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1.
During the last glaciation of northern Ellesmere Island many areas remained ice-free. A caribou antler from deglacial-marine sediments in Clements Markham Inlet dates 8,415 ± 135 B.P. (S-2501). If locally derived it places caribou at the northern limit of their contemporary range at the onset of deglaciation in this area. Immediately to the south, on the Hazen Plateau, ice remained at its limit until c . 8,000 B.P. Therefore, this antler may indicate the presence of caribou during full glacial time.  相似文献   

2.
Kylen Lake, located within the Toimi drumlin field, is critically positioned in relation to Late Wisconsin glacial advances, for it lies between the areas covered by the Superior and St. Louis glacial lobes between 12,000 and 16,000 yr B.P. The pollen and plant-macrofossil record suggests the presence of open species-rich “tundra barrens” from 13,600 to 15,850 yr B.P. Small changes in percentages of Artemisia pollen between 14,300 and 13,600 yr B.P. appear to be artifacts of pollen-percentage data. Shrub-tundra with dwarf birch, willow, and Rhododendron lapponicum developed between 13,600 and 12,000 yr B.P. Black and white spruce and tamarack then expanded to form a vegetation not dissimilar to that of the modern forest-tundra ecotone of northern Canada. At 10,700 B.P. spruce and jack pine increased to form a mosaic dominated by jack pine and white spruce on dry sites and black spruce, tamarack, and deciduous trees such as elm and ash on moist fertile sites. At 9250 yr B.P. red pine and paper birch became dominant to form a vegetation that may have resembled the dry northern forests of Wisconsin today. The diagram terminates at 8410 ± 85 yr B.P. Climatic interpretation of this vegetational succession suggests a progressive increase in temperature since 14,300 yr B.P. This unidirectional trend in climate contrasts with the glacial history of the area. Hypotheses are presented to explain this lack of correspondence between pollen stratigraphy and glacial history. The preferred hypothesis is that the ice-margin fluctuations were controlled primarily by changes in winter snow accumulation in the source area of the glacier, whereas the vegetation and hence the pollen stratigraphy were controlled by climatic changes in front of the ice margin.  相似文献   

3.
Pollen data from core KW31 recovered off the mouth of the Niger River (3°31′1N–05°341E; 1181 m water depth) provide an exceptional record of vegetation changes in the West African lowlands between 40,000 and 3500 cal yr B.P. The highly diverse microflora testify for the permanency of rain and secondary forests in the Niger river catchment, at least as gallery formations along rivers, during the last glacial period when dry conditions occurred in relation to enhanced trade-wind circulation. The direct consequence of the post-glacial warming and the correlative increase in monsoon fluxes over West Africa was the increase in forest diversity and the expansion of rain and secondary forests on the nearby continent. Comparison between KW31 pollen record and continental pollen data from 5°S to 25°N allows the evaluation of migration rates of tropical forest populations throughout North West Africa at the beginning of the Holocene and the vegetation response to the shift toward aridity recorded widely at the end of the African Humid Period around 4000 cal yr B.P.  相似文献   

4.
Along the northern coasts of Ellesmere Island, at least two glaciations are recognized on the basis of morphostratigraphy. The early Holocene ice limit lay only 5 to 60 km beyond present glaciers due to constraints imposed by aridity and calving. This limited ice advance likely extended beyond any Wisconsinan glacial limit. Marine limits established during, retreat from the last glacial maximum reach 148 m a.s.l. In contrast, earlier, more extensive glaciations inundated the coastlines and are associated with former relative sea levels now reaching 286 m a.s.l. Correlation of these pre-Wisconsinan glaciations is based upon amino acid ratios. However, this approach is severely limited by slow rates of racemization, a lack of in situ samples, and complex thermal histories owing to multiple transgressions. Models favoring extensive regional glaciation of northern Ellesmere Island and Greenland must include a glacioclimatic scenario recognizing the constraint that aridity places on glaciation. We suggest that the large ice volume associated with the oldest recognized glaciation relates to a period of reduced sea-ice cover, possibly >400,000 BP, and may correlate with an interglacial stage of the marine oxygen isotope record.  相似文献   

5.
Seasonal and annual pollen deposition over an eight-year period (1981-89) in the snow layers of the Agassiz Ice Cap, Ellesmere Island, was investigated. The annual and seasonal layers were determined from the snow stratigraphy. Thirteen to thirty-five kg were sampled for each layer. Most of the pollen grains deposited on the ice cap originate either from the high Arctic regional tundra, or from the boreal forest and low Arctic tundra over 1,000 km to the south. The regional pollen types show large variations throughout the year and the maximum concentration is normally found in the late summer-early autumn snow. The exotic types usually show less seasonal variation, although with slight maximum in the spring or summer layers. The annual regional and exotic pollen influx to the ice cap can be highly variable. These variations are tentatively related to climatic parameters.  相似文献   

6.
A 14-m-thick section of marine and nonmarine sediments of the Gubik Formation of northern Alaska, exposed in bluffs near Ocean Point on the Colville River, has been studied by means of pollen analysis. Pollen from the marine sediments, of probable late Pliocene age, records a boreal forest of spruce and birch with minor amounts of alder in the adjacent terrestrial vegetation. Pine and perhaps true fir were probably at or near their northern limit here, but hemlocks and hardwoods were absent. The suggested environment for the Arctic Slope during the time represented by the marine sediments is similar to that of present-day Anchorage. Pollen floras from the overlying fluvial strata, of early or middle Pleistocene age, record predominantly herbaceous taxa indicating tundra conditions probably more severe than those of the present day. These deposits were most likely contemporaneous with glacial conditions in the Brooks Range to the south. Pollen of woody taxa (spruce, alder, birch, heaths) is rare through most of the section, although birch and alder percentages similar to those found in modern river sediments indicate an interstadial or interglacial warming in midsection. Inland climates during glacial episodes may have been similar to those of the present Arctic coast.  相似文献   

7.
Vegetation history during the Holocene is interpreted from the pollen and sedimentary records of nine sections of peat deposits located in sedge tundra at sites in the northern and northwestern parts of the Prince William Sound region. Basal radiocarbon ages of the deposits are between 10,015 and 580 yr B.P. Modern surface pollen data from these and 25 additional sites, ranging from lowlands to an altitude of 675 m in the alpine tundra, were used to aid in the interpretation of the fossil records. Both frequency and influx pollen diagrams of the oldest section disclose a sequence of communities beginning with sedge tundra, containing thickets of willow and alder, followed by alder, which became predominant at about 8300 yr B.P. Later, alder declined, and an inferred growth of sedge tundra and the establishment of colonies of mountain hemlock and Sitka spruce with some western hemlock occurred about 2680 yr B.P. Finally, regrowth of sedge tundra accompanied by the development of forest communites took place over the past 2000 yr. The influence of glacier advances on the vegetation in the fjords occurred during Neoglacial episodes dated at 3200–2500 yr B.P. and during recent centuries. Regional Holocene tectonic activity was also an influential factor, especially at the time of the 1964 earthquake.  相似文献   

8.
Links between southern and northern hemisphere climates during the Late Quaternary are poorly known, partly due to the scarcity of continuous climatic records in the southern tropics. Pollen and diatom evidence from Lake Tritrivakely (19°47′S) provides information on vegetational and hydrological changes in the central highlands of Madagascar over the past 40,000 yr. Most of the record reflects natural environmental variability since humans arrived on the island ca. 2000 yr B.P. During glacial times, the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric CO2levels. In the lake, a positive mean annual hydrologic balance, from 38,000 to 36,000 and from 17,500 to 9800 cal yr B.P., coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics. A negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate. The last glacial maximum was cool and dry. The deglacial warming occurred in two steps. The first step, accompanied by an increase in wetness, occurred abruptly at ca. 17,000 cal yr B.P., about two millennia earlier than in the northern hemisphere. It is abundantly documented in southern terrestrial data. The second step, at 15,000 cal yr B.P., was in phase with the first major temperature change in the northern hemisphere.  相似文献   

9.
Pollen and plant macrofossils preserved in lake sediment from Lake West Okoboji, Dickinson County, Iowa, indicate how the vegetation of that area changed during the late glacial and postglacial. A closed coniferous forest, dominated by spruce and larch trees, produced the Picea-Larix pollen assemblage zone. Fir trees were a minor constituent of this forest; pine trees were probably absent. Black ash trees increased in abundance at Lake West Okoboji and by 13,500 yr ago were an important constituent of the forest. The sediment accumulation rate and the pollen influx were low throughout this time. Birch and alder pollen peaked in abundance approximately 11,800 yr ago. Pollen influx increased rapidly as birch and alder replaced coniferous trees on the uplands. A deciduous forest, containing abundant oak and elm trees, replaced the birch-alder-coniferous forest. This forest inhabited northwestern Iowa from approximately 11,000 to 9000 yr B.P. Nonarboreal species became prevalent between approximately 9000 and 7700 yr B.P. as prairie began to replace deciduous forest on the uplands. Charred remains of Amorpha canescens and other upland species attest to the presence of prairie fires as an aid in establishing prairie and destroying the forest. The pollen influx declined. The warmest, driest part of the postglacial occurred in northwestern Iowa from approximately 7700 to 3200 yr ago. Lake level fell 9 to 10 m, and prairie extended to the edge of the lake. Wet-ground weeds inhabited areas near lake level which were alternately flooded, then dry. Pollen influx was approximately 100 grains/cm2/yr during the driest time in this dry interval.Deciduous trees, particularly oaks, returned after approximately 3200 yr B.P. Prairie continued to occupy the uplands but trees were more common in the lowlying wet areas. Settlement by Europeans in northwestern Iowa about 1865 is marked by an increase in weed pollen. Macrofossil deposition changed in 1910 in response to the stabilization of lake level.  相似文献   

10.
Given the difficulty of separating the three Picea species—P. glauca, P. mariana, and P. rubens (white, black, and red spruce)—in the pollen record, little is known about their unique histories in eastern North America following deglaciation. Here we report the first use of a classification tree analysis (CART) to distinguish pollen grains of these species. It was successfully applied to fossil pollen from eight sites in Maine and one in Massachusetts. We focused on the late glacial/early Holocene (14,000 to 8000 cal yr B.P.) and the late Holocene (1400 cal yr B.P. to present)—the two key periods since deglaciation when Picea has been abundant in the region. The result shows a shift from a Picea forest of P. glauca and P. mariana in the late glacial to a forest of P. rubens and P. mariana in the late Holocene. The small number of P. rubens grains identified from the late glacial/early Holocene samples (<5%) suggests that that species was either absent or rare at most of the sites. The occurrence and distribution of the three species do not reveal any geographic or temporal trend during late glacial time, but the data suggest that they were distributed in local patches on the landscape. The results of this study indicate that the recent population expansion of Picea (1000 to 500 cal yr B.P.) was likely the first time since deglaciation that P. rubens was abundant in the region.  相似文献   

11.
A 22 m long sediment core from Lake Yamozero on the Timan Ridge in northern Russia has provided evidence of intriguing climatic shifts during the last glacial cycle. An overall shallowing of the lake is reflected in the lower part of the cores, where pollen indicates a transition from glacial steppe vegetation to interstadial shrub-tundra. These beds are capped by a well-defined layer of compact clay deposited in relatively deep water, where pollen shows surrounding spruce forests and warmer-than-present summer temperatures. The most conservative interpretation is that this unit represents the last interglacial period. However, a series of Optical Stimulated Luminescence (OSL) dates suggests that it corresponds with the Early Weichselian Odderade interstadial (MIS 5a). This would imply that the Odderade interstadial was just as warm as a normal interglacial in this continental part of northern Europe. If correct, then pollen analysis, as a correlation tool, is less straightforward and the definition of an interglacial is more complex than previously thought. We discuss the validity and possible systematic errors of the OSL dates on which this age model is based, but conclude they really indicate a MIS 5a age for the warm period. Above the clay is an unconformity, most likely reflecting a period of subaerial exposure implying dry conditions. Deposition of silt under fluctuating cold climates in the Middle Weichselian continued until a second gap in the record at c . 40 kyr BP. The lake basin started to fill up again around 18 kyr BP.  相似文献   

12.
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13.
A narwhal (Monodon monoceros) tusk from 34 m above sea level and located at 82°N on the northwest coast of Ellesmere Island has been radiocarbon dated at 6,830 ± 50 B.P. It was collected from a narrow coastal strip which is isolated from the adjacent Arctic Ocean by glacier ice, ice shelf and multiyear pack ice. The specimen represents an early Holocene range extension of 400–700 km over the present. Because the narwhal requires abundant open water to survive, the Holocene tusk is an important independent item of proxy data on palaeoclimatic change. Contemporary migration routes are directly related to seasonal sea ice in the inter-island channels of the central Canadian Arctic archipelago. The presence of a narwhal on the northwest Ellesmere Island coast at 6,830 ± 50 B. P. suggests that sea ice and ice-shelf conditions were more favourable at that time. A comprehensive chronological framework for late Quaternary and Holocene geomorphic/climatic events from northern Ellesmere Island records a warm early Holocene characterized by abundant driftwood entry into the high Arctic. This was followed by a mid-Holocene climatic deterioration during which the ice shelves of the Ellesmere coast formed. Therefore, the narwhal tusk is further evidence that a period of maximum postglacial warmth occurred during the early Holocene in the Canadian high Arctic.  相似文献   

14.
Sediments from two limnic basins in a sub-continental region of northern Norway are investigated for pollen, plant macrofossils and bivalves at a high stratigraphical resolution. The basins are located at 280 and 400 m a.s.l. on the S-SW slope of Mt. Skrubben (848 m a.s.l.). The bio- and lithostratigraphical records are interpreted in terms of immigration and establishment of forest tree species and climate. The mountain was deglaciated at both sites at c . 9200-9100 BP ( c . 10200-10100 cal. BP). Betula pubescens forest stands established at c . 8900 BP ( c . 9900 cal. BP). The first Pinus sylvestris individuals may have established at or near the lowermost investigation site at c . 8700-8600 BP ( c . 9700-9600 cal. BP), while P. sylvestris forest stands were present at 400 m a.s.l. c . 7700 BP ( c . 8500 cal. BP) and expanded in area and/or density from c . 7200 BP. Macrofossils of P. sylvestris occur in strata with a pine pollen influx as low as c . 200 grains cm -2 yr -1 . The immigration of P. sylvestris to the western part of northern Fennoscandia may thus have happened earlier than formerly interpreted from pollen analyses alone, where influx values as low as c . 200 grains cm -2 yr -1 would not be considered as indicative of local presence. Alnus incana established at approximately the same time as P. sylvestris . During the period from deglaciation to c . 6600 BP ( c . 7300 cal. BP), calcareous limnic sediments were deposited in the basins, with Chara species and a Sphaeriidae fauna consisting of Pisidium hibernicum , P. lilljeborgii , P. milium , P. nitidum , P. waldeni and P. casertanum . During the period c . 8800-8500 BP, P. nitidum prevailed, and almost pure CaCO 3 sediments were formed. We interpret the environment as dry with a low influx of both minerogenic particles and humic substances. The period 8800-8500 BP (9800-9500 cal. yr BP) is interpreted as a period with warm and dry summers and cold and dry winters.  相似文献   

15.
A new extended pollen and charcoal record is presented from Lake Euramoo, Wet Tropics World Heritage rainforest of northeast Queensland, Australia. The 8.4-m sediment core taken from the center of Lake Euramoo incorporates a complete record of vegetation change and fire history spanning the period from 23,000 cal yr B.P. to present. The pollen record is divided into five significant zones; 23,000–16,800 cal yr B.P., dry sclerophyll woodland; 16,800–8600 cal yr B.P., wet sclerophyll woodland with marginal rainforest in protected pockets; 8600–5000 cal yr B.P., warm temperate rainforest; 5000–70 cal yr B.P., dry subtropical rainforest; 70 cal yr B.P.–AD 1999, degraded dry subtropical rainforest with increasing influence of invasive species and fire.The process of rainforest development appears to be at least partly controlled by orbital forcing (precession), though more local environmental variables and human activity are also significant factors. This new record provides the opportunity to explore the relationship between fire, drought and rainforest dynamics in a significant World Heritage rainforest region.  相似文献   

16.
Wood macrofossil remains of alder and willow/poplar have been recovered from a sediment sequence in the valley of the Turker Beck in the Vale of Mowbray, North Yorkshire. These remains have yielded radiocarbon dates early in the Devensian Late Glacial (14.7–14k cal a bp ), equivalent to the early part of the Greenland Interstadial (GI-1e) of the GRIP ice-core record. These are the earliest dates recorded for the presence of alder in the Late Glacial in the British Isles. Associated biological remains have provided a palaeoenvironmental record for this early part of the Greenland Interstadial, generally indicative of open environments dominated by herbaceous taxa on both the wetland and dryland surfaces. However, stands of alder, birch and willow woodland were also present, and indicate the possibility that such tree species survived in cryptic refugia in Britain as elsewhere in northern Europe during the Last Glacial Maximum. The absence of alder pollen at Turker Beck, in a sequence in which its macrofossil remains are relatively abundant, lends support to the view that pollen can be a poor indicator of the presence of tree species in Late Glacial sequences in northern and western Europe.  相似文献   

17.
A peat layer beneath two till beds was found at Seitevare in Swedish Lapland. The pollen flora layer indicates deposition during the later part of the Eemian Interglacial. The vegetation consisted of open forests predominated by pine and birch, mixed with alder and spruce. The sedimentation took place in a small oligotrophic-dystrophic water basin with a pH about 5, according to the diatom flora. In an adjacent section, silty sediments with pollen indicating interstadial vegetation (birches, herbs) are covered by one till bed. These sediments are tentatively correlated with the Peräpohjola Interstadial in northern Finland and north-eastern Sweden. The lithostratigraphy indicates one pre-Eemian and probably three Weichselian glacial advances.  相似文献   

18.
A new ice core (TALDICE) drilled at Talos Dome (East Antarctica, Ross Sea sector) preserves a ca. 250 ka long record of palaeoclimate and atmospheric history. We investigate dust variability and provenance at the site during glacial periods and the Holocene through the Sr–Nd isotopic composition of ice core dust and potential source areas (PSA). We provide new isotopic data on dust sources from Victoria Land such as regoliths, glacial drifts, aeolian sands and beach deposits. Some of these sources are located at high altitude and are known to have been ice free throughout the Pleistocene. The major features of the TALDICE dust record are very similar to those from central East Antarctica. During glacial times, South America was the dominant dust supplier for Talos Dome as well as for the entire East Antarctic plateau. Conversely, during the Holocene the principal input of mineral dust at Talos Dome probably derives from proximal sources which are the ice‐free areas of northern Victoria Land, located at similar altitude with respect to the drilling site. Atmospheric mobilisation of dust from these neighbouring areas and transport inland to Talos Dome can be ultimately associated with advection of maritime air masses from the Pacific/Ross Sea region. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

19.
Paleoecological (pollen, phytolith, and wood) analyses of sediments, radiocarbon dated 33,000 to 26,000 yr B.P., from two sites in Ecuadorian Amazonia provide data that suggest a cooling of ca. 7.5°C below present in equatorial lowlands from 33,000 to 30,000 yr B.P. A period of warning followed in which novel species assemblages, a blend of montane and lowland floral components, persisted for at least 4000 years. These data of forest community change, from sites lying within the postulated glacial rain forest Napo refugium, provide the strongest paleoecological refutation of the refugial hypothesis yet obtained. The large temperature depression at ca. 30,000 yr B.P. allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum (LGM) of the northern hemisphere, freezing temperatures would have been experienced in parts of lowland Amazonia between 25,000 and 18,000 B.P.  相似文献   

20.
A 56 cm long core covering ca. 9,000 years was obtained from a small, unnamed lake (78°29.5'N, 4 76°46.8'W) at 295 m elevation, innermost Baird Inlet, Ellesmere Island. The vegetation is high arctic, but locally relatively rich, with areas of Cassiope tetragona–Empetrum–Vaccinium uliginosum heath on the steep slopes above the lake. Cores were taken from the basin centre through ca. 15 m of water. Basal sandy-silt is overlain by 51 cm of laminated silty-gyttja and fine detritus gyttja grading upwards to loose algal gyttja. Three radiocarbon dates indicate rates of sedimentation from 0.07 to 0.04 mm per year. Pollen concentration varies from 1,500 to 2,000 per cm3 in the lower part of the core to 100 to 200 in the upper part; calculated pollen influx is between less than one and 14 grains per cm2 per year. Four local pollen assemblage zones reflect an early pioneer phase (grass-sedge- Oxyria ) in the lowermost organic sediment, followed by the appearance and spread of Salix some 8,000 years ago and then, some 7,000 years ago, by a rise in Ericales. A period of vegetational stability with dwarf-shrub heaths follows, lasting for over 3,000 years. The topmost zone shows some increase in indicators of bare ground and fell-field vegetation ( Saxifraga, Ranunculus, Papaver , Caryophyllaceae, Dryas ) and decline in pollen influx, hence deterioration of local conditions during the last 3,500 to 4,000 years.  相似文献   

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