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1.
《Quaternary Science Reviews》2007,26(3-4):500-516
We use lake sediment records from an epishelf lake on Alexander Island to provide a detailed picture of the Holocene history of George VI Ice Shelf (GVI-IS). Core analyses included; micropaleontology (diatoms/foraminifera), stable isotope (δ18O, δ13C), geochemistry (total organic carbon (TOC), total nitrogen (TN), C/N ratios) and grain-size analyses. These data provide robust evidence for one period of past ice shelf absence during the early Holocene. The timing of this period has been constrained by 10 AMS 14C dates performed on mono-specific foraminifera samples. These dates suggest that GVI-IS was absent between c 9600 cal yr BP and c 7730 cal yr BP. This early Holocene collapse immediately followed a period of maximum Holocene warmth that is recorded in some Antarctic ice cores and coincides with an influx of warmer ocean water onto the western Antarctic Peninsula (AP) shelf at c 9000 cal yr BP. The absence of a currently extant ice shelf during this time interval suggests that early Holocene ocean-atmosphere variability in the AP was greater than that measured in recent decades.  相似文献   

2.
New pollen, micro-charcoal, sediment and mineral analyses of a radiocarbon-dated sediment core from the Serra Sul dos Carajás (southeast Amazonia) indicate changes between drier and wetter climatic conditions during the past 25,000 yr, reflected by fire events, expansion of savanna vegetation and no-analog Amazonian forest communities. A cool and dry last glacial maximum (LGM) and late glacial were followed by a wet phase in the early Holocene lasting for ca. 1200 yr, when tropical forest occurred under stable humid conditions. Subsequently, an increasingly warm, seasonal climate established. The onset of seasonality falls within the early Holocene warm period, with possibly longer dry seasons from 10,200 to 3400 cal yr BP, and an explicitly drier phase from 9000 to 3700 cal yr BP. Modern conditions with shorter dry seasons became established after 3400 cal yr BP. Taken together with paleoenvironmental evidence from elsewhere in the Amazon Basin, the observed changes in late Pleistocene and Holocene vegetation in the Serra Sul dos Carajás likely reflect large-scale shifts in precipitation patterns driven by the latitudinal displacement of the Inter-Tropical Convergence Zone and changes in sea-surface temperatures in the tropical Atlantic.  相似文献   

3.
A high-resolution record of paleostorm events along the French Mediterranean coast over the past 7000 years was established from a lagoonal sediment core in the Gulf of Lions. Integrating grain size, faunal analysis, clay mineralogy and geochemistry data with a chronology derived from radiocarbon dating, we recorded seven periods of increased storm activity at 6300–6100, 5650–5400, 4400–4050, 3650–3200, 2800–2600, 1950–1400 and 400–50 cal yr BP (in the Little Ice Age). In contrast, our results show that the Medieval Climate Anomaly (1150–650 cal yr BP) was characterised by low storm activity.The evidence for high storm activity in the NW Mediterranean Sea is in agreement with the changes in coastal hydrodynamics observed over the Eastern North Atlantic and seems to correspond to Holocene cooling in the North Atlantic. Periods of low SSTs there may have led to a stronger meridional temperature gradient and a southward migration of the westerlies. We hypothesise that the increase in storm activity during Holocene cold events over the North Atlantic and Mediterranean regions was probably due to an increase in the thermal gradient that led to an enhanced lower tropospheric baroclinicity over a large Central Atlantic–European domain.  相似文献   

4.
The paleoclimatic record of Juréia Paleolagoon, coastal southeastern Brazil, includes cyclic and gradual changes with different intensities and frequencies through geological time, and it is controlled by astronomical, geophysical, and geological phenomena. These variations are not due to one single cause, but they result from the interaction of several factors, which act at different temporal and spatial scales. Here, we describe paleoenvironmental evidence regarding climatic and sea level changes from the last 9400 cal yr BP at the Juréia Paleolagoon — one of the main groups of protected South Atlantic ecosystems. Geochemical evidences were used to identify anomalies from multi-proxy analyses of a paleolagoon sediment core. The anomalies of centennial scale were correlated to climate and transgression–regression cycles from the Holocene period. Decadal scale anomalous oscillations in the Quaternary paleolagoon sediments occur between 9400 and 7500 cal yr BP, correlated with long- and short-term natural events, which generated high sedimentation rates, mainly between 8385 and 8375 cal yr BP (10 cm/yr). Our results suggest that a modern-day short-duration North Atlantic climatic event, such as the 8.2 ka event, could affect the environmental equilibrium in South America and intensify the South American Summer Monsoon.  相似文献   

5.
《Quaternary Science Reviews》2003,22(5-7):541-554
The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000 cal yr BP. At 8000–4500 cal yr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800 cal yr BP. From 4500 cal yr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ18O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.  相似文献   

6.
《Quaternary Science Reviews》2007,26(13-14):1871-1883
Multi-proxy palaeoenvironmental studies of nine sediment sequences from four areas in north-western Russia reveal significant changes in climate, lake productivity and vegetation during the Lateglacial and early Holocene that show some degree of correlation with changes reconstructed from sites throughout the North Atlantic region. At Lake Nero in the Rostov-Jaroslavl’ area, which is outside the maximum limit of the Scandinavian Ice Sheet, sedimentation recommenced shortly after 15 cal ka BP in response to increases in temperature and humidity during Greenland Interstadial 1 (GI-1; Bølling-Allerød). However, climatic amelioration during GI-1 was slow to increase lake organic productivity or trigger large-scale changes in much of northwestern Russia. In general, this region was characterised by long-lasting lake-ice cover, low lake productivity, soil erosion, and dwarf shrub and herb tundra until the end of Greenland Stadial 1 (GS-1; Younger Dryas). At some sites, distinct increases in lake organic productivity, mean summer temperatures and humidity and the expansion of forest trees coincide with rapid warming at the beginning of the Holocene and the increasing influence of warm air masses from the North Atlantic. At other sites, particularly on the Karelian Isthmus, but also in Russian Karelia, the delayed response of limnic and terrestrial environments to early Holocene warming is likely related to the cold surface waters of the Baltic Ice Lake, the proximity of the Scandinavian Ice Sheet and associated strengthened easterlies, and/or extensive permafrost and stagnant ice. These multi-proxy studies underscore the importance of local conditions in modifying the response of individual lakes and their catchments.While Lateglacial vegetation was dominated by Betula nana and Salix shrubs and various herbs, pollen and plant macrofossils suggest that Betula pubescens trees became established as early as 14–13 cal ka BP in the Rostov-Jaroslavl’ area. In general, our data sets suggest that trees migrated from the southeast to the west and then spread later to the northeast and northwest, paralleling the direction of ice retreat, with Betula pubescens immigrating first, followed by Pinus sylvestris and Picea abies. However, palaeoecological records from Lake Terebenskoye in the Valdai Highlands suggest that the arrival of Picea abies preceded other trees in that area and that it colonised tundra communities as early as 12 cal ka BP. Since Lateglacial vegetation change in north-western Russia was time-transgressive, independent measures of palaeoclimate (e.g., chironomid-based palaeotemperature estimates) are needed for this region.  相似文献   

7.
Germania Havn Sø is located at the outermost coast of northeastern Greenland. According to radiocarbon dating, the lake basin was deglaciated in the early Holocene, around 11,000 cal yr BP. At that time the lake was a marine bay, but the lake was isolated soon after deglaciation at ~ 10,600 cal yr BP. The marine fauna was species-poor, indicating harsh conditions with a high sedimentation rate and lowered salinity due to glacial meltwater supply. The pioneer vegetation around the lake was dominated by mosses and herbs. Deposition of relatively coarse sediments during the early Holocene indicates erosion of the newly deglaciated terrain. Remains of the first woody plant (Salix herbacea) appear at 7600 cal yr BP and remains of other woody plants (Salix arctica, Dryas octopetala, Cassiope tetragona and Empetrum nigrum) appear around one millennium later. Declining concentrations of D. octopetala and the caddis fly Apatania zonella in the late Holocene probably imply falling summer temperatures. Only moderate changes in the granulometric and geochemical record during the Holocene indicate relatively stable environmental settings in the lake, which can probably be explained by its location at the outer coast and the buffering effect of the neighboring ocean.  相似文献   

8.
A lake-level record of Lake Ledro (northern Italy) spans the entire Holocene with a chronology derived from 51 radiocarbon dates. It is based on a specific sedimentological approach that combines data from five sediment profiles sampled in distinct locations in the littoral zone. On a millennial scale, the lake-level record shows two successive periods from 11,700 to 4500 cal yr BP and from 4500 cal yr BP to the present, characterized by lower and higher average lake levels, respectively. In addition to key seasonal and inter-hemispherical changes in insolation, the major hydrological change around 4500 cal yr BP may be related to a non-linear response of the climate system to orbitally-driven gradual decrease in insolation. The Ledro record questions the notion of an accentuated summer rain regime in the northern Mediterranean borderlands during the boreal insolation maximum. Moreover, the Ledro record highlights that the Holocene was punctuated by successive centennial-scale highstands. Correlations with the Preboreal oscillation and the 8.2 ka event, and comparison with the atmospheric 14C residual record, suggest that short-lived lake-level fluctuations developed at Ledro in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity.  相似文献   

9.
《Quaternary Science Reviews》2005,24(12-13):1463-1478
The aragonite mineralogy and geochemistry of the mollusc faunas preserved at Navan and Bearbrook, Ontario, serve as proxies of original seawater chemistry. The composite section spanning 12,980–10,980 cal yr BP includes the Younger Dryas (YD) paleoclimatic oscillation. Oxygen isotopes demonstrate the onset of cooling with the YD event, in addition to the lowering of marine values by the influx of isotopically light glacial meltwater from Lake Agassiz. Impact of cooling and dilution is reduced or eliminated with the start of the Holocene, when water temperatures and salinities for Champlain Sea (CS) seawater were 8–16 °C and 27–34 ppt, respectively. Overall, oxygen isotope values deceased to −3.5% during the YD mainly due to freshening by glacial meltwater. Carbon isotopes confirm the rise in atmospheric CO2 concentration at the YD–Holocene transition. Marine strontium isotope values for the Allerød–YD–earliest Holocene range from 0.709151 (16,210 cal yr BP) to 0.709145 (12,980 cal yr BP) and 0.709142 (10,950 cal yr BP). The oceanographic changes recorded for the CS are in agreement with the evolutionary phases of Lake Agassiz and deglaciation dynamics of the Laurentide Ice Sheet. The volume and direction of meltwater discharge from Lake Agassiz alternated between the Gulf of Mexico during the Allerød, via the Great Lakes through the CS to the North Atlantic during the YD, and back to the Gulf of Mexico during the early Holocene, but with diminished impact.  相似文献   

10.
《Quaternary Science Reviews》1999,18(8-9):1021-1038
Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ18O trends that are almost exactly out of phase with coupled δ18O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ18O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ18O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ18O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ18O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ18O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ18O, probably indicative of warm and/or dry conditions. Exceptionally low δ18O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ18O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ18O and extension rate. Instead, its extension rate correlates roughly with δ13C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ18O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ18O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ18O and extension rate. Had drip-water availability been the dominant control on δ18O at this semi-arid site then higher δ18O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.  相似文献   

11.
《Quaternary Science Reviews》2007,26(15-16):1999-2011
A multi proxy sediment core record on the continental margin off western Svalbard, European Arctic, reflects large climatic and oceanographic oscillations at the Lateglacial–early Holocene transition. Based on studies of planktonic foraminifera, their stable oxygen and carbon isotopic composition and ice rafted debris, we have reconstructed the last 14 cal. ka BP. The period 14–13.5 cal. ka BP was characterized by highly unstable climatic conditions. Short-lived episodes of warming alternated with meltwater pulses and enhanced iceberg rafting. This period correlates to a regional warming of the northern North Atlantic. An overall decrease in meltwater took place during the deglaciation (14–10.8 cal. ka BP). The late Younger Dryas and subsequent transition into the early Holocene is characterized by a reduced flux of planktonic foraminifera and increased iceberg rafting. A major warming took place from 10.8 to 9.7 cal. ka BP, the influence of meltwater ceased and the flux of warm Atlantic Water increased. From 9.7 to 8.8 cal. ka BP, the western Svalbard margin surface waters were significantly warmer than today. This warm period, the thermal maximum, was followed by an abrupt cooling at 8.8. cal. ka BP, caused by an increased influence of Arctic Water from the Arctic Ocean. The results document that the European Arctic was very sensitive to climatic and oceanographic changes at the end of the last glacial and during the Holocene.  相似文献   

12.
The study of a 300-cm-thick exposed lacustrine sediment section in the Hedong village in Zhaoqing area which is located in sub-tropical west Guangdong Province in South China, demonstrates that the lacustrine sedimentary sequence possibly contains evidence for exploring variation of Asian monsoon climate. Multi-proxy records, including the humification intensity, total organic carbon, and grain size fractions, reveal a general trend towards dry and cold conditions in the late Holocene that this is because of a decrease in solar insolation on an orbital scale. Three intensified Asian summer monsoon (ASM) intervals (∼3300–3000 cal yr BP, ∼2600–1600 cal yr BP, and ∼900–600 cal yr BP), and three weakened ASM intervals (∼4000–3300 cal yr BP, ∼3000–2600 cal yr BP, and ∼1600–900 cal yr BP) are identified. Our humification record (HDcal) shows a good correlation on multi-centennial scale with the tree ring Δ14C record, a proxy of solar activity. A spectral analysis of HDcal reveals four significant cycles, i.e., ∼1250 yr, 300 yr, 110 yr, and 70 yr, and most of these cycles are related to the solar activity. Our findings indicate that solar output and oceanic–atmospheric circulation probably have influenced the late Holocene climate variability in the study region.  相似文献   

13.
《Quaternary Science Reviews》2007,26(5-6):705-731
Sediment cores from two mountain lakes (Lake Grusha at 2413 m a.s.l. and Ak-Khol at 2204 m a.s.l.) situated in the Tuva Republic (southern Siberia, Russia), just north of Mongolia, were studied for chironomid fossils in order to infer post-glacial climatic changes and to investigate responses of the lake ecosystems to these changes. The results show that chironomids are responding both to temperature and to changing lake depth, which is regarded as a sensitive proxy of regional effective moisture. The post-glacial history of this mountain region in Central Asia can be divided into seven successive climatic phases: the progressive warming during the last glacial–interglacial transition (ca 15.8–14.6 cal kyr BP), the warm and moist Bølling-Allerød-like interval (ca 14.6–13.1 cal kyr BP), the cool and dry Younger Dryas-like event (ca 13.1–12.1 cal kyr BP), warmer and wetter conditions during ca 12.1–8.5 cal kyr BP, a warm and dry phase ca 8.5–5.9 cal kyr BP, cold and wet conditions during ca 5.9–1.8 cal kyr BP, as well as cold and dry climate within the last 1800 years. The chironomid records reveal patterns of climatic variability during the Late-glacial and Holocene, which can be correlated with abrupt climatic events in the North Atlantic and the Asian monsoon-dominated regimes. Apparently, the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon, as well as the influence of alpine glacier meltwater supply. It is possible that monsoon tracks could have reached the southwest Tuva, resulting in an increase in precipitation at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP, whereas cyclonic westerlies from the North Atlantic were likely responsible for considerable moisture transport accompanying the global Neoglacial cooling at ca 5.9–1.8 cal kyr BP. These events suggest the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Some discrepancies compared with other reconstructions from Central Asia may be associated with regional (spatial) differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape. In this paper, we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate-induced changes in water depth of lakes in Central Asia.  相似文献   

14.
The Pantanal is the world's largest tropical wetland and a biodiversity hotspot, yet its response to Quaternary environmental change is unclear. To address this problem, sediment cores from shallow lakes connected to the Upper Paraguay River (PR) were analyzed and radiocarbon dated to track changes in sedimentary environments. Stratal relations, detrital particle size, multiple biogeochemical indicators, and sponge spicules suggest fluctuating lake-level lowstand conditions between ~ 11,000 and 5300 cal yr BP, punctuated by sporadic and in some cases erosive flood flows. A hiatus has been recorded from ~ 5300 to 2600 cal yr BP, spurred by confinement of the PR within its channel during an episode of profound regional drought. Sustained PR flooding caused a transgression after ~ 2600 cal yr BP, with lake-level highstand conditions appearing during the Little Ice Age. Holocene PR flood pulse dynamics are best explained by variability in effective precipitation, likely driven by insolation and tropical sea-surface temperature gradients. Our results provide novel support for hypotheses on: (1) stratigraphic discontinuity of floodplain sedimentary archives; (2) late Holocene methane flux from Southern Hemisphere wetlands; and (3) pre-colonial indigenous ceramics traditions in western Brazil.  相似文献   

15.
《Quaternary Science Reviews》2007,26(15-16):1989-1998
A synthesis of the early Holocene climatic development in the North Atlantic region is presented, based on three previously published lake records from southern Greenland (Lake N14), Iceland (Lake Torfadalsvatn) and the Faroe Islands (Lake Lykkjuvøtn). The interval 11 500–8500 cal BP has been divided into five phases with respect to the inferred strength of the North Atlantic Current (NAC) and Irminger Currents (IC). Phase 1 (11 500–10 750 cal BP) was characterised by the first establishment of the NAC and IC in the vicinity of the studied sites, interrupted by the Preboreal Oscillation around 11 200 cal BP. Phase 2 (10 750–10 100 cal BP) was marked by a further warming step in southern Greenland rather concordant with a change into colder and more variable winters on the Faroe Islands. It is proposed that this could partly be related to a series of melt water outbursts disturbing the thermohaline circulation in the eastern Atlantic Ocean, resulting in a warming trend in the western region. During Phase 3 (10 100–9400 cal BP) the strength of the IC reaching northwestern Iceland intensified. A more stable regime in surface circulation was established at the onset of Phase 4 (9400–8900 cal BP) in southern Greenland and was followed by a change towards further warm conditions on Iceland at the onset of Phase 5 (8900–8500 cal BP).  相似文献   

16.
《Gondwana Research》2014,26(4):1357-1368
Oceanic Anoxic Event 2 (OAE 2) during the Cenomanian–Turonian (C/T) transition caused stepwise marine extinctions. Using organic compounds, stable carbon and oxygen isotopes, and foraminifera from three depth-transect sections in northern Spain, this study revealed repeated anoxic/euxinic events coinciding with warming and stepwise extinctions of planktonic and/or benthic foraminifera within intermediate to surface waters in the proto-North Atlantic during the C/T transition. Those short-duration euxinic events occurred four times: at 93.95 Ma, marked by the extinction of Rotalipora greenhornensis; at 93.90 Ma, marked by the extinction of Rotalipora cushmani; at the mid-time maximum of the plateau of the δ13C of carbonates (93.70 Ma); and at the time of the C/T boundary (93.55 Ma). Furthermore, the main benthic foraminiferal extinctions occurred during the first and second euxinic events in the upper slope, during the second and third euxinic events in the outer to middle shelf, and during the third and fourth events in the middle shelf. The main euxinic events in each section also showed a progression to the shallow shelf. The main anoxia–extinction events occurred in the upper slope and outer shelf then moved to the middle shelf. The shallowest section had relatively weak anoxia and a proportionally low extinction rate. These new findings indicate that foraminiferal extinctions started from the intermediate water and the continental slope and then moved to the continental shelf. This was the result of the repeated progression of euxinic–anoxic water from the upper slope to the middle shelf on the eastern continental margin of the proto-North Atlantic four times during a 400 kyr period, to the end of the Cenomanian.  相似文献   

17.
The sediment core NP05‐71GC, retrieved from 360 m water depth south of Kvitøya, northwestern Barents Sea, was investigated for the distribution of benthic and planktic foraminifera, stable isotopes and sedimentological parameters to reconstruct palaeoceanographic changes and the growth and retreat of the Svalbard–Barents Sea Ice Sheet during the last ～16 000 years. The purpose is to gain better insight into the timing and variability of ocean circulation, climatic changes and ice‐sheet behaviour during the deglaciation and the Holocene. The results show that glaciomarine sedimentation commenced c. 16 000 a BP, indicating that the ice sheet had retreated from its maximum position at the shelf edge around Svalbard before that time. A strong subsurface influx of Atlantic‐derived bottom water occurred from 14 600 a BP during the Bølling and Allerød interstadials and lasted until the onset of the Younger Dryas cooling. In the Younger Dryas cold interval, the sea surface was covered by near‐permanent sea ice. The early Holocene, 11 700–11 000 a BP, was influenced by meltwater, followed by a strong inflow of highly saline and chilled Atlantic Water until c. 8600 a BP. From 8600 to 7600 a BP, faunal and isotopic evidence indicates cooling and a weaker flow of the Atlantic Water followed by a stronger influence of Atlantic Water until c. 6000 a BP. Thereafter, the environment generally deteriorated. Our results imply that (i) the deglaciation occurred earlier in this area than previously thought, and (ii) the Younger Dryas ice sheet was smaller than indicated by previous reconstructions.  相似文献   

18.
《Quaternary Science Reviews》2007,26(11-12):1650-1669
We reconstruct the vegetational history of the southern side of the Alps at 18,000–10,000 cal yr BP using previous and new AMS-dated stratigraphic records of pollen, stomata, and macrofossils. To address potential effects of climatic change on vegetation, we compare our results with independent paleoclimatic series (e.g. isotope and chironomid records from the Alps and the Alpine forelands). The period before 16,000 cal yr BP is documented only at the lowland sites. The previous studies used for comparison with our new Palughetto record, however, shows that Alpine deglaciation must have started before 18,000–17,500 cal yr BP south of the Alps and that deglaciated sites were colonized by open woods and shrublands (Juniperus, tree Betula, Larix, Pinus cembra) at ca 17,500 cal yr BP. The vegetational history of a new site (Palughetto, 1040 m a.s.l.) is consistent with that of previous investigations in the study region. Our results show three conspicuous vegetational shifts delimited by statistically significant pollen zones, at ca 14,800–14,400, 13,300–12,800 and 11,600–11,200 cal yr BP. At sites situated above 1000 m a.s.l. (e.g. Palughetto, Pian di Gembro) forests expanded in alpine environments at ca 14,500 cal yr BP (onset of Bølling period, GI-1 in the Greenland ice record). At the same time, rather closed treeline communities of the lowlands were replaced by dense stands of Pinus sylvestris and Betula. These early forests and shrublands consisted of Larix, P. cembra, Juniperus, P. sylvestris, Pinus mugo, and Betula, and had become established at ca 16,000 cal yr BP, probably in response to a temperature increase. If combined with other records from the Southern Alps, our data suggest that treeline ascended by ca 800–1000 m in a few centuries at most, probably as a consequence of climatic warming at the beginning of the Bølling period. At 13,100–12,800 cal yr BP the onset of a long-lasting decline of P. sylvestris was accompanied by the expansion of Quercus and other thermophilous tree taxa below ca 600 m a.s.l. This vegetational change was probably induced by a shift to warmer climatic conditions before the onset of the Younger Dryas, as indicated by independent paleoclimatic records. Only a few centuries later, at ca 12,700–12,500 cal yr BP, an expansion of herbaceous taxa occurred in the lowlands as well as at higher altitudes, documenting an opening of forested habitats. This change coincided with the beginning of the Younger Dryas cooling (GS-1), which according to the paleoclimatic series (e.g. oxygen isotope series), started at 12,700–12,600 cal yr BP and lasted for about 1000 years. Environments south of the Alps responded markedly to climatic warming at the onset of the Holocene (11,600–11,500 cal yr BP). Thermophilous trees that had declined during the Younger Dryas re-expanded very rapidly in the lowlands and reached the high altitude sites below ca 1500 m a.s.l. within a few centuries at most. Our study implies that the synchronous vegetational changes observed over wide areas were probably a consequence of abrupt climatic shifts at the end of the Last Glacial Maximum (LGM) and during the Lateglacial. We emphasize that important vegetational changes such as the expansion of forests occurred millennia before the onset of similar processes in northwestern and central Europe.  相似文献   

19.
Located on a mountain pass in the west-central Pyrenees, the Col d'Ech peat bog provides a Holocene fire and vegetation record based upon nine 14C (AMS) dates. We aim to compare climate-driven versus human-driven fire regimes in terms of frequency, fire episodes distribution, and impact on vegetation. Our results show the mid-Holocene (8500–5500 cal yr BP) to be characterized by high fire frequency linked with drier and warmer conditions. However, fire occurrences appear to have been rather stochastic as underlined by a scattered chronological distribution. Wetter and colder conditions at the mid-to-late Holocene transition (4000–3000 cal yr BP) led to a decrease in fire frequency, probably driven by both climate and a subsequent reduction in human land use. On the contrary, from 3000 cal yr BP, fire frequency seems to be driven by agro-pastoral activities with a very regular distribution of events. During this period fire was used as a prominent agent of landscape management.  相似文献   

20.
At present, the climate in south-west Scandinavia is predominantly controlled by westerlies carrying moist Atlantic air, which forms a main source of precipitation in all seasons. Past variations in the ratio between precipitation and evaporation (effective precipitation) from terrestrial sites, however, may indicate changes in the degree of maritime influence. Palaeoclimatic archives in this region are thus ideally situated to study past changes in atmospheric circulation patterns. In this study, multi-proxy comparisons of records from three peat bogs and two lakes are used to reconstruct regional-scale variations of effective precipitation in south-west Sweden during the Late Holocene. The total aeolian sediment influx into two peat bogs is used as a proxy for storm activity. The frequency of storm phases increases strongly after ca 2500 cal yrs BP. Dry conditions occur on a regional scale around 4800–4400, 2000–1700, 1300–1000, 700–500 and 300–100 cal yrs BP. In addition, a comparison to winter precipitation reconstructed from four Norwegian glaciers shows similar variations during the past ca 2000 years. This indicates that the climate in both regions was controlled by large-scale atmospheric circulation dynamics of the North Atlantic region. The strong variability of effective precipitation and storm activity after ca 2500 cal yrs BP indicates a highly variable climate.  相似文献   

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