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1.
The longest continuous Amazonian palynological record (ca 7010 yrs B.P. to present) from Lake Ayauchi, Ecuador, reveals species-by-species abundance changes during a period of climatic change. Pollen influx from a wet tropical rain forest was found to be high, 1×104–105 grains cm-2 yr-1, although mature forest taxa were poorly represented. Horizons of laminated sediments and weathered gyttja, dated to ca 4200–3150 B.P., evidence a period of reduced net water availability. During this period Ficus, Alchornea and Palmae pollen representation appears to decline, although there is no evidence of a major forest compositional change. The lake was reduced to a shallow, possibly seasonal, pool. Zea cultivation was recorded between ca 2850 B.P., (the earliest paleoecological record to date in the Amazon basin) and ca 800 B.P. It is suggested that Zea was cultivated on exposed lake sediment within the crater at times of low water levels. The abandonment of Zea cultivation may have been due to rising water levels or social unrest.  相似文献   

2.
On the basis of distribution maps showing the first pollen occurrences in the Holocene of the well-known climate indicators Hedera, Ilex and Viscum as well as data for Corylus, a series of maps have been prepared that show summer and winter isotherms at various time intervals during the Holocene. From these maps climate curves for Amsterdam, the Netherlands have been set out. These were compared with curves for the Eemian at the same site. In both of these warm periods there is evidence for increased seasonality in the early phases which were relatively continental. Changes in insolation could account for such differences. Summer optima occurred earlier than winter optima. Changes in land-sea distribution are important, especially with regard to the patterns in winter climate. During the latter half of the Eemian, the climate was distinctly more oceanic than in the Holocene. Early in the Holocene, an influx of warm ocean water resulted in higher winter temperatures in the Gulf of Biscay, the Irish Sea, and areas east of Skagerrak-Kattegat. Temperature decline after the climatic optimum was greatest in the north, i.e. at 60°N, where a depression in the order of 2°C in summer and 2–3°C in winter occurred. Temperature decline was less farther south, i.e. at ca. 50°N, where a distinct west-east gradient in temperature change can be observed.  相似文献   

3.
4.
Early Pliocene lacustrine sediments from the Ptolemais Basin, northern Greece, exhibit a climatically induced cyclicity, which can be correlated with the orbital cycle of precession. Superimposed on precession-controlled cycles, palynological time-series data reveal a higher-order climatic cyclicity with periodicities of ∼10, ∼2.5, and ∼1.5 ka. These millennial-scale vegetation changes are likely to reflect changes of the soil-moisture gradient on mountain slopes, caused by fluctuations in orographic winter-precipitation. Such fluctuations corroborate the concept of a NAO-like North Atlantic climatic teleconnection during Early Pliocene times. The periodicities are similar to those of climate oscillations inferred from Quaternary records. The occurrence of millennial-scale cyclicity in a time interval when the Northern Hemisphere was essentially ice-free, implies an ultimate forcing mechanism that operated independently of changes in the thermohaline circulation in the Atlantic Ocean. The cycles are likely to be related to long-period variations in solar activity.  相似文献   

5.
We present a pollen record from a fen at the eastern shore of Brazo Sur south of Lago Argentino, Argentina (core BRS 1/06, 50°34′54″S, 72°54′52″W, 198 m a.s.l.). The coring site is located in the transition zone from humid grass steppe to Andean Nothofagus forest. With the exception of a mid-Holocene sand layer, the record covers the interval between 13,350 and 1,700 cal b.p., indicating that there could have been no recurrence of Late-glacial conditions in Brazo Sur, as has been suggested in the literature as having been contemporary with the Younger Dryas interval in the northern hemisphere. The Late-glacial and early Holocene periods at Brazo Sur are characterized by fluctuations between humid grass-dominated steppe associations and drier ones dominated by Asteraceae and Ericaceae. At the start of the Holocene, the pollen influx increased. Nothofagus dominance began by 7,700 cal b.p. probably due to increasing moisture. Present-day closed Nothofagus forest developed around 2,500 cal b.p., related to a further increase in moisture.  相似文献   

6.
John G. Farmer 《Hydrobiologia》1994,290(1-3):39-49
The chemical record in Loch Lomond sediments deposited since the end of the last Ice Age provides evidence of the Flandrian marine transgression some 5500–7000 14C years B.P., sedimentation rates and the influence of man's local activities, environmental pollution and its sources since the onset of the Industrial Revolution, and of elemental mobility linked with the reduction-diffusion-oxidation cycle of early sedimentary diagenesis. Information derived from vertical profiles of halogen elements bromine and iodine, radionuclides 14C and 210Pb, heavy metals lead, zinc, and cadmium, stable lead isotopes 206Pb and 207Pb, and redox-sensitive elements manganese, iron and arsenic is reviewed and assessed.  相似文献   

7.
Detailed botanical (microfossil and macroremain), zoological and geochemical analyses (major and trace elements including C, Al, S, Ca, Fe, P, As, Zn, U, Ba and Rare Earth Elements) of organic deposits provide new insights into Early Holocene environmental change in the Kreekrak area (southwestern Netherlands). The age assessment of the record is based on high resolution AMS 14C wiggle-match dating (WMD). For the first time an AMS 14C WMD based chronology covering the Late Glacial/Holocene transition and early Preboreal is introduced for a site in The Netherlands.The Kreekrak botanical record reflects the end of the Younger Dryas to early Boreal and can be well correlated with pollen records from other sites in The Netherlands and Belgium. The palaeo-topography showed that the Kreekrak deposits formed in an abandoned channel of the River Schelde. Around ca. 11,490 cal BP, at the end of the Late Glacial/Holocene transition, infilling of the lake started with predominantly organic deposits in slowly running water. As a result of the warmer climate the area became forested with birch and poplar during the Friesland Phase (ca. 11,490-11,365 cal BP). Biological productivity of the lake and its surroundings increased. Aquatic vegetation developed in the lake, while shrubs of willow, reed swamps and grasslands fringed the shores. Precipitation increased, which caused a rise in the lake water table and an increase in the supply of oxic surface (= river) water into the Kreekrak lake. During this period, the Kreekrak lake was fed by inflowing river water, run-off, precipitation and seepage of Fe-rich groundwater. Around ca. 11,435 cal BP the water became stagnant probably as result of a total cut-off of the river channel. Inflow of river water ceased, while the supply of reduced Fe-rich groundwater became dominant. During the Rammelbeek Phase (ca. 11,365-11,250 cal BP), the climate was more continental and the abundance of grasslands and open herbaceous vegetation increased. Biological productivity remained high. In the lake, the supply of Fe-rich groundwater continued, the water level slightly decreased but aquatic vegetation remained present. At the end of the Rammelbeek Phase a sudden reduction in the supply of Fe-rich reduced groundwater caused a lowering of the groundwater level in the area, resulting in the development of a hiatus. Due to this hiatus, the Late Preboreal (11,250-10,710 cal BP) is absent from the record. During the early Boreal (10,710-10,000 cal BP) the landscape became densely forested and accumulation of peat in the former lake resumed due to a slowly rising groundwater level. The Boreal was a relatively stable period with low sedimentation rates.The combination of palaeobotanical and geochemical analyses in the Kreekrak record shows a close interrelation between landscape development and geochemistry. It appears that the environmental development of this area during the Late Glacial/Holocene transition and Early Holocene was largely influenced (directly or indirectly) by major climatic changes that occurred during this period, which determined local phenomena such as the composition and density of the vegetation, occurrence of seepage and river activity. Further research of this type has the potential to develop the application of major- and trace element geochemistry in palaeoenvironmental reconstructions.  相似文献   

8.
In some continental and island sites in the western Mediterranean basin, the Holocene vegetation and climate dynamics seem to show the same patterns in time and space. Nevertheless, different synchronous scenarios have been proposed from other south European, North African and Near Eastern pollen records from around the Mediterranean basin. Striking similarities and synchronisms have been found between Sicily and the Balearic Islands. These islands, although under different bioclimatic influences, show similarities in the main trends of vegetation and climate dynamics during the Post-glacial. Lago di Pergusa is the only natural inland lake in Sicily and because of its geographical location, has a good potential sensitivity to the climatic changes of the Mediterranean basin. Likewise, coastal sediments from Minorca and Majorca, the Balearic Islands, have similar peculiarities. The present-day environmental situation, now that most of the natural vegetation in these islands has disappeared, has been brought about either by a climatic trend towards increasing aridity or an increase in human activities. It seems clear that prehistoric human people alone could not have caused all the environmental changes recorded in the last millennia in both places.  相似文献   

9.
Dinoflagellate cysts, pollen and charred cuticle fragments from two sediment cores (giant piston Calypso core MD03-2708 and giant gravity Casq core MD03-2708CQ) collected off the Ogooué River mouth, Gabon (01°10.33′S-08°19.01′E, 920 m water depth) have been analysed to identify the direction and timing of marine-terrestrial environmental changes over western equatorial Africa during the last 26,000 yr. Changes in the proxy records indicate that both terrestrial and oceanic domains off Gabon were impacted synchronously by significant climate changes during the last glacial-interglacial transition and Holocene. Leading into, and during, the Last Glacial Maximum (LGM), heterotrophic dinoflagellate cysts and Ca X-ray fluorescence intensity suggest nutrient enrichment off the river mouth, while pollen records indicate expansions of open forest, savannah woodland, and the Afromontane forest in the catchment area. From the deglaciation to mid-Holocene, however, a marked decrease in Brigantedinium spp. as well as in Poaceae, Cyperaceae and Podocarpus pollen reflects a reduction in nutrient supply to the coastal ocean in parallel with a reduction in grassland, herbaceous communities and Afromontane forest within the catchment. A sharp decline or even disappearance of heterotrophic species during this period is almost contemporaneous with an appearance of Operculodinium aguinawense, reflecting enhanced river influence in the study area. A marked increase of Rhizophora (mangrove) pollen during this transition also indicates eustatic sea-level rise after the LGM which forced a major expansion of the mangrove ecosystem across the gradually submerging shelf. The combination during the late Holocene of a reoccurrence of heterotrophic dinoflagellate cysts with increased Poaceae, Cyperaceae and Podocarpus pollen indicates enhanced nutrient supply to the ocean concordant with a shift to cooler/drier conditions over the Gabon basin. This appears to be linked to a deterioration of the monsoon system induced by the low sea-surface temperatures in the tropical Atlantic Ocean in association with decreased summer insolation.  相似文献   

10.
A 2700-year high resolution pollen record from annually-varved Sugan Lake in the Qaidam Basin at 2793 m a.s.l was obtained to examine vegetation and climatic change on the NE Tibetan Plateau. Pollen data shows that Sugan Basin was constantly covered by open desert-steppe vegetation dominated by Chenopodiaceae, Artemisia, Poaceae and Ephedra. However, large variations in Artemisia/Chenopodiaceae (A/C) ratios suggest regional moisture fluctuations over the last 2700 years, including a dry and relatively stable climate prior to 300 AD, relatively wet climate from 300 to 1200 AD with variability during 1100-1200 AD, and unstable climate since 1200 AD with relatively moister climate during 1250-1400 AD and 1700-1800 AD. However, other proxies (varve thickness, Chironomid taxa, isotopes of oxygen in precipitated carbonate) show fresher water when regional moisture was lower inferred from A/C ratio. This inconsistency suggests the possible difference of in-lake lithology/environment and regional moisture change. Fresh water into the lake from ice melting on the surrounding mountains might have contributed to the in-lake lithology and environment variation. The effective moisture changes in the Basin are in opposite phases to snow accumulation records from Dunde ice core (5325 m a.s.l) and to the monsoon intensity inferred from Dongge Cave, suggesting that the regional topography might have played an important role in mediating moisture changes at regional scale. Pollen data from Sugan Lake shows the shift of moisture at 1200 AD, from stable to variable conditions. This event is well correlated with other paleoclimate proxies in China and other parts of the world; however, the mechanisms behind these patterns require further investigation.  相似文献   

11.
This study focuses on diatom assemblages occurring in cores of Late-glacial and Holocene deposits retrieved from the mouth of the lower Rega River valley, of the southern coast of the Baltic Sea. Sediment samples from four cores were the subject of the present study. Diatom-inferred environmental characteristics, e.g., water level; water salinity (conductivity), trophic status and pH, within each core are presented. Diatom assemblage zones (DAZ) were distinguished, based on differences in the distribution of particular ecological groups. Each DAZ appears to be related to environmental changes during the deposition of a given sediment interval. The Late-glacial (Allerød) sediments originated in a shallow lake with increasing concentrations of solutes and nutrients. The Holocene record begins in the early Atlantic Chronozone and the diatoms point to weakly brackish-water sediments deposited in a shallow water environment. During the period of 8,500–5,800 cal year BP sedimentation took place in a shallow embayment and/or lagoon. From ca. 5,800 cal year BP onwards sedimentation took place in a peat bog environment alternating with Aeolian deposition. Changes in diatom community structure imply a close relationship with the climate-controlled eustatic rise of the ocean level and its consequence Littorina transgression. As with other southern Baltic Sea localities, brackish-water diatoms appear in the sediments, signaling the onset of marine transgression somewhat earlier than previously accepted. Differences and similarities in diatom assemblages and the palaeogeographic development of nearby regions within the Baltic Sea basin and lagoons (coastal areas) from different geographic regions, are also discussed.  相似文献   

12.
The palynological sequence from core ND-1 on the Song Hong delta in the northern Vietnam reveals the climate change during the last deglaciation. The identified pollen in the core is dominated by Castanopsis (Lithocarpus), Elaeocarpus, Ficus, Piperaceae and Quercus. High percentages of temperate taxa including the conifers Dacrydium, Podocarpus, Pinus, Cupressaceae, Txodiaceae and Cryptomeria, and broad-leaved taxa of Carpinus, Alnus, Juglans, Carya, Ulmus, Fagus, Ilex, Castanea, Quercus, which have a mainly upland distribution, is possibly associated with a lowering of the montane vegetation boundary. On the other hand, intensified river influence can also result in an increase in the temperate allochthonous taxa. After analyzing the magnitude of river influence on pollen assemblages during the sedimentary environment evolution, we extracted the climate information and used this to reconstruct climate change during the last deglaciation. A cooler climate during 14.5-10.9 cal. kyr BP than at present is indicated by higher percentage of temperate types than at present under a similar river influence on the floodplain. The Younger Dryas cooling event is recognized by widely developed grasslands between 12.9 and 11.6 cal. kyr BP, adding to the evidences for this period from both terrestrial and marine records in regions influenced by the East Asian monsoon. Two other cooling periods, 9.4-9.0 cal. kyr BP and 6.5-5.2 cal. kyr BP, are indicated by increases of temperate pollen taxa such as Cryptomeria, Alnus, Quercus, and Castanea.  相似文献   

13.
In 1998, an ancient shipping wharf was brought to light in Pisa, Italy. The shipwrecks found there showed human activity from the Etruscan to the late Roman Empire periods. Sandy sediments burying the ships and related materials show four main periods of catastrophic floods separated by thin muddy layers pointing to phases between flooding episodes. Pollen analyses in these fine grained deposits showed: 1) significant percentages of Abies and Fagus during the pre-Roman period; 2) elements of mixed oak woodland together with hygrophilous plants in pollen spectra dominated by herbs, in the Roman interflood deposits. On the whole, the pollen data show a vegetational change which may be attributed to different climatic conditions, supporting the hypothesis that the Roman period was warmer, even though a significant human influence cannot be excluded. Numerous fresh water genera suggest that the area was a poorly drained alluvial plain. Integration between pollen and sedimentological analyses allows some inferences on the dynamics of the warm Roman period and indicates that the warm climate was punctuated by century-scale hydro-climatic crises triggering catastrophic floods. Communicated by J.-L. de Beaulieu  相似文献   

14.
Pollen, microscopic charcoal, palaeohydrological and dendrochronological analyses are applied to a radiocarbon and tephrochronologically dated mid Holocene (ca. 8500–3000 cal b.p.) peat sequence with abundant fossil Pinus (pine) wood. The Pinus populations on peat fluctuated considerably over the period in question. Colonisation by Pinus from ca. 7900–7600 cal b.p. appears to have had no specific environmental trigger; it was probably determined by the rate of migration from particular populations. The second phase, at ca. 5000–4400 cal b.p., was facilitated by anthropogenic interference that reduced competition from other trees. The pollen record shows two Pinus declines. The first at ca. 6200–5500 cal b.p. was caused by a series of rapid and frequent climatic shifts. The second, the so-called pine decline, was very gradual (ca. 4200–3300 cal b.p.) at Loch Farlary and may not have been related to climate change as is often supposed. Low intensity but sustained grazing pressures were more important. Throughout the mid Holocene, the frequency and intensity of burning in these open PinusCalluna woods were probably highly sensitive to hydrological (climatic) change. Axe marks on several trees are related to the mid to late Bronze Age, i.e., long after the trees had died.  相似文献   

15.
The last 7000 years of environmental history for Paoay Lake and its surrounding landscape is examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating. Basal sediments contain shells of Cerithiidae and the saline‐tolerant diatom Diploneis indicating that this was an estuarine environment before becoming a freshwater lake after 6000 bp . Pollen analysis shows that submontane forests, characterized by Pinus pollen, underwent a major disturbance around 5000 years ago, recovering to previous levels by 1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although the highest levels occur in the earlier part of the record, between 6500 and 5000 years ago. An aspect of the project was to examine whether there is evidence of land clearance and agricultural development in the region during the late Holocene. While a clear signal of human impact in the record remains equivocal, there appears to be a correspondence between submontane forest decline and mid‐Holocene ocean data that depict warmer and possibly drier conditions for the region. The study highlights the vulnerability of these montane forests to forecasts of a warmer and drier climate in the near future.  相似文献   

16.
The Weichselian Late-glacial marks the transition from the Weichselian Late Pleniglacial to the warmer Holocene. During this period climate changed rapidly, as did vegetation and the abiotic landscape. For analysis of the relationship between these changes, accurate chronologies are essential. In the Netherlands, a compilation has been made of over 250 palynologically investigated sites relating to this period. A regional zonation has been constructed based on common trends in vegetation development. On palynological grounds, not only the classical Late-glacial sub-division but also minor changes can be recognized within the Allerod, Late Dryas and Early Preboreal. After a critical evaluation of the available radiocarbon dates this zonation has been attached to the14C timescale. Palynological changes which can be attributed to episodes of climatic cooling are recorded at 12 100, 11 500, 10 950 and 9950 B.P. The chronological framework made it possible to compare vegetation development with other proxy records. Climatic events recorded in oxygen-isotope records from Swiss lake sediments and the Greenland ice cores are also reflected in the regional vegetation development in the Netherlands. The regional trends in vegetation development expressed by the pollen zones are considered to be related to changes in climate, while anomalies reflect other, local environmental changes.  相似文献   

17.
A continuous palynological record from the marine core GeoB1016-3 from the Angola Basin reveals the regional vegetation and climate history of the last 300 ka. Pollen and spores found at the studied site have their source areas in the different vegetation zones of the adjacent part of the West African continent. Those vegetation zones comprise tropical rain forest, coastal mangrove swamp, Miombo woodland, dry forest, Afromontane forest, desert and semi-desert. The main pollen transport agent is the southeast trade wind system. Ocean currents also partly play a role in transporting pollen and spores. During the interglacial periods, ocean currents also transported palynomorphs southward. During the glacial periods, increased trade winds are indicated by high influx of pollen and spores and high pollen percentages of Poaceae and taxa from desert and semidesert vegetations. Reconstruction of the geographical position of palaeo-vegetation zones shows that the northern boundary of the Namib Desert did not move north of 12°S during the last 300 ka. This implies that northward shifts of the Angola-Benguela Front did not pass the latitude of 12°S.  相似文献   

18.
We reconstruct long-term vegetation/paleoclimatic trends, spanning the last 18 million years, in Alaska, Yukon and far western Northwest Territories. Twenty-one average percentage spectra for pollen and spores are assembled from eight surface/subsurface sections. The sections are dated independently or by correlation. Pollen and spore ratios indicate the direction of change in vegetation and climatic parameters — growing season temperature (Test), tree canopy density (Cest) and paludification at study sites (Pest). A global warm peak ca. 15 Ma is shown by the abundance of thermophilous taxa, including Fagus and Quercus. A temperature decline immediately following 15 Ma parallels climatic reconstructions based on marine oxygen isotopes. Subsequent declines correlate to the Messinian event and the onset of late Pliocene-Pleistocene glaciation. After 7 Ma herbs and shrubs become more important elements of the palynological assemblages, suggesting a more continental, colder/drier climate. However, a late Pliocene warm interval is evident. Vegetation/climatic changes during the early to late Miocene show synchrony with, and are most economically attributable to, global events. After 7 Ma, vegetation/climate change is attributed primarily to latest Miocene-to-Pleistocene uplift of the Alaska Range and St. Elias Mts. The continuing influence of global climatic patterns is shown in the late Pliocene warm interval, despite uplift to the south. The opening of the Bering Strait ca. 3 Ma may have moderated the climate in the study area.  相似文献   

19.
The Holocene sedimentary record of core ZX-1, recovered west of mid-Holocene Chenier ridges on the monsoon-controlled Southern Yangtze delta, eastern China, consists of lagoon, salt marsh, upper tidal flat, and limnic facies, reflecting low-energy depositional environments. Holocene deposits mainly originated from the surrounding Tai Lake drainage basin. The temporal variation of most geochemical element percentages corresponds with the climatic phases inferred from the pollen record, i.e., the relatively low values of SiO2, Na2O, CaO and high values of Al2O3, K2O, MgO, Fe2O3, FeO, FeO + Fe2O3 and TiO2 generally concur with the warm and humid climate, and vice versa. Three geochemical indices − Al2O3/Na2O, K2O/Na2O, and CaO/K2O − are found to be sensitive to past precipitation in the monsoon-controlled southern Yangtze River delta. Based on fine sediment analysis of core ZX-1, the samples deposited under a dry climate with a mean annual precipitation (MAP) < 500–900 mm tend to have Al2O3/Na2O values less than 12.2, and the samples deposited under a moist climate with an MAP 1000–1800 mm mostly have Al2O3/Na2O values higher than 12.2. Similarly, the K2O/Na2O boundary value is 2.0. However, for CaO/K2O, the dry climate sediments likely show values higher than 0.9, and those wet climate ones generally have values less than 0.9. This geochemical response suggests the potential application of these indices in the interpretation of palaeoclimate variation of monsoon-controlled eastern China.

Holocene climatic variation history is reconstructed for the southern Yangtze delta. From 8000 to 7000 yr BP, regional climate demonstrated frequent fluctuations, with warm and wet periods (8000–7700; 7500–7200 yr BP) alternating with cool and less humid periods (7700–7500; 7200–7000 yr BP). From 7000 to 6000 yr BP, the climate was relatively warm and humid. Since then, it had turned cool and dry, climaxing in an intense cold event around 4000 yr BP. After the cold event, it became warm and humid until around 2500 yr BP.  相似文献   


20.
A pollen diagram from a site in the Esbjerg area, western Denmark, is used for reconstruction of the Holocene vegetational and environmental history there. During the Atlantic there was a parallel development of the landscape to that of other areas in Jylland (Jutland). From the late Neolithic onwards the development took its own course related to the approaching North Sea, which periodically inundated parts of the Esbjerg area. The record reflects landscape development in a formerly marine valley where sediments seem to be missing from parts of the Bronze Age and the early Iron Age. Consequently the landscape development during these times is only reflected in glimpses in the vegetation record, which shows gradually more open woodland and increasing human impact. During the late part of the Iron Age, Viking period and Middle Ages, the woodland was diverse in taxa but became increasingly open, finally reaching a stage during which there may have been too little wood even for daily use. At the same time the use of the land intensified. During the Sub-Atlantic, the Esbjerg area offered good natural resources with extensive grazing areas in the marine marshes in addition to good possibilities for farming and use of the woodland on higher ground, but devastating floods occurred.  相似文献   

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