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1.
A consensus on Holocene climate variability at the modern northern fringe of the East Asian summer monsoon (EASM) region remains elusive. Here, we present a pollen-based reconstruction of vegetation history and associated climate variations of a sediment core from Huangqihai Lake, central Inner Mongolia. During 10.7 to 8.8 cal kaBP, typical steppe with small patches of forest dominated the lake area, suggesting a moderately wet climate, followed by ameliorating climatic conditions until 8.0 cal kaBP as deduced by the expansion of forest. Typical steppe recovered the lake area between 8.0 and 7.2 cal kaBP, reflecting a deterioration of climatic conditions; in combination with other proxy records in the study region, we noticed that severe aridity was prevailed in the lake area between 8.0 and 7.6 cal kaBP. During 7.2 to 3.2 cal kaBP, abundant tree pollen indicated dominance of forest-steppe around the lake, marking regionally wet conditions. A notable absence of broadleaved trees after 5.2 cal kaBP reveals a slight drying trend, and climate deterioration from 4.5 to 4.1 cal kaBP might be linked to the 4.2 ka event. After 3.2 cal kaBP, a transition to steppe was associated with dry conditions in the region. Based on our pollen record and prior paleoclimatic reconstructions in the Huangqihai Lake region, there was a generally-accepted, stepwise shift to a wet climate during the early Holocene, an overall humid climate from 7.2 to 3.2 cal kaBP, and then severe drought for the rest of the Holocene. Moreover, regional comparisons among pollen records derived from lakes situated in the temperate steppe region suggested a roughly synchronous pattern of vegetation and climate changes during the Holocene and demonstrated an intensified EASM during the middle Holocene.  相似文献   

2.
Palaeoclimatic records derived from a variety of independent proxies provide evidence of post‐glacial changes of temperature and soil moisture in northern Fennoscandia. We use pollen percentage, pollen influx, stomatal and chironomid records from Toskaljavri, a high‐altitude lake in northern Finland, to assess how treelines and alpine vegetation there have responded to these climate changes. The evidence suggests that the cool, moist climate of the early Holocene supported birch forest in the area 9600 cal. yr BP onwards and that a rise of temperature triggered the immigration of pine at 8300 cal. yr BP. At 6100–4000 cal. yr BP altitudinal treeline in the area was formed by pine, in contrast to the modern situation where mountain birch reaches a higher elevation. Alpine vegetation also demonstrates clear changes. Plant communities typical of dry, oligotrophic heaths of northern Fennoscandia expanded during the dry climatic period at 7000–4000 cal. yr BP and decreased in response to cooler and moister conditions after 4000 cal. yr BP. Alpine plant communities favouring moist sites show an inverse pattern, expanding after a change towards moister climate after 4000 cal. yr BP. In a redundancy analysis (RDA), a statistically significant proportion of the variability in the total chironomid assemblages was captured by changes in the pollen types reflecting alpine vegetation typical of moist sites. Although chironomid community changes appeared to follow the major patterns in the alpine vegetation succession, the present study does not support a direct link between the changing treeline position and chironomid stratigraphy. Rather, the data indicate that the terrestrial and aquatic environments have each responded directly to the same ultimate cause, namely changing Holocene climate. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

3.
Vegetation dynamics during the Younger Dryas-Holocene transition in the extreme northern taiga zone of the Usa basin, northeastern European Russia, were reconstructed using plant macrofossil and pollen evidence from a sediment core from Lake Llet-Ti. The pollen stratigraphy during the Younger Dryas (about 12 500-11 500 cal. yr BP) is characterized by pollen types indicative of treeless arctic vegetation, whereas the macrofossil evidence shows the occurrence of scattered spruce and birch trees around the lake. The Younger Dryas-early Holocene transition is characterized by a rapid increase in vegetation density, including an increase in the birch population, followed by the expansion of the spruce population at about 10 000 cal. yr BP. Dense spruce-birch forest dominated until 5000 cal. yr BP. Our results contribute to the debate about the Lateglacial environments in northern Russia, and illustrate the importance of plant macrofossil records in Lateglacial vegetation reconstructions.  相似文献   

4.
Lake sedimentary records that allow documentation of the distinct climatic and environmental shifts during the early part of the Last Termination are scarce for northern Europe. This multi‐proxy study of the sediments of Atteköpsmosse, southwest Sweden, therefore fills an important gap and provides detailed information regarding past hydroclimatic conditions and local environmental responses to climatic shifts. Lake infilling started c. 15.5 cal. ka BP, but low aquatic productivity, cold summer lake water temperatures, unstable catchments, and scarce herb and shrub vegetation prevailed until c. 14.7–14.5 cal. ka BP. Inflow of warmer air masses and higher July air temperatures favoured a rise in aquatic productivity and lake water summer temperatures, and the establishment of a diverse herb, shrub and dwarf shrub vegetation, which also included tree birch c. 14.5 cal. ka BP. Freshening of the moisture source region c. 13.7–13.6 cal. ka BP does not seem to have had a large impact on the ancient lake and its catchment, as lake aquatic productivity increased further and lake water summer temperatures and minimum mean July air temperatures remained around 12–14 °C. In contrast, further freshening of the moisture source region c. 13 cal. ka BP triggered a decrease in lake productivity, drier conditions and lower lake water summer temperatures. Macroscopic finds of tree Betula and Pinus sylvestris at 13–12.8 cal. ka BP demonstrate the presence of these trees in the lake's catchment. The transition into the Holocene (11.6–11.5 cal. ka BP) is marked by a change in chironomid assemblages and by a rise in lake water summer temperatures and aquatic productivity. These changes were followed by the re‐establishment of a diverse aquatic and terrestrial vegetation, including tree birch and Pinus sylvestris at 11.4 cal. ka BP.  相似文献   

5.
Here we present Holocene organic carbon, nitrogen, sulphur, carbon isotope ratio and macrofossil data from a small freshwater lake near Sisimiut in south‐west Greenland. The lake was formed c. 11 cal ka BP following retreat of the ice sheet margin and is located above the marine limit in this area. The elemental and isotope data suggest a complex deglaciation history of interactions between the lake and its catchment, reflecting glacial retreat and post‐glacial hydrological flushing probably due to periodic melting of local remnant glacial ice and firn areas between 11 and 8.5 cal ka BP. After 8.5 cal ka BP, soil development and associated vegetation processes began to exert a greater control on terrestrial–aquatic carbon cycling. By 5.5 cal ka BP, in the early Neoglacial cooling, the sediment record indicates a change in catchment–lake interactions with consistent δ13C while C/N exhibits greater variability. The period after 5.5 cal ka BP is also characterized by higher organic C accumulation in the lake. These changes (total organic carbon, C/N, δ13C) are most likely the result of increasing contribution (and burial) of terrestrial organic matter as a result of enhanced soil instability, as indicated by an increase in Cenococcum remains, but also Sphagnum and Empetrum. The impact of glacial retreat and relatively subdued mid‐ to late Holocene climate variation at the coast is in marked contrast to the greater environmental variability seen in inland lakes closer to the present‐day ice sheet margin. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

6.
Pollen-assemblage data from a sediment core from Hulun Lake in northeastern Inner Mongolia describe the changes in the vegetation and climate of the East Asian monsoon margin during the Holocene. Dry steppe dominated the lake basin from ca. 11,000 to 8000 cal yr BP, suggesting a warm and dry climate. Grasses and birch forests expanded 8000 to 6400 cal yr BP, implying a remarkable increase in the monsoon precipitation. From 6400 to 4400 cal yr BP, the climate became cooler and drier. Chenopodiaceae dominated the interval from 4400 to 3350 cal yr BP, marking extremely dry condition. Artemisia recovered 3350-2050 cal yr BP, denoting an amelioration of climatic conditions. Both temperature and precipitation decreased 2050 to 1000 cal yr BP as indicated by decreased Artemisia and the development of pine forests. During the last 1000 yr, human activities might have had a significant influence on the environment of the lake region. We suggest that the East Asian summer monsoon did not become intensified until 8000 cal yr BP due to the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoon precipitation on millennial to centennial scales would be related to ocean-atmosphere interactions in the tropical Pacific.  相似文献   

7.
Major Holocene monsoon changes in continental Southeast Asia are reconstructed from analysis of 14C-dated changes in pollen and organic/inorganic carbon in sediment cores taken from permanent, closed-basin, volcanic lakes in Ratanakiri Province, northeastern Cambodia. Analysis focuses on the nature and timing of monsoon changes, inferred from changes in vegetation and lake conditions. These data provide the first well-dated palynological record, covering most of the Holocene and continuous up to the present, from a terrestrial site in mainland Southeast Asia. The record from a 15-m core retrieved from Kara Lake, representing the last 9300 years, shows that the late Glacial conditions ended about 8500 14C yr B.P., more than 1000 years later than sites in southwest China. Summer monsoon intensity increased over the period ca. 8400–5300 14C yr B.P., similar to most other sites in the Asian monsoon region. A subsequent expansion of secondary forests at the expense of dense semievergreen forests suggest a drier climate leading to more frequent fire disturbance. After ca. 3500 14C yr B.P. disturbance frequency may have increased further with increasing seasonality. From ca. 2500 14C yr B.P. to the present, dense forest has recovered in a mosaic with annually burned dry forest, but climate may not be the main control on local vegetation dynamics in the late Holocene.  相似文献   

8.
Climate models suggest that the global warming during the early to mid‐Holocene may have partly resulted from the northward advance of the northern treeline and subsequent reduction of the planetary albedo. We investigated the Holocene vegetation history of low arctic continental Nunavut, Canada, from a radiocarbon‐dated sediment core from TK‐2 Lake, a small‐lake ca. 200 km north of the limit of the forest‐tundra. The pollen and loss‐on‐ignition data indicate the presence of dwarf shrub tundra in the region since the beginning of organic sedimentation at ca. 9000 cal. yr BP with dominance of Betula, especially since 8700 cal. yr BP. At 8100–7900 cal. yr BP the dominance of the shrub tundra was punctuated by a transient decline of Betula and coincident increases of Ericaceae undiff., Vaccinium‐type, and Gramineae. This suggests an abrupt disturbance of the Betula glandulosa population, approximately simultaneously with the sudden 8200 cal. yr BP event in the North Atlantic. However, in the absence of other sites studied in the area, linkage to the 8200 cal. yr BP event remains tentative. The lack of any evidence of forest‐tundra in the region constrains the northern limit of the mid‐Holocene advance of the forest‐tundra boundary in central northern Canada. Consequently, our results show that the climate models imposing a mid‐Holocene advance of the limit of the forest‐tundra to the arctic coast of Canada may have overestimated the positive climatic feedback effects that can result from the replacement of tundra by the boreal forest. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

9.
While single pollen records are widely used in reconstructing the environment for nearby prehistoric settlements, they are less helpful when addressing large‐scale issues of variation in human settlement patterns. In order to assess the impact of vegetation change on regional prehistoric settlement and subsistence patterns in an ecotone sensitive area, we inferred the general change in main vegetation types based on palaeobotanical investigations from across northernmost Fennoscandia. Tundra vegetation was predominant during the Lateglacial and earliest parts of the Holocene. Maritime birch forests rich in ferns started to expand c. 11 000 cal. a BP and became dominant from 10 000 cal. a BP. Pine expanded from the NE of the investigation area and pine‐birch forest dominated in the inland around 8000 cal. a BP. A gradual degeneration of forest towards more open birch woodland started c. 6000 cal. a BP with the most marked change around 3500 cal. a BP. Along the northern outer coast, this eventually led to open heathland. Comparison with the archaeological setting suggests a general correlation between low forest cover and extensive mobility patterns, while widespread and varied forest cover appear to have led to a more sedentary way of life. The background for this is arguably that the forested landscapes hosted a larger diversity of resources within a shorter foraging distance, while areas and periods with low forest cover required longer travels to obtain the desired prey and materials.  相似文献   

10.
We reconstructed a 10,500-yr fire and vegetation history of a montane site in the North Cascade Range, Washington State based on lake sediment charcoal, macrofossil and pollen records. High-resolution sampling and abundant macrofossils made it possible to analyze relationships between fire and vegetation. During the early Holocene (> 10,500 to ca. 8000 cal yr BP) forests were subalpine woodlands dominated by Pinus contorta. Around 8000 cal yr BP, P. contorta sharply declined in the macrofossil record. Shade tolerant, mesic species first appeared ca. 4500 cal yr BP. Cupressus nootkatensis appeared most recently at 2000 cal yr BP. Fire frequency varies throughout the record, with significantly shorter mean fire return intervals in the early Holocene than the mid and late Holocene. Charcoal peaks are significantly correlated with an initial increase in macrofossil accumulation rates followed by a decrease, likely corresponding to tree mortality following fire. Climate appears to be a key driver in vegetation and fire regimes over millennial time scales. Fire and other disturbances altered forest vegetation at shorter time scales, and vegetation may have mediated local fire regimes. For example, dominance of P. contorta in the early Holocene forests may have been reinforced by its susceptibility to frequent, stand-replacing fire events.  相似文献   

11.
Plant macrofossils from 38 packrat middens spanning the last ~ 33,000 cal yr BP record vegetation between ~ 650 and 900 m elevation along the eastern escarpment of the Sierra San Pedro Mártir, northern Baja California. The middens span most of the Holocene, with a gap between ~ 4600 and 1800 cal yr BP, but coverage in the Pleistocene is uneven with a larger hiatus between 23,100 and 14,400 cal yr BP. The midden flora is relatively stable from the Pleistocene to Holocene. Exceptions include Pinus californiarum, Juniperus californica and other chaparral elements that were most abundant > 23,100 cal yr BP and declined after 14,400 cal yr BP. Despite being near the chaparral/woodland-desertscrub ecotone during glacial times, the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record, and this is corroborated by a nearby semi-continuous pollen stratigraphy from lake sediments. Regular appearance of C4 grasses and summer-flowering annuals since 13,600 cal yr BP indicates occurrence of summer rainfall equivalent to modern (JAS average of ~ 80–90 mm). This casts doubt on the claim, based on temperature proxies from marine sediments in the Guaymas Basin, that monsoonal development in the northern Gulf and Arizona was delayed until after 6200 cal yr BP.  相似文献   

12.
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   

13.
Bolshaya Imandra, the northern sub-basin of Lake Imandra, was investigated by a hydro-acoustic survey followed by sediment coring down to the acoustic basement. The sediment record was analysed by a combined physical, biogeochemical, sedimentological, granulometrical and micropalaeontological approach to reconstruct the regional climatic and environmental history. Chronological control was obtained by 14C dating, 137Cs, and Hg markers as well as pollen stratigraphy and revealed that the sediment succession offers the first continuous record spanning the Lateglacial and Holocene for this lake. Following the deglaciation prior to c. 13 200 cal. a BP, the lake's sub-basin initially was occupied by a glacifluvial river system, before a proglacial lake with glaciolacustrine sedimentation established. Rather mild climate, a sparse vegetation cover and successive retreat of the Scandinavian Ice Sheet (SIS) from the lake catchment characterized the Bølling/Allerød interstadial, lasting until 12 710 cal. a BP. During the subsequent Younger Dryas chronozone, until 11 550 cal. a BP, climate cooling led to a decrease in vegetation cover and a re-advance of the SIS. The SIS disappeared from the catchment at the Holocene transition, but small glaciers persisted in the mountains at the eastern lake shore. During the Early Holocene, until 8400 cal. a BP, sedimentation changed from glaciolacustrine to lacustrine and rising temperatures caused the spread of thermophilous vegetation. The Middle Holocene, until 3700 cal. a BP, comprises the regional Holocene Thermal Maximum (8000–4600 cal. a BP) with relatively stable temperatures, denser vegetation cover and absence of mountain glaciers. Reoccurrence of mountain glaciers during the Late Holocene, until 30 cal. a BP, presumably results from a slight cooling and increased humidity. Since c. 30 cal. a BP Lake Imandra has been strongly influenced by human impact, originating in industrial and mining activities. Our results are in overall agreement with vegetation and climate reconstructions in the Kola region.  相似文献   

14.
The new pollen record from the upper 12.75 m of a sediment core obtained in Lake Ladoga documents regional vegetation and climate changes in northwestern Russia over the last 13.9 cal. ka. The Lateglacial chronostratigraphy is based on varve chronology, while the Holocene stratigraphy is based on AMS 14C and OSL dates, supported by comparison with regional pollen records. During the Lateglacial (c. 13.9–11.2 cal. ka BP), the Lake Ladoga region experienced several climatic fluctuations as reflected in vegetation changes. Shrub and grass communities dominated between c. 13.9 and 13.2 cal. ka BP. The increase in Picea pollen at c. 13.2 cal. ka BP probably reflects the appearance of spruce in the southern Ladoga region at the beginning of the Allerød interstadial. After c. 12.6 cal. ka BP, the Younger Dryas cooling caused a significant decrease in spruce and increase in Artemisia with other herbs, indicative of tundra‐ and steppe‐like vegetation. A sharp transition from tundra‐steppe habitats to sparse birch forests characterizes the onset of Holocene warming c. 11.2 cal. ka BP. Pine forests dominated in the region from c. 9.0 to 8.1 cal. ka BP. The most favourable climatic conditions for deciduous broad‐leaved taxa existed between c. 8.1 and 5.5 cal. ka BP. Alder experiences an abrupt increase in the local vegetation c. 7.8 cal. ka BP. The decrease in tree pollen taxa (especially Picea) and the increase in herbs (mainly Poaceae) probably reflect human activity during the last 2.2 cal. ka. Pine forests have dominated the region since that time. Secale and other Cerealia pollen as well as ruderal herbs are permanently recorded since c. 0.8 cal. ka BP.  相似文献   

15.
江汉平原沔城M1孔的沉积特征与古环境重建   总被引：6，自引：0，他引：6  
通过对江汉平原沔城M1孔湖泊沉积物进行沉积特征、粒度、14C年代、孢粉分析,重建了晚冰期以来该地区古环境、古气候演化的过程和序列:晚冰期后期有一扩张期,气候温湿并出现湖泊相沉积;晚冰期末期气候温凉偏干、河流环境;全新世初期(10-8.9kaBP),气候转向温湿;全新世大暖期(8.9-3.5kaBP),总体上气候温暖湿润,其中6.8-4.9kaBP是最宜期,4.9-4.8kaBP和4.4-4.2kaBP为两次降温事件,4.8kaBP积水湖盆开始形成,3.9-3.5kaBP为云梦泽鼎盛期;全新世晚期(3.5-1.7kaBP),气温较大暖期有所下降,其中3.5-2.5kaBP温凉偏湿,2.5-1.7kaBP较为温湿,湖泊较为稳定,1.7kaBP开始,云梦泽萎缩,钻孔所在位置出露水面。   相似文献   

16.
A lake sediment record from the Friedländer Groβe Wiese in northeast Germany was studied to reconstruct summer temperature changes associated with changes in vegetation development during the Weichselian Lateglacial. The record was analysed for pollen, chironomids, and oxygen and carbon isotopes of lake marl. The combination of radiocarbon dates, the presence of the Laacher See Tephra and correlation of lithological and palynological changes with other records from the region indicated that the record encompassed the Allerød to the early Holocene. Pollen assemblages reflect development of birch and later pine‐dominated forests during the Allerød, comparable to other sites in the region. Chironomid‐inferred mean July air temperatures (C‐IT) for this period range between ～14.0 and 14.8°C. A temporary decrease in C‐IT of ～1°C, a negative shift in the isotope records, and a minor decline of birch may correspond to Greenland Interstadial 1b. Even though the transition to the Younger Dryas appears to be affected by reworking and redeposition processes, a drop in C‐IT to ～11.1°C is reconstructed for the later part of the Younger Dryas, while it appears that pine locally persisted in the region. Comparison with a nearby pollen record further indicates a local expansion of wetland grasses during this period. At the transition to the Holocene, C‐IT increased to ～15.7°C, while birch and pine forests re‐expanded. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

17.
《Quaternary Science Reviews》2007,26(17-18):2229-2246
A sediment core recovered from Garba Guracha, a glacial lake at 3950 m altitude in the Bale Mountains of Ethiopia, at the boundary of the Ericaceous and Afroalpine vegetation belts, provides a 16,700-year pollen record of vegetation response to climatic change. The earliest vegetation recorded was sparse and composed mainly of grasses, Amaranthaceae–Chenopodiaceae and Artemisia, indicating an arid climate. At 13,400 cal BP, Amaranthaceae–Chenopodiaceae pollen declined sharply and Cyperaceae increased, suggesting a change to moister conditions. The Younger Dryas interval is represented by a small increase in Artemisia and reduced Cyperaceae, indicating aridity. Just after the start of the Holocene (11,200 cal BP), the upper altitudinal limit of the Ericaceous belt rose, and woody Ericaceous vegetation extended across the Sanetti plateau, in response to increased moisture and temperature. The marked change from clastic to organic lake sedimentation at this time reflects the increase in woody vegetation cover in the lake catchment, accompanied by soil stabilisation, and increased leaf litter and soil humus content. From about 6000 cal BP, and especially after 4500 cal BP, mid-altitude dry Afromontane Juniper–Podocarpus forests developed on the northern slopes of the mountains in response to reduced rainfall in a shortened wet season. Erica shrub and forest decreased in area and altitude, and the Afroalpine ecosystem expanded on the plateau. Podocarpus declined from about 2000 cal BP, as Juniperus increased to its present dominance at 2500–3300 m altitude. Human impact on the high-altitude Afroalpine and Ericaceous vegetation has been relatively minor, confirming that the endemic biodiversity of the Ethiopian mountains is a legacy of natural Holocene vegetation change, following repeated expansion and contraction of the upland ecosystems during the Quaternary.  相似文献   

18.
Charcoal particles are widespread in terrestrial and lake environments of the northern temperate and boreal biomes where they are used to reconstruct past fire events and regimes. In this study, we used botanically identified and radiocarbon-dated charcoal macrofossils in mineral soils as a paleoecological tool to reconstruct past fire activity at the stand scale. Charcoal macrofossils buried in podzolic soils by tree uprooting were analyzed to reconstruct the long-term fire history of an old-growth deciduous forest in southern Québec. Charcoal fragments were sampled from the uppermost mineral soil horizons and identified based on anatomical characters. Spruce (Picea spp.) fragments dominated the charcoal assemblage, along with relatively abundant wood fragments of sugar maple (Acer saccharum) and birch (Betula spp.), and rare fragments of pine (Pinus cf. strobus) and white cedar (Thuja canadensis). AMS radiocarbon dates from 16 charcoal fragments indicated that forest fires were widespread during the early Holocene, whereas no fires were recorded from the mid-Holocene to present. The paucity of charcoal data during this period, however, does not preclude that a fire event of lower severity may have occurred. At least eight forest fires occurred at the study site between 10,400 and 6300 cal yr B.P., with a dominance of burned conifer trees between 10,400 and 9000 cal yr B.P. and burned conifer and deciduous trees between 9000 and 6300 cal yr B.P. Based on the charcoal record, the climate at the study site was relatively dry during the early Holocene, and more humid from 6300 cal yr B.P. to present. However, it is also possible that the predominance of conifer trees in the charcoal record between 10,400 and 6300 cal yr B.P. created propitious conditions for fire spreading. The charcoal record supports inferences based on pollen influx data (Labelle, C., Richard, P.J.H. 1981. Végétation tardiglaciaire et postglaciaire au sud-est du Parc des Laurentides, Québec. Géographie Physique et Quaternaire 35, 345-359) of the early arrival of spruce and sugar maple in the study area shortly after deglaciation. We conclude that macroscopic charcoal analysis of mineral soils subjected to disturbance by tree uprooting may be a useful paleoecological tool to reconstruct long-term forest fire history at the stand scale.  相似文献   

19.
This paper investigates a detailed well‐dated Lateglacial floristic colonisation in the eastern Baltic area, ca. 14 000–9000 cal. a BP, using palynological, macrofossil, loss‐on‐ignition, and 14C data. During 14 000–13 400 cal. a BP, primarily treeless pioneer tundra vegetation existed. Tree birch (Betula sect. Albae) macro‐remains and a high tree pollen accumulation rate indicate the presence of forest‐tundra with birch and possibly pine (Pinus sylvestris L.) trees during 13 400–12 850 cal. a BP. Palaeobotanical data indicate that the colonisation and development of forested areas were very rapid, arising within a period of less than 50 years. Thus far, there are no indications of conifer macrofossils in Estonia to support the presence of coniferous forests in the Lateglacial period. Signs of Greenland Interstadial 1b cooling during 13 100 cal. a BP are distinguishable. Biostratigraphic evidence indicates that the vegetation was again mostly treeless tundra during the final colder episode of the Lateglacial period associated with Greenland Stadial 1, approximately 12 850–11 650 cal. a BP. This was followed by onset of the Holocene vegetation, with the expansion of boreal forests, in response to rapid climatic warming. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
李卓仑  王乃昂  李育  程弘毅 《冰川冻土》2013,35(6):1481-1489
通过对花海古湖泊沉积剖面8.42~0.405 m沉积物样品的矿物和化学元素测定,分析了沉积物中盐类矿物含量及化学元素K/Na比值的变化情况,结合已有的年代地层结果,重建了花海古湖泊10.47~5.5 cal ka BP湖水盐度变化. 结果表明：花海湖泊全新世湖相沉积阶段中,除个别层位以硫酸盐类矿物沉积为主外,早全新世(10.47~8.87 cal ka BP)和中全新世(8.87~5.5 cal ka BP)均以碳酸盐盐类矿物沉积为主,并且早全新世时期K/Na高于中全新世时期,揭示了早全新世时期湖水盐度高于中全新世时期. 这一结果与该湖泊沉积过程所揭示的湖泊水位变化、粒度等揭示的有效湿度变化具有一致性,表明花海湖泊早、中全新世湖水盐度的高低可以指示其湖泊水位的变化,并间接反映了有效湿度的变化. 结合花海湖泊晚全新世湖泊萎缩、气候干旱的特点,该区域早、中、晚全新世气候干湿变化变化模式可以概况为早全新世降水增强、气候呈现由干向湿的转变,中全新世有效湿度最大,晚全新世气候干旱. 这种全新世气候干湿变化模式有别于西风区,亦与季风区不完全相同,呈现出了一种季风-西风过渡带全新世气候干湿变化的模式.  相似文献   

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