Sea averaged - 1 8 ° C during the last glaciation, which is 8°C colder than today. Given the temperature dependence of sea-ice thickening rates published by Crary (1960) for the Arctic, it is possible that sea ice could have grounded in the shallow water of the Chukchi Sea in as little as 300 yr, giving an initial thickness of up to 60 m for the marine ice sheet (Ackerman, 1988). Ice in the Bering Sea may have originated as highland ice on Chukchi Peninsula, or as marine ice from the Chukchi Sea that spilled through Bering Strait and eroded north-south through valleys across Chukchi Peninsula. In any case, a calving ice wall fed by highland glaciers in western Beringia and by a marine ice sheet in northern Beringia may have extended for 1000 km along the continental shelf edge of southern Beringia from Kamchatka Peninsula to the Gulf Anadyr, as shown in Fig. 7.

5. Ice gates and sea gates across Beringia

The standard model of Beringian glaciation depicted in Fig. 5 fits the mammoth steppe concept of Guthrie (1990), in which the high unglaciated latitudes of Northern Hemisphere continents was a continuous cold, dry, windy, grassy landscape that supported large grazing animals and their

predators, most of which became extinct during the Holocene environmental transformation. During the last glacial maximum, the mammoth steppe extended continuously from the Scandinavian Ice Sheet in Europe to the Laurentide Ice Sheet in Canada, and there was no marine ice sheet on the Arctic continental shelves of Eurasia and North America that transgressed central Beringia. However, if any of the Beringian glaciation models in Figs. 6-8 are correct, the mammoth steppe was closed across central Beringia by an "ice gate" that blocked migrations during glacial stadials and by a "sea gate" that blocked migrations during glacial interstadials. If the high Arctic environment was indeed cold, dry and windy, marine ice transgressions from the Arctic continental shelf would have been onto permafrost, with ice ablation largely by sublimation, as in the dry valleys of Antarctica today (Stuiver et al., 1981 ), so glacial erosion and deposition processes would have been attenuated or inactive. A record of postglacial isostatic rebound in regions where coastal erosion was slow and fast ice was absent during interglacials may be the only evidence for these cold-based marine ice sheets. The continuity of a mammoth steppe across Beringia has been questioned by Colinvaux (1980), Cwynar and Ritchie (1982), Ritchie and Cwynar (1982), Ritchie (1984) and Colinvaux and West (1984), who argued from pollen data in eastern Beringia that only a present-day high Arctic polar desert or barren tundra existed during Pleistocene glacial maxima, that the glacial tundra plant species could not support large grazing animals, and that dated large animal fossils are absent from the record during full glacial times. Guthrie (1989) has addressed each of these criticisms. A possible resolution of the controversy may be found in Beringian glaciation models depicted in Figs. 6-8, especially Fig. 8 for the last glaciation. If migration of plants and animals across central Beringia was possible only during the short intervals between glacial stadials and interstadials, when the ice gate and the sea gate were both open because the marine ice sheet was either advancing or retreating and sea level was either falling or rising, then eastern Beringia would be much more isolated than western Beringia during full glacials. Harsh

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full glacial conditions in eastern Beringia could have taken their toll on the reduced grassy flora and megafauna that managed to cross the land bridge during these brief intervals and subsequently became isolated in a full-glacial Alaskan refugium. An important test of the ice gate and sea gate concept would be the archaeological record of human migration from Asia into the Americas (Morlan, 1987). Paleolithic hunter-gatherer societies would follow the grazing animals, which followed the grassy plants across central Beringia when both the ice gate and the sea gate were open. Therefore, the time available for human migration across the land bridge would be even shorter because the mammoth steppe environment would have to become established in central Beringia before Paleolithic man could make the crossing. The archaeological record is therefore more valuable than the faunal and floral record as a barometer for restricted migration across the Beringian land bridge during the last glaciation. Archaeology is poised for a paradigm shift in its search to unlock the history of human migration from Asia and subsequent dispersion in the Americas (Adovasio et al., 1988). The standard paradigm holds that Beringia was a land bridge opened to human migration during Pleistocene glaciations, which is about 90% of the Pleistocene, but for some unknown reason only plants and animals made the crossing until after the last glacial maximum, when evidence for human habitation first appeared in Alaska and when Clovis sites proliferated in North America, especially south of the ice-free corridor between the retreating Laurentide and Cordilleran ice sheets (Mead and Meltzer, 1985; Bryan, 1986; Carlisle, 1988; Martin and Klein, 1989; Agenbroad et al., 1990; Guthrie, 1990; Bonnichsen and Turnmire, 1991). The standard paradigm was held together by a synthesis combining Clovis archaeological data (Bonnichsen and Turnmire, 1991), with dental traits linking prehistoric and modern native people of Northeast Asia to those of the Americas (Turner, 1985, 1986, 1987), and a classification of language dispersal among American Indians (Greenberg, 1987), all pointing to 12 ka for initial peopling of the Americas. This synthesis began to unravel, with

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perceptible convergence toward a new synthesis, at the First World Summit Conference on the Peopling of the Americas at the University of Maine in 1989 (Morell, 1990). The need for a new paradigm became compelling with the successful defense by Parenti (1993) of his doctoral thesis that the Pedra Furada rockshelter, one of several hundred in northeastern Brazil, has a coherent record of human habitation dating from 50 to 5 ka (Bahn, 1993). Validation of the Pedra Furada chronology adds credibility to other pre-Clovis sites in the Americas, notably Monte Verde in central Chile with habitation ranging from firm dates of 13 ka in upper layers to less firm dates of 33 ka in deeper layers (Dillehay, 1989) and the Meadowcroft rockshelter in western Pennsylvania, which has numerous firm dates as old as 16 ka (Adovasio et al., 1990). The Clovis culture is characterized by stone artifacts designed for hunting and processing large animals, and Clovis people are commonly cited as being partly responsible for Late Quaternary extinctions, notably of mammoths. Pre-Clovis Americans may have survived mainly on plant or shellfish harvesting (Morell, 1990), with hunting restricted to smaller animals and using weapons preserved only under unusual conditions, such as the 29,600 year old bone projectile imbedded in the ankle of an ancient horse in New Mexico (Allison, 1992). The standard paradigm for peopling the Americans maintains that human habitation of western Beringia dates from the last glacial maximum, and consisted primarily of the 18-10 ka Diuktai Cave site on the Lena River near Yakutsk, the 16 10 ka Ushki Lake sites on Kamchatka Peninsula, and several undated sites on the southeastern Chukchi Peninsula which have affinities to both the Diuktai and Ushki sites, and to various archaeological sites in Alaska (Dikov, 1988; Ackerman, 1988). However, Mochanov (1977) insists that the Diuktai site was occupied as early as 35 ka, and that human occupation of the Diring site on the Lena River 140 km upstream from Yakutsk is an astounding 1.8 3.4 m.y. old (Hall, 1992). This predates the 1.7-2.7 m.y. old Oldowan sites in Africa, and leads Mochanov (see Hall, 1992) to propose that Siberia was the cradle of humanity because it was "the edge of existence"

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where "stress influenced mutation" to produce "adaptive peaks" that stimulated the manufacture of tools and the use of fire. The 35 ka age of the Diuktai site has been accepted by West (1981) and Michael (1984), and Miiller-Beck (1982) believes that Upper Paleolithic societies in western Beringia could have reached the Bering Sea between 40 and 30 ka. The crossing of central Beringia into eastern Beringia took place in three waves, beginning 14,000 yr ago, according to Turner (1985, 1986, 1987), based on the worldwide rate of dental evolution. The question arises, if western Beringia was inhabited by man at least 35,000 yr ago and perhaps for over 3,000,000 yr, why is there no record of human habitation in Alaska more than 12,000 yr old (Hamilton, 1989)? Prior to the Late Quaternary glaciation cycles beginning 900,000 yr ago, migration was probably blocked by the sea in Bering Strait, which first opened just over three million years ago (Hopkins, 1972). If the conventional model of ice-age Beringia depicted in Fig. 5 is correct, however, the central Beringian land bridge must have been a broad highway for human migration during most of the last 900,000 yr. Migration could have been prevented during most of that time only if the land bridge was closed by an "ice gate" across central Beringia during glacial stadials and by a "sea gate" across central Beringia during glacial interstadials and interglaciations. The ice gate and the sea gate were opened only for brief intervals between stadials and interstadials. By necessity, human migration had to follow migration of plants and animals in order to survive, so the ice gate and the sea gate would be open to human migration for only a fraction of the time that it was opened for plant and animal migration. This would explain why eastern and western Beringia shared most of the same Pleistocene flora and fauna, whereas Alaska has a younger and less diverse archaeological record than either Siberia or the rest of the Americas. Human populations that crossed the land bridge had difficulty surviving in Alaska once the ice gate and the sea gate closed, but groups that continued their migration through the ice-free corridor between the Laurentide and Cordilleran ice sheets were able to disperse into the Americas and thrive.

Nichols (1990) proposed a linguistic survival and dispersal model for migration through the Americas that allows a single Beringian entry dated at 50 ka or ten entries beginning at 35 ka, for example, but it rules out the Greenberg (1987) linguistic model of three migrations waves beginning at 12 ka (see Simpson, 1992). Her model agrees with the age of South American archaeological sites and pre-Clovis archaeological sites in North America because 12,000 yr is not enough time to produce the observed linguistic diversity. Szathmary (1993) holds that genetic data on native American populations reveals a diversity that is a poor fit to the "three wave" Clovis model, but fits much better with a model in which migrations across Beringia began much earlier with genetic diversity already existing in the migrating populations or arising from isolation after an initially homogeneous population dispersed over the Americas (see Hall, 1993a). If the three-wave model is assumed to be true, mitochondrion DNA in blood samples from Indians in both North and South America points to four genetically distinct population groups in an initial "wave" that migrated between 21 and 14 ka (Horai et al., 1993; Gibbons, 1993; Baer, 1993). The old synthesis of linguistic, dental and archaeological evidence held that migration into the Americas began only 12,000 yr ago. The linguistic and archaeological legs supporting that tripod have been kicked away, causing the synthesis to collapse. The rapid state of flux in seeking a new synthesis for peopling the Americas was a central theme of the First World Conference on Prehistoric Mongoloid Dispersals held in Tokyo in 1992 (Hall, 1993b). Disciplines represented at the conference included archaeology, physical and cultural anthropology, human genetics, ethnology, geomorphology, isotope chemistry and computer science. With no consensus yet in site, various models of Beringian glaciation have a place in this interdisciplinary mix, and can make a contribution to the synthesis that finally emerges.

6. Discussion

Quaternary's grand unsolved problem is whether or not Quaternary glaciation cycles produced an

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Arctic Ice Sheet that formed when sea ice thickened, grounded and domed on Arctic continental shelves, and expanded seaward as a floating ice shelf and expanded landward as a continental ice sheet. If it did, a marine ice sheet may have transgressed central Beringia from the Chukchi Sea to the Bering Sea, becoming an ice gate blocking migration across the Beringian land bridge during glacial maxima, and being replaced by a sea gate that blocked migration during glacial minima. Central Beringia is more like a gateway than a land bridge in this view, and a distinction must be made between marine glaciation of central Beringia and highland glaciation in eastern and western Beringia. The distinguishing characteristic of marine glaciations is that marine ice transgression is very rapid, because initially ice accumulates on both top and bottom surfaces of rapidly thickening sea ice with no ice ablation, and marine ice recession is also rapid because marine ice sheets are relatively low so a minor rise in the snowline causes a major negative shift in mass balance and ice ablation is accelerated by calving when sea level rises. Four models of Beringian glaciation were examined. The minimum glaciation model in Fig. 5 is now widely accepted (Hopkins et al., 1982). However, it does not fit well with pre-Clovis records in human habitation in the Americas, with the diversity of languages and mitochondrion DNA in native American populations, and with the contention by Yuri Mochanov and others, as reported by Hall (1992), that eastern Siberia has the oldest archaeological site in Eurasia. The Grosswald (1988) model of Beringian glaciation in Fig. 6 is least likely because it requires a vast ice-dammed lake in central Beringia and an ice shelf over the southwestern Bering Sea, both of which are glaciologically improbable and lack supporting evidence in the sedimentary record. The maximum glaciation model in Fig. 7 requires a marine ice sheet on the Arctic continental shelf of Siberia and Alaska that transgressed onto the Arctic Coastal Plain and poured through Bering Strait to merge with highland glaciers in Alaska and Siberia and end as an ice shelf calving into the Pacific Ocean along the Aleutian Islands. If such extensive glaciation existed, it would have

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caused enough glacioisostatic depression of the Arctic Coastal Plain of Alaska to cause the Wainwrightian or the Pelukian transgressions from 540 to 125 ka, and would have produced a vast ice-dammed lake in interior Alaska. Local glaciation can account for known interior ice-dammed lakes that drained when glaciers retreated during the Pelukian transgression (Hamilton, 1991; Hamilton and Brigham-Grette, 1991). The model for the last glaciation in Fig. 8 is compatible with marine ice transgression onto the Arctic Coastal Plain of Alaska early in the glaciation cycle, allowing the Simpsonian transgression from 80 to 70 ka (Dinter et al. (1990), possibly during glacioisostatic rebound after marine ice retreated, and marine or highland ice transgression across central Beringia as far as Saint Lawrence Island (BrighamGrette et al., 1992; Heiser et al., 1992), but prior to the last glacial maximum if mammoths lived on Wrangel Island from 20 to 4 ka (Vartanyan et al., 1993). If Wrangel Island was glaciated at the last glacial maximum, the highland glaciation in Fig. 1 should be accompanied by the marine glaciation in Fig. 8. If snowlines lower enough to produce the maximum highland glaciation in Fig. 5, our ice-sheet model will produce the maximum marine glaciation in Fig. 7. Field studies can decide which of the four models should be abandoned. Since the models present different degrees of transgression across central Beringia by marine ice and water during Pleistocene glaciation cycles, four critical field tests are recommended: ( 1) Determine the Pleistocene history of Wrangel Island. If Wrangel Island was inhabited by mammoths during the last glacial maximum, it is unlikely that a marine ice sheet existed in the Chukchi Sea and transgressed central Beringia far enough to become an ice gate closing off human migration. This is the easiest and least costly field test. If Wrangel Island was glaciated, three other field tests become necessary. (2) Drill into Pleistocene deposits on the continental shelf of the Chukchi Sea. This will establish whether marine ice sheets developed there during Pleistocene glaciation cycles, including histories of advance and retreat, especially southward during the last glaciation cycle when human migrations

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across central Beringia t o o k place. This is necessary for an initial e v a l u a t i o n o f the glaciation m o d e l in Fig. 8. (3) Drill into Pleistocene d e p o s i t s o n b o t h the c o n t i n e n t a l shelf a n d the o c e a n basins o f the Bering Sea. This will reveal w h e t h e r lacustrine sediments o r till exist on the shelf a n d w h e t h e r ice-rafted glacial d e p o s i t s exist in the basins. I f not, the m o d e l s in Figs. 6 - 8 c a n be d i s c a r d e d ; otherwise each o f the three m o d e l s can be distinguished by m a p p i n g the d i s t r i b u t i o n o f eolian, fluvial, lacustrine a n d glacial d e p o s i t s over time. (4) M a p the d i s t r i b u t i o n , elevation a n d age o f Pleistocene lacustrine sediments in central A l a s k a . This will establish w h e t h e r or n o t the large iced a m m e d lake in Fig. 7 ever existed. M a p p i n g will require a d e t e r m i n a t i o n o f w h e t h e r layers o f icy silt o r silty ice, called " y e d o m a " in the R u s s i a n literature, qualifies as a lacustrine sediment. These layers are w i d e s p r e a d in central A l a s k a , where they are often i n t e r p r e t e d as being loess sheets.

Acknowledgements This c o n t r i b u t i o n is an e x p a n d e d version o f a t e r m p a p e r b y Beverly H u g h e s for the course, M o d e l s in A r c h a e o l o g y , t a u g h t by R o b s o n Bonnichsen, w h o e n c o u r a g e d its p u b l i c a t i o n . D e t a i l e d a n d c o n s t r u c t i v e criticisms were p r o v i d e d by J o h n A n d r e w s , C a r l Benson, Julie B r i g h a m Grette, M i k h a i l G r o s s w a l d , T h o m a s H a m i l t o n , D a v i d H o p k i n s , Peter Lea, a n d A n d e r s Solheim, even when they disagreed with o u r conclusions. This is an E P S C O R p u b l i c a t i o n .

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