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1.
The concept that Rannoch Moor, the centre of the Younger Dryas (YD), West Highland Icefield, was deglaciated as early as 12.5 cal ka BP is discussed in the light of radiocarbon dates and varve sequences from outlet glaciers of this icefield, and climate change during the YD. The maximum positions of three YD glaciers were reached after 11.6–11.8 cal ka BP (Lomond), and after 11.8–11.9 cal ka BP (Spean and Treig) indicating that ice remained on Rannoch Moor until long after c.12.5 cal ka BP, and possibly until the YD/Holocene transition at c.11.7 cal ka BP. Further, the Spean glacier dammed a proglacial lake in Lochaber for at least 495 varve years over a period that included the deposition of the Vedde Ash (c.12.1 cal ka BP) and a late YD ash layer (c. 11.7–11.2 cal ka BP), a thesis at variance with supposed early YD deglaciation. Recent examination of this issue using 10Be exposure age determinations from Rannoch Moor is equivocal. In view of the presence of hard water algae at the sampling site on Rannoch Moor it is recommended that the ‘early’ 14C dates from Rannoch Moor need to be further reassessed using chronological constraints provided by dated microtephra, and a collaborative radiocarbon dating programme.  相似文献   

2.
《Quaternary Research》2014,81(3):445-451
Some scholars have argued that the formation and outburst of an ancient dammed lake in the Jishi Gorge at ca. 3700 cal yr BP resulted in the destruction of Lajia, the site of a famous prehistoric disaster in the Guanting Basin, upper Yellow River valley, China. However, the cause of the dammed lake and the exact age of the dam breaching are still debated. We investigated ancient landslides and evidence for the dammed lake in the Jishi Gorge, including dating of soil from the shear zone of an ancient landslide, sediments of the ancient dammed lake, and loess above lacustrine sediments using radiocarbon and optically stimulated luminescence (OSL) dating methods. Six radiocarbon dates and two OSL dates suggested that the ancient landslides and dammed lake events in the Jishi Gorge probably occurred around 8100 cal yr BP, and the ancient dammed lake was breached between 6780 cal yr BP and 5750 cal yr BP. Hence, the outburst of the ancient dammed lake in the Jishi Gorge was unrelated to the ruin of the Lajia site, but likely resulted in flood disasters in the Guanting Basin around 6500 cal yr BP.  相似文献   

3.
This study presents a high-resolution multi-proxy investigation of sediment core MD03-2601 and documents major glacier oscillations and deep water activity during the Holocene in the Adélie Land region, East Antarctica. A comparison with surface ocean conditions reveals synchronous changes of glaciers, sea ice and deep water formation at Milankovitch and sub-Milankovitch time scales. We report (1) a deglaciation of the Adélie Land continental shelf from 11 to 8.5 cal ka BP, which occurred in two phases of effective glacier grounding-line retreat at 10.6 and 9 cal ka BP, associated with active deep water formation; (2) a rapid glacier and sea ice readvance centred around 7.7 cal ka BP; and (3) five rapid expansions of the glacier–sea ice systems, during the Mid to Late Holocene, associated to a long-term increase of deep water formation. At Milankovich time scales, we show that the precessionnal component of insolation at high and low latitudes explains the major trend of the glacier–sea ice–ocean system throughout the Holocene, in the Adélie Land region. In addition, the orbitally-forced seasonality seems to control the coastal deep water formation via the sea ice–ocean coupling, which could lead to opposite patterns between north and south high latitudes during the Mid to Late Holocene. At sub-Milankovitch time scales, there are eight events of glacier–sea ice retreat and expansion that occurred during atmospheric cooling events over East Antarctica. Comparisons of our results with other peri-Antarctic records and model simulations from high southern latitudes may suggest that our interpretation on glacier–sea ice–ocean interactions and their Holocene evolutions reflect a more global Antarctic Holocene pattern.  相似文献   

4.
Easter Island (SE Pacific, 27°S) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Although the general climate evolution of the south Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial–interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by “El Niño-like” dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2–0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake.  相似文献   

5.
Excavations at Bonneville Estates Rockshelter, Nevada recovered rodent remains from stratified deposits spanning the past ca. 12,500 14C yr BP (14,800 cal yr BP). Specimens from horizons dating to the late Pleistocene and early Holocene include species adapted to montane and moist and cool habitats, including yellow-bellied marmot (Marmota flaviventris) and bushy-tailed woodrat (Neotoma cinerea). Shortly after 9000 14C BP (10,200 cal yr BP) these mammals became locally extinct, or nearly so, taxonomic diversity declined, and the region became dominated by desert woodrats (Neotoma lepida) and other species well-adapted to xeric, low-elevation settings. The timing and nature of changes in the Bonneville Estates rodent fauna are similar to records reported from nearby Homestead and Camels Back caves and provide corroborative data on terminal Pleistocene–early Holocene environments and mammalian responses to middle Holocene desertification. Moreover, the presence of northern pocket gopher (Thomomys talpoides) at Bonneville Estates adds to a sparse regional record for that species and, similar to Homestead Cave, it appears that the ca. 9500 14C yr BP (10,800 cal yr BP) replacement of the northern pocket gopher by Botta's pocket gopher in the Great Salt Lake Desert vicinity was also in response to climate change.  相似文献   

6.
《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   

7.
A sediment core from Lake Arapisto, Finland, was examined for fossil diatom assemblages to reconstruct changes in Holocene nutrient availability. Our aim was to investigate the long-term relationship between lake trophic status and climate by comparing the diatom-based phosphorus reconstruction with paleoclimatic proxies. Our results showed that the cold early Holocene was characterized by elevated nutrient conditions concurrent with newly exposed fertile ground. As the climate rapidly warmed and ice sheet further retreated, the catchment vegetation developed, which resulted in decreased nutrient flux into the lake. The Holocene Thermal Maximum (HTM), between ~ 8000 and 4000 cal yr BP, was characterized by oligotrophic conditions, which may have been caused by low effective precipitation and stable watershed vegetation. After the HTM, the lake became more productive. There was no particular increase in the trophic state that could be connected to more recent human influence. Although lake productivity has been shown to be affected by temperature, our record indicated that the nutrient dynamics were driven by complex interactions between changes in temperature, precipitation, catchment, and in-lake processes. Understanding of long-term nutrient dynamics and the associated processes can help in resolving relationships between lake productivity and climate during past and present climate changes.  相似文献   

8.
《Quaternary Science Reviews》2007,26(3-4):463-478
The changes in flow and character of the warm Atlantic Water through the last 17,500 cal yr are reconstructed from the distribution of benthic foraminifera species, planktonic and benthic foraminifera abundances, stable oxygen isotopes and lithology in two cores from the western and northern shelf of Svalbard. The results show almost continuous presence of Atlantic Water at the shelf areas since >14,500 cal yr BP. The Bølling and Allerød intervals stand out as periods of highest bottom waters temperatures. The strong inflow of saline, but chilled Atlantic Water during the early Holocene was followed by cooling and freshening of the bottom waters during the mid- and late Holocene. The two records reveal synchronous oceanographic changes that are closely tied to changes in the flow of Atlantic Water recorded further south in the Nordic seas. The early Holocene warming was not just an effect of higher solar insolation, but was also due to increased heat flux from the stronger Atlantic Water inflow driven by wind force and/or thermohaline circulation.  相似文献   

9.
《Quaternary Science Reviews》2007,26(11-12):1610-1620
The primary objective of the present study is to identify major phases of alluviation in the Indian region since the abrupt Deglacial intensification of the monsoon (∼15 cal ka BP) on the basis of analysis of 68 radiocarbon dates from two major hydro-geomorphic regions of India: the Central Ganga Basin (CGB) and the Deccan Peninsula (DP). The recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences. The clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using OxCal version 4.0.1 and CALPAL (version May 2006) software packages.The summed probability plots reveal that periods of alluviation in the CGB, represented by three clusters (13.9–12.3, 11.9–11.2 and 9.8–9.0 cal ka BP) occur roughly before the onset of Early Holocene monsoon optimum phase. Two other clusters occur in the intervals 3.6–2.8 and 1.1–0.9 cal ka BP. The peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision. This period also shows clustering of radiocarbon dates of the abandoned channels. In comparison, 14C dates from DP alluvial units form clusters at 16.4–14, 12.8–11.2, 10.8–8.9, 8.1–6.7 and 5.1–3.9 cal ka BP, indicating an association with the Deglacial–Early Holocene humid phase. Alluviation in the DP appears to have continued, more or less, uninterrupted till the middle of the Holocene epoch. The beginning and end of the discernible gap in the radiocarbon dates of CGB (9.0–3.6 cal ka) broadly corresponds with the two well-established short-term events of the Holocene, 8.2 and 4.2 ka cal BP. In comparison, the prominent gap of DP radiocarbon dates (3.9–2.1 ka cal BP) approximately begins with the 4.2 ka cal BP short-term event (onset of aridity) and ends with the 2.0 ka cal BP enhanced monsoon event.Notwithstanding the inter-regional differences in the fluvio–sedimentary response in the India region, the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the Indian subcontinent were intimately linked to long-term fluctuations in the monsoon strength during the Late Quaternary.  相似文献   

10.
The environmental conditions of the Szczecin Bay, which existed prior to Szczecin Lagoon, have been reconstructed on the basis of the stable carbon and oxygen isotope (18O and 13C) analysis and radiocarbon dates obtained for subfossil shells of Cerastoderma (Cardium) glaucum. The shells in the collected core were well preserved in their life positions, representing a geochemical record of past temperature variation over the middle Holocene. Three major periods with different thermal conditions have been distinguished in the interval ~ 6000–4300 cal yr BP, when the important Littorina regional transgression took place. During the first period, 6000–5250 cal yr BP, water temperature decreased by 1.4°C, and then remained constant over the second period (5250–4750 cal yr BP). In contrast, during the third period (4750–4300 cal yr BP) both δ-values were highly variable and the mean summer temperature (March–November) increased by about 3.5°C. During first two periods, δ18O and δ13C were significantly correlated, indicating stability of the environmental conditions.  相似文献   

11.
《Quaternary Science Reviews》2007,26(19-21):2661-2673
Extensive areas in the southern part of the Duero Tertiary Basin (Central Spain) are covered by aeolian sands. Presently, the aeolian system is relict but in its origin and development it can be described as a “wet aeolian system”. Climatic and environmental changes during the Holocene are typified by alternating humid and arid periods. These are recorded in the sedimentary record as either organic-rich sandy palaeosols or clean aeolian sand, respectively. Palaeosol dating (12 radiocarbon dated samples) and stratigraphical and sedimentological analysis of several dunefields in quarries and boreholes allow the distinction of four periods of palaeosol development since the Allerød.Aeolian sediments commonly rest on fluvial deposits, which were themselves the major source area for aeolian sands. These fluvial deposits have an age of about 14,000 cal yr BP. The first phase of aeolian activity postdates these fluvial sediments and has an upper age of about 12,000–11,700 cal yr BP, probably corresponding to the last cold oscillation of the Lateglacial (Younger Dryas). The second phase ranges from about 11,500 to 9500 cal yr BP, during which period the majority of dunes in the Tierra de Pinares area formed. This is also a major phase of aeolian activity in other areas of the Iberian Peninsula. A third and probably discontinuous phase of aeolian activity took place between 6800 and about 3000 cal yr BP. The age for this phase is supported by the presence of Visigothic burial sites covered by aeolian sands. The presence of charred material and degraded slipfaces clearly indicate stabilisation by vegetation and the final degradation of the aeolian system at the end of the fourth aeolian phase (990–540 cal yr BP). Minor aeolian activity has also occurred subsequently in this area, since aeolian sand movement was even reported in the 20th century.The aeolian phases can be tentatively correlated with aeolian phases in Europe. Aeolian activity tends to occur regionally during specific time-intervals, especially in dunefields with little human disturbance. This argues for a broad climatic forcing in Holocene aeolian accumulation, such has been previously suggested for the little ice age. The precise timing of these phases, however, is not strictly coincident, probably due to the delayed responses of aeolian environments to climatic and subsequent vegetation change.  相似文献   

12.
In order to understand human response to Holocene ashfall events, tephra layers found in archaeological sites along the upper Limay River basin, Northern Patagonia, Argentina were bracketed with radiocarbon dates and correlated with tephra from a lacustrine sediment core and from outcrops and the archaeological evidence was analyzed.A dark tephra associated with seismic activity was identified in the El Trébol rockshelter filling interstices between fallen blocks together with remains of human activity and bones of extinct fauna, marking a seismo-volcanic event occurring between 11 758 and 12 866 cal yr BP. This same seismic event affected the Cuyín Manzano site, where the roof collapse made the site uninhabitable for a time. A white tephra, present in Epullán Grande, Epullán Chica and Traful I caves is correlated with Nahuel Huapi tephra (NHT), equivalent to Laya's Río Blanco and Río Pereyra members, (Río Pireco Formation). NHT is considered to have been derived from the same eruptive event, with dates ranging between ca. 1950–2500 cal yr BP. A dark tephra from Ortega's cave and several tephra from Puerto Tranquilo I rockshelter ranging between 521 and 2069 cal yr BP show how the mobile hunters–gatherers of Northern Patagonia were able to cope with the changing circumstances.  相似文献   

13.
《Quaternary Science Reviews》2007,26(5-6):793-807
The environmental history of a talus-derived rockglacier located in northern Norway has been reconstructed through the Last Glacial–Interglacial transition based on two cores retrieved from an adjacent lake. The methods used to quantify sedimentary properties include rock magnetism, grain size analyses, loss-on-ignition (LOI) and bulk density, which when combined has enabled an unmixing of the various sediment components and their corresponding sources. Rockglaciers signify mean annual air temperatures (MAAT) of −4 °C or colder, but little is known about their dynamical response to changing thermal regimes. We document here for the first time that a permafrost regime did exist in northern Norway during the lateglacial period, and that it required a lowering equivalent of at least 7 °C compared to present-day MAAT. The lake sediments suggest that the rockglacier existed prior to the local deglaciation of the Fennoscandian Ice Sheet (>14 800 cal yr BP), and continued its expansion until the end of the Younger Dryas whereupon it became fossil. The cool climate of the lateglacial was intersected by brief warming spells that caused a systematic release of sedimentladen meltwater from the rockglacier. During the Holocene the minerogenic influx to the lake was driven by spring snowmelting, which are related to the magnitude of winter precipitation. Three phases are recognised: (1) 9800–6500 cal yr BP when wet winters prevailed, (2) 6500–4000 cal yr BP with dry winters, and (3) the last 4000 cal yr BP with a return to wetter winters.  相似文献   

14.
《Quaternary Science Reviews》2007,26(11-12):1455-1475
This paper uses a palynological sequence to examine the Holocene (8390–160 cal yr BP) environmental history of the Sierra de Baza (Granada, southeastern Spain) with the goal of establishing the mechanisms exerting control over vegetation change. During the period ca 8390–6320 cal yr BP, Pinus dominated the pollen spectra, indicating a forested landscape over the high-elevation areas of the Sierra. From ca 6320–3800 cal yr BP, an expansion of deciduous oaks and other broad-leaf trees took place. After an optimum around 5800–5600 cal yr BP, mesophytes decreased in the 3800–2560 cal yr BP interval while a fire-prone scrub became established. The main loss of forest accompanied the spread of thorny matorral after ca 2560 cal yr BP. Overall, this mountain region has shown itself to be sensitive to a range of influences, among which a continental climate that has become increasingly arid over the last 5000 years, the scarcity of soils suitable for cultivation, a geology that includes sources of copper and other metals and, especially, the incidence of grazing as well as the repeated appearance of fires during the last 4000 years, are highlighted. The history of the vegetation of the Sierra de Baza seems clearly influenced by changes in local economy. Here we discuss how ecological transitions have interacted with cultural changes, with emphasis on the locally highly populated Chalcolithic (5700–4400 cal yr BP) and Argaric (4400–3550 cal yr BP) periods, as well as the Iberian period (3200–2220 cal yr BP). The sierra was abandoned during the Iberian Period which was, paradoxically, when the highest human impact on mountain vegetation is noticeable.  相似文献   

15.
《Quaternary Science Reviews》2007,26(17-18):2128-2151
After the first emergence following deglaciation, relative sea level rose by 10 m in western Norway and culminated late in the Younger Dryas (YD). The relative sea-level history, reconstructed by dating deposits in isolation basins, shows a sea-level low-stand between ∼13 640 and 13 080 cal yr BP, a 10 m sea-level rise between ∼13 080 and 11 790 cal yr BP and a sea-level high-stand between ∼11 790 and 11 550 cal yr BP. Shortly after the YD/Holocene boundary, sea level fell abruptly by ∼37 m. The shorelines formed during the sea-level low-stand in the mid-Allerød and during the sea-level high-stand in the YD have almost parallel tilts with a gradient of ∼1.3 m km−1, indicating that hardly any isostatic movement has taken place during this period of sea-level rise. We conclude that the transgression was caused by the major re-advance of the Scandinavian Ice Sheet that took place in western Norway during the Lateglacial. The extra ice load halted the isostatic uplift and elevated the geoid due to the increased gravitational attraction on the sea. Our results show that the crust responded to the increased load well before the YD (starting ∼12 900 cal yr BP), with a sea-level low-stand at 13 640 cal yr BP and the subsequent YD transgression starting at 13 080 cal yr BP. Thus, we conclude that the so-called YD ice-sheet advance in western Norway started during the Allerød, possibly more than 600 years before the Allerød/YD transition.  相似文献   

16.
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   

17.
《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   

18.
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   

19.
《Quaternary Science Reviews》2007,26(17-18):2281-2300
We review Late Cenozoic climate and environment changes in the western interior of China with an emphasis on lacustrine records from Lake Qinghai. Widespread deposition of red clay in the marginal basins of the Tibetan Plateau indicates that the Asian monsoon system was initially established by ∼8 Ma, when the plateau reached a threshold altitude. Subsequent strengthening of the winter monsoon, along with the establishment of the Northern Hemisphere ice sheets, reflects a long-term trend of global cooling. The few cores from the Tibetan Plateau that reach back a million years suggest that they record the mid-Pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles.During Terminations I and II, strengthening of the summer monsoon in China's interior was delayed compared with sea level and insolation records, and it did not reach the western Tibetan Plateau and the Tarim Basin. Lacustrine carbonate δ18O records reveal no climatic anomaly during MIS3, so that high terraces interpreted as evidence for extremely high lake levels during MIS3 remain an enigma. Following the Last Glacial Maximum (LSM), several lines of evidence from Lake Qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr BP, which was followed by a brief cold reversal, possibly corresponding to the Younger Dryas event in the North Atlantic region. Maximum warming occurred about 10 000 cal yr BP, accompanied by increased monsoon precipitation in the eastern Tibetan Plateau. Superimposed on this general pattern are small-amplitude, centennial-scale oscillations during the Holocene. Warmer than present climate conditions terminated about 4000 cal yr BP. Progressive lowering of the water level in Lake Qinghai during the last half century is mainly a result of negative precipitation–evaporation balance within the context of global warming.  相似文献   

20.
《Quaternary Science Reviews》2007,26(13-14):1736-1758
This paper presents a high-resolution lake-level record for the Holocene at Lake Accesa (Tuscany, north-central Italy) based on a range of sedimentological techniques validated in previous studies, with a chronology derived from 43 radiocarbon dates and four tephra layers. It gives evidence of centennial-scale fluctuations with major highstands at ca 11 500, 11 100, 10 200, 9400, 8200, 7300, 6200, 5700–5200, 4850, 4200, 3400, 2600, 1200 and 400 cal BP. Except for the Early Holocene until ca 10 500 cal BP, this pattern of hydrological changes appears to be in agreement with the regional pattern established for west-central Europe. Correlations with the Preboreal oscillation and the 8.2 ka event as well as with the atmospheric 14C residual series suggest that lake-level fluctuations developed at Accesa in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity. For the period after 4500 cal BP, correlations with other palaeohydrological records from central Italy indicate that lake-level changes reconstructed at Accesa were mainly driven by climatic variations while anthropic activities and local geomorphological factors only played a secondary role. The Accesa lake-level record also highlights millennial-scale variations with a maximal lowstand at ca 9200–7700 cal BP contemporaneous with Sapropel event 1 in the Mediterranean. It was followed by generally higher lake-level conditions. This appears to be the opposite of that observed in Sicily (southern Italy) where a lake-level maximum developed at ca 9000–8200 cal BP and was followed by a general trend towards aridification. These opposite patterns were interpreted as contrasting hydrological responses to orbitally induced changes in summer insolation. This interpretation has to be tested by further lake-level studies in the central Mediterranean region. Finally, correlations between major lowstands and periods of maximal representation of Quercus ilex point to convergences between climate oscillations and Holocene vegetation history in the Accesa region. However, the maximal representation of Abies during the first half of the Holocene, including a time window where lake level reached a minimal level, suggests a more subtle impact of seasonality processes.  相似文献   

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