北美东部威斯康星中、晚期古环境与气候变化的甲虫化石证据

北美Laurentide冰盖于末次冰期的大部分时间在北美东北部一直存在，冰盖的活动对北美东部冰期时的环境和气候有很大的影响。甲虫分析表明，北美东部中纬度地区在威斯康星中、晚期的气候以寒冷为主，中间分别在42000aB.P.,34000aB.P.和28000aB.P.出现3次明显的气候变暖。此区威斯康星中期气候冷暖波动的幅度和出现的时间可以与同期的格陵兰冰芯记录及北大西洋Heinrich事件对比。     

A record of late Quaternary vegetation from Davis Lake,southern Puget Lowland,Washington

Pollen and macrofossil analyses of a core spanning 26,000 yr from Davis Lake reveal late Pleistocene and Holocene vegetational patterns in the Puget Lowland. The core ranges lithologically from a basal inorganic clay to a detritus gyttja to an upper fibrous peat and includes eight tephra units. The late Pleistocene pollen sequence records two intervals of tundra-parkland vegetation. The earlier of these has high percentages of Picea, Gramineae, and Artemisia pollen and represents the vegetation during the Evans Creek Stade (Fraser Glaciation) (ca. 25,000–17,000 yr B.P.). The later parkland interval is dominated by Picea, Tsuga mertensiana, and Gramineae. It corresponds to the maximum ice advance in the Puget Lowland during the Vashon Stade (Fraser Glaciation) (ca. 14,000 yr B.P.). An increase in Pinus ontorta pollen between the two tundra-parkland intervals suggests a temporary rise in treeline during an unnamed interstade. After 13,500 yr B.P., a mixed woodland of subalpine and lowland conifers grew at Davis Lake during a period of rapid climatic amelioration. In the early Holocene, the prolonged expansion of Pseudotsuga and Alnus woodland suggests dry, temperate conditions similar to those of present rainshadow sites in the Puget Lowland. More-mesic forests of Tsuga eterophylla, Thuja plicata, and Pseudotsuga, similar to present lowland vegetation, appeared in the late Holocene (ca. 5500 yr B.P.).     

Early Wisconsinan (MIS 4) Arctic ground squirrel middens and a squirrel-eye-view of the mammoth-steppe

Fossil arctic ground squirrel (Spermophilus parryii) middens were recovered from ice-rich loess sediments in association with Sheep Creek-Klondike and Dominion Creek tephras (ca 80 ka) exposed in west-central Yukon. These middens provide plant and insect macrofossil evidence for a steppe-tundra ecosystem during the Early Wisconsinan (MIS 4) glacial interval. Midden plant and insect macrofossil data are compared with those previously published for Late Wisconsinan middens dating to ～25–29¹⁴C ka BP (MIS 3/2) from the region. Although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils, the co-occurrence of steppe-tundra plants and insects (e.g., Elymus trachycaulus, Kobresia myosuroides, Artemisia frigida, Phlox hoodii, Connatichela artemisiae) provides evidence for successive reestablishment of the zonal steppe-tundra habitats during cold stages of the Late Pleistocene. Arctic ground squirrels were well adapted to the cold, arid climates, steppe-tundra vegetation and well-drained loessal soils that characterize cold stages of Late Pleistocene Beringia. These glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of Alaska and Yukon, including the Klondike, where they are absent today. Arctic ground squirrels have endured numerous Quaternary climate oscillations by retracting populations to disjunct “interglacial refugia” during warm interglacial periods (e.g., south-facing steppe slopes, well-drained arctic and alpine tundra areas) and expanding their distribution across the mammoth-steppe biome during cold, arid glacial intervals.     

Sedimentology and palynology of Middle Wisconsinan deposits in the Pecatonica River valley,Wisconsin and Illinois

The bedrock valley of the Pecatonica River north of Freeport, Illinois, contains a thick valley-fill complex of alluvium and drift. Within the valley, loess-capped benches surround hills of silty Illinoian drift. Beneath these benches lie thick deposits of poorly sorted stony silt interbedded with thin lenses of silt, sand, and organic-rich loam. Channel deposits and peat cap the diamicton in places. We interpret the stony silts as solifluction debris shed from silty slopes within the valley-fill during the Early or Middle Wisconsinan (Altonian). Top and bottom radiocarbon dates from a 2.5-m section of peat overlying the diamicton are 26,820 ± 200 and 40,500 ± 1700 yr B.P., respectively. We informally refer to the stony silts, channel sediments, and peat as the “Martintown unit.” Geomorphic position, sediment input, and macrofossils suggest that the dated peat was deposited in a floodplain pond (oxbow?). The pollen record from the peat indicates that a boreal forest dominated this area during the Middle Wisconsinan (late Altonian and Farmdalian). Two pollen zones are recognized: a basal Zone I with Pinus slightly more abundant than Picea and with few herbs and shrubs, and an upper Zone II dominated by Picea and with a larger representation of herbaceous and shrub taxa. Little displacement of vegetation zones is indicated, even though ice advanced to within 100 km of the site during the time of peat accumulation. Because of the problems involved in clearly defining Middle Wisconsinan forest-tundra in mid-latitudes by using analogs of Holocene forest-tundra in high latitudes, caution is required in making geomorphic inferences solely from vegetation data. Together, though, pollen and sediment data indicate that during the Middle Wisconsinan, Pecatonica hillslopes progressed through a sequence of instability-stability-instability related to climatic fluctuations.     

Paleoecology of a >90,000-year lacustrine sequence from Fog Lake, Baffin Island, Arctic Canada

A 137 cm sediment core from Fog Lake, a small upland basin that was not glaciated during the Late Wisconsinan, reveals the following stratigraphic succession: (I) basal diamicton, (II) laminated silts, (III) compacted dewatered gyttja, (IV) stratified minerogenic sediments, and (V) overlying gyttja. Units I–III, and most of unit IV, were deposited prior to the Holocene; unit V represents approximately the last 8000 ¹⁴C years. Alternate luminescence dating techniques (TL and IRSL) place unit III in latter oxygen isotope stage (OIS) 5, >74,000 years BP. A suite of 20 AMS ¹⁴C dates suggests that much of the post-OIS 5 paleoenvironmental record is characterized by either extremely low continuous sediment accumulation, or by one or more episodes of more rapid sedimentation. Diverse pollen and spore assemblages have concentrations that are unexpectedly high throughout the pre-Holocene sediments (>10,000 grains cm⁻³), by comparison to the Holocene (5000 grains cm⁻³). Exceedingly low sediment accumulation rates appear to have resulted in the concentration of atmospherically derived pollen and spores. Diatom assemblages record the evolution of the lake's chemistry and past algal production. Several abrupt changes occurred synchronously in both the terrestrial and aquatic components of the system, and these appear to represent ecological responses to pre-Holocene climatic variability. The sediments of Fog Lake provide the first integrated paleoecological record older than Holocene for a vast region of the Canadian Arctic, thereby refining our understanding of long-term arctic ecosystem ontogeny.     

Ice sheet extent and early deglacial history of the southwestern sector of the Greenland Ice Sheet

The offshore and coastal geomorphology of southwest Greenland records evidence for the advance and decay of the Greenland Ice Sheet during the Last Glacial Maximum. Regional ice flow patterns in the vicinity of Sisimiut show an enlarged ice sheet that extended southwestwards on to the shelf, with an ice stream centred over Holsteinsborg dyb. High level periglacial terrain composed of blockfield and tors is dated to between 101 and 142 ka using ²⁶Al and ¹⁰Be cosmogenic exposure ages. These limit the maximum surface elevation of the Last Glacial Maximum ice sheet in this part of southwest Greenland to ca 750–810 m asl, and demonstrate that terrain above this level has been ice free since MIS 6. Last Glacial Maximum ice thickness on the coast of ca 700 m implies that the ice sheet reached the mid to outer continental shelf edge to form the Outer Hellefisk moraines. Exposure dates record ice surface thinning from 21.0 to 9.8 ka, with downwasting rates varying from 0.06 to 0.12 m yr⁻¹. This reflects strong surface ablation associated with increased air temperatures running up to the Bølling Interstadial (GIS1e) at ca 14 ka, and later marine calving under high sea levels. The relatively late retreat of the Itilleq ice stream inland of the present coastline is similar to the pattern observed at Jakobshavn Isbræ, located 250 km north in Disko Bugt, which also retreated from the continental shelf after ca 10 ka. We hypothesise that the ice streams of West Greenland persisted on the inner shelf until the early Holocene because of their considerable ice thickness and greater ice discharge compared with the adjacent ice sheet.     

Abrupt vegetation changes during the last glacial to Holocene transition in mid‐latitude South America

A pollen record from the Huelmo site (ca. 41°30′S) shows that vegetation and climate changed at millennial time‐scales during the last glacial to Holocene transition in the mid‐latitude region of western South America. The record shows that a Nothofagus parkland dominated the landscape between 16 400 and 14 600 ¹⁴C yr BP, along with Magellanic Moorland and cupressaceous conifers. Evergreen North Patagonian rainforest taxa expanded in pulses at 14 200 and 13 000 ¹⁴C yr BP, following a prominent rise in Nothofagus at 14 600 ¹⁴C yr BP. Highly diverse, closed canopy rainforests dominated the lowlands between 13 000 and 12 500 ¹⁴C yr BP, followed by the expansion of cold‐resistant podocarps and Nothofagus at ca. 12 500 and 11 500 ¹⁴C yr BP. Local disturbance by fire favoured the expansion of shade‐intolerant opportunistic taxa between 10 900 and 10 200 ¹⁴C yr BP. Subsequent warming pulses at 10 200 and 9100 ¹⁴C yr BP led to the expansion of thermophilous, summer‐drought resistant Valdivian rainforest trees until 6900 ¹⁴C yr BP. Our results suggest that cold and hyperhumid conditions characterised the final phase of the Last Glacial Maximum (LGM), between 16 400 and 14 600 ¹⁴C yr BP. The last ice age Termination commenced with a prominent warming event that led to a rapid expansion of North Patagonian trees and the abrupt withdrawal of Andean ice lobes from their LGM positon at ca. 147 000 ¹⁴C yr BP. Hyperhumid conditions prevailed between 16 400 and 13 000 ¹⁴C yr BP, what we term the ‘extreme glacial mode’ of westerly activity. This condition was brought about by a northward shift and/or intensification of the southern westerlies. The warmest/driest conditions of the last glacial–interglacial transition occurred between 9100 and 6900 ¹⁴C yr BP. During this period, the westerlies shifted to an ‘extreme interglacial mode’ of activity, via a poleward migration of stormtracks. Our results indicate that a highly variable climatic interval lasting 5500 ¹⁴C years separate the opposite extremes of vegetation and climate during the last glacial‐interglacial cycle, i.e. the end of the LGM and the onset of the early Holocene warm and dry period. Copyright © 2003 John Wiley & Sons, Ltd.     

Vegetation and climate history in the Laptev Sea region (Arctic Siberia) during Late Quaternary inferred from pollen records

Paleoenvironmental records from a number of permafrost sections and lacustrine cores from the Laptev Sea region dated by several methods (¹⁴C-AMS, TL, IRSL, OSL and ²³⁰Th/U) were analyzed for pollen and palynomorphs. The records reveal the environmental history for the last ca 200 kyr. For interglacial pollen spectra, quantitative temperature values were estimated using the best modern analogue method. Sparse grass-sedge vegetation indicating arctic desert environmental conditions existed prior to 200 kyr ago. Dense, wet grass-sedge tundra habitats dominated during an interstadial ca 200–190 kyr ago, reflecting warmer and wetter summers than before. Sparser vegetation communities point to much more severe stadial conditions ca 190–130 kyr ago. Open grass and Artemisia communities with shrub stands (Alnus fruticosa, Salix, Betula nana) in more protected and moister places characterized the beginning of the Last Interglacial indicate climate conditions similar to present. Shrub tundra (Alnus fruticosa and Betula nana) dominated during the middle Eemian climatic optimum, when summer temperatures were 4–5 °C higher than today. Early-Weichselian sparse grass-sedge dominated vegetation indicates climate conditions colder and dryer than in the previous interval. Middle Weichselian Interstadial records indicate moister and warmer climate conditions, for example, in the interval 40–32 kyr BP Salix was present within dense, grass-sedge dominated vegetation. Sedge-grass-Artemisia-communities indicate that climate became cooler and drier after 30 kyr BP, and cold, dry conditions characterized the Late Weichselian, ca 26–16 kyr BP, when grass-dominated communities with Caryophyllaceae, Asteraceae, Cichoriaceae, Selaginella rupestris were present. From 16 to 12 kyr BP, grass-sedge communities with Caryophyllaceae, Asteraceae, and Cichoriaceae indicate climate was significantly warmer and moister than during the previous interval. The presence of Salix and Betula reflect temperatures about 4 °C higher than present at about 12–11 kyr BP, during the Allerød interval, but shrubs were absent in the Younger Dryas interval, pointing to a deterioration of climate conditions. Alnus fruticosa, Betula nana, Poaceae, and Cyperaceae dominate early Holocene spectra. Reconstructed absolute temperature values were substantially warmer than present (up to 12 °C). Shrubs gradually disappeared from coastal areas after 7.6 kyr BP when vegetation cover became similar to modern. A comparison of proxy-based paleoenvironmental reconstructions with the simulations performed by an Earth system model of intermediate complexity (CLIMBER-2) show good accordance between the regional paleodata and model simulations, especially for the warmer intervals.     

Temperature variability and vertical vegetation belt shifts during the last ∼50,000 yr in the Qilian Mountains (NE margin of the Tibetan Plateau, China)

A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around ∼46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by ∼4-7°C than today. Markedly higher temperatures were inferred from higher arboreal pollen frequencies between ∼13,000 and ∼7,000 yr ago with a Holocene temperature optimum and a maximal Picea-Betula mixed-forest expansion between ∼9,000 and ∼7000 yr ago, when temperatures exceeded the present-day conditions by at least 1-2°C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.     

Early Holocene climate signals from stable isotope composition of ice wedges in the Chara Basin,northern Transbaikalia,Russia

Stable isotope composition of syngenetic and epigenetic ice wedges, radiocarbon age, and pollen spectra of the surrounding deposits were studied during long term investigations at the "Belyi Klyuch" site on the first(6-8 m height) terrace of the Chara River(720 m.a.s.l.) in northern Transbaikalia to assess climatic conditions during ice-wedge formation. It was revealed that Holocene ice wedges had been formed from 10 to 7.5 ka 14 C BP. The isotope composition(δ~(18)O, δ~2 H) of relict ice wedges is the lightest and amounts-23‰ and-185‰, correspondingly. The isotopic compositions of ice lenses from sandy loam above ice wedges are-15.7‰, and-133‰; of small ice wedge in peat and sand are-15.3‰ and-117.9‰, accordingly.Interpretation of the ice wedge isotope composition has yielded that mean winter temperatures during cold stages of Holocene optimum were lower than today, during warm stages they were close to modern ones. During the coldest stages of Holocene optimum the total annual freezing index varied from-5100 to-5700 ℃ degree days, i.e. 300-600 ℃ degree days colder than during extremely severe modern winters. The total annual thawing index varied from 1300 to 1800 ℃ degree days, which was slightly higher than modern ones.     

Late Quaternary sediments and stratigraphy on the continental shelf off Troms and west Finnmark, northern Norway

Lithologic, paleontologic, and chronostratigraphic investigation of 13 gravity cores indicates the following environmental evolution: a high- (mid-) arctic period with a slight influx of ice-rafted debris occurred during the early middle Weichselian followed by a mid- (high-) arctic environment with a high influx of iceberg-rafted debris during the remainder of the middle Weichselian. The continental ice sheet probably did not extend beyond the inner shelf during middle Weichselian and a minimum relative sea level was ca. −120 m. A low-arctic environment occurred during (parts of) the late Weichselian with an initial winnowing of the sediments. The Norwegian Current entered the area during this substage. A high- (mid-) boreal environment occurred during the Holocene with high winnowing activity in the early Holocene. Winnowing is still very active on the shallower banks in contrast to the deeper banks where it has ceased. Relatively high percentages of carbonate in the form of biogenic skeletal remains occur in the Holocene sediments.     

Late Pleistocene and Holocene climate and geomorphic histories as interpreted from a 23,000 C yr B.P. paleosol and floodplain soils, southeastern West Virginia, USA

Field, micromorphological, pollen, whole soil (XRF), and stable isotope geochemical methods were used to evaluate the latest Pleistocene to Holocene climate record from a floodplain-terrace system in southeastern West Virginia. A late Pleistocene (22,940 ± 150 ¹⁴C yr B.P.) silt paleosol with low-chroma colors formed from fluviolacustrine sediment deposited during the last glacial maximum (Wisconsinan) and records a cooler full-glacial paleoclimate. Fluvial gravel deposited between the latest Pleistocene and earliest Holocene (prior to 6360 ± 40 ¹⁴C yr B.P.) was weathered in the middle Holocene under warmer, drier climate conditions, possibly correlated with the Hypsithermal and Altithermal Events of the eastern and southwestern United States, respectively. The glacial to interglacial climate shift is recorded by: (1) changes from a poorly drained landscape with fine-textured soil, characterized by high organic C and redoximorphic features related to Fe removal and concentration, to a well-drained, coarse-textured setting without gley and with significant argillic (Bt) horizon development; (2) changes from a high Zr and Ti silt-dominated parent material to locally derived, coarse fluvial gravels lower in Zr and Ti; (3) a shift from dominantly conifer and sedge pollen in the paleosol to a modern oak/hickory hardwood assemblage; and (4) a shift in δ¹³C values of soil organic matter from −28‰ to −24‰ PDB, suggesting an ecosystem shift from cooler, C3-dominated flora to one that was mixed C3 and C4, but still predominantly composed of C3 plants. A root-restrictive placic horizon developed between the late Pleistocene silt paleosol and the overlying fluvial gravel because of the high permeability contrast between the two textures of soil materials. This layer formed a barrier that effectively isolated the Pleistocene paleosol from later Holocene pedogenic processes.     

Late Quaternary contourites and glaciomarine sedimentation in the Fram Strait

Two sites in the eastern Fram Strait, the Vestnesa Ridge and the Yermak Plateau, have been surveyed and sampled providing a depositional record over the last glacial‐interglacial cycle. The Fram Strait is the only deep‐water connection from the Arctic Ocean to the North Atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction. On the Vestnesa Ridge, the western Svalbard margin, a sediment drift was identified in 1226 m of water. Gravity and multicores from the crest of the drift recovered turbidites and contourites. ¹⁴C dating indicates an age range of 8287 to 26 900 years BP (Early Holocene to Late Weichselian). The Yermak Plateau is characterized by slope sediments in 961 m of water. Gravity and multicores recovered contourites and hemipelagites. ¹⁴C ages were between 8615 and 46 437 years BP (Early Holocene to mid‐Weichselian). Downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid‐Weichselian, suggesting variable sea ice extent, productivity and polynyas present even during the Last Glacial Maximum. Four layers of ice‐rafted debris were also identified and correlated within the cores. These events occurred ca at 9, 24 to 25, 26 to 27 and 43 ka, asynchronous with Heinrich layers in the wider north‐east Atlantic and here interpreted as reflecting instability in the Svalbard/Barents Ice sheet and the northward advection of warm Atlantic water during the Late Weichselian. The activity of the ancestral West Spitsbergen Current is interpreted using mean sortable silt records from the cores. On the Vestnesa Ridge drift the modern mass accumulation rate, calculated using excess ²¹⁰Pb, is 0·076 g cm^?2 year^?1. On the Yermak Plateau slope the modern mass accumulation rate is 0·053 g cm^?2 year^?1.     

Changing patterns in the Holocene pollen record of northeastern North America: A mapped summary

By mapping the data from 62 radiocarbon-dated pollen diagrams, this paper illustrates the Holocene history of four major vegetational regions in northeastern North America. Isopoll maps, difference maps, and isochrone maps are used in order to examine the changing patterns within the data set and to study broad-scale and long-term vegetational dynamics. Isopoll maps show the distributions of spruce (Picea), pine (Pinus), oak (Quercus), herb (nonarboreal pollen groups excluding Cyperaceae), and birch + maple + beech + hemlock (Betula, Acer, Fagus, Tsuga) pollen at specified times from 11,000 BP to present. Difference maps were constructed by subtracting successive isopoll maps and illustrate the changing patterns of pollen abundances from one time to the next. The isochrone maps portray the movement of ecotones and range limits by showing their positions at a sequence of times during the Holocene. After 11,000 BP, the broad region over which spruce pollen had dominated progressively shrank as the boreal forest zone was compressed between the retreating ice margin and the rapidly westward and northward expanding region where pine was the predominant pollen type. Simultaneously, the oak-pollen-dominated deciduous forest moved up from the south and the prairie expanded eastward. By 7000 BP, the prairie had attained its maximum eastward extent with the period of its most rapid expansion evident between 10,000 and 9000 BP. Many of the trends of the early Holocene were reversed after 7000 BP with the prairie retreating westward and the boreal and other zones edging southward. In the last 500 years, man's impact on the vegetation is clearly visible, especially in the greatly expanded region dominated by herb pollen. The large scale changes before 7000 BP probably reflect shifts in the macroclimatic patterns that were themselves being modified by the retreat and disintegration of the Laurentide ice sheet. Subsequent changes in the pollen and vegetation were less dramatic than those of the early Holocene.     

Late‐glacial to Holocene environmental changes and climate variability: evidence from Voldafjorden,western Norway

Sedimentological, micropalaeontological (benthic foraminifers and dinoflagellate cysts), stable isotope data and AMS ¹⁴C datings on cores and surface samples, in addition to acoustic data, have been obtained from Voldafjorden, western Norway. Based on these data the late glacial and Holocene sedimentological processes and variability in circulation and fjord environments are outlined. Glacial marine sedimentation prevailed in the Voldafjorden between 11.0 kyr and 9.2 kyr BP (radiocarbon years). In the later part of the Allerød period, and for the rest of the Holocene, there was deposition of fine‐grained normal marine sediments in the fjord basin. Turbidite layers, recorded in core material and on acoustic profiles, dated to ca. 2.1, 6.9–7.6, ca. 9.6 and ca. 11.0 kyr BP, interrupted the marine sedimentation. The event dated to between 6.9 and 7.6 kyr BP probably corresponds to a tsunami resulting from large‐scale sliding on the continental margin off Norway (the Storegga Tsunami). During the later part of the Allerød period, Voldafjorden had a strongly stratified water column with cold bottom water and warm surface water, reaching interglacial temperatures during the summer seasons. During the Younger Dryas cold event there was a return to arctic sea‐surface summer temperatures, possibly with year‐round sea‐ice cover, the entire benthic fauna being composed of arctic species. The first strong Holocene warming, observed simultaneously in bottom and sea‐surface temperature proxies, occurred at ca. 10.1 kyr BP. Bottom water proxies indicate two cold periods, possibly with 2°C lowering of temperatures, at ca. 10.0 (PBO 1) and at 9.8 kyr BP (PBO 2). These events may both result from catastrophic outbursts of Baltic glacial lake water. The remainder of the Holocene experienced variability in basin water temperature, indicated by oxygen isotope measurements with an amplitude of ca. 2°C, with cooler periods at ca. 8.4–9.0, 5.6, 5.2, 4.6, 4.2, 3.5, 2.2, 1.2 and 0.4–0.8 kyr BP. Changes in the fjord hydrology through the past 11.3 kyr show a close correspondence, both in amplitude and timing of events, recorded in cores from the Norwegian Sea region and the North Atlantic. These data suggest a close relationship between fjord environments and variability in large‐scale oceanic circulation. Copyright © 2001 John Wiley & Sons, Ltd.     

Pre-glacial and interglacial pollen records over the last 3 Ma from northwest Canada: Why do Holocene forests differ from those of previous interglaciations?

We synthesize pollen spectra from eleven dated stratigraphic sections from central and northern Yukon. Palaeomagnetic and tephra dating indicates the earliest assemblages, representing closed canopy Pinus and Picea forest, are middle-late Pliocene age. More open forest conditions, indicated by increased Poaceae and with evidence of permafrost, are dated at ca 3 Ma. While Pinus pollen is abundant at 3 Ma, it is reduced in records after 2.6 Ma, and subsequent Pleistocene interglacial forest records are repeatedly dominated by Picea, along with Alnus and small but significant amounts of Abies. Surface sample comparisons indicate that Abies was more widespread and abundant in past interglaciations than at present and that Middle-Pleistocene Picea–Abies forest grew in the northern Yukon Porcupine Basin, 500 km beyond modern Abies limits. In contrast, Pinus, which occurs today in southern and central Yukon, was not a significant component of these Pleistocene interglacial forests. Late-Holocene pollen assemblages with rare Abies and high Pinus are the most distinct in the past 2.6 Ma. Possible factors driving Holocene difference are paleoclimate, paludification, changes in megafaunal herbivory and an unusual fire regime. Anthropogenic burning is a factor unique to the Holocene, and if it is shown to be important in this case, it would challenge our notion of what constitutes boreal wilderness.     

Late-Glacial and Early Holocene Environmental and Climatic Change at Lake Tambichozero, Southeastern Russian Karelia

High-resolution lithostratigraphy, mineral magnetic, carbon, pollen, and macrofossil analyses, and accelerator mass spectrometry ¹⁴C measurements were performed in the study of a sediment sequence from Lake Tambichozero, southeastern Russian Karelia, to reconstruct late-glacial and early Holocene aquatic and terrestrial environmental changes. The lake formed ca. 14,000 cal yr B.P. and the area around the lake was subsequently colonized by arctic plants, forming patches of pioneer communities surrounded by areas of exposed soil. A minor rise in lake productivity and the immigration of Betula pubescens occurred ca. 11,500 cal yr B.P. The rise in summer temperatures probably led to increased melting of remnant ice and enhanced erosion. The distinct increase in lake productivity and the development of open Betula-Populus forests, which are reconstructed based on plant macrofossil remains, indicate stable soils from 10,600 cal yr B.P. onward. Pinus and Picea probably became established ca. 9900 cal yr B.P.     

The last interglacial as represented in the glaciochemical record from Mount Moulton Blue Ice Area,West Antarctica

Understanding climate during the last interglacial is critical for understanding how modern climate change differs from purely naturally forced climate change. Here we present the first high-resolution ice core record of the last interglacial and transition to the subsequent glacial period from Antarctica and the first glaciochemical record for this period from West Antarctica. Samples were collected from a horizontal ice trench in the Mt. Moulton Blue Ice Area (BIA) in West Antarctica and analyzed for their soluble major anions (Cl^?, NO₃^?, SO₄^2-), major and trace elements (Na, Mg, Ca, Sr, Cd, Cs, Ba, La, Ce, Pr, Pb, Bi, U, As, Al, S, Ti, V, Cr, Mn, Fe, Co, Cu, Zn) and water hydrogen isotopes (δD). The last interglacial is characterized by warmer temperatures (δD), weakened atmospheric circulation (dust elements, seasalts aerosols), decreased sea ice extent (Na, nssSO₄^2-) and decreased oceanic productivity (nssSO₄^2-). A combined examination of Mt. Moulton seasalts, dust, nssSO₄^2- and δD records indicates that the last interglacial was extremely stable compared to glacial age climate events and it ended through a long period of gradual cooling unlike that projected for future Holocene climate.     

Ice wedge casts and past permafrost distribution in North AmericaFossile eiskeile und dauerfrost in der vergangenheit in NordamerikaPseudomorphoses des coins de glace et distribution du permafrost au passé en Amérique du Nord

Ice wedge casts are the most accurate and widespread indicators of past permafrost. Many ice wedge casts exist in Alaska, some in areas of existing ice wedges. In addition to indicating paleotemperature conditions and a wider distribution of permafrost in Wisconsinan time than now, casts in Alaska also indicate permafrost in Iliinoian and pre-lllinoian time. Hundreds of ice wedge casts are now known in temperate North America and are described from about 22 widespread localities coast to coast in Canada and United States. Permafrost existed in late Wisconsinan time, 20,000 to 10,000 years ago, along the glacial border in temperate United States. Later permafrost formed north of the glacial border as the continental ice sheet withdrew exposing drift to the rigorous periglacial climate. Ice wedge casts indicate that the ? 7 °C mean annual air isotherm was about 2000 km farther south in late Wisconsinan time than now.     

Plant and insect remains from the Wisconsinan interstadial/stadial transition at Wedron, north-central Illinois

Organic material exposed within a small swale fill in Pit 6 of the Wedron Silica Sand Co. near Wedron in LaSalle County, Illinois, includes well-preserved pollen, plant macrofossils, and insect remains. This material occurs in slackwater sediment in the lower part of the Peddicord Formation, which was deposited as existing valleys were dammed by fluvial aggradation during the initial late Wisconsinan advance of Laurentide ice into the Wedron area. Wood from the organic horizon has a radiocarbon age of 21,460 ± 470 yr B.P. (ISGS-1486). The pollen spectrum is dominated by Picea, Pinus, and Cyperaceae. Plant macrofossils comprise a mix of boreal-forest taxa, including Picea, Larix laricina, and the moss Campylium stellatum; subarctic species including Betula glandulosa, Empetrum nigrum, and Selaginella selaginoides; along with the predominantly arctic Vaccinium uliginosum var. alpinum, Dryas integrifolia, and Rhododendron lapponicum. The insect fauna contains the western montane ground beetle Opisthius richardsoni; several arctic-subarctic ground beetles including Diacheila polita, Helophorus sibiricus, and Pterostichus (Cryobius) caribou; and a diverse assemblage of insects that today inhabit the boreal forest. We interpret the biotic record to record a phase in the transition from closed boreal forest to open tundra as climatic conditions deteriorated in advance of continental glaciation.