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1.
Sediment sequences retrieved from Lake Medvedevskoye (60°13'N; 29°54'E) and Lake Pastorskoye (60°13'N; 30°02'E), Karelian Isthmus, northwestern Russia, were analysed for lithology, pollen and diatom stratigraphy, total organic carbon content and mineral magnetic parameters. Age control for both sequences was provided by AMS 14C measurements and the Vedde Ash tephra. The reconstructed climatic and environmental development shows the deglaciation of the sites and the establishment of sparse shrub and herb/grass vegetation before 12650 cal. yrs BP ('Allerød'; GI-1a). Steppe tundra and cold, dry conditions prevailed until about 11000 cal. yrs BP, i.e. throughout the 'Younger Dryas' (GS-1) and the earliest Holocene. The establishment of open Picea-Pinus-Betula forest around the lakes at about 11 000 cal. yrs BP coincides with the first distinct change towards gradually warmer and more humid climatic conditions. Boreal forest with Picea, Pinus, Betula, Alnus incana and Corylus was present at the lower altitude site between c. 10700 and 10200 cal. yrs BP, while open Betula-Pinus forest continued to dominate the vegetation around the higher altitude site. After a short, possibly colder, phase around 10200-10000 cal. yrs BP, which is expressed by a marked reduction in vegetation cover and decreased lake productivity, climatic conditions became significantly warmer and possibly more humid. Boreal forest with Pinus, Betula, Picea, Alnus incana, Corylus and Ulmus became widespread in the region after 10000 cal. yrs BP. The delayed environmental response of the lakes and their catchment to hemispheric warming at the Pleistocene/Holocene boundary may be explained by a sustained blocking of westerly air masses due to the presence of the Scandinavian ice sheet and associated strengthened easterlies and anticy-clonic circulation and/or extensive permafrost.  相似文献   

2.
Changes in tree-line, mean July temperature (T jul ) and mean annual precipitation (P ann ) for the last 10 200 cal. yr BP are reconstructed on the basis of pollen and plant macrofossils preserved in lake sediments from two sites near the present-day tree-line in Troms, northern Norway. Quantitative climate reconstructions are performed using pollen-climate transfer functions based on WA-PLS regression. Early Holocene Betula pubescens forests were gradually replaced by Pinus sylvestris at Dalmutladdo (355 m a.s.l.) starting about 7000 cal. yr BP. The local presence of pine woodland at that time is supported by finds of stomata and plant macrofossils and by high pollen accumulation rates. Until about 4000 cal. yr BP the P. sylvestris tree-line was 250-300 m higher than today, suggesting T jul about 2.0°C higher than at present. The later part of the Holocene has a cooler and moister climate and an increasing development of mires and fern-rich vegetation, as shown by increases of Sphagnum and fern spores and the re-establishment of B. pubescens woodland. The reconstructed T jul from the two sites shows similar trends to previously published data, with T jul 1-2°C warmer between 9500 cal. yr BP and 2000 cal. yr BP T jul . Maximum T jul values occur between 8500 and 4500 cal. yr BP, after which there is a gradual decrease in T jul .  相似文献   

3.
A high-resolution Younger Dryas–late Holocene record of climate and environment from the Malangen fjord has been established on the basis of two marine sediment cores. Five pollen-spore assemblage zones have been defined covering the period c . 11 500 cal. yr BP (10 200 14C yr BP) to c . 1600 cal. yr BP (1600 14C yr BP) with a hiatus of c . 2000 cal. years between c . 10 200 and 8100 cal. yr BP (9000 and 7300 14C yr BP). The Holocene vegetation development from pioneer vegetation to forest development, identified in the marine pollen record, correlates well with pollen records from terrestrial sections of northern Norway. The marine pollen record was also correlated directly with marine proxy records of the bottom water temperature investigated in the same sediment cores. Correlation between the marine and terrestrial proxies suggests that changes in the influx of warm Atlantic Water to the fjord led to an instant change in the vegetation of the surrounding land area. The results thus support a strong link between marine and atmospheric mean climatic states in the North Atlantic region throughout the Holocene.  相似文献   

4.
The Late Quaternary climate history of the Larsemann Hills has been reconstructed using siliceous microfossils (diatoms, chrysophytes and silicoflagellates) in sediment cores extracted from three isolation lakes. Results show that the western peninsula, Stornes, and offshore islands were ice‐covered between 30 000 yr BP and 13 500 cal. yr BP. From 13 500 cal. yr BP (shortly after the Antarctic Cold Reversal) the coastal lakes of the Larsemann Hills were deglaciated and biogenic sedimentation commenced. Between 13 500 and 11 500 cal. yr BP conditions were warmer and wetter than during the preceding glacial period, but still colder than today. From 11 500 to 9500 cal. yr BP there is evidence for wet and warm conditions, which probably is related to the early Holocene climate optimum, recorded in Antarctic ice cores. Between 9500 and 7400 cal. yr BP dry and cold conditions are inferred from high lake‐water salinities, and low water levels and an extended duration of nearshore sea‐ice. A second climate optimum occurred between 7400 and 5230 cal. yr BP when stratified, open water conditions during spring and summer characterised the marine coast of Prydz Bay. From 5230 until 2750 cal. yr BP sea‐ice duration in Prydz Bay increased, with conditions similar to the present day. A short return to stratified, open water conditions and a reduction in nearshore winter sea‐ice extent is evident between 2750 and 2200 cal. yr BP. Simultaneously, reconstructions of lake water depth and salinity suggests relatively humid and warm conditions on land between 3000 and 2000 cal. yr BP, which corresponds to a Holocene Hypsithermal reported elsewhere in Antarctica. Finally, dry conditions are recorded around 2000, between 760 and 690, and between 280 and 140 cal. yr BP. These data are consistent with ice‐core records from Antarctica and support the hypothesis that lacustrine and marine sediments on land can be used to evaluate the effect of long‐term climate change on the terrestrial environment. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

5.
The last ca. 20,000 yr of palaeoenvironmental conditions in Podocarpus National Park in the southeastern Ecuadorian Andes have been reconstructed from two pollen records from Cerro Toledo (04°22'28.6"S, 79°06'41.5"W) at 3150 m and 3110 m elevation. Páramo vegetation with high proportions of Plantago rigida characterised the last glacial maximum (LGM), reflecting cold and wet conditions. The upper forest line was at markedly lower elevations than present. After ca. 16,200 cal yr BP, páramo vegetation decreased slightly while mountain rainforest developed, suggesting rising temperatures. The trend of increasing temperatures and mountain rainforest expansion continued until ca. 8500 cal yr BP, while highest temperatures probably occurred from 9300 to 8500 cal yr BP. From ca. 8500 cal yr BP, páramo vegetation re-expanded with dominance of Poaceae, suggesting a change to cooler conditions. During the late Holocene after ca. 1800 cal yr BP, a decrease in páramo indicates a change to warmer conditions. Anthropogenic impact near the study site is indicated for times after 2300 cal yr BP. The regional environmental history indicates that through time the eastern Andean Cordillera in South Ecuador was influenced by eastern Amazonian climates rather than western Pacific climates.  相似文献   

6.
This article is a detailed pollen analysis and accurate AMS chronology of the Lateglacial of two coastal sites in western Norway. The area was deglaciated around 14 600 cal. yr BP or shortly before. The earliest vegetation was open, with a pioneer mosaic of vegetation on mineral soils, including snowbed communities, and plants on wind-blown ridges. Later, more stable vegetation developed with Empetrum as an important constituent. Scattered tree birches were established in the area in the last part of the Bølling/Allerød (GI-1). The pollen record from Vassnestjern indicates three short-lasting cold periods: c . 14 050 to 13 900, 13 800 to 13 700 and 13 150 to 13 000 cal. yr BP. It has been suggested that the last-mentioned period, detected at both sites, corresponds with the Gerzensee/Killarney Oscillation. From about 12 750 cal. yr BP, the vegetation was affected by the Younger Dryas (GS-1) cooling, which caused the vegetation to break up and humus-soil communities to disappear. In the early Holocene, the humus-soil communities re-established and open birch forests developed. This Lateglacial vegetation development is broadly similar to the reconstructed vegetation development in other parts of southwestern Norway.  相似文献   

7.
Inferred mean annual temperatures (Tann) from a radiocarbon-dated fossil pollen stratigraphy obtained from Lake Trehörningen, southwest Sweden, provide evidence of climate variability during the Holocene in the boreal-nemoral zone of Sweden. The record indicates low early Holocene temperatures, followed by a rising trend and maximum temperature values from about 7000 to 4000 cal. yr BP. During the later part of the Holocene until present day, a cooling trend is reflected in the temperature record. At about 8200 cal. yr BP, temperatures temporarily drop, and at 8000 cal. yr BP the rising temperature trend is recovered. Inferred mid-Holocene temperatures are about 2.5-3°C higher than at present, and also higher than other pollen-inferred mid-Holocene temperatures of 1.5-2.5°C further to the north and east in Scandinavia. The reconstructed long-term climate pattern in Trehörningen has an overall consistency with temperature reconstructions from Scandinavia, suggesting a regional climate link for the Holocene variability in Sweden. Holocene climate trends in Trehörningen also show a remarkably similar pattern with the classic work on postglacial climate change by Sernander (1893, Sernander 1909), Andersson (1909) and von Post (1924).  相似文献   

8.
To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we analysed sediment pollen, conifer stomata, and plant macrofossils from Lake Kharinei, a tundra lake near the treeline in the Pechora area. We present quantitative summer temperature reconstructions from Lake Kharinei and Lake Tumbulovaty, a previously studied lake in the same region, using a pollen–climate transfer function based on a new calibration set from northern European Russia. Our records suggest that the early-Holocene summer temperatures from 11,500 cal yr BP onwards were already slightly higher than at present, followed by a stable Holocene Thermal Maximum (HTM) at 8000–3500 cal yr BP when summer temperatures in the tundra were ca. 3°C above present-day values. A Picea forest surrounded Lake Kharinei during the HTM, reaching 150 km north of the present taiga limit. The HTM ended with a temperature drop at 3500–2500 cal yr BP associated with permafrost initiation in the region. Mixed spruce forest began to disappear around Lake Kharinei at ca. 3500 cal yr BP, with the last tree macrofossils recorded at ca. 2500 cal yr BP, suggesting that the present wide tundra zone in the Pechora region formed during the last ca. 3500 yr.  相似文献   

9.
Analysis of pollen, spores, macrofossils, and lithology of an AMS 14C-dated core from a subarctic fen on the Kenai Peninsula, Alaska reveals changes in vegetation and climate beginning 14,200 cal yr BP. Betula expansion and contraction of herb tundra vegetation characterize the Younger Dryas on the Kenai, suggesting increased winter snowfall concurrent with cool, sunny summers. Remarkable Polypodiaceae (fern) abundance between 11,500 and 8500 cal yr BP implies a significant change in climate. Enhanced peat preservation and the occurrence of wet meadow species suggest high moisture from 11,500 to 10,700 cal yr BP, in contrast to drier conditions in southeastern Alaska; this pattern may indicate an intensification and repositioning of the Aleutian Low (AL). Drier conditions on the Kenai Peninsula from 10,700 to 8500 cal yr BP may signify a weaker AL, but elevated fern abundance may have been sustained by high seasonality with substantial snowfall and enhanced glacial melt. Decreased insolation-induced seasonality resulted in climatic cooling after 8500 cal yr BP, with increased humidity from 8000 to 5000 cal yr BP. A dry interval punctuated by volcanic activity occurred between 5000 and 3500 cal yr BP, followed by cool, moist climate, coincident with Neoglaciation. Tsuga mertensiana expanded after ~ 1500 cal yr BP in response to the shift to cooler conditions.  相似文献   

10.
In order to study the stability and dynamics of mountain rainforest and paramo ecosystems, including the biodiversity of these ecosystems, the Holocene and late Pleistocene climate and fire variability, and human impact in the southeastern Ecuadorian Andes, we present a high‐resolution pollen record from El Tiro Pass (2810 m elevation), Podocarpus National Park. Palaeoenvironmental changes, investigated by pollen, spores and charcoal analysis, inferred from a 127 cm long core spanning the last ca. 21 000 cal. yr BP, indicate that grass‐paramo was the main vegetation type at the El Tiro Pass during the late Pleistocene period. The grass‐paramo was rich in Poaceae, Plantago rigida and Plantago australis, reflecting cold and moist climatic conditions. During the early Holocene, from 11 200 to 8900 cal. yr BP, subparamo and upper mountain rainforest vegetation expanded slightly, indicating a slow warming of climatic conditions during this period. From 8900 to 3300 cal. yr BP an upper mountain rainforest developed at the study site, indicated by an increase in Hedyosmun, Podocarpaceae, Myrsine and Ilex. This suggests a warmer climate than the present day at this elevation. The modern subparamo vegetation became established since 3300 cal. yr BP at El Tiro Pass. Fires, probably anthropogenic origin, were very rare during the late Pleistocene but became frequent after 8000 cal. yr BP. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

11.
Hager Pond, a mire in northern Idaho, reveals at least five pollen zones since sediments formed after the last recession of continental ice (>9500 yr BP). Zone I (>9500-8300 yr BP) consists mainly of diploxylon pine, plus low percentages of Abies, Artemisia, and Picea. SEM examination of conifer pollen at selected levels in the zone reveals that Pinus albicaulis, P. monticola, and P. contorta are present in unknown proportions. The zone resembles modern pollen spectra from the Abies lasiocarpa-P. albicaulis association found locally today only at high elevation. Presence of whitebark pine indicates a cooler, moister climate than at present, but one which was rapidly replaced in Zone II (8300-7600 yr BP) by warmer, drier conditions as inferred by prominence of grass with diploxylon pine. Zone III (7600-3000 yr BP) was probably dominated by Pseudotsuga menziesii, plus diploxylon pine and prominent Artemisia and denotes a change in vegetation but continuation of the warmer drier conditions. Beginning at approximately 3000 yr BP Picea engelmannii, Abies lasiocarpa, and/or A. grandis and diploxylon pine were dominants and the inferred climate became cooler and moister concomitant with Neoglaciation. The modern climatic climax (Zone V), with Tsuga heterophylla as dominant, has emerged in approximately the last 1500 yr.  相似文献   

12.
A sediment core from Lake BC01 (75°10.945′N, 111°55.181′W, 225 m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae and Salix, typical of a High Arctic plant community, whereas the Arctic herb Oxyria underwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000 cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000 cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000 cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000 cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500 cal yr BP but then decreased substantially by 3500 cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.  相似文献   

13.
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   

14.
Chironomid remains from a mid-elevation lake in the Sierra Nevada, California, were used to estimate quantitative summer surface water temperatures during the past ∼15,000 yr. Reconstructed temperatures increased by ∼3°C between lake initiation and the onset of the Holocene at ∼10,600 cal yr BP (calibrated years before present). Temperatures peaked at 6500 cal yr BP, displayed high variability from 6500 to 3500 cal yr BP, and stabilized after 3500 cal yr BP. This record generally tracks reconstructed Santa Barbara Basin sea surface temperatures (SSTs) through much of the Holocene, highlighting the correspondence between SST variability and California land temperatures during this interval.  相似文献   

15.
We present a late glacial pollen record (17,700 to 8500 cal yr BP) from a Lake Naleng sediment core. Lake Naleng is located on the southeastern Tibetan Plateau (31.10°N 99.75°E, 4200 m) along the upper tree-line. Variations in the summer monsoon are evident from shifts in vegetation that correspond to late glacial climate trends from other monsoon-sensitive regions. Alpine steppe was recorded between 17,700 and 14,800 cal yr BP, indicating low effective moisture at the study site. Expansion of alpine meadows followed by advances in the position of tree-line around Lake Naleng suggest that climate became warmer and wetter between ∼ 14,800 and 12,500 cal yr BP, probably representing an enhancement of the Asian monsoon. Climatic cooling and reduced effective moisture are inferred from multivariate analysis and the upward retreat of tree-line between ∼ 12,500 and 11,700 cal yr BP. The timing and nature of these shifts to warm, wet and then cold, dry climatic conditions suggest that they correspond to the Bølling/Allerød and Younger Dryas intervals. Abies-Betula forests, representing warm and moist conditions, spread during the early Holocene.  相似文献   

16.
《Quaternary Science Reviews》2005,24(1-2):105-121
Pollen analysis of radiocarbon-dated lake sediment from northern Vancouver Island, southwest British Columbia reveals regional changes in forest vegetation over the last 12,200 14C yr (14,900 cal yr). Between at least 12,200 and 11,700 14C yr BP (14,900–13,930 cal yr BP), open woodlands were dominated by Pinus contorta, Alnus crispa, and various ferns. As P. contorta decreased in abundance, Alnus rubra and more shade-tolerant conifers (i.e., Picea and Tsuga mertensiana) increased. Increases in T. mertensiana, P. contorta, and A. crispa pollen accumulation rates (PARs) between 10,600 and 10,400 14C yr BP (11,660–11,480 cal yr BP) reflect a cool and moist climate during the Younger Dryas chronozone. Orbitally induced warming around 10,000 14C yr BP (11,090 cal yr BP) allowed the northward extension of Pseudotsuga menziesii, although Picea, Tsuga heterophylla, and A. rubra dominated early Holocene forests. By 7500 14C yr BP (8215 cal yr BP), shade-tolerant T. heterophylla was the dominant forest tree. Cupressaceae (Thuja plicata and Chamaecyparis nootkatensis) was present by 7500 14C yr BP but reached its maximum after 3500 14C yr BP (3600 cal yr BP), when a cooler and wetter regional climate facilitated the development of temperate rainforest. The highest rates of vegetation change are associated with Lateglacial climate change and species with rapid growth rates and short life spans.  相似文献   

17.
Core MD95‐2011 was taken from the eastern Vøring Plateau, near the Norwegian coast. The section between 250 and 750 cm covers the time period from 13 000 to 2700 cal. yr BP (the Lateglacial and much of the Holocene). Samples at 5 cm intervals were analysed for fossil diatoms. A data‐set of 139 modern sea‐surface diatom samples was related to contemporary sea‐surface temperatures (SSTs) using two different numerical methods. The resulting transfer functions were used to reconstruct past sea‐surface temperatures from the fossil diatom assemblages. After the cold Younger Dryas with summer SSTs about 6°C, temperatures warmed rapidly to about 13°C. One of the fluctuations in the earliest Holocene can be related to the Pre‐Boreal Oscillation, but SSTs were generally unstable until about 9700 cal. yr BP. Evidence from diatom concentration and magnetic susceptibility suggests a change and stabilization of water currents associated with the final melting of the Scandinavian Ice Sheet at c. 8100 cal. yr BP. A period of maximum warmth between 9700 and 6700 cal. yr BP had SSTs 3–5°C warmer than at present. Temperatures cooled gradually until c. 3000 cal. yr BP, and then rose slightly around 2750 cal. yr BP. The varimax factors derived from the Imbrie & Kipp method for sea‐surface‐temperature reconstructions can be interpreted as water‐masses. They show a dominance of Arctic Waters and Sea Ice during the Younger Dryas. The North Atlantic current increased rapidly in strength during the early Holocene, resulting in warmer conditions than previously. Since about 7250 cal. yr BP, Norwegian Atlantic Water gradually replaced the North Atlantic Water, and this, in combination with decreasing summer insolation, led to a gradual cooling of the sea surface. Terrestrial systems in Norway and Iceland responded to this cooling and the increased supply of moisture by renewed glaciation. Periods of glacial advance can be correlated with cool oscillations in the SST reconstructions. By comparison with records of SSTs from other sites in the Norwegian Sea, spatial and temporal changes in patterns of ocean water‐masses are reconstructed, to reveal a complex system of feedbacks and influences on the climate of the North Atlantic and Norway.  相似文献   

18.
Pollen and macrofossil investigations and radiocarbon datings were carried out at a bog in the Khibiny mountains and the northernmost bog in European Russia on the Rybachiy Peninsula (69°98'N) on the western part of the Kola Peninsula. Peat accumulation on the Kola Peninsula started at c . 8500–7500 BP. Pinus sylvestris reached its present northern limit on the peninsula by 7000 BP, while 6000–5000/4500 BP was a time of maximal progress of birch forest tundra up to the Barents Sea shoreline. Alnus ineana grew up to the Rybachiy Peninsula c . 40 km north of its present-day northern limit. By c , 5500/5300 BP Picen ohovata had immigrated to the Khibiny mountains. After 5000/4500 BP the forested area had retreated in the northern part of the Kola Peninsula and the tundra belt bordering the Barents Sea shore was formed. By 3500 BP spruce had reached its modern northern limit.  相似文献   

19.
A multiproxy study of palaeoceanographic and climatic changes in northernmost Baffin Bay shows that major environmental changes have occurred since the deglaciation of the area at about 12 500 cal. yr BP. The interpretation is based on sedimentology, benthic and planktonic foraminifera and their isotopic composition, as well as diatom assemblages in the sedimentary records at two core sites, one located in the deeper central part of northernmost Baffin Bay and one in a separate trough closer to the Greenland coast. A revised chronology for the two records is established on the basis of 15 previously published AMS 14C age determinations. A basal diamicton is overlain by laminated, fossil-free sediments. Our data from the early part of the fossiliferous record (12 300–11 300 cal. yr BP), which is also initially laminated, indicate extensive seasonal sea-ice cover and brine release. There is indication of a cooling event between 11 300 and 10 900 cal. yr BP, and maximum Atlantic Water influence occurred between 10 900 and 8200 cal. yr BP (no sediment recovery between 8200 and 7300 cal. yr BP). A gradual, but fluctuating, increase in sea-ice cover is seen after 7300 cal. yr BP. Sea-ice diatoms were particularly abundant in the central part of northernmost Baffin Bay, presumably due to the inflow of Polar waters from the Arctic Ocean, and less sea ice occurred at the near-coastal site, which was under continuous influence of the West Greenland Current. Our data from the deep, central part show a fluctuating degree of upwelling after c . 7300 cal. yr BP, culminating between 4000 and 3050 cal. yr BP. There was a gradual increase in the influence of cold bottom waters from the Arctic Ocean after about 3050 cal. yr BP, when agglutinated foraminifera became abundant. A superimposed short-term change in the sea-surface proxies is correlated with the Little Ice Age cooling.  相似文献   

20.
Quantitative information on vegetation and climate history from the late glacial-Holocene on the Tibetan Plateau is extremely rare. Here, we present palynological results of a 4.30-m-long sediment record collected from Koucha Lake in the Bayan Har Mountains, northeastern Tibetan Plateau. Vegetation change has been traced by biomisation, ordination of pollen data, and calculation of pollen ratios. The application of a pollen-climate calibration set from the eastern Tibetan Plateau to Koucha Lake pollen spectra yielded quantitative climate information. The area was covered by alpine desert/steppe, characteristic of a cold and dry climate (with 50% less precipitation than today) between 16,700 and 14,600 cal yr BP. Steppe vegetation, warm (∼ 1°C higher than today) and wet conditions prevailed between 14,600 and 6600 cal yr BP. These findings contradict evidence from other monsoon-influenced areas of Asia, where the early Holocene is thought to have been moist. Low effective moisture on the northeastern Tibetan Plateau was likely due to high temperature and evaporation, even though precipitation levels may have been similar to present-day values. The vegetation changed to tundra around 6600 cal yr BP, indicating that wet and cool climate conditions occurred on the northeastern Tibetan Plateau during the second half of the Holocene.  相似文献   

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