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1.
This paper presents quantitative climate estimates for the last millennium, using a multi-proxy approach with pollen and lake-level data from Lake Joux (Swiss Jura Mountains). The climate reconstruction, based on the Modern Analogue Technique, indicates warmer and drier conditions during the Medieval Warm Period (MWP). MWP was preceded by a short-lived cold humid event around AD 1060, and followed by a rapid return around AD 1400 to cooler and wetter conditions which generally characterize the Little Ice Age (LIA). Around AD 1450 (solar Spörer minimum), the LIA attained a temperature minimum and a summer precipitation maximum. The solar Maunder minimum around AD 1690 corresponded at Joux to rather mild temperatures but maximal annual precipitation. These results generally agree with other records from neighbouring Alpine regions. However, there are differences in the timing of the LIA temperature minimum depending on the proxy and/or the method used for the reconstruction. As a working hypothesis, the hydrological signal associated with the MWP and LIA oscillations at Lake Joux may have been mainly driven by a shift around AD 1400 from positive to negative NAO modes in response to variations in solar irradiance possibly coupled with changes in the Atlantic meridional overturning circulation.  相似文献   

2.
On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

3.
The Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget is documented by sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. Six high-amplitude reflectors within the lacustrine drape can be correlated to periods of enhanced inter- and underflow deposition in sediment cores. Based on the synthesis of major environmental changes in the NW Alps and on the age-depth model covering the past 7500 years in Lake Le Bourget, periods of enhanced Rhone River flood events in the lake can be related to abrupt climate changes and/or to increasing land use since c. 2700 cal. yr BP. For example, significant land use under rather stable climate conditions during the Roman Empire may be responsible for large flood deposits in the northern part of Lake Le Bourget between AD 966 and 1093. However, during the Little Ice Age (LIA), well-documented major environmental changes in the catchment area essentially resulted from climate change and formed basin-wide major flood deposits in Lake Le Bourget. Up to five 'LIA-like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget are documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP. These abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels. Correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation.  相似文献   

4.
Egesen moraines throughout the Alps mark a glacial advance that has been correlated with the Younger Dryas cold period. Using the surface exposure dating method, in particular the measurement of the cosmogenic nuclide 10Be in rock surfaces, we attained four ages for boulders on a prominent Egesen moraine of Great Aletsch Glacier, in the western Swiss Alps. The 10Be dates range from 10 460±1100 to 9040±1020 yr ago. Three 10Be dates between 9630±810 and 9040±1020 yr ago are based upon samples from the surfaces of granite boulders. Two 10Be dates, 10 460±1100 and 9910±970 yr ago, are based upon a sample from a quartz vein at the surface of a schist boulder. In consideration of the numerous factors that can influence apparently young 10Be dates and the scatter within the data, we interpret the weighted mean of four boulder ages, 9640±430 yr (including the weighted mean of two 10Be dates of the quartz vein), as a minimum age of deposition of the moraine. All 10Be dates from the Great Aletsch Glacier Egesen moraine are consistent with radiocarbon dates of nearby bog‐bottom organic sediments, which provide minimum ages of deglaciation from the moraine. The 10Be dates from boulders on the Great Aletsch Glacier Egesen moraine also are similar to 10Be dates from Egesen moraines of Vadret Lagrev Glacier on Julier Pass, in the eastern Swiss Alps. Both the morphology of the Great Aletsch Glacier Egesen moraine and the comparison with 10Be dates from the inner Vadret Lagrev Egesen moraine support the hypothesis that the climatic cooling that occurred during the Younger Dryas cold episode influenced the glacial advance that deposited the Great Aletsch Glacier Egesen moraine. Because of the large size and slow response time of Great Aletsch Glacier, we suggest that the Great Aletsch Glacier Egesen moraine was formed during the last glacial advance of the multiphased Egesen cold period, the Kromer stage, during the Preboreal chron. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

5.
The last millennium is a key period for understanding environmental change in eastern Africa, as there is clear evidence of marked fluctuations in climate (effective moisture) that place modern concern with future climate change in a proper context, both in terms of environmental and societal impacts and responses. Here, we compare sediment records from two small, nearby, closed crater lakes in western Uganda (Lake Kasenda and Lake Wandakara), spanning the last 700 (Wandakara) and 1200 years (Kasenda) respectively. Multiproxy analyses of chemical sedimentary parameters (including C/N ratios, δ13C of bulk organic matter and δ13C and δ18O of authigenic carbonates) and biotic remains (diatoms, aquatic macrofossils, chironomids) suggest that Kasenda has been sensitive to climate over much of this period, and has shown substantial fluctuations in conductivity, while Wandakara has a more muted response, likely due to the increasing dominance of human activity as a driver of change within the lake and catchment over the length of our record. Evidence from both records, however, supports the idea that lake levels were low from ～AD 700–1000 AD, with increasing aridity from AD 1100–1600, and brief wet phases around AD 1000 and 1400. Wetter conditions are recorded in the 1700s, but drought returned by the end of the century and into the early 1800s, becoming wetter again from the mid-1800s. Comparison with other records across eastern Africa suggests that while some events are widespread (e.g. aridity beginning ～ AD 1100), at other times there is a more complex spatial signature (e.g. in the 1200s to 1300s, and from the 1400s to 1600s). This study highlights the important role of catchment-specific factors (e.g. lakemorphometry, catchment size, and human impact) in modulating the sensitivity of proxies, and lake records, as indicators of environmental change, and potential hazards when regional inference is based on a single site or proxy.  相似文献   

6.
Abandoned shorelines are an important archive used to constrain past fluctuations in the hydrological balance of lakes around the globe. Within Australia, the shorelines preserved at Lake George, NSW, form one of the few shoreline archives in the south-east of the continent that record palaeoenvironmental conditions throughout the late Quaternary. Here, we examined and tested the lake-level record for Lake George constructed in the 1970s by dating a well-preserved shoreline sequence at Luckdale, on the lake's eastern shore, using single-grain optically stimulated luminescence (OSL) dating. Ten stratigraphic units were identified, and these suggest a late Quaternary highstand for Lake George in MIS 3, with fluctuations superimposed upon an overall drying trend throughout MIS 2 and into the present. At Luckdale, the highest four shoreline-associated units were deposited ~13 to 19 m above lake base and date to between 39 ± 2 and 29 ± 1 ka ago. Our study pushes back the timing of maximum lake depth at Lake George to at least MIS 3, rather than MIS 2. The overall drying trend is supported by similar reductions in both Riverine Plain fluvial activity and other associated lake-level records from within the Murray basin.  相似文献   

7.
The history of variations in water level of Lake Constance, as reconstructed from sediment and pollen analysis of a sediment sequence from the archaeological site of Arbon-Bleiche 3, shows an abrupt rise in lake level dendrochronologically dated to 5375 yr ago (5320 yr relative to AD 1950). This event, paralleled by the destruction of the Neolithic village by fire, provoked the abandonment of this prehistoric lake-shore location established in the former shallow bay of Arbon-Bleiche, and was the last of a series of three episodes of successively higher lake level, the first occurring at 5600-5500 cal yr B.P. The dendrochronologically dated rise event was synchronous with an abrupt increase in atmospheric 14C. This supports the hypothesis of an abrupt climate change forced by varying solar activity. Moreover, the three successive episodes of higher lake level between 5600 and 5300 cal yr B.P. at Arbon-Bleiche 3 coincided with climatic cooling and/or changes in moisture conditions in various regions of both hemispheres. This period corresponds to the mid-Holocene climate transition (onset of the Neoglaciation) and suggests inter-hemispheric linkages for the climate variations recorded at Arbon-Bleiche 3. This mid-Holocene climate reversal may have resulted from complex interactions between changes in orbital forcing, ocean circulation and solar activity. Finally, despite different seasonal hydrological regimes, the similarities between lake-level records from Lake Constance and from Jurassian lakes over the mid-Holocene period point to time scale as a crucial factor in considering the possible impact of climate change on environments.  相似文献   

8.
Pollen analyses of three sediment cores performed on the archaeological lake dwellings of Chindrieux and Tresserve (Savoie, France), on the eastern shore of Lake Bourget, enable the reconstruction of vegetation history and human impact during the Bronze Age. Results show a good correlation between lake-level fluctuations and the evolution of anthropogenic indicators. The end of the Neolithic period and the early Bronze Age are characterised by a discreet occupation of the Lake Bourget surroundings, in spite of a warmer climatic condition. The middle Bronze Age, contemporaneous with wetter and cooler conditions, is related to a decrease in pollen of cultivated plants and weed. Farming activities reappear in the late Bronze Age and human impact increases suddenly at the very end of the late Bronze Age. Three anthropogenic phases are clearly visible: a short decrease in anthropogenic indicators and a high lake-level phase occur in the middle of this period of intense farming development. A new decrease in human impact appears at the beginning of the Iron Age.  相似文献   

9.
A continuous sediment record since 12.3 cal ka bp from Lake Wuxu (south‐eastern Tibetan Plateau) was investigated in terms of the Holocene evolution of the Indian Summer Monsoon. The molar C/N ratio and stable C isotope were used to identify the source of the organic matter as well as climate conditions. The evolution of Lake Wuxu was summarized wihtin two periods. During the first period (early to mid‐Holocene), the lake received increased fluvially transported materials, reflecting variation in the summer monsoon with solar insolation. The lake level declined and water residence time increased because of reduced river discharge during the second period (late Holocene) corresponding to a weakening of the summer monsoon. The organic material revealed a major contribution from lake primary productivity, which showed identical patterns with a high‐resolution isotope record from Dongge Cave, as well as total solar irradiance. Our record from Lake Wuxu indicates that the Holocene evolution of the Indian Summer Monsoon has been driven by the solar forcing at decadal/centennial to millennial time scales. Furthermore, an abrupt decline in the monsoon was detected at around 4.0 cal ka bp , which is probably caused by an increased frequency of EI Nino‐Southern Oscillation events. Copyright © 2019 John Wiley & Sons, Ltd.  相似文献   

10.
Geochemical methods (major elements and Sr, Nd isotopes) have been used to (1) characterize Lake Le Bourget sediments in the French Alps, (2) identify the current sources of the clastic sediments and estimate the source variability over the last 600 years. Major element results indicate that Lake Le Bourget sediments consist of 45% clastic component and 55% endogenic calcite. In addition, several individual flood levels have been identified during the Little Ice Age (LIA) on the basis of their higher clastic content (> 70%).Potential sources of Lake Le Bourget clastic sediments have been investigated from Sr and Nd isotope compositions. The sediments from the Sierroz River and Leysse River which are mainly derived from the Mesozoic Calcareous Massifs are characterised by lower 87Sr/86Sr ratios and slightly lower ?Nd(0) ratios than the Arve River sediments which are derived from the Palaeozoic Mont-Blanc External Crystalline Massifs. The Rhône River appears to have been the main source of clastic sediments into the lake for the last 600 years, as evidenced by a similar Sr and Nd isotopic compositions analyzed in core B16 sediments (87Sr/86Sr = 0.719, ?Nd(0) = − 10) and in the sediments of the Rhône River (87Sr/86Sr = 0.719, ?Nd(0) = − 9.6).The isotopic signatures of flood events and background samples from core B16 in Lake Le Bourget are also similar. This indicates that prior to ∼ 1800, the inputs into the lake have remained relatively homogeneous with the proportion of clastic component mainly being a function of the palaeohydrology of the Rhone River. Early human modification (deforestation and agriculture) of the lake catchment before the 1800s appears to have had little influence on the source of clastic sediments.  相似文献   

11.
The most accurate source of information on lake-level fluctuations in the Great Lakes is the historical record from lake-level gauges. Although it can be semiquantitatively extended back into the late 1700's, the historical record is too short to recognize long-term patterns of lake-level behavior. To extend the historical record, information must be obtained from the Great Lakes geologic record. Such information includes the elevation and age of geomorphic features and stratigraphic sequences.

One of the longest geologic records of late Holocene lake-level variation is preserved in a beach-ridge complex along the southern shore of Lake Michigan called the Toleston Beach. This strandplain contains over 150 beach ridges that arc across northwestern Indiana and fan out into northeastern Illinois. Each ridge was formed during the fall from a high lake level, and the elevation of the foreshore deposits in each ridge provides information on the upper physical limit of lake level over the past 4000 years. Three scales of quasi-periodic lake-level variation were determined by radiocarbon-dating basal peats of wetlands between the ridges and by measuring the elevation of foreshore (swash) deposits within ridges. These three scales are: (1) a short-term and small-scale fluctuation of 25 to 35 years with a range of about 0.5 to 0.6 m; (2) an intermediate-term and meso-scale fluctuation of 140 to 160 years and a range of about 0.8 to 0.9 m; and (3) a long-term and large-scale fluctuation of 500 to 600 years and a range of 1.8 to 3.7 m. The short-term and intermediate-term fluctuations are reflected in the historical record.

An increase in the rate of shoreline progradation from east to west across Indiana's shoreline causes differential preservation of the lake-level fluctuations. That is, groups of four to six ridges in the western part of the strandplain that formed in response to the small-scale fluctuations combine eastward into single ridges and groups of ridges representing the meso-scale fluctuations. The large-scale fluctuations produced the most dramatic response in the western part of the Toleston Beach. Here, following each high stand, individual spits prograded southward off of a bedrock headland. The successive spit extensions created several small lakes landward of the spits and started the 20 km eastward stream-mouth deflection of the Grand Calumet River across Indiana's western lakeshore.  相似文献   


12.
Vegetation and lake-level data from the archaeological site of Tresserve, on the eastern shore of Lake Le Bourget (Savoie, France), are used to provide quantitative estimates of climatic variables over the period 4000–2300 cal BP in the northern French Pre-Alps, and to examine the possible impact of climatic changes on societies of the Bronze and early Iron Ages. The results obtained indicate that phases of higher lake level at 3500–3100 and 2750–2350 cal BP coincided with major climate reversals in the North Atlantic area. In west-central Europe, they were marked by cooler and wetter conditions. These two successive events may have affected ancient agricultural communities in west-central Europe by provoking harvest failures, more particularly due to increasing precipitation during the growing season. However, archaeological data in the region of Franche-Comté (Jura Mountains, eastern France) show a general expansion of population density from the middle Bronze Age to the early Iron Age. This suggests a relative emancipation of proto-historic societies from climatic conditions, probably in relation to the spread of new modes of social and economic organisation.  相似文献   

13.
Vegetation and lake-level data from the sites of Lake Morat (Switzerland) and Lake Annecy (eastern France) are used to provide quantitative estimates of climatic variables over the period 6600-5500 cal. yr BP in the northern Subalpine zone, and to test the method of climate reconstruction based on modern pollen analogues constrained by lake-level data. The results obtained from both sites suggest: (1) that phases of higher lake level coincided with a cooling and a shortening of the growing season and an increase in annual precipitation, and (2) that during climatic reversals summer was more affected than winter. They also provide evidence of a sensitive method of reconstruction of climatic variables capable of capturing minor climatic oscillations such as reflected by weakly pronounced lake-level fluctuations. However, comparison of results obtained at Lake Morat and Lake Annecy, with each other and with results from other sites in central Europe, also clearly indicates that either anthropogenic disturbances of the vegetation cover or an overrepresentation of taxa due to particular site exposition can bias the reconstruction of variations in climatic parameters. In addition to pluri-centennial climatic oscillations, both the Lake Morat and Lake Annecy records show a general trend toward temperature cooling over the period considered. This may be related to a reduction in summer insolation induced by orbital factors. Finally, the quantitative estimates of climatic parameters reconstructed at Lake Morat and Lake Annecy help to increase our understanding of the possible impact of climatic oscillations on ancient agricultural communities.  相似文献   

14.
Several abrupt climate events during the Holocene, including the widely documented oscillation at 8.2 thousand years before present (ka), are attributed to changes in the North Atlantic thermohaline circulation. Additional mechanisms, such as interactions between atmospheric circulation, ice-sheet dynamics, and the influence of solar irradiance, also have been proposed to explain abrupt climatic events, but evidence remains elusive. This study presents evidence from multi-proxy analyses on the Holocene sediments of Eleanor Lake, interior British Columbia. Climatic inferences from our decadal-resolution record of biogenic silica (BSi) abundance are supported by changes in diatom and pollen assemblages from the same core and correlations with existing regional climate records. The BSi record reveals abrupt and persistent climatic shifts at 10.2, 9.3, and 8.5 ka, the latter two of which are coeval with major collapses of the Laurentide Ice Sheet. The record also reveals a short-term cooling at 8.2 ka that is distinct from the 8.5 ka event and similar in magnitude to several other late-Holocene coolings. BSi is correlated with solar-irradiance indices (r = 0.43–0.61), but the correlation is opposite in sign to that expected from direct solar forcing and weakens after 8 ka. Possible mechanisms causing the abrupt and persistent climate changes of the early Holocene include 1) sudden losses of ice and proglacial lake extent, causing a shift in the meridional structure of atmospheric circulation, 2) a possible link between solar minima and El Niño-like conditions that are correlated with warm spring temperature in interior British Columbia, and 3) the influence of solar irradiance variability on the position of the polar jet, possibly via effects on the strength of the glacial anticyclone.  相似文献   

15.
Variations in fossil diatom assemblages and their relationship with global and Indian monsoon climate changes for the last 600,000 yr were investigated using a core of ancient lake (Paleo-Kathmandu Lake) sediments drilled at the Kathmandu Basin, Nepal Himalaya. Chronological scales of the core were constructed by tuning pollen wet and dry index records to the SPECMAP δ18O stack record. Examinations of biogenic silica contents and fossil diatom assemblages revealed that variations in productivity and compositions of diatom assemblages were closely linked with global and Indian monsoon climate changes on glacial and interglacial time scales. When summer monsoonal rainfall increased during interglacials (interstadials), diatom productivity increased because of increased inputs of terrestrial nutrients into the lake. When summer monsoonal rainfall reduced and/or winter monsoonal aridification enhanced during glacials (stadials), productivity of the diatoms decreased and lake-level falling brought about changes in compositions of diatom assemblages. Monospecific assemblages by unique Cyclotella kathmanduensis and Puncticulata versiformis appeared during about 590 to 390 ka. This might be attributed to evolutionary fine-tuning of diatom assemblages to specific lake environmental conditions. Additionally, low-amplitude precessional variations in monsoon climate and less lake-level changes may have also allowed both species to dominate over the long periods.  相似文献   

16.
The Alps play a pivotal role for glacier and climate reconstructions within Europe. Detailed glacial chronologies provide important insights into mechanisms of glaciation and climate change. We present 26 10Be exposure dates of glacially transported boulders situated on moraines and ice‐moulded bedrock samples at the Belalp cirque and the Great Aletsch valley, Switzerland. Weighted mean ages of ～10.9, 11.1, 11.0 and 9.6 ka for the Belalp, on up to six individual moraine ridges, constrain these moraines to the Egesen, Kartell and Schams stadials during Lateglacial to early Holocene times. The weighted mean age of ～12.5 ka for the right‐lateral moraine of the Great Aletsch correlates with the Egesen stadial related to the Younger Dryas cooling. These data indicate that during the early Holocene between ～11.7 and ～9.2 ka, glaciers in the Swiss Alps seem to have been significantly affected by cold climatic conditions initiated during the Younger Dryas and the Preboreal Oscillation. These conditions resulted in glacier margin oscillations relating to climatic fluctuations during the second phase of the Younger Dryas – and continuing into Boreal times – as supported by correlation of the innermost moraine of the Belalp Cirque to the Schams (early) Holocene stage. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

17.
Palaeoenvironmental and archaeological data from Arbon Bleiche, Lake Constance (Switzerland) give evidence of a rapid rise in lake‐level dated by tree‐ring and radiocarbon to 5320 cal. yr BP. This rise event was the latest in a series of three successive episodes of higher lake‐level between 5550 and 5300 cal. yr BP coinciding with glacier advance and tree‐limit decline in the Alps. This west‐central European climate change may have favoured the quick burial and the preservation of the Alpine Iceman recently found in the Tyrolean Alps. It has possible equivalents in many records from various regions in both hemispheres dating to 5600–5000 cal. yr BP and corresponds to global cooling and contrasting patterns of hydrological changes. This major mid‐Holocene climate event marks the Hypsithermal/Neoglaciation transition possibly resulting from a combination of different factors including orbital forcing, changes in ocean circulation and variations in solar activity. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

18.
《Quaternary Science Reviews》2007,26(5-6):678-689
A high-resolution study was performed on varved sediments from Lake Lehmilampi in eastern Finland. Varve data was collected by digital image analysis using standard 1.8 mm thick samples impregnated in epoxy and X-rayed. Climatic variability is imprinted on varve properties (varve thickness and accumulation of mineral and organic matter) during the last 2000 years. The cumulative counting error of the varve record is estimated as 2.3%. Qualitative comparison of varve parameters and residual Δ14C constructed from tree-rings revealed close correspondence between the two records, suggesting solar forcing on lake sedimentation. Classical climatic periods of the last millennia, Medieval Climate Anomaly (1060–1280 in the varve record) and Little Ice Age (cooler phases culminating in 1340, 1465, 1545, 1680, 1850 and also in 1930 in the varve record) are clearly evident in the varve record. At present the physical link between solar activity levels and lake sedimentation has not been established.  相似文献   

19.
The European Alps are very sensitive and vulnerable to climate change. Recent improvements in Alpine glacier length records and climate reconstructions from annually laminated sediments of Alpine Lake Silvaplana give the opportunity to investigate the relationship between these two data sets of Alpine climate. Two different time frames are considered: the last 500–1000 years as well as the last 7400 years. First, we found good agreement between the two different climate archives during the past millennium: mass accumulation rates and biogenic silica concentration are largely in phase with the glacier length changes of Mer de Glace and Unterer Grindelwaldgletscher, and with the records of glacier length of Grosser Aletschgletscher and Gornergletscher. Secondly, the records are compared with temporally highly resolved data of solar activity. The Sun has had a major impact on the Alpine climate variations in the long term, i.e. several centuries to millennia. Solar activity varies with the Hallstatt periodicity of about 2000 years. Hallstatt minima are identified around 500, 2500 and 5000 a. Around these times grand solar minima (such as the Maunder Minimum) occurred in clusters coinciding with colder Alpine climate expressed by glacier advances. During the Hallstatt maxima around 0, 2000 and 4500 a, the Alpine glaciers generally retreated, indicating a warmer climate. This is supported by archaeological findings at Schnidejoch, a transalpine pass in Switzerland that was only accessible when glaciers had retreated. On shorter timescales, however, the influence of the Sun cannot be as easily detected in Alpine climate change, indicating that in addition to solar forcing, volcanic influence and internal climate variations have played an important role. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
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   

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