首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 62 毫秒
1.
Holocene sedimentation patterns and environmental development in Aarhus Bay, Denmark, were reconstructed based on proxy analyses of two sediment cores (M1 and M5). Together, the two cores offer an opportunity to examine the history of the area during the past c. 10 000 years. The investigation consisted of acoustic mapping and multi-proxy analyses of the sediment cores including macrofossils, sediment physical properties, sediment accumulation rates, grain size, and X-ray fluorescence elemental counts. Radiocarbon dating of the two sediment successions revealed that they cover the periods c. 10 000–3700 cal. a BP (M1) and c. 4400 cal. a BP to the present (M5). The data from the M1 site indicate the presence of a near-shore lake environment between c. 10 000 and 9000 cal. a BP. The first intrusion of marine water into the area is dated to c. 9000 cal. a BP. In the following c. 1300 years, brackish-water conditions prevailed in the area characterized by a mixture of taxa from marine, limnic and terrestrial habitats, reflecting a shallow estuarine environment. Around 7700 cal. a BP full marine conditions were established, accompanied by a marked increase in sedimentation rates. The changes to full marine conditions and higher sedimentation rates are probably due to a significant sea-level rise leading to flooding of former land areas and intensified erosion. A subsequent distinct decrease in sedimentation rates around 6350 cal. a BP is presumably linked to a previously documented sea-level drop about this time. Continuous sedimentation ceased around 3700 cal. a BP in the central part of the bay, most probably due to a major sea-level lowering involving widespread erosion. In the eastern and deeper part of the bay, sedimentation continued until today. Fully marine conditions prevailed there for at least the last 4400 years.  相似文献   

2.
Pollen analysis of a 33.21 m deep sediment core from Surinsar lake in Jammu region has revealed that between 9,500 and 7,700 yr BP (Pollen zone SL-I), the mixed oak-broad-leaved/chirpine forest occurred in the region under a warm and humid climate. The record of aquatic plants viz. Potamogeton, Typha and freshwater alga Botryococcus in appreciable numbers denotes the existence of the lake since the Early Holocene. Subsequently, mixed chirpine/oak-broad-leaved forests appeared in the area around 7,700 to 6,125 yr BP (Pollen zone SL-II) with the dominance of chirpine (Pinus cf. roxburghii) by the onset of cool and dry climate, attributed to decrease in monsoon rainfall. The expansion of oak and its broad-leaved associates between 6,125 and 4,330 yr BP (Pollen zone SL-III) suggests that the climate turned moderately humid and warm, presumably due to enhanced monsoonal effect. The region has witnessed a brief spell of pluvial environment between 4,330 and 4,000 yr BP (Pollen zone SL-IV) as evidenced by the presence of sandy layer at 15.4–14 m depths. The period of 4,000 to 2,100 yr BP (Pollen zone SL-V) is marked by the prevalence of cool and dry climate as depicted by a sharp decline in oak (Quercus cf. incana) and other broad-leaved taxa and a concurrent increase in chirpine. From 2,100 to 800 yr BP (Pollen zone SL-VI) no palaeofloristic inferences could be drawn due to paucity of pollen, however, the presence of sandy deposit at the corresponding level in lithocolumn implies a pluvial episode by this time. Since 800 yr BP to Present (Pollen zone SL-VII) the slight advance in the oak reflects the ameliorating trend of climate, despite the existing cool and dry climate.  相似文献   

3.
This paper attempts to relate current knowledge of sea-level history in Beringia to the Broecker-van Donk “Termination” concept of climatic and sea-level history. The Einahnuhtan transgression is thought to represent Termination III, which according to Broecker and van Donk, took place about 225,000 y.a. The Kotzebuan transgression is thought to represent a positive fluctuation that modulated the generally falling sea level during the ensuing 100,000 yr. Sea level probably fell to about ?135 m in the Bering Sea area during the maximum phase of the penultimate glaciation. The two Pelukian shorelines probably represent Termination II (about 125,000 yr BP in the Broecker-van Donk chronology) and one of the two positive fluctuations that modulated the generally falling sea level of early Wisconsinan time, about 105,000 and 80,000 y.a. according to Broecker and van Donk. Another positive modulation brought sea level to at least ?20 m, about 30,000 y.a. Sea level evidently fell to between ?90 and ?100 m during the late Wisconsinan regression, but a substantial part of the outer Bering shelf remained submerged. Submerged shoreline features at ?38m, ?30 m, ?24 to ?20 m, and ?12 to ?10 m represent stillstands or slight regressions that modulated Termination I, the late Wisconsinan, and early Holocene recovery of sea level.  相似文献   

4.
Many coastal lakes were inundated by both the Storegga tsunami (7000 14C yr BP) and the mid-Holocene sea-level rise (the Tapes transgression) in western Norway. The tsunami eroded lake bottoms and deposited graded and/or massive beds of sand, rip-up clasts, and coarse plant material. By contrast, when the rising sea entered the lakes, it deposited only gyttja, silt and fine sand, without causing much erosion of the underlying lake sediments. Storegga tsunami deposits in some coastal lakes were interpreted previously as ordinary marine sediments from the Tapes transgression. Our reinterpretation of these deposits shows that the transgression maximum phase was reached after 6500 yr BP, more than 1000 yr later than previously inferred for the coast of Sunnmøre. The new data cannot be combined in a shoreline diagram without showing the 6000 yr BP and 7000 yr BP shorelines as slightly warped. © 1998 John Wiley & Sons, Ltd.  相似文献   

5.
Andersson, S. & Schoning, K. 2010: Surface wetness and mire development during the late Holocene in central Sweden. Boreas, Vol. 39, pp. 749–760. 10.1111/j.1502‐3885.2010.00157.x. ISSN 0300‐9483. Late Holocene mire development and surface wetness changes have been studied in a small mixed mire located in central Sweden. Today the mire is characterized by a mainly ombrotrophic centre dominated by Sphagnum mosses, with Carex content increasing towards the more minerotrophic mire margins. Two peat sequences extracted from the central ombrotrophic part were investigated for stratigraphy, humification, testate amoebae analysis, C/N ratio and δ13C and δ15N stable isotopes. Three main stages of mire development are identified, with the first stage, between c. 4200 and 2600 cal. yr BP, characterized by water‐logged conditions, suggesting a minerotrophic fen stage. The second stage, between c. 2600 and 1000 cal. yr BP, is characterized by more ombrotrophic conditions and Sphagnum‐dominated vegetation. The onset of the prominent change at c. 2600 cal. yr BP could have been initiated by climate change coincident with a change in solar activity. The last stage, between c. 1000 and 50 cal. yr BP, is dominated by more ombrotrophic conditions, suggesting increased precipitation. This study shows that the response of hydrological proxies in a mixed mire during its development towards more ombrotrophic conditions might result in conflicting results, a finding that needs to be considered in palaeoenvironmental reconstructions from mires that change between ombrotrophic and minerotrophic settings.  相似文献   

6.
Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to 14C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP.  相似文献   

7.
Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious,as is the great rapidity(<104 yr)of the sea-level rise.Such extreme glacioeustatic events,if real,would have been global and affected all continents.In South Australia,up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin.The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise.In the central Adelaide Rift Complex,diamictite with faceted and striated clasts occurs at the top of the Elatina Formation<1-2 m beneath the early Ediacaran Nuccaleena Formation’cap carbonate’.One hundred kilometres to the south,~30 m of siltstone and sandstone followed by^6 m of clast-poor diamictite with clasts 10+cm long occur between tidal rhythmites and the cap carbonate.Three hundred kilo metres further south,~70 m of siltsto ne,dolo mitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata.Hence the rhythmites were deposited during a high stand(interstadial or interglacial),not during post-glacial sea-level rise.Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition,and water depth and other palaeoenvironmental factors are uncertain,casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition.The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations.A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate,and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 10^5-10^6 yr.This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California,and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates.It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.  相似文献   

8.
This paper presents the results of an investigation into Holocene relative sea-level (RSL) change, isostatic rebound and ice sheet dynamics in Disko Bugt, West Greenland. Data collected from nine isolation basins on Arveprinsen Ejland, east Disko Bugt, show that mean sea level fell continuously from ca. 70 m at 9.9 ka cal. yr BP (8.9 ka 14C yr BP) to reach a minimum of ca. −5 m at 2.8 ka cal. yr BP (2.5 ka 14C yr BP), before rising to the present day. A west–east gradient in isostatic uplift across Disko Bugt is confirmed, with reduced rebound observed in east Disko Bugt. However, RSL differences (up to 20 m at 7.8 ka to 6.8 ka cal. yr BP (7 ka to 6 ka 14C yr BP)) also exist within east Disko Bugt, suggesting a significant north–south component to the area’s isostatic history. The observed magnitude and timing of late Holocene RSL rise is not compatible with regional forebulge collapse. Instead, RSL rise began first in the eastern part of the bay, as might be expected under a scenario of crustal subsidence caused by neoglacial ice sheet readvance. The results of this study demonstrate the potential of isolation basin data for local and regional RSL studies in Greenland, and the importance of avoiding data compilations from areas where the isobase orientation is uncertain. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

9.
Multiproxy analysis of three littoral cores from western New Caledonia supports the hypothesis that the main controlling factors of environmental changes are sea-level change, ENSO variability and extra-tropical phenomena, such as the Medieval Warm Period (MWP) marked by a tendency for La Niña-like conditions in the tropical Pacific. The record starts during the late Holocene sea-level rise when the terrestrial vegetation indicated wet and cool conditions. The site was a coastal bay definitely transformed into a freshwater swamp at around 3400 cal yr BP, after the rapid drawdown of sea level to its current level. Sediments and foraminiferal assemblages indicated subsequent episodes of freshwater infillings, emersion or very high-energy conditions, likely related to climatic changes and mostly controlled by ENSO variability. Between 2750 and 2000 cal yr BP, relatively dry and cool climate prevailed, while wetter conditions predominated between ca. 1800 and 900 cal yr BP. The Rhizophoraceae peak between ca. 1080 and 750 cal yr BP, coeval with the MWP, may indicate a global phenomenon. Microcharcoal particles present throughout the record increased after 1500 cal yr BP, suggesting an anthropogenic source. From ca. 750 cal yr BP the appearance of current type of vegetation marks the human impact.  相似文献   

10.
Coupled measurements of δ18O and accelerator mass spectrometry (AMS) 14C in a particular species of planktonic foraminifera may be used to calculate sea-level estimates for the last deglaciation. Of critical importance for this type of study is a knowledge of the seasonality of foraminiferal growth, which can be provided by δ18O measurements of modern shells (core tops, plankton tows). Isotopic (δ18O, AMS-14C dating) and faunal records (transfer function sea surface temperature) were obtained from two cores in the North Atlantic at about 37°N. The locations were chosen to obtain high sedimentation rate records removed from the major ice-melt discharge areas of the last deglaciation. Based upon Globigerina bulloides data, four δ18O-based sea-level estimates were calculated: −67 ± 7 m at 12,200 yr B.P. and −24 ± 8 m at about 8200 yr B.P. for core SU 81-18; −83 ± 10 m at 12,200 yr B.P. and −13 ± 11 m at about 8500 yr B.P. for core SU 81-14. Using a second working hypothesis concerning the seasonability of G. bulloides growth, it is suggested that the sea-level rose by about 40 m during the millennium which followed 14,500 yr B.P.  相似文献   

11.
From the Sellevollmyra bog at Andøya, northern Norway, a 440‐cm long peat core covering the last c. 7000 calendar years was examined for humification, loss‐on‐ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle‐match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410–6380, 6230–6050, 5730–5640, 5470–5430, 5340–5310, 5270–5100, 4790–4710, 4890–4820, 4380–4320, 4220–4120, 4000–3810, 3610–3580, 3370–3340 (regionally 2850–2750; in Sellevollmyra a hiatus between 2960–2520), 2330–2220, 1950, 1530–1450, 1150–840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla‐4 tephra layer started some decades before the eruption event.  相似文献   

12.
During the course of a study of historical water wells along the Israeli shore, which has been in progress since 1984, an innovative method for investigating sea-level fluctuations was developed. Eighteen ancient water wells were re-dug, 14 by the author, and four by archaeologists. Most of the re-dug wells are found at archaeological sites located a very short distance inland from the present shoreline. Evidence of ancient ground-water levels found in the wells directly reflects on historic eustatic sea-level changes, and the rate at which the end of the post-glacial transgression advanced. A critical question concerns the durability and life span of these wells, as the true age is very important for the accurate reconstruction of a sea-level curve. The Pre-Pottery Neolithic well of Atlit-Yam, which is the oldest known well in the world (ca. 8000 yr old), enabled the most accurate sea-level reconstruction for early Holocene times. Sea-level rise during that period of the Atlit-Yam site was of the order of 20 mm yr−1 at the beginning, slowing to 6–8 mm yr−1 at the abandonment of this site at ca. 7.5 ka BP, when it was flooded by the advancing and rising sea. After reaching its present level, sea-level fluctuations for the past 2.5 millennia were not greater than 1.5 to 2 m. © 1997 John Wiley & Sons, Ltd.  相似文献   

13.
Two sand wedge structures and their host sediments, from Jonzac in SW France, were successfully dated using Optically Stimulated Luminescence (OSL) measurements on both small aliquots and single grains of quartz from the 180–212 μm size fraction. One of the sand wedges clearly contains primary infilling. However, grain‐size analysis and field observations do not clearly indicate whether the other feature represents a primary sand wedge or a composite sand wedge with primary and secondary infilling. OSL results and the geological setting justify using the Central Age Model (CAM) for the calculation of age estimates. Grain‐size analysis and detailed investigations of OSL results revealed the contamination of one sand wedge sample with host sediment. However, age calculation using the Finite Mixture Model (FMM) provided what is considered to be a reliable age estimate for the contaminated sample. The age estimates for all samples correspond to Marine Isotope Stage (MIS) 3. While fine‐grained sediments were deposited in the middle of MIS 3 (c. 43–55 ka), the sand wedges unexpectedly correspond to the end of this period (c. 33 ka) or the onset of MIS 2 (c. 27 ka). The sand wedges were probably formed during intense but short cold periods, possibly correlated with a Heinrich event (H2 and/or H3). The results help us to assess how effective luminescence dating is on sand wedges and the limitations involved in correlating sand wedge ages with Heinrich events, and contribute to the debate on the timing of cryogenic formation processes and the permafrost distribution in SW France.  相似文献   

14.
The radiocarbon ages of mollusc shells from the Bogenfels Pan on the hyper arid southern coast of Namibia provide constraints on the Holocene evolution of sea level and, in particular, the mid-Holocene highstand. The Bogenfels Pan was flooded to depths of 3 m above mean sea level (amsl) to form a large subtidal lagoon from 7300 to 6500 calibrated radiocarbon years before present (cal yr BP). The mollusc assemblage of the wave sheltered lagoon includes Nassarius plicatellus, Lutraria lutraria, and the bivalves Solen capensis and Gastrana matadoa, both of which no longer live along the wave-dominated southern Namibian coast. The radiocarbon ages of mollusc shell from a gravely beach deposit exposed in a diamond exploration trench indicate that sea level fell to near or 1 m below its present-day position between 6500 and 4900 cal yr BP. The rapid emergence of the pan between 6500 and 4900 cal yr BP exceeds that predicted by glacio-isostatic models and may indicate a 3-m eustatic lowering of sea level. The beach deposits at Bogenfels indicate that sea level rose to 1 m amsl between 4800 and 4600 cal yr BP and then fell briefly between 4600 and 4200 cal yr BP before returning to 1 m amsl. Since 4200 cal yr BP sea level has remained within one meter of the present-day level and the beach at Bogenfels has prograded seaward from the delayed arrival of sand by longshore drift from the Orange River. A 6200 cal yr BP coastal midden and a 600 cal yr BP midden 1.7 km from the coast indicate sporadic human utilization of the area. The results of this study are consistent with previous studies and help to refine the Holocene sea-level record for southern Africa.  相似文献   

15.
A reconstruction of deglaciation and associated sea-level changes on northern James Ross Island, Antarctic Peninsula, based on lithostratigraphical and geomorphological studies, shows that the initial deglaciation of presently ice-free areas occurred slightly before 7400 14C yr BP. Sea-level in connection with the deglaciation was around 30 m a.s.l. A glacier readvance in Brandy Bay, of at least 7 km, with the initial 3 km over land, reached a position off the present coast at ca. 4600 yr BP. The culmination of the advance was of short duration, and by 4300 yr BP the coastal lowlands again were ice-free. A distinct marine level at 16–18 m a.s.l. was contemporaneous with or slightly post-dates the Brandy Bay advance, thus indicating the relative sea-level around 4600–4500 yr BP. Our results from James Ross Island confirm that over large areas in this part of Antarctica the last deglaciation occurred late. © 1997 John Wiley & Sons, Ltd.  相似文献   

16.
We present here the results of pollen analysis of two sequences of about 8.06 m and 11.90 m length, originating from two adjacent peat bogs in the southern part of Transylvania province, Romania (155 and 122 pollen spectra). The vegetation record, which is supported by 17 14C dates, begins in the Late Glacial interstadial when forest recolonisation began with the development of Pinus, without a pioneer Betula phase. Picea began to expand from regional refuges. After a well‐defined Younger Dryas, the Holocene opens with the expansion of Betula, Ulmus and Picea, followed, at about 10 400 cal. yr BP, by Fraxinus, Quercus and Tilia. The Corylus optimum is correlated with the Atlantic chronozone (after 8600 cal. yr BP). The local establishment of Carpinus occurred at about 6500 cal. yr BP, with a maximum at about 5700 cal. yr BP. Fagus pollen is regularly recorded after 8200 cal. yr BP. This taxon became dominant at about 3700 cal. yr BP. The first indications of human activities appear at around 7200 cal. yr BP. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

17.
Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23–21 kyr BP, Vendsyssel was part of an ice‐dammed lake where the Ribjerg Formation was deposited c. 27–23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice‐marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45–50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re‐advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20–19 kyr BP in a large ice‐dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice‐contact fans, which make up the Jyske Ås and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north–south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re‐advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice‐dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19–18 kyr BP. Related to this stage of deglaciation, eight ice‐marginal positions have been identified based on the distribution of large tunnel‐valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice‐contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel‐valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine‐grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice‐proximal environment, and the infill consists of alternating layers of fine‐ to coarse‐grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.  相似文献   

18.
Two sedimentary cores with pollen, charcoal and radiocarbon data are presented. These records document the Late‐glacial and Holocene dry forest vegetation, fire and environmental history of the southern Cauca Valley in Colombia (1020 m). Core Quilichao‐1 (640 cm; 3° 6′N, 76° 31′W) represents the periods of 13 150–7720 14C yr BP and, following a hiatus, from 2880 14C yr BP to modern. Core La Teta‐2 (250 cm; 3° 5′N, 76° 32′W) provides a continuous record from 8700 14C yr BP to modern. Around 13 150 14C yr BP core Quilichao‐1 shows an active Late‐glacial drainage system and presence of dry forest. From 11 465 to 10 520 14C yr BP dry forest consists mainly of Crotalaria, Moraceae/Urticaceae, Melastomataceae/Combretaceae, Piper and low stature trees, such as Acalypha, Alchornea, Cecropia and Celtis. At higher elevation Andean forest comprising Alnus, Hedyosmum, Quercus and Myrica was common. After 10 520 14C yr BP the floral composition of dry forest changed, with extensive open grass vegetation indicative of dry climatic conditions. This event may coincide with the change to cool and dry conditions in the second part of the El Abra stadial, an equivalent to the Younger Dryas. From 8850 14C yr BP the record from La Teta indicates dry climatic conditions relative to the present, these prevailing up to 2880 14C yr BP at Quilichao and to 2720 14C yr BP at La Teta. Severe dryness reached maxima at 7500 14C yr BP and 4300 14C yr BP, when dry forest reached maximum expansion. Dry forest was gradually replaced by grassy vegetation, reaching maximum expansion around 2300 14C yr BP. After 2300 14C yr BP grassy vegetation remains abundant. Presence of crop taxa (a.o. Zea mays), disturbance indicators (Cecropia) and an increase in charcoal point to the presence of pre‐Columbian people since 2300 14C yr BP. After 950 14C yr BP, expansion of secondary forest taxa may indicate depopulation and abandonment of previously cultivated land. After 400 14C yr BP, possibly related to the Spanish conquest, secondary forest expanded and charcoal concentrations increased, possibly indicating further reduction of cultivated land. During the past century, Heliotropium and Didymopanax became abundant in an increasingly degraded landscape. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
Palynological studies undertaken in the mountainous regions of the northwest of the Iberian Peninsula are few in number and have been concerned largely with the second half of the Holocene. New pollen data from two Galician sierras, the Courel and Queixa Sierras, provide a 10000‐yr record of vegetation and climate change. In the Courel Sierra before 9750 yr BP, Laguna Lucenza (1420 m a.s.l.) reflects a period of open landscape covered by Poaceae and heliophilous plants, which may be correlated with the Younger Dryas. The onset of the Holocene is characterised by the expansion of oak woodland, prior to 9300 yr BP, following a short phase of birch along with the gradual decline of pine. The oak values reach a peak at 8350 ± 80 yr BP. Towards 8800 yr BP Corylus begins to expand, followed by Alnus (7500 yr BP) and Ulmus. During this period, the Fraga pollen assemblage (Queixa Sierra, 1360 m a.s.l.) indicates Betula woodland surrounding the site, masking the regional predominance of oak. After 5000 yr BP there is a gradual decrease in arboreal pollen values in both Sierras. Castanea appears in Laguna Lucenza (Courel Sierra) at 4075 ± 75 yr BP. There is widespread deforestation during the last 4000 yr. During this period the presence of large quantities of microcharcoal particles points to the occurrence of fire. The reduction in forest is associated with the arrival of cultivation at 4000 yr BP at low altitudes in the Queixa Sierra. At higher altitudes the first agricultural activity is dated at later than 2000 yr BP. This coincides with the first record of cereal cultivation at high altitude in the Courel Sierra. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号