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1.
A cool period from about 11000 to 10 500 BP (11 to 10.5 ka) is recognized in pollen records from the southern Great Lakes area by the return of Picea and Abies dominance and by the persistence of herbs. The area of cooling appears centred on the Upper Great Lakes. A high-resolution record (ca. 9 mm/y) from a borehole in eastern Lake Erie reveals, in the same time interval, this pollen anomaly, isotope evidence of meltwater presence (a — 3 per mil shift in 18O and a +1.1 per mil shift in 13C), increased sand, and reduced detrital calcite content, all suggesting concurrent cooling of Lake Erie. The onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream Main Lake Algonquin during the first eastward discharge of glacial Lake Agassiz. Termination of the cooling coincides with drainage of Lake Algonquin, and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area. It is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in Lake Algonquin. The deep mixing would have greatly increased the thermal conductive capacity of this extensive lake, causing suppression of summer surface lakewater temperatures and reduction of onshore growing-degree days. Alternatively, a rapid flow of meltwater, buoyed on sediment-charged (denser) lakewater, may have kept the lake surface cold in summer. Other factors such as wind-shifted pollen deposition and possible effects from the Younger Dryas North Atlantic cooling could have contributed to the Great Lakes climatic reversal, but further studies are needed to resolve their relative significance.Contribution to Climo Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate ProgramGeological Survey of Canada Contribution 58 890  相似文献   

2.
Postglacial fire history has been reconstructed for eastern Canada from charcoal-influx anomalies from 30 sites taken from a lacustrine charcoal database. The reconstruction exhibits coherent patterns of fire occurrence in space and time. The early Holocene is characterised by high fire incidence. There is a major change to much lower occurrence slightly after 8 ka BP. A return to more fire appears after 3 ka BP. This sequence does not fit with the hydro-climatic reconstruction deduced from lake level reconstructions for northeastern North America, which indicates a dry early and mid-Holocene, and a wet late-Holocene. Fire occurrence however closely matches summer relative humidity inferred from δ18O. The differences between fire frequency and lake level history, are due to changes in the seasonality of precipitation and drought frequency. Lake levels are essentially controlled by winter precipitation while summer precipitation controls fire occurrence. The early Holocene before 8–7.5 ka BP experienced dry summers due to higher solar radiation and dry adiabatic winds from the residual Laurentide Ice Sheet. The middle Holocene was dominated by wet summers due to stability of the Atlantic air mass over eastern Canada. After 2.5 ka BP, summers became drier, albeit not as fire-conducive as during the early Holocene. Late-Holocene summers conducive to fire are explained by more frequent incursions of dry Cool Pacific or Cold Arctic air masses over eastern Canada. Received: 25 January 1999 / Accepted: 14 December 1999  相似文献   

3.
We inferred the Holocene paleoclimate history of the northeastern Yucatan Peninsula, Mexico, by studying stratigraphic variations in stable isotopes (δ 18O and δ 13C) and lithologic properties (organic matter and carbonate content) in sediment cores taken in 6.3 and 16.2 m of water from Lake Punta Laguna. We present a simple model to explain the lithologic and isotopic variations, and discuss the inferred paleoclimate history in terms of its relation to ancient Maya cultural development. We find evidence for lower lake level and drier climate at about the same time as each major discontinuity in Maya cultural history: Preclassic Abandonment (150–250 A.D.), Maya Hiatus (534 to 593 A.D.), Terminal Classic Collapse (750–1050 A.D.), and Postclassic Abandonment (mid-fifteenth century). Although these broad temporal correlations suggest climate played a role in Maya cultural evolution, chronological uncertainties preclude a detailed analysis of climate changes and archaeologically documented cultural transformations.  相似文献   

4.
A previously unpublished record of lake levels from Lake Naivasha, Kenya from 1880 to 1976 has been analysed and shows little similarity to the level record from nearby Lake Victoria. Level changes from year to year of the two lakes show no significant correlation (at 5%) and spectral analysis of the two records shows no common significant peaks. Both lakes show significant correlations between their level changes and the strength of the North Atlantic winter circulation, with the correlation coefficients in opposing directions indicating important, but different, large scale climatic links.Lake Naivasha's major level increases occur during May and September. Lake Victoria's level increases mainly in May with a small December increase. East African rainfall is generally during April and November, corresponding with Lake Victoria's changes. Rainfall records from Kenyan highland areas, however, show an August rainfall peak and little rainfall in November. Rainfall amounts from Equator, a highland meteorological station, for July, August and September are highly correlated (at 1% significance level) with the change in Naivasha's level during September. Winds at the highland stations are westerly during August while the lower level stations experience the drier S.E. Trades. The level changes of Lake Naivasha indicate changes in the extent of the penetration of moist air from West Africa between the Trade winds and the 200 mb easterly jet.  相似文献   

5.
Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt.%Corg, %CaCO3, and % biogenicsilica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the 13C of modern lacustrine organic sources and the 13C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000–13,500 yr BP) and lake water was freshunder the associated outflow conditions. The Pleistocene/Holocene transition (13,500–7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP.Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system.  相似文献   

6.
Glacier hazards threaten societies in mountain regions worldwide. Glacial lake outburst floods (GLOFs) pose risks to exposed and vulnerable populations and can be linked in part to long-term post-Little Ice Age climate change because precariously dammed glacial lakes sometimes formed as glaciers generally retreated after the mid-1800s. This paper provides an interdisciplinary and historical analysis of 40?years of glacier hazard management on Mount Hualcán, at glacial Lake 513, and in the city of Carhuaz in Peru’s Cordillera Blanca mountain range. The case study examines attempted hazard zoning, glacial lake evolution and monitoring, and emergency engineering projects to drain Lake 513. It also analyzes the 11 April 2010 Hualcán rock-ice avalanche that triggered a Lake 513 GLOF; we offer both a scientific assessment of the possible role of temperature on slope stability and a GIS spatial analysis of human impacts. Qualitative historical analysis of glacier hazard management since 1970 allows us to identify and explain why certain actions and policies to reduce risk were implemented or omitted. We extrapolate these case-specific variables to generate a broader socio-environmental framework identifying factors that can facilitate or impede disaster risk reduction and climate change adaptation. Facilitating factors are technical capacity, disaster events with visible hazards, institutional support, committed individuals, and international involvement. Impediments include divergent risk perceptions, imposed government policies, institutional instability, knowledge disparities, and invisible hazards. This framework emerges from an empirical analysis of a coupled social-ecological system and offers a holistic approach for integrating disaster risk reduction and climate change adaptation.  相似文献   

7.
Recently, and perhaps most threatening, Lake Victoria water level has been receding at an alarming rate. A recent study suggested the possibility of the expanded hydroelectric power station in Uganda. However, since the lake receives 80% of its refill through direct rainfall and only 20% from the basin discharge, climatic contributions cannot be ignored, since the 80% water is directly dependant on it. It is therefore necessary to investigate climatic contribution to the declining Lake Victoria water level observed over a long period, i.e., 30 years. This contribution uses 30 years period anomalies for rainfall, river discharge and lake level changes of stations within Lake Victoria basin to analyse linear and cyclic trends of climate indicators in relation to Lake levels. Linear trend analysis using the Student’s t test indicate a decreasing pattern in rainfall anomalies, with the slope being statistically similar to those of water levels at both Kisumu, Maziba and Jinja stations for the same period of time (1976–1999), thus showing a strong correlation. On the other hand, cyclic trend analysis using Discrete Fourier Transform (DFT) shows cyclic period of water level to coincide with those of droughts and rainfall. The strong relationship between climatic indicators of drought and rainfall on one-hand and lake levels on the other hand signifies the need to incorporate climate information in predicting, monitoring and managing lake level changes.  相似文献   

8.
A model of lake ice was coupled with a model of lake temperature and evaporation to assess the possible effect of ice cover on the late-Pleistocene evaporation rate of Lake Lahontan. The simulations were done using a data set based on proxy temperature indicators and features of the simulated late-Pleistocene atmospheric circulation over western North America. When a data set based on a mean-annual air temperature of 3° C (7° C colder than present) and reduced solar radiation from jet-stream induced cloud cover was used as input to the model, ice cover lasting 4 months was simulated. Simulated evaporation rates (490–527 mm a–1) were 60% lower than the present-day evaporation rate (1300 mm a–1) of Pyramid Lake. With this reduced rate of evaporation, water inputs similar to the 1983 historical maxima that occurred in the Lahontan basin would have been sufficient to maintain the 13.5 ka BP high stand of Lake Lahontan.  相似文献   

9.
The East Australian Current (EAC) is the western boundary current of the south Pacific gyre transporting warm tropical waters to higher southern latitudes. Recent modelling shows that the partial separation of the EAC (~32°S) and the coupled formation of the Tasman Front (~34°S) are caused by a steep gradient in the zonally integrated wind stress curl. Analysis of oxygen isotope ratios (δ18O) in the planktonic foraminifer, Globigerinoides ruber, from sediment cores from the Coral Sea and Tasman Sea indicates that the EAC separation shifted northward to between 23 and 26°S during the last glacial. We suggest these results indicate a significant change in the Pacific wind stress curl during the glacial. Given recent evidence for El Niño-like conditions in the Pacific during the last glacial, with a reduction in the east–west sea surface temperature (SST) gradient, we suggest that weaker trade winds combined with more northerly, stronger westerlies were associated with a change to the wind stress curl, which repositioned the EAC separation and Tasman Front. In contrast, by ~11 ka BP, the EAC separation was forced south of 26°S. This southward shift was synchronous with a rapid warming of tropical SSTs, and the onset of a La Niña-like SST configuration across the tropical Pacific. It appears that the south Pacific trade winds strengthened accordingly, causing the EAC to readjust its flow. This readjustment of the EAC marks the onset of modern surface-ocean circulation in the southwest Pacific, but the present EAC transport was only achieved in the late Holocene, after 5 ka BP.  相似文献   

10.
On mudflow hazardous Bashkara lake   总被引：1，自引：0，他引：1  
In this paper, a glacial mudflow hazardous lake, Bashkara, is described. Its depth is 32 m and volume exceeds 494400 m3. Quite intensive deformations and destructive processes at the end of Bashkara glacier are noted. The lake debacle probability is consistently growing. A new way to prevent the glacial mudflow that can be caused by the lake debacle is proposed. This way supposes conducting of the uncontrolled lake discharging by digging, filling of the lake with moraine materials, formatting of the lake surrounding dam, and making a drainage outlet in this dam.  相似文献   

11.
Central Mexico contains a large number of lake basins offering opportunities for climatic reconstruction. The area has, however, also been the focus for human settlement since the time of the earliest occupation of the Americas, as well as being subject to tectonic and volcanic activity. A number of methodological issues arise including the susceptibility of common palaeoecological proxies (pollen, diatoms) to multiple forcing factors and problems of obtaining reliable chronologies. Published lake records indicate that the last 1,500 years have been marked by strong climatic variability, superimposed on continuing high levels of anthropogenic impact. Dry conditions, probably the driest of the Holocene, are recorded over the period 1400 to 800 14C yr BP (ca. AD 700–1200). Climatic change over the last 1,000 years is not well represented, but there are indications of drier conditions corresponding to the ‘Little Ice Age’ of mid- to high latitudes. A range of mechanisms (e.g. solar cycles, ENSO variability) have been proposed to explain climatic variability over the last 1,500 years, but current lake records are inadequate to test these. The developing dendroclimatology for the Mexican highlands and the rich historical archives of the Hispanic period (from AD 1521) offer new opportunities and challenges to palaeolimnologists.  相似文献   

12.
From direct measurements of the vertical turbulent fluxes of momentum, heat, and water vapor over shallow water, the drag and bulk aerodynamic coefficients are computed and compared with those obtained via the profile technique. The results show thatC D =C T =C E =1.2×10?3, with variability among their mean values being ? 20 %. All measurements were taken at 8 m above the water surface at Lake Hefner, Oklahoma. The measurement platform was a rectangular tower located within the lake and approximately 1 km from the nearest shore. The depth of the lake at the tower was 8 m.  相似文献   

13.
The long-term ice record (from 1964 to 2008) of an Arctic lake in northern Europe (Lake Kilpisj?rvi) reveals the response of lake ice to climate change at local and regional scales. Average freeze-up and ice breakup occurred on 9 November and 19 June, respectively. The freeze-up has been significantly delayed at a rate of 2.3 d per decade from 1964 onward (P?<?0.05). No significant change has taken place in ice breakup. Annual average ice thickness has become smaller since the mid-1980s (P?<?0.05). Air temperature during the early ice season significantly affected the ice thickness. The freeze-up date exhibits the highest correlation with the 2-month average daily minimum air temperature centered at the end of October, while the ice breakup date exhibits the highest correlation with the 2-month average daily maximal air temperature centered in mid May. A 1°C increase in the surface air temperature corresponds to a freeze-up later by 3.4?days and an ice breakup earlier by 3.6?days. Snow cover is a critical factor in lake-ice climatology. For cumulative November to March precipitation of less than 0.13?m, the insulating effect of the snow dominated, while higher rates of precipitation favored thicker ice due to the formation of snow ice. Variations in ice records of Lake Kilpisj?rvi can serve as an indicator of climate variations across the northern Europe. The North Atlantic Oscillation (NAO) does not significantly affect the ice season there, although both the local air temperatures and winter precipitation contain a strong NAO signal.  相似文献   

14.
To interpret past vegetation and climate changes from pollen data, we need to reveal the degree of similarity between modern analogues and fossil pollen spectra, which would help us predict the future climate and vegetation. Ninety surface pollen samples across six vegetation zones along an altitudinal gradient from 460 to 3510 m and 44 fossil samples at Caotan Lake were collected in the central Tianshan Mountains, northern Xinjiang, China. Discriminant analyses results, fossil pollen and phytolith assemblages were then used to reconstruct palaeovegetation and palaeoclimate in the area. The 90 surface samples were divided into six pollen zones (alpine cushion, alpine and subalpine meadow, montane Tianshan spruce forest, forest-steppe ecotone, Artemisia desert, typical desert), corresponding to the major vegetation types in the area. These zones follow a climatic gradient of increasing precipitation with increasing elevation. Paleovegetation reconstructed from 44 fossil pollen assemblages through discriminant analysis reflects the regional vegetation shifted from typical desert to Artemisia desert since 4640 cal. year BP in the Caotan Lake wetland. The fossil pollen and phytolith record also reveal the arid climate has not fundamentally changed in the period. But a dry-wet-dry local climate oscillation since 2700 cal. year BP has a fundamental influence on local wetland vegetation dynamics and peat accumulation of the Caotan wetland. Modern wetland landscape and surface pollen assemblages from the Ebinur Lake Wetland Nature Reserve provide further evidence for ferns and Betula growing in the Caotan Lake wetland during the historical period.  相似文献   

15.
 Annual precipitation, July and January temperatures were reconstructed from a continuous Holocene pollen sequence from the Middle Atlas, Morocco, using the best modern analogues method. The reconstructions show a clear difference between the early and late Holocene: from ∼10 ka to ∼6.5 ka the climate was drier and warmer than during the period since 6.5 ka. The average value of annual precipitation was ∼870 mm until 6.5 ka, then rose to ∼940 mm. Between 10 ka and 6.5 ka January and July temperatures were about 4 °C higher than the present. Both temperatures show a marked decrease between 7 ka and 6 ka. After 6.5 ka July and January temperatures fluctuated between 21 and 23 °C, and 2.5 and 5 °C respectively. January temperatures show a period of intermediate values (∼3.5 °C) between 4 ka and 5.5 ka. The reconstructed climate values generally match palaeolimnological data from the same core, which show five intervals of low lake level during the Holocene. They are also consistent with regional-scale COHMAP simulated palaeoclimate that shows contrasting patterns of rainfall variation between the northwesternmost part of Africa and the intertropical band. Received: 7 July 1997 / Accepted: 28 May 1998  相似文献   

16.
Summary The wind regime of the Lake Tekapo Basin is examined with reference to the interaction of multi-scale local, regional and synoptic circulations. Analysis of the historical wind direction record from Mt John identifies airflow from three principal directions to most frequently affect the study area. Both seasonal and diurnal trends in the frequency of each directional category are identified, which reflect the influence of local thermal forcings and seasonal changes in synoptic circulation on the Lake Tekapo windfield. Meteorological observations from a network of automatic weather stations and anemographs within the study area identified Lake Tekapo to generate its own circulation system, a lake/land breeze. This combines with the larger scale valley wind, which during ideal conditions continues after sunset in the upper reaches of the lake catchment. During light to moderate foehn northwesterly conditions, the combined lake breeze/valley wind system remains decoupled from the prevailing synoptic airstream. Towards evening when local thermal circulations weakened, a channelised foehn airstream often becomes dominant over the entire field area. Observations made during this investigation have a number of applied implications with respect to air pollution dispersion modelling and forecasting within alpine lake basins.With 7 Figures  相似文献   

17.
Z. Guo  T. Liu  J. Guiot  N. Wu  H. Lü  J. Han  J. Liu  Z. Gu 《Climate Dynamics》1996,12(10):701-709
Three loess sections in the Loess Plateau of China have been studied to characterize the variations of the East Asian monsoon climate in the Late Quaternary period. Paleo-weathering profiles based on two weathering indices reveal a series of spatially correlative intervals of lower weathering intensity (LW), in the last glacial and late penultimate glacial loess, indicating significantly cooler or/and drier conditions. Most of them are identified at or near the transitional boundaries between loess and soil units, suggesting that climatic conditions favorable for LW events tended to occur at or near major climatic boundaries. Twenty-eight radiocarbon and thermoluminescence measurements, combined with Kukla's magnetic susceptibility age model date these events at ∼14, ∼21, ∼29, ∼38, ∼52, ∼71, ∼135, and ∼145 ka BP, with ∼5–10 ka frequency inlaid within the orbitally induced ∼20-ka periodicity. The ages of the first six events in the last glacial period are therefore close to those of the coarse-grained Heinrich layers in the North Atlantic Ocean, which resulted from massive discharges of icebergs. The results indicate that the climate in the area affected by the East Asian monsoon has experienced high-frequency changes more or less synchronous with the Heinrich events. These high-frequency changes have been reported by Porter and An, based on the grain-size time series from the Luochuan loess section. Our data also reveal that similar events also occurred during the penultimate glaciation. The mechanisms linking the Heinrich events and the East Asian monsoon climate may be similar to that driving the Younger Dryas event which has been identified in a large number of records in China and in the surrounding oceans. Received: 16 October 1995 / Accepted: 10 May 1996  相似文献   

18.
An attempt is made to simulate the Pleistocene glacial cycles with a numerical model of the Northern Hemisphere ice sheets. This model treats the vertically-integrated ice flow along a meridian, including computation of bedrock adjustment and temperature distribution in the ice. Basal melt water is traced and controls ice-mass discharge. The model produces asymmetric glacial cycles, even when it is not forced. Model parameters can be chosen such that cycles with a duration of about 100 000 yr occur. Due to the production of basal melt water and bedrock sinking, deglaciations are very rapid. The occurrence of glacial cycles in the model is a stable feature, but thephase of the cycles is very sensitive to the model parameters. The main conclusion is that ice-sheet dynamics may provide an explanation for the Pleistocene glacial cycles. However, the ‘predictability’ of the ice-volume record appears to be small.  相似文献   

19.
A regional model of the atmosphere (version 4 of the NCAR mesoscale model, MM4) was used to assess whether lake-effect precipitation was a significant component of the late-Pleistocene hydrologic budgets of Lakes Lahontan and Bonneville. Control simulations for January and July of 1979 were made using MM4, and the Pleistocene highstand surface areas of the lakes were added to the model and the simulations repeated. In the January simulations, 18% of the moisture added to the modeled atmosphere by Lake Lahontan returned to the Lahontan basin as precipitation, while 32% of the water evaporated from Lake Bonneville fell as precipitation over the Bonneville basin. In the July simulations, 7% of the moisture added to the modeled atmosphere by Lake Lahontan returned to the Lahontan basin as precipitation, and 4% of the water evaporated from Lake Bonneville fell as precipitation over the Bonneville basin. An additonal January simulation was made with the lake surface areas set at onehalf their highstand extents (the average surface area 20 to 15 ka BP). Results from this simulation were similar to the simulation with the highstand lakes, indicating lake-effect precipitation could have been a significant component of the hyrologic budgets of the lakes before and during the highstand period.The National Center for Atmospheric Research is sponsored by the U.S. National Science Foundation Offprint requests to: SW Hostetler  相似文献   

20.
Observations and numerical modelling of Lake Ontario breezes   总被引：1，自引：0，他引：1  
Abstract

Analysis of two years of land‐based data shows that the Lake Ontario breeze develops on 30% of the days during summer. It typically develops in mid‐morning and persists until the late evening when it is replaced by a well developed land‐breeze regime. Simulations of 4 cases with the Colorado State University mesoscale model show good agreement with observations and suggest that local lake breezes are strongly influenced by adjacent water bodies (e.g. Lake Erie), the elongated shape of the lake, and the large‐scale wind direction. With gradient flows across the long axis of the lake, a broad band of along‐lake flow develops during the afternoon (easterly winds during southerly gradient flows and westerly winds during northerly gradient flows). Furthermore, during west‐to‐northwesterly gradient flow a nocturnal cyclonic eddy is predicted at the western end of the lake. These results imply that wind‐field models applied in the vicinity of Lake Ontario should incorporate the entire lake in their modelling domain.  相似文献   

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