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Research 76 (2011) 383–392

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Quaternary Research

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Late glacial and landscapes of central Beringia

Anatoly V. Lozhkin a,⁎, Patricia Anderson b, Wendy R. Eisner c, Tatiana B. Solomatkina a a North East Interdisciplinary Science Research Center, Branch, Russian Academy of Sciences, Magadan, 685000, b Earth & Space Sciences, Quaternary Research Center, University of Washington, Seattle, WA, 98195-1310, USA c Department of Geography, University of Cincinnati, Cincinnati, OH 45221-0131, USA article info abstract

Article history: New palynological and sedimentological data from St. Lawrence Island present a rare view into late-glacial Received 5 August 2010 and Holocene environments of the central Bering . The late glaciation was a time of dynamic Available online 7 September 2011 landscape changes in south-central Beringia, with active thermokarst processes, including the formation and drainage of thaw lakes. The presence of such a wet, unstable substrate, if widespread, probably would have Keywords: had an adverse impact on food sources and mobility for many of the large populations. The Bering Land Bridge establishment of Betula shrub on the island suggests late-glacial summers that were warmer than Thermokarst processes Peat present, consistent with regional paleoclimatic interpretations. However, the increasing proximity to the Late glaciation , as postglacial sea levels rose, modified the intensity of warming and prevented the establishment Holocene of deciduous forest as found in other areas of Beringia at this time. The mid- to late Holocene is marked by more stable land surfaces and development of Sphagnum and Cyperaceae peat deposits. The accumulation of organic deposits, decline of shrub Betula, and decrease in thermokarst disturbance suggest that conditions were cooler than the previous. A recent decline in peat accumulation at the study sites may relate to local geomorphology, but similar decreases have been noted for other . © 2011 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Quaternary of . Pioneering investigations by Colinvaux (1967a) on St. Lawrence Island (Fig. 1) provided the first insight into the No environmental history of Beringia has been more difficult to vegetation and climate history of the . The pollen record is obtain than that from the central Bering Land Bridge (in sensu Young, thought to span the last ~18,000 14C yr (~21,500 cal yr BP), but it is 1982; Fig. 1). Understanding of this key biotic crossroad has been poorly dated. The herb-dominated full-glacial vegetation is replaced pieced together from ancient surfaces found in marine cores (Elias et by Betula shrub tundra, presumably marking initial climate al., 1996, 1997), terrestrial deposits on islands within the Bering Sea amelioration during the late glaciation. The Holocene was divided (Colinvaux, 1967a,b; Heusser, 1973,1978; Parrish, 1979; Lozhkin et into two intervals with the earlier pollen zone radiocarbon dating to al., 1998), and continental records from lands bordering the current 5650±275 (I-993; ~6500 cal yr BP) and containing moderate pollen (Hopkins et al., 1960; Colinvaux, 1964; Matthews, 1974; percentages of the exotics Alnus and Picea. Colinvaux proposed that Colinvaux, 1981; Ager, 1982; Hopkins et al., 1982; Anderson, 1985; this assemblage represented the post-glacial thermal maximum. The Lozhkin et al., 1996; Hoefle et al., 2000; Brubaker et al., 2001; Elias, second zone characterized the island's contemporary tundra, which 2001; Goetcheus and Birks, 2001; Anderson et al., 2002; Ager, 2003; is dominated by herbaceous species with Betula nana and shrub Ager and Phillips, 2008). Much of the fascination with central Beringia Salix (Young, 1971). has focused on the (LGM) and to a lesser extent Little follow-up work has been done on the Bering Strait the late glaciation (~18,000–12,000 14C; ~21,500–13,900 cal yr BP), islands after their initial paleoenvironmental exploration in the with questions relating to the of the landscapes that facilitated 1960s and 1970s. However, they remain one of the few sources or hampered movements of early human and large mammal of data for describing late Quaternary environments in the heart populations between (e.g., Hopkins et al., 1982; Guthrie, of the Beringian subcontinent. In 1991, we sampled two 2001; Ager and Phillips, 2008). Not surprisingly, the scientific allure of Holocene–late glacial exposures on northern St. Lawrence Island the Bering Strait region drew the earliest research into the late (Lozhkin et al., 1996). Although these sites present a spatially limited view, the palynological and sedimentological data hint at the dynamic and changing landscapes which characterized this ⁎ Corresponding author at: Earth & Space Sciences, Box 35-1310, University of – Washington, Seattle, WA 98195, USA. Fax: +1 206 543 3836. portion of the Bering Land Bridge during the latest glacial E-mail address: [email protected] (A.V. Lozhkin). transition.

0033-5894/$ – see front matter © 2011 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2011.08.003 384 A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392

Figure 1. Map of St. Lawrence Island showing the locations of the study sites. The inset map indicates the position of St. Lawrence Island within Bering Strait.

Study area and study sites diverse herb-Salix tundra, with Salix polaris and S. pulchra being the most common woody species in the local vegetation. Graminoids Sea-level history are especially abundant with Alopecurus alpinus typical of boggy areas. Although the relief is modest, south-facing slopes support St. Lawrence Island is located in the middle of the Bering Land tilesii, A. borealis, Salix , and S. ovalifolia. Salix arctica, Bridge to the south of modern day Seward and Chukchi peninsulas S. polaris, S. reticulata, S. ovalifolia, and S. pulchra are found near the (Fig. 1). Various relative sea-level (RSL) reconstructions offer slightly lagoon beach. different paleogeographic histories, but in all examples the advent of late-glacial warming resulted in the shift of St. Lawrence Island from Study sites interior central Beringia during the LGM to an isolated island by the earliest Holocene (Hopkins, 1982; Elias et al., 1996; Lozhkin, 2002; Field work concentrated on the far northwestern tip of the island, Manley, 2002; Brigham-Grette et al., 2004). Lozhkin's (2002) map where a small strait connects the Bering Sea to Aghnaghak Lagoon shows a significant reduction in the extent of the southern Land (Fig. 1). Peat has accumulated in lowlying areas throughout northern Bridge between 18,000 and 15,000 14C yr BP (~21,500–18,300 cal yr St. Lawrence Island to an elevation of ~1 to 1.5 m a.s.l. The Aghnaghak BP; −50 m RSL) with the coast line approaching southern St. site (AG; 63° 39′ 30″ N, 171° 32′ W) is a 400-m-long exposure along Lawrence Island. He also indicated that a narrow meandering strait the south shore of the strait. Here marine sediments of Oligocene age separated the Asian and American continents by ~12,000 14CyrBP form a 25-m-high cliff. A 2-m-thick moraine overlies portions of the (~13,900 cal yr BP; −37 m RSL). At this time, St. Lawrence remained a Oligocene deposits. Although undated, it is likely that the moraine is part of the North American mainland, but its western edge bordered of late Wisconsinan age (D. Hopkins, 1991, personal communication). directly on Anadyr Strait. Elias et al. (1996), however, proposed that a A ~7.3-m-thick layer of peat, silt, and sand overlies the moraine. A permanent seaway was not established until 11,000 14Cyr BP second exposure, informally named Naskak (NA; 63° 39′ N, 171° 29′ (~12,900 cal yr BP). continued rising and by ~10,000 to W), was discovered 2.2 km to the east of AG. The 2.6-m-thick NA 9500 14C yr BP (~11,500 to 10,800 cal yr BP; −20 m RSL) the island exposure is located on the eastern beach of a small lake. The lake is was formed, but its area was 1.5–2 times greater than today (Lozhkin, separated from the Bering Sea by a spit of sand and pebbles that is 2002). The island's present-day configuration was achieved by ~150 m wide. ~8500 14C yr BP (~9500 cal yr BP). By 5000 14 C yr BP (~5800 cal yr We also excavated a test pit (63° 37′ N, 171° 38′ W) ~7– BP), near-modern sea levels were attained in the Bering Strait region 10 km from the modern coastline. We uncovered buried tree (Elias et al., 1996). trunks ~30–40 cm in diameter and ~5 m length at 35 cm depth. Wood from one tree trunk was radiocarbon-dated to 775±30 14 Cyr Modern vegetation BP (~700 cal yr BP; Table 1). Modern Bering Sea storms are severe and the associated surge waves are capable of carrying such large- Today the treeless vegetation of St. Lawrence Island includes an sized wood several km into the island's interior. Alternatively, abundance of herb taxa, ericads, eight species of Salix, and Betula humans could have transported the trees, but the logs were nana, the latter restricted to the south–southeastern part of the unworked and unburned, implying a noncultural transportation island (Young, 1971). Modern vegetation near the field sites is mechanism. We continued excavations to a depth of 100–110 cm, A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392 385

Table 1 Radiocarbon dates for study sites, St. Lawrence Island.

Depth (cm) 14C dates Calibrated ages cal yr BP Lab number Material 14CyrBP (2 σ range and midpoint)

Aghnaghak exposure 50–55 3575±90 3640–4140 MAG-1402 Peat 3890 85–90 4320±20 4840–4960 MAG-1401 Peat 4900 110–115 5205±70 5760–6180 MAG-1394 Peat 5970 160–165 5290±40 5940–6190 MAG-1393 Peat 6065 190–195 5560±50 6280–6440 MAG-1392 Peat 6360 222–227 9735±290 10,280–12,060 MAG-1391 Peat 11,170 232–234 10,750±120 12,410–12,930 Beta-47006 Plant detritus 12,670 Naskak exposure 60–65 995±115 690–1170 MAG-1400 Peat 930 83–92 3815±45 3270–3560 MAG-1399 Peat 3415 98–103 4920±20 5600–5700 MAG-1398 Peat 5650 122–127 5140±40 5750–5990 MAG-1397 Peat 5870 148–153 5345±30 6000–6270 MAG-1396 Peat 6135 161–166 5570±40 6300–6440 MAG-1395 Peat 6370 Peat 7 km from northern shore 35 775±30 670–730 MAG-1403 Wood 690 105–115 1995±100 1710–2300 MAG-1404 Peat 2005

where a bulk peat sample was dated to 1995±100 14 Cyr BP not able to be radiocarbon dated. However, the high percentages of (~2000 cal yr BP). Excavations stopped because of excessive ground Betula pollen suggest that zones AG1 and NA1 correspond to the water seeping through the peat. Zone that characterized both eastern and western Beringia during the late glaciation (e.g., Lozhkin et al., 1993; Ager and Phillips, 2008). Methods Depositional environments at the study sites suggest that the landscapes were more unstable as compared to those of the Holocene. Exposure faces were thawed and cleaned prior to collecting Unit 1 at NA (Table 3, Fig. 3) is colluvium, whereas the lower 318 cm palynological samples. Although sampling intervals varied depending of the AG section (Table 2; Fig. 2) represents permafrost processes on sedimentology, care was taken to extract subsamples of no more (see next section). Although the pre-Holocene section at AG is than 5 cm and 2 cm thickness for radiocarbon and pollen analysis, relatively thick (~500 cm), thaw lake deposits can accumulate respectively. Pollen samples were prepared following standard PALE quickly, as can colluvium. Therefore, the lower sediments may only (1994) procedures. Pollen percentages (Figs. 2–3) are based on a sum represent a portion of the late glaciation. of all identified and unidentified terrestrial pollen grains, with the Differences exist in the AG and NA pollen diagrams (Figs. 2 and 3), exception of Cyperaceae. This taxon was removed from the pollen reflecting variations in local plant communities. However, the sites sum, because it forms the matrix of the peaty sediments. Cyperaceae share general trends that more likely indicate broader environmental and spores are given as percentages of the pollen sum. Subsums are changes. The basal zone of the AG diagram is characterized by an illustrated as percentages of arboreal and nonarboreal pollen plus unusual assemblage that is co-dominated by Betula, Ericales, and spores. The MAG radiocarbon dates (Table 1) were obtained using a pollen. These are generally the highest percentages for Betula scintilization method. Bulk peat samples were examined carefully to and Ericales in the section. Sphagnum spores are moderately high. insure no Tertiary coal, sometimes found in materials from the Bering Zone AG2 marks a transition to lower pollen percentages of Betula and Strait region, was included in the samples. Calibrated ages were Ericales, with variable but increasing trends in Poaceae and to a lesser calculated following Stuiver and Reimer (1993; INTCAL04 curve, 5.0 extent Alnus and Salix. Sphagnum spores peak at a 215 cm, and version). Cyperaceae pollen shows a slight increase. Zone AG3, whose central portion was barren of pollen, has a general increasing then decreasing Results trend in Poaceae pollen. Betula pollen percentages decline from zone AG2 but increase again in upper zone AG3. Salix and Alnus percentages The upper portions of the AG and NA exposures are dominated by increase and then decline, with Ericales pollen being variable. peats (Tables 2–3), which are capped by poorly developed and/or Cyperaceae percentages increase, whereas Sphagnum spores decrease. sand containing degraded plant remains. Radiocarbon dates indicate Coniferous pollen appears more consistently than previously. Zone that peat accumulation began during the early Holocene at AG but not AG4 is dominated by Poaceae and Cyperaceae pollen. Betula until the mid-Holocene at NA. Lowermost units at both sites are percentages are lower than in upper AG3, whereas Alnus pollen is dominated by organic-poor sands and silts, and the sediments were generally greater. Salix pollen shows an increasing trend throughout 386 A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392

Figure 2. Percentage diagrams of pollen and spores from the Aghnaghak section showing: a) main pollen taxa and Sphagnum; and b) minor pollen taxa, aquatics, and other spores. Three general lithological units were defined: 1) mixed and sand; 2) peat; and 3) silt containing plant remains, ice wedge, and segregation ice. The bottom 318 cm of the section is not illustrated as they contained no pollen. See Table 2 for more details on lithology.

the zone, whereas Poaceae generally decreases. Conifer pollen At the NA site, zone NA1 has high pollen percentages of Poaceae appears fairly consistently but in low amounts. The uppermost zone and Ericales with moderate percentages of Betula and Cyperaceae. has variable percentages of all main taxa. Sphagnum spores are the highest in the core (except for ~107 cm). A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392 387

Figure 3. Percentage diagrams of pollen and spores from the Naskak section showing: a) main pollen taxa; and b) minor taxa and spores. Three general lithological units were defined: 1) mixed sand and organic layers; 2) peat; and 3) mixed sand, clay, pebbles, and cobbles. See Table 3 for more details on lithology.

Zone NA2 is marked by increases in percentages of Poaceae and Holocene and late glacial landscapes, northern St. Lawrence Island Cyperaceae pollen with a decrease in Ericales and Betula. Alnus pollen is variable but with a peak at 120 cm. In zone NA3, graminoid pollen Land surface history decreases and Ericales increases with variable Salix and generally decreased Betula. Alnus is somewhat higher. Zone NA4 has only 2 Lithological changes in the AG and NA exposures suggest northern samples; graminoid pollen is variable but other taxa are similar to St. Lawrence Island experienced two stages of terrain development: zone NA3. Conifer pollen, particularly Pinus, is present in trace 1) pre-Holocene with active landscape processes; and 2) Holocene amounts throughout the core. with more stable surfaces. Radiocarbon dates (Table 1) at both sites 388 A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392

Table 2 Table 3 Sediment Description of the Aghnaghak Exposure. Sediment description of the Naskak exposure.

Depth from Sediment type Depth from Sediment type surface (cm) surface (cm)

0–7 Modern soil, fine-grained, gray sand with organic remains 0–59 Medium-grained sand; gray; horizontal lenses and layers of 7–22 Fine-grained, brown sand; ungulating contacts within the sand particulate organics (probably aeolian); unit includes lenses of fine-grained gray 59–89 Cyperaceae or graminoid peat, from brown to black in color; sand and layers of particulate organics compacted, lenses of silt; massive cryostructure in peat 22–52 Cyperaceae peat; black-brown in color (aerobic conditions); 89–166 Cyperaceae peat; brown in color; fibrous; scattered horizontal compact with lenses of light gray, fine-grained sand layers include medium- to large-grained sand; lenses of 52–210 Cyperaceae peat; brown in color; fibrous; massive cryostructure, segregation ice that are 5–10 cm thick; sand layer between peat includes fragments of Salix 153 and 157 cm 210–232 Sphagnum peat 166–171 Medium-grained sand; gray 232–234 Horizontal layer of silt with abundant remains of detrital plants 171–260 Medium-grained sand; brown-gray; often interlayered with sandy 234–412 Silt, gray, with remains of stems of herbs and branches of small clay that is dark gray; unit includes many angular pebbles shrubs (~0.5 cm diameter); silt includes thin layers with (1–5 cm diameter) and cobbles (15 cm diameter); massive incompletely crosshatched cryo-structure and segregation ice; cryostructure vertical ice wedge with width near at top of ~80 cm and ~2–7cm wide near the bottom; ice-wedge cross-cuts entire unit 412–417 Coarse-grained sand; brown-gray in color 417–452 Silt; gray; horizontal layers of segregation ice; 1–2 mm thickness graminoid meadows (Webber et al., 1980). If the landscape is 452–492 Silt; dark gray with scattered remains of branches of shrubs; particularly active, one or more additional thaw lakes might migrate 1.5 cm diameter over the original basin. 492–630 Silt; gray; with rounded gravel and small pebbles The AG stratigraphy suggests the local development of at least one – 630 730 Medium-grained sand; brown-gray; with lenses of black silt and and perhaps up to three thaw lakes (Table 2). The 234–412 cm brown medium-grained sand; upper contact with the silt is irregular sequence is the most classic, with ice wedges and segregation ice in silty sediments and a mix of herb stems and branches of small shrubs. The pollen spectra contain comparatively uniform percentages, also a characteristic of thaw lake deposits (Anderson, 1982). The overlying indicate times of peat accumulation and landscape stability during the detrital layer (232–234 cm) probably represents a near-shore locality second stage (Tables 2 and 3). Radiocarbon dates are absent for the reflecting subsequent shoreline slumping as the basin expanded. A first stage. However, radiocarbon dates and sediment stratigraphy at second layer of segregation ice (417–452 cm), overlain by coarse sand AG indicate greatest disturbance prior to ~10,000–11,000 14CBP (412–417 cm), perhaps denotes another lake-forming episode that is (~11,500–12,900 cal yr BP), and the pollen stratigraphy (zone AG1) only partially preserved. A possible third lake-forming event, also only places the sediments as probably late glacial in age. The lower sandy partially preserved, is characterized by a detrital layer between 452 unit at NA is colluvium, which likely represents slumping from the and 492 cm with silt and sandy silt below it. These deposits, though, nearby moraine. Radiocarbon dates at NA indicate that these do not contain any cryostructure. sediments accumulated some time prior to the mid-Holocene. Although unconformities may exist at both sites, the radiocarbon However, comparison of pollen zones NA1 and AG1 (moderate pollen (AG) and pollen stratigraphy (AG, NA) indicate that more stabilized percentages of Betula and Poaceae; high percentages of Ericales, low land-surfaces existed perhaps by the early Holocene and certainly by percentages of Cyperaceae, Salix and Alnus) suggests that deposition the mid-Holocene. The accumulation of Sphagnum peat at AG began was likely sometime during the late glaciation. While the colluvium at ~9700 14C yr BP (~11,200 cal yr BP), and thaw lake activity subse- NA indicates some instability to the local landscape, the sedimentary quently has been absent (note: thermokarst lakes are present today sequence at AG provides stronger evidence for a dynamic landscape, on St. Lawrence, being found predominantly to the southeast of the one characterized by active permafrost processes and development of study area). Thermokarst activity is sensitive to a variety of forcing thermokarst lakes. factors including the thickness and continuity of permafrost, Thermokarst or thaw lakes develop with melting of ice-rich characteristics of the active layer, topography, and climate. Even in permafrost and consequent collapsing of land surfaces (Hopkins, modern landscapes it is difficult to assess which factors are most 1949). This type of lake formation requires: 1) “substantial quantities significant in causing thaw lake drainage (Hinkel et al., 2007). Thus of ground ice” (Hopkins and Kidd, 1988, p. 791) typically consisting of reasons for the local changes in thermokarst processes remain ice wedges or segregation ice; and 2) the presence of ponded surface- uncertain, but the paleoenvironmental data strongly suggest that water. Today surface waters typically pool in one of three settings: more stabilized land-surfaces existed perhaps by the early Holocene 1) intersecting ice-wedges; 2) damming caused by disturbance of and certainly by the mid-Holocene on northern St. Lawrence Island. local drainage; or 3) coalescing of small basins resulting from the The upper portions of the peats at AG and NA incorporate an breakdown of low-centered ice-wedge polygons (Rex, 1961; Harry increasingly greater component of sand, the peats finally being and French, 1983; Hopkins and Kidd, 1988). These small pools expand replaced by sandy units. The most recent radiocarbon date from NA as the waters warm the ground both under and adjacent to the suggests that this change occurred after ~900 14C yr (~825 cal yr) ago. incipient thaw lake. Once the basin is of sufficient size, wind-driven At AG, this sand has an aeolian origin, likely related to the waves undercut the banks, resulting in basin expansion and establishment of the sand spit that today separates the site from the consequent slumping of vegetated surfaces into the lake. This Bering Sea. The spit may also be the source for the upper sands at NA. movement can cause thaw lakes to drain quickly, especially when These local geomorphic processes are probably the main influences on they migrate into streams or other lakes. Such a geomorphic history the reduction in peat, but they may also be acting in concert with provides a distinctive stratigraphy (Hopkins and Kidd, 1988). Basal more hemispheric-scale factors that have influenced peat accumula- sediments, which often overlie ice-wedge pseudomorphs, are a mix of tion (see Discussion). sand, silt, and organic detritus, the latter originating when surface vegetation erodes into the enlarging basin. Overlying sediments vary Vegetation history from fine-grained, bedded silts (central basin) to mixed silts and organics rich in macrofossils (near-shore). Once drained, the surface Even though thaw-lake deposits provide mixed palynological may stabilize, and in tundra settings it is often associated with assemblages, the pollen spectra remain sufficiently distinct to infer A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392 389 the general characteristics of the vegetation (Anderson, 1982; ZonesAG3,AG4,NA2,andNA3areassociatedwiththe Katamura et al., 2006). At AG and NA, the late-glacial pollen accumulation of Cyperaceae peat, initiation of which was dated at assemblages (zones AG1 and NA1) reflect the presence of shrub both sites to ~5600 14C yr BP (~6400 cal yr BP). These pollen Betula–ericales–graminoid tundra. This interpretation of shrub Betula assemblages reflect the widespread growth of moist graminoid tundra is consistent with that for Flora Lake (Colinvaux, 1967a), and a communities, as indicated by the dominance of Cyperaceae and similar vegetation is documented in relatively nearby Alaskan and Poaceae pollen with traces of other mesic taxa, such as Ranuncula- Chukotkan sites (Anderson et al., 2002, 2004; Ager and Phillips, 2008). ceae, Rumex, Polemonium, and Rubus chamaemorus. Although rela- Shrub Betula today is restricted to southern St. Lawrence Island, but tively high values (~20%) are found in some samples, most Betula the high pollen percentages in zones AG1 and NA1 suggest the shrub pollen percentages are sufficiently low (generally b20%) to indicate was probably present in northern parts of the island and possibly in that the shrubs were rare or absent from the study area by the mid- adjacent areas of the Land Bridge. The AG and NA diagrams are Holocene (Anderson and Brubaker, 1986). Climate simulations for unusual in that they have higher percentages of Sphagnum spores and 6000 cal yr BP indicate that summers remained warmer than present ericaceous pollen as compared to mainland sites. These odd spectra in mainland areas of Beringia (Bartlein et al., 1998). For St. Lawrence may be the result of sediment mixing or over-representation of the Island, however, the occurrence of near-modern sea-levels would local vegetation. They may also represent a variant of shrub Betula have depressed summer temperatures and could account for a decline tundra particularly rich in Ericales. Nonetheless, the landscape of in the island's shrub Betula population. northern St. Lawrence Island was likely a mosaic of wet to mesic As for other mid-Holocene woody taxa represented in the pollen tussock tundra and thaw lakes. Although organic remains are few in diagrams, shrub Alnus does not grow today on St. Lawrence Island, the AG and NA deposits, high percentages of Ericales pollen and and, like Betula, its pollen percentages are low enough to suggest that Sphagnum spores found in Holocene records often mark the growth of the pollen reflects long-distance wind transport to the sites. The same organic soils, perhaps suggesting a relatively productive environment influence accounts for the Pinus and Picea pollen, the plants arriving during the late glaciation. near present-day coastal sites ~6000–5000 14Cyr BP (~6800– Although the 10,750 14C date at the upper zone AG1 boundary 5700 cal yr BP; Anderson and Brubaker, 1994; Anderson et al., 2002; suggests the establishment of Betula shrub tundra prior to the early Ager, 2003; Anderson et al., 2004). Pollen percentages of Salix and Holocene, the homogeneous nature of thaw lake deposits yields Ericales are generally low, but are sufficient to indicate their presence erroneous radiocarbon dates. The single mid-Holocene radiocarbon on the mid-Holocene landscape. date at Flora Lake (Colinvaux, 1967a) equally is of little help in The late Holocene is characterized by the continued accumulation determining the specific timing of shrub Betula expansion on St. of Cyperaceae peat (zones AG4 and NA3) which eventually is replaced Lawrence Island. The herb-to-shrub-Betula transition has been dated by aeolian sands or soil (zones AG5 and NA 4). Zones AG4 and NA3 as early as ~14,000 14C (~17,400 cal yr BP) in western Alaska (Ager, display spectra with high percentages of Salix and/or Ericales pollen. 1982; Anderson, 1985; Ager, 2003) and ~12,500–12,000 14C (14,600– From ~5000 14C yr BP (5700 cal yr BP) to perhaps ~1000 14CyrBP 13,900 cal yr BP) in eastern Chukotka (Anderson et al., 2002). More (~930 cal yr BP), moist graminoid communities likely continued to recent studies suggest that the bulk sediment dates from Alaska likely dominate the area but with greater presence of Salix or Ericales placed this vegetation change too early. For example, a new record thickets. As during other periods, the change in vegetation probably from the Kotzebue Sound drainage, which uses an age model based on reflects shifts in edaphic conditions related to local factors, such as plant macrofossils, suggests that Betula shrub tundra became drainage. Alternatively, the increase in these taxa might possibly established closer to 12,000 14C yr BP (~13,900 cal yr BP; Abbott et indicate cooler conditions associated with the Neoglaciation (Mann et al., 2010). AMS dates from other areas of Alaska indicate late-glacial al., 1998). For example, an increase in boggy areas or persistence of amelioration began between ~13,000 and 11,800 14C yr BP (~15,700– snow bed communities would provide suitable habitats for these 13,600 cal yr BP) with the transition likely occurring earlier in areas plants. The most recent pollen spectra (zones AG5 and NA1) are near Bering Strait (Ager and Phillips, 2008). Radiocarbon ages on the variable and reflect the shift to sandy depositional environments. Siberian side remain consistent at ~12,400 14C yr BP (~14,500 cal yr While local factors associated with the development of the sand spit BP), regardless of sample type used in dating (Lozhkin, 1997). Given are probably the main influence on the pollen record, the reduction in the general similarity of pollen assemblages and the location of the recent peat formation is consistent with broader-scale trends noted study sites (i.e., still attached to the North American mainland), it below. seems probable that the expansion of shrub Betula in the vicinity of St. Lawrence Island occurred at least by ~13,000–12,000 14Cyr BP Discussion (~15,700–13,900 cal yr BP); i.e., approximately the time of vegetation change in other areas bordering central Beringia. The AG and NA sites yield only the smallest of spatial views into Chronology becomes more reliable for the AG exposure with late-glacial and Holocene landscapes of central Beringia, but when stabilization of the landscape and initiation of Sphagnum peat growth combined with other studies from the region provide an opportunity ~9700 yr BP (~11,200 cal yr BP; zone AG2). The accumulation of to explore various aspects of the history of the Land Bridge. We Sphagnum peat indicates locally less dynamic active-layer processes, consider three aspects of that environmental history: thermokarst as compared to the late glaciation. By the early Holocene, Betula shrub activity, peat formation, and the postglacial thermal maximum tundra no longer characterized the transBeringian vegetation. (PGTM). Deciduous forest (dominated by in eastern Beringia and Larix in western Beringia) and/or high shrub Betula–Alnus–Salix Thaw lakes and thermokarst tundra characterized much of the region between 11,000– 9000 14C yr BP (~12,900–10,200 cal yr BP; Edwards et al., 2005). The appearance of thaw lakes on St. Lawrence Island during the However, Betula shrub tundra apparently remained important near late glaciation is perhaps not surprising. Thermokarst terrain is the study sites during the earliest Holocene. Betula height cannot be characteristic of many low-lying areas of the Arctic and Subarctic such determined from pollen morphology, and no macrofossils were as coastal and flood plains (Hopkins, 1949; Tomirdiaro, 1982), and discovered that could clarify the size of the shrub. However, the although St. Lawrence represents a high point on the exposed Land boggy nature of the landscape and cooling climates caused by rising Bridge, its topographic relief is modest (generally b80 m with sea levels that separated St. Lawrence Island from mainland Beringia maximum elevation of ~870 m). Furthermore, the late glaciation is a at this time suggest that shrubs were likely of modest size. dynamic time, with most of the related paleoenvironmental changes 390 A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392 being capable of enhancing the initiation and/or persistence of Lozhkin et al., 2011). Inasmuch as our study sites may be taken as thermokarst processes. The onset of thermokarst activity reflects general indicators of paleoenvironments of St. Lawrence, the AG disturbances related to changes in thermal regimes in areas of frozen record suggests that peats developed somewhat later on the island ground (French, 1996). Often this disturbance is the result of shifts in than in other areas of Beringia, with organics beginning to accumulate air temperature, but increased seasonal precipitation can also alter by the late glacial–early Holocene transition. Whether this shift in active-layer thicknesses thereby beginning landscape changes asso- depositional environments is a function of local factors or of climate is ciated with freeze–thaw cycles (e.g., Pavlov, 1996). As the seas uncertain. Kremenetski et al. (2003) in their study of peat develop- encroached and the Land Bridge diminished, local maritime influences ment in the Western Siberian lowlands concluded that peat initiation, would have increased, bringing additional sources of moisture to the which began ~9800–9000 14Cyr BP (~11,200–10,200 cal yr BP) island. Hemispheric-scale shifts in atmospheric circulation as related declined during the PGTM (note: in this region the thermal maximum to the changing seasonal distribution of insolation, melting of dates to ~7200–3700 14C yr BP; ~8100–4500 cal yr BP), because of continental ice sheets, and reduction of sea ice in the North Pacific warm, dry conditions. The PGTM in Beringia, placed between ~11,000 impacted both regional and local climates, resulting in warmer and and 9000 14Cyr BP (~12,900–10,200 cal yr BP cal yr BP), saw moister conditions than during full-glacial times (Bartlein et al., 1991; relatively abundant peat development in western Beringia, leading Ager, 2003). A Beringian-wide marker of this late-glacial climatic Lozhkin et al. (2011) to suggest that effective moisture during the amelioration is the replacement of the LGM tundra by shrub Betula PGTM was greater than originally thought (see Edwards et al., 2005). tundra. The shift in vegetation not only confirms summers that were The increase in Apiaceae pollen at AG (zone AG2) may indicate the warmer and/or wetter than previously, but it also indicates a major establishment of vegetation similar to maritime tundra of Alaska and alteration of land-surface characteristics as compared to the LGM the , a vegetation associated with cool, foggy summers (Anderson et al., 2004; Edwards et al., 2005). The increasing and mild, snowy winters (Ager, 2003). Here again rising sea levels seasonality and greater ground moisture (from both atmospheric probably dampened the affects of increasing temperature, thereby sources and from changes in the active layer related to more extensive enhancing land-surface stabilization and peat accumulation. vegetation cover) could easily have been responsible for increased The appearance of Cyperaceae peat at both AG and NA during the thaw-lake activity documented at AG. mid-Holocene indicates a continuation of stable land-surfaces on the It is impossible to say whether the AG site is representative of the island, a rising ground water level, and an increase in nutrient larger central Beringian lowland during the late glaciation. However, availability (Moore et al., 1991). Ages for peat deposits also pollen and plant macrofossil analyses of peaty horizons in cores from correspond to an interval of renewed growth in western Beringia at the Chukchi and Bering seas indicate that these areas were likely ~5000 14C yr BP (~5800 cal yr BP; Lozhkin et al., 2011). By this time, dotted by small ponds (Elias et al., 1996), which were perhaps of the size and configuration of St. Lawrence Island were near modern, as thermokarst origin. Thermokarst processes also shaped landscapes was that of the coasts of the bordering continents. The establishment along the bordering mainlands. For example, in northern Alaska thaw of evergreen conifers in these central Beringian borderlands repre- lakes began forming after ~12,000 14C yr BP (~13,900 cal yr BP; sents an increase in effective moisture and boggy landscapes as McCulloch and Hopkins, 1966; Hopkins and Robinson, 1979; compared to the late glaciation. For example, increased snow depths Anderson, 1982). In western Beringia, thermokarst activity, which is brought Pinus pumila to far eastern Chukotka by the mid-Holocene characteristic of yedoma-alas relief, began ~12,500 14Cyr BP (Anderson et al., 2002). Wetter conditions enhanced the migration of (~14,600 cal yr BP; Lozhkin, 1991; Sher et al., 1979) This timing Picea mariana into western Alaska, where land surfaces paludified and contrasts to other areas farther west in , where formation of muskegs formed, processes also seen at this time in north-central alas dates to ~11,500 to 10,000 14C yr BP (~13,400–11,500 cal yr BP; Alaska (Anderson and Brubaker, 1994; Anderson et al., 2004). Lake- Katamura et al., 2006). If in fact polygonal relief and thaw lakes were level studies from interior Alaska indicate that the mid-Holocene was common across central Beringia, perhaps creating a landscape similar characterized by moist conditions (Edwards et al., 2001). Anomaly to modern-day coastal plains in Alaska and Siberia, the inundation of maps of modern circulation patterns associated with this study the Land Bridge would not have been the only factor that limited suggest that precipitation would also have been greater than present movement of people and herd animals. The shift to such a wet, in areas bordering Bering Strait. Thus, regional climatic patterns and unstable substrate with related changes in vegetation would certainly increased local maritime influences suggest that effective moisture have had an adverse impact on food sources for populations of these during the mid-Holocene was likely greater than previously. Such large . Wet terrain also presented dangers for the largest conditions promoted the development of an organic-rich landscape fauna, as attested to by the trapping and drowning of in on the island, and the boggy terrain apparently persisted to the latest Siberia (e.g., the Kirgilyakh ; Shilo et al., 1983). Such Holocene. changes perhaps restricted movement to river valleys during the Several regional studies have noted the general decline in peatland warmest months, especially if graminoid tussocks, growth forms initiation or peat accumulation since the mid-Holocene (Western common in this type of terrain, were abundant in interfluvial areas. Siberian Lowland; Peteet et al., 1998; northern , Gorham et al., 2007; western Beringia, Lozhkin et al., 2011). If the Peat formation in central Beringia trends at the study sites are not just reflecting local geomorphic processes, the St. Lawrence data suggest a decrease in peat The postglacial restructuring of Beringian landscapes included the accumulation during the latest Holocene, possibly the result of drier widespread development of peatlands during the latter part of the and/or cooler climates (Charman et al., 2009; Kremenetski et al., late glaciation into the early Holocene (Kaplina and Lozhkin, 1982; 2003). Lozhkin and Postolenko, 1989; Gorham et al., 2007; Lozhkin et al., 2011). Gorham et al. (2007) in a survey of North American peatland Postglacial thermal maximum history showed that the earliest peats in eastern and central Beringia dated to ~12,500 14C yr BP (~14,600 cal yr BP) and became particu- Paleobotanical evidence from eastern and western Beringia larly abundant by ~8600 14 C yr BP (~9600 cal yr BP). Western suggests that the PGTM occurred ~11,000–9000 14C yr BP (~12,900– Beringian peat development also began ~12,500 14C yr BP and is at 10,200 cal yr BP cal yr BP) and was characterized by the widespread a maximum between ~11,000 and 8000 14C BP (~12,900–8900 cal yr appearance of deciduous woodland (dominated by Populus in the east BP) with secondary peaks between ~6000–5000 14 C yr BP (~6800– and Larix in the west) and/or high shrub tundra (Edwards et al., 2005). 5800 cal yr BP) and ~4000–3000 14 C yr BP (~4500–3200 cal yr BP; The implication of the vegetation reconstructions is that this A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392 391 deciduous biome was widespread, perhaps encompassing all or most also thank Tom Ager and Mary Edwards for their insightful reviews of Beringia. Testing this idea is difficult because of the limited data which greatly improved the manuscript. available from the flooded Land Bridge. However, no plant macrofossil data exist for wooded landscapes on St. Lawrence Island or on the Chukchi or Bering shelves during either the late glaciation or early References Holocene (Binney et al., 2009). The strongest support for a postglacial Abbott, M.B., Edwards, M.E., Finney, B.P., 2010. A 40,000-year record of environmental thermal optimum in central Beringia is provided by insect remains change from Burial Lake in northwest Alaska. Quaternary Research 74, 156–165. which indicate warmer than present summer temperatures by Ager, T.A., 1982. Vegetational history of western Alaska during the Wisconsin glacial ~11,500–11,000 14C yr BP (~13,400–12,900 cal yr BP; Elias et al., interval and Holocene. In: Hopkins, D.M., Matthews Jr., J.V., Schweger, C.E., Young, – 1996; Kurek et al., 2009). Inferences drawn from the St. Lawrence S.B. (Eds.), Paleoecology of Beringia. Academic Press, New York, pp. 75 93. Ager, T.A., 1983. Holocene vegetational . In: Wright Jr., H.E. (Ed.), Late paleobotanical data are more suggestive than definitive. The relatively Quaternary Environments of the . : The Holocene, Vol. 1. University of high percentages of Betula pollen in AG (up to 35%) and NA (up to Minnesota Press, Minneapolis, pp. 128–141. 25%) records, as compared to late Holocene spectra, would suggest Ager, T.A., 2003. Late Quaternary vegetation and climate history of the central Bering land bridge from St. Michael Island, western Alaska. Quaternary Research 60, that shrub Betula was well established along southern portions of 19–32. central Beringia. Unfortunately, we do not know if these spectra Ager, T.A., Phillips, R.L., 2008. Pollen evidence for late Bering land bridge include all of the late glaciation, but the 10,750 14Cyr BP environments from Norton Sound, northeastern Bering Sea, Alaska. Arctic, Antarctic, and Alpine Research 40, 451–461. (~12,700 cal yr BP) date from the uppermost detrital level suggests Anderson, P.M., 1982. Reconstructing the Past: The Synthesis of Archeological and at least part of this record encompasses the regional thermal Palynological Data, Northern Alaska and Northwestern . Brown University, maximum. Increased thermokarst activity, as indicated by the suite PhD dissertation. Anderson, P.M., 1985. Late Quaternary vegetational change in Kotzebue Sound area, of thaw lakes at AG, is often associated with warmer summers and northwestern Alaska. Quaternary Research 24, 307–321. greater seasonal precipitation (French, 1966). The restriction of Anderson, P.M., Brubaker, L.B., 1986. Modern pollen assemblages from northern Alaska. thermokarst lakes to the late glaciation (i.e., the birch zones of AG1 Review of Palaeobotany and Palynology 46, 273–291. Anderson, P.M., Brubaker, L.B., 1994. Vegetation history of northcentral Alaska: a and NA1) is further suggestive that climate was warmer than present mapped summary of late-Quaternary pollen data. Quaternary Science Reviews 13, on the island. However, local conditions related to the flooding Land 71–92. Bridge mitigated any insolation-driven warming that at the time were Anderson, P.M., Kotov, A.N., Lozhkin, A.V., Trumpe, M.A., 2002. Palynological and primarily responsible for the rising summer temperatures (Edwards radiocarbon data from late Quaternary deposits of Chukotka. In: Anderson, P.M., Lozhkin, A.V. (Eds.), Late Quaternary Vegetation and Climate of Siberia and the et al., 2005). Young (1978) noted that arctic lowland tundra today is (Palynological and Radiocarbon Database). NOAA and Russian found no more than 300 km from the sea and requires the moderating Academy of Sciences, North East Science Center, Magadan, pp. 35–79. influences of a maritime climate. Thus, while insolation forcing may Anderson, P.M., Edwards, M.E., Brubaker, L.B., 2004. Results and paleoclimatic fi implications of 35 years of paleoecological research in Alaska. In: Gillespie, A.R., have increased temperatures suf ciently for the spread of shrub Porter, S.C., Atwater, B.F. (Eds.), The Quaternary Period in the United States Betula in south-central Beringia, the cool oceanic summers would not Development in Quaternary Science, 1. Elsevier, New York, pp. 427–440. support the establishment of a cool deciduous forest. While it is Bartlein, P.J., Anderson, P.M., Edwards, M.E., McDowell, P.F., 1991. A framework for interpreting paleoclimatic variations in eastern Beringia. Quaternary International possible that woodlands occurred in more interior parts of central 10–12, 73–83. Beringia, the low-lying nature of the terrain and possible extensive Bartlein, P.J., Anderson, K.H., Anderson, P.M., Edwards, M.E., Mock, C.J., Thompson, R.S., thermokarst activity, at least during the early PGTM (if St. Lawrence Wegg, R.S., Webb III, T., Whitlock, C., 1998. Paleoclimate simulations for North America over the past 21,000 years: features of the simulated climate and Island is indicative of a broader region), may have presented a much comparisons with paleoenvironmental data. Quaternary Science Reviews 6–7, wetter landscape than found in the forested valleys of eastern and 549–586. western Beringia. Binney, H.A., Willis, K.J., Edwards, M.E., Shonil, S.A., Anderson, P.M., Andreev, A.A., Blaauw, M., Damblon, F., Haesaerts, P., Kienast, F., Kremenetski, K.V., Krivonogov, Colinvaux (1967a) originally proposed the mid-Holocene as the S.K.,Lozhkin,A.V.,MacDonald,G.M.,Novenko, E.Yu., Oksanen, P., Sapelko, T.V., climatic optimum based on increases in pollen of Picea and Alnus in Valiranto, M., Vazhenina, L.N., 2009. The distribution of late-Quaternary woody Flora Lake sediments. The rise in these taxa approximate times of taxa in northern : evidence from a new macrofossil database. Quaternary – incursion of these species into southwestern Alaska and the Kotzebue Science Reviews 28, 2445 2464. Brigham-Grette, J., Lozhkin, A.V., Anderson, P.M., Glushkova, O.Yu., 2004. Paleoenvir- Sound drainage (Ager, 1982,1983; Anderson, 1985; Brubaker et al., onmental conditions before and during the last glacial maximum. In: Madsen, D.B. 2001; Ager, 2003), although shrub Alnus (Duschekia fruticosa) (Ed.), Entering America: Northeast and Beringia before the Last Glacial – established in areas of southern Chukotka by ~12,000 14Cyr BP Maximum. The University of Utah Press, Salt Lake City, pp. 29 61. Brubaker, L.B., Anderson, P.M., Hu, F.S., 2001. Vegetation ecotone dynamics in (~13,900 cal yr BP). Thus, the increases of these taxa in the Flora Lake southwest Alaska during the late Quaternary. Quaternary Science Reviews 20, record are likely the result of long-distance wind transport, with 175–188. sources on the Alaskan side. Evidence of a mid-Holocene “Hypsither- Colinvaux, P.A., 1964. The environment of the Bering land bridge. Ecological – ” Monographs 34, 297 329. mal is somewhat ambiguous in the Pribilof Islands, which lie to the Colinvaux, P.A., 1967a. A long pollen record from St. Lawrence Island, Bering Sea south of St. Lawrence. Changes in the pollen records are subtle and (Alaska). Palaeogeography, Palaeoclimatology, Palaeoecology 3, 29–48. suggest both a slight cooling during the late Holocene and no change Colinvaux, P.A., 1967b. Quaternary vegetational history of arctic Alaska. In: Hopkins, D.M. (Ed.), The Bering Land Bridge. Stanford University Press, Stanford, pp. 207–231. at all (Parrish, 1979). Heusser (1973, 1978) inferred a mid-Holocene Colinvaux, P.A., 1981. Historical ecology of Beringia: the south land bridge coast of thermal maximum for the , but this interpretation also St. Paul Island. Quaternary Research 16, 18–36. is based on quite subtle changes to the pollen records. Data from AG Edwards, M.E., Mock, C.J., Finney, B.P., Barber, V.A., Bartlein, P.J., 2001. Potential analogues for paleoclimatic variations in eastern interior Alaska during the past and NA do not support a mid-Holocene thermal maximum but rather 14,000 yr: atmospheric-circulation controls of regional temperature and moisture suggest persistently cool climates. responses. Quaternary Science Reviews 20, 189–202. Edwards, M.E., Brubaker, L.B., Anderson, P.M., Lozhkin, A.V., 2005. Functionally novel biomes: a response to past warming in Beringia. Ecology 86, 1696–1703. Elias, S.A., 2001. Mutual climatic range reconstructions of season temperatures based Acknowledgments on beetle assemblages in eastern Beringia. Quaternary Science Reviews 20, 77–91. Elias, S.A., Short, S.K., Nelson, C.H., Birks, H.H., 1996. The life and times of the Bering land This work was supported by the Far East Branch, Russian Academy – З У bridge. Nature 382, 60 63. of Sciences (projects 09-I-OH -11; 09-II- O-08-003), the Russia Elias, S.A., Short, S.K., Birks, H.H., 1997. Late Wisconsin environments of the Bering land Foundation for Fundamental Science (project 06-05-64129), and the bridge. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 293–308. U.S. National Science Foundation. We acknowledge the great help French, H.M., 1996. The Periglacial Environment, Second Edition. Longman, London. fi Goetcheus, V.G., Birks, H.H., 2001. Full-glacial upland tundra vegetation preserved provided by David Hopkins and Julie Brigham-Grette during eld under tephra in the , , Alaska. Quaternary work. We thank Julya Korzun for help in preparing the figures. We Science Reviews 20, 135–147. 392 A.V. Lozhkin et al. / Quaternary Research 76 (2011) 383–392

Gorham, E., Lehman, C., Dyke, A., Janssens, J., Dyke, L., 2007. Temporal and spatial Lozhkin, A.V., Anderson, P.M., Eisner, W.R., Rovako, L.G., Hopkins, D.M., Brubaker, L.B., aspects of peatland initiation following in North America. Quaternary Colinvaux, P.A., Miller, M.C., 1993. Late Quaternary lacustrine pollen records from Science Reviews 26, 300–311. southwestern Beringia. Quaternary Research 39, 314–324. Guthrie, R.D., 2001. Origin and causes of the mammoth : a story of cloud cover, Lozhkin, A.V., Anderson, P.M., Eisner, W.R., Hopkins, D.M., Brubaker, L.B., 1996. The tooth pits, buckles, and inside-out Beringia. Quaternary Science change of vegetation cover of western Alaska during the last 18000 years. In: Reviews 20, 549–574. Bichkiv, Y.M. (Ed.), Quaternary Climates and Vegetation of Beringia. North East Harry, D.G., French, H.M., 1983. The orientation and of thaw lakes: southwest Interdisciplinary Science Research Institute, Far East Branch, Russian Academy of Banks Island, Canadian Arctic. Permafrost 4th International Conference, pp. 456–461. Sciences, Magadan, pp. 31–42 (In Russian). Heusser, C.J., 1973. Postglacial vegetation on Umnak Island, Aleutian Islands, Alaska. Lozhkin, A.V., Hopkins, D.M., Solomatkina, T.B., Eisner, W.R., Brigham-Grette, J., 1998. Review of Palaeobotany and Palynology 15, 277–285. Radiocarbon dates and palynological characteristics of peat from St. Lawrence Heusser, C.J., 1978. Postglacial vegetation on , Aleutian Islands, Alaska. Island, Alaska. In: Simakov, K.V. (Ed.), Environmental Changes in Beringia during Bulletin of the Torrey Botanical Club 105, 18–23. the Quaternary. North East Science Center, Far East Branch, Russian Academy of Hinkel, K.M., Jones, B.M., Eisner, W.R., Cuomo, C.J., Beck, R.A., Frohn, R., 2007. Methods Sciences, Magadan, pp. 9–27. In Russian. to assess natural and anthropogenic thaw lake drainage on the western arctic Lozhkin, A.V., Anderson, P.M., Vazhenina, L.N., 2011. Younger Dryas and early Holocene coastal plain of northern Alaska. Journal of Geophysical Research 112, FO2S16. age peats from the north of Far East Russia. Quaternary International 237 (1–2), doi:10.1029/2006JF000584. 54–64. Hoefle, C., Edwards, M.E., Hopkins, D.M., Mann, D.H., Ping, C.-L., 2000. The full-glacial Manley, W.F., 2002. Post-glacial Flooding of the Bering Land Bridge: A Geospatial environment of the northern Seward Peninsula, Alaska, reconstructed from the Animation. INSTAAR, University of Colorado. http://instaar.colorado.edu/qgisl/ 18,000-year old Kitluk paleosol. Quaternary Research 53, 143–153. bering_land_bridge. Hopkins, D.M., 1949. Thaw lakes and thaw sinks in the Imuruk Lake area, Seward Mann, D.H., Crowell, A.L., Hamilton, T.D., Finney, B.P., 1998. Holocene geologic and Peninsula, Alaska. Journal of Geology 57, 119–131. climatic history around the Gulf of Alaska. Arctic Anthropology 35, 112–131. Hopkins, D.M., 1982. Aspects of the paleogeography of Beringia during the late Matthews Jr., J.V., 1974. Quaternary environments at Cape Deceit (Seward Peninsula, Pleistocene. In: Hopkins, D.M., Matthews Jr., J.V., Schweger, C.E., Young, S.B. (Eds.), Alaska): evolution of a tundra . Geological Society of America Bulletin 85, Paleoecology of Beringia. Academic Press, New York, pp. 3–28. 1353–1385. Hopkins, D.M., Kidd, J.G., 1988. Thaw lake sediments and sedimentary environments. McCulloch, D.S., Hopkins, D.M., 1966. Evidence for an early recent warm interval in Permafrost Fifth International Conference Proceedings (August 25), pp. 790–795. northwestern Alaska. Geological Society of America Bulletin 77, 1089–1108. Hopkins, D.M., Robinson, S.W., 1979. Radiocarbon dates from the Beaufort and Chukchi Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, Second edition. Blackwell Sea coasts. U.S. Geological Survey Circular, 804-B, pp. 44–47. Scientific Publications, Oxford. Hopkins, D.M., McNeil, F.S., Leopold, E.B., 1960. The coastal plain at Nome, Alaska: a late PALE, 1994. Research protocol for PALE paleoclimates of Arctic Lakes and Estuaries. Cenozoic type section for the Bering Strait region. Report of the 11th International PAGES Workshop Report Series 94–1. Bern. Geological Congress, 4, pp. 46–57. Parrish, L., 1979. A Record of Holocene Climatic Changes from St. George Island, Hopkins, D.M., Matthews Jr., J.V., Schweger, C.E., Young, S.B. (Eds.), 1982. Paleoecology Pribilofs. The Ohio State University, M.S. Thesis. of Beringia. Academic Press, New York. Pavlov, A.V., 1996. Permafrost-climatic monitoring of Russia: analysis of field data and Kaplina, T.N., Lozhkin, A.V., 1982. History of the vegetation evolution at coastal forecast. Polar Geography and Geology 20, 44–64. lowlands of Yakutia during Holocene. In: Velichko, A.A., Spasskaya, I.I., Khotinsky, Peteet, D., Andreev, A., Bardeen, W., Mistretta, F., 1998. Long-term arctic peatland N.A. (Eds.), Evolution of the Environment of the USSR Territory during the Late dynamics, vegetation and climate history of the Pur-Taz region, western Siberia. Pleistocene and Holocene. Nauka, Moscow, pp. 207–220 (In Russian). Boreas 27, 115–126. Katamura, F., Fukuda, M., Bosikov, N.P., Desyatkin, R.V., Nakamura, T., Moriizumi, J., Rex, R.W., 1961. Hydrodynamic analysis of circulation and orientation of lakes in 2006. Thermokarst formation and vegetation dynamics inferred from a palyno- northern Alaska. In: Raasch, G.O. (Ed.), Geology of the Arctic, v. 2. University of logical study in central Yakutia, eastern Siberia, Russia. Arctic, Antarctic, and Alpine Toronto Press, Toronto, pp. 1021–1043. Research 38, 561–570. Sher, A.V., Kaplina, T.N., Giterman, R.E., Lozhkin, A.V., Arkhangelov, A.A., Kiselyov, S.V., Kremenetski, K.V., Velichko, A.A., Borisova, O.K., MacDonald, G.M., Smith, L.C., Frey, K.E., Kouznesov, Yu.V., Virina, E.I., Zazhigin, 1979. Scientific Excursion on Problem “Late Orlova, L.A., 2003. Peatlands of the Western Siberian lowlands: current knowledge Cenozoic of the Lowland” Tour XI The Guide XIV Pacific Science Congress. on zonation, carbon content and late Quaternary history. Quaternary Science USSR Academy of Sciences, Moscow. Reviews 22, 703–723. Shilo, N.A., Lozhkin, A.V., Titov, E.E., Schumilov, Yu.V., 1983. The Kirgilyakh Mammoth. Kurek, J., Cwynar, L.C., Ager, T.A., Abbott, M.B., Edwards, M.E., 2009. Late Quaternary Nauka, Moscow. paleoclimate of western Alaska inferred from fossil chironomids and its relation to Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon vegetation histories. Quaternary Science Reviews 28, 799–811. calibration program. Radiocarbon 35, 215–230. Lozhkin, A.V., 1991. The questions of geochronology and paleogeography of the Tomirdiaro, S.V., 1982. Evolution of lowland landscapes in northeastern Asia during late Quaternary period of the northeast of the USSR. Geology of the Transition Zone Quaternary time. In: Hopkins, D.M., Matthews Jr., J.V., Schweger, C.E., Young, S.B. between the and Ocean in Northeast USSR. North East Interdisciplinary (Eds.), Paleoecology of Beringia. Academic Press, New York, pp. 29–37. Science Research Center, Far East Branch, USSR Academy of Sciences, Magadan, pp. Webber, P.J., Miller, P.C., Chapin III, F.S., McCown, B.H., 1980. The vegetation: pattern 47–59 (In Russian). and succession. In: Brown, J., Tieszen, L.L., Bunnell, F.L. (Eds.), An Arctic Ecosystem Lozhkin, A.V., 1997. Environmental history of Beringia during the late Pleistocene and The Coastal Tundra at Barrow, Alaska. Dowden Hutchinson & Ross, Stroudsburg, Holocene: some results of joint Russian–American research. In: Gagiev, M.Kh. (Ed.), pp. 186–218. Late Pleistocene and Holocene of Beringia. North East Science Center, Far East Young, S.B., 1971. The vascular flora of Saint Lawrence Island, with special reference to Branch, Russian Academy of Sciences, Magadan, pp. 5–22 (In Russian). floristic zonation in the arctic regions. Contribution from the Gray Herbarium 201, Lozhkin, A.V., 2002. Boundaries of Beringia during the Late Pleistocene and Holocene. In: 11–115. Simakov, K.V. (Ed.), Quaternary Paleogeography of Beringia. North East Science Young, S.B., 1978. Phytogeography of the . In: Yurtsev, B.A. (Ed.), Center, Far East Branch, Russian Academy of Sciences, Magadan, pp. 4–12 (In Russian). The Arctic Floristic Region. Nauka, Leningrad, pp. 105–126. In Russian. Lozhkin, A.V., Postolenko, G.A., 1989. New data about the environmental evolution of Young, S.B., 1982. The vegetation of land-bridge Beringia. In: Hopkins, D.M., Matthews the mountain region of the Kolyma region during the late Anthropogene. Doklady Jr., J.V., Schweger, C.E., Young, S.B. (Eds.), Paleoecology of Beringia. Academic Press, Akademii Nauk 307 (5), 1184–1188 (In Russian). New York, pp. 179–191.