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1.
The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The total inorganic carbon content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the Medieval Climate Anomaly in southeastern Patagonia. At the beginning of the Little Ice Age the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century.  相似文献   

2.
The relationship between surface sediment diatom assemblages and measured limnological variables in fifty eight lake samples from the south Bolivian Altiplano was examined by constructing a diatom-water chemistry dataset. Analysis of this dataset by canonical correspondence analysis revealed that salinity and ionic concentration accounted for a significant amount of the variation in the distribution of the diatom assemblages. Two methods weighted-averaging regression and calibration, and by-class mean percentage table were used to established a transfer function for future reconstruction of past lake water salinity and ionic concentration in the southern Bolivian Altiplano. Weighted-averaging regression and calibration with inverse deshrinking provided a better model for the water chemistry reconstructions in this region.  相似文献   

3.
Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km2 of catchment area and 40 km2 of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. 210Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr–1, which also was confirmed by 137Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (LF), susceptibility of anhysteretic remanent magnetism (ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/LF) had much more information to reveal environmental changes. The ARM/LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.  相似文献   

4.
The study was undertaken as part of a wider palaeoecological investigation of Late glacial and Holocene lake sediments from a site on the exposed Atlantic coast of the Shetland Islands. The diatom data presented here define a sequence of assemblages, commencing at c. 15.8 cal ka BP, which reflects lithological variation in the section, in particular the Late glacial alternation of minerogenic and more organic horizons. Cliff retreat caused drainage of the lake sometime after c. 4.0 cal ka BP. Almost all taxa recorded are small benthic and tychoplanktonic diatoms: Fragilaria (sensu lato), Achnanthes (s.l.) and some Navicula spp. predominate in the Late glacial. Different benthos become dominant in the Holocene, but no plankton developed. Stauroforma was the commonest genus present, and results indicate a relationship between the occurrence of two types, Stauroforma A and Stauroforma B, and the severity of prevailing environmental conditions. The lithology and associated assemblages suggest a sequence including the classic north European Bølling and Allerød' warmer periods, followed by the Loch Lomond Stadial. Subsequently, the temporal diatom succession resembles the pattern described in modern linear transects across the circumpolar treeline in north America and Asia, both in type of assemblage and some dominant species.  相似文献   

5.
The Cenozoic continental sedimentary basins in eastern China are rich in lacustrine source rocks. Based on their paleogeographic location and fossils, these basins can be grouped into inland and near-shore basins. Before the collision of India and Eurasia about 45 million years ago, the inland basins were dominated by arid climates that were unfavorable for the substantial preservation of oil source rocks. In contrast, the contemporary near-shore basins experienced alternating arid and humid climates, probably induced by sea level changes, which produced conditions that favoured the formation and preservation of oil source rocks. With the rise of the Tibetan Plateau, the Asian monsoon was initiated or significantly intensified in the Late Eocene. This, in turn, changed the arid climates in the inland basins to humid or to alternating arid and humid, providing ideal conditions for the formation and preservation of lacustrine source rocks in the inland basins. The evidence suggests that Tibetan uplift played a crucial role in the generation and preservation of the Cenozoic lacustrine source rocks in eastern China.This is the third paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.  相似文献   

6.
Chrysophyte algae produce siliceous resting cysts (stomatocysts) that are becoming an increasingly useful class of paleoecological indicator microfossils. This paper provides a review of the role that stomatocysts play in the life cycle and reproductive ecology of freshwater planktonic chrysophytes. Such information provides paleolimnologists with greater insight into the ecology of the vegetative, planktonic growth phase of species contributing stomatocysts to lacustrine microfossil assemblages. Specific chrysophyte reproductive characteristics discussed include: temporal dynamics of vegetative growth and encystment, cyst induction, cyst survivorship, germination requirements and recruitment strategies. This information serves as an introduction to a special issue of the Journal of Paleolimnology which is dedicated to the Application of Chrysophyte Stomatocysts in Paleolimnology.This is the first in a series of four papers published in this dedicated issue entitled Application of Chrysophyte Stomatocysts in Paleolimnology. Dr. C. D. Sandgren served as guest editor for these papers.  相似文献   

7.
Measurements of anhysteretic (ARM) and isothermal (IRM) remanences in cores 6 and 8 from Big Moose Lake reveal evidence for changes in magnetic mineralogy and grain size within and between the two cores. It is proposed that changes in the strength and demagnetization characterizations of the IRM reflect the accumulation of atmospherically deposited magnetic minerals resulting from industrial processes. The record of magnetite deposition especially in Core 6 parallels that for coal soot at the site. In both cores, the record of haematite deposition parallels that for several anthropogenic indicators.This is the fourth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.  相似文献   

8.
Subfossil chironomid assemblages were used to infer long-term water quality changes in south-central Ontario shield lakes, which are currently being impacted by anthropogenic eutrophication, acid rain, and recent climate change. Using a transfer function developed to infer average end-of-summer volume-weighted hypolimnetic oxygen (avg VWHO), a top-bottom paleolimnological approach was used to reconstruct pre-industrial (pre-1850) deepwater oxygen conditions. Comparison with present-day (top surface sediments) chironomid-based inferences of avgVWHO results suggest that hypolimnetic oxygen levels are presently similar to natural, pre-industrial (bottom sediments) levels in most lakes. Approximately half of the study lakes recorded an increase in hypolimnetic oxygen since the 19th century. Inferred changes in avgVWHO correlate well with our results from another chironomid-based oxygen model which reconstructs the Anoxic Factor (AF). When study lakes are separated according to their hydrological status (i.e., natural versus managed), lakes with a dam at their outlet and seasonally managed lake levels had significantly different changes in avgVWHO compared to lakes with natural hydrology. Lakes with a dam at their outlet generally recorded increases in avgVWHO, while natural hydrology lakes mostly recorded declines. There was no relationship between inferred changes in avgVWHO and the density of cottage and resort development along the shorelines of lakes. Changes in dissolved organic carbon (DOC) possibly related to recent climate changes may also be affecting deepwater oxygen, however patterns of change are very subtle.  相似文献   

9.
Argentine literature from different disciplines dealing with lake sediments is presented in chronological order and briefly discussed. Quaternary and pre-Quaternary deposits are considered. Papers reveal five periods of production and three major regions of paleolimnological research.This is the first in a series of papers published in this issue on Paleolimnology in Southern South America. Dr C. A. Fernández served as guest editor for these papers.  相似文献   

10.
Sedimentation rate data of Engstromet al. (1991) were analyzed in relation to the elevation of the eight ponds studied. The ... asynchroneity of the stratigraphic changes among the lakes ... that they noted was transformed into a pattern of time-transgressive change of sedimentation rate with elevation. Sediment units representing increasing (and decreasing) sedimentation rate and accelerating (and decelerating) sediment accumulation were correlated from pond to pond, suggesting that the sediment-producing condition somehow moved upslope over time at a rate of 3.4 to >5 m yr–1. Climatic data appear not to explain this monotonic pattern, but dendrochronological data and elk population numbers suggest that ungulate foraging patterns might. Engstromet al. (1991) concluded that ...the lake-sediment records in the northern range do not convincingly show systematic direct or indirect effects of ungulate grazing during the history of the Park. Reinterpretation of these sediment data suggests that detailed reanalysis might lead to a different conclusion.  相似文献   

11.
As part of English Natures Lakes Flagship Project to address adverse environmental impacts on selected, important lakes, a proposal has been made to dredge Aqualate Mere. The site has experienced rapid, recent sedimentation thought to be derived from a nearby canal. The aim of this study has been to determine the recent sedimentation history of the site in order to assess the possibility of the disposal to land of its sediments and the efficacy of this form of lake restoration. A predominantly clayey silt layer was found across the lake beneath which darker, organic-rich sediments were noted. This transition may represent the input of canal-derived sediments, although it may reflect other environmental changes at this time. The radiometric dating technique employed was unable to date this sediment boundary. A further change in the characteristics of the upper part of the clayey silt layer may represent an additional influence of the canal. Heavy metal levels were modest, whereas nutrient levels were relatively high and some pesticides were detected. Topsoil erosion supplying nutrients and other compounds associated with agriculture have been an important source of the lower layers of the clayey silt sediments in particular. The highest levels of most pollutants were found in the finer sediments in the uppermost (post-1950s) part of the sediment profile. These sediments appear to reflect a change in the characteristics of the sediments of the canal, which was associated with a change in the nature of its water supply. The key geochemical properties of the sediments should not preclude the land-based disposal of dredged materials under current UK regulations for waste management. Accurate estimation of sediment quantities was limited, as the interface between the recent and underlying sediments was not positively identified at all sample points.  相似文献   

12.
Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and 18O and 13C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.  相似文献   

13.
The utility of classifying chrysophyte stomatocysts by their characteristic honeycomb and ridge patterns is questioned, because a strikingly similar expanding pattern appears on the surface of ionized polymer gels during osmotical swelling as a result of simple physical forces. The rapid accumulation of silicate into a spherical cyst inside a chrysophyte cell appears to be as a physical process sufficiently similar to result in an analogous pattern in microscopic scale. Chrysophyte stomatocysts that possess honeycomb or ridge patterns could be regarded as frozen moments of the pattern evolution during the silicate gel phase. As a consequence, such structures should have little taxonomical value.  相似文献   

14.
The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have 13C and 18O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high 13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in 13C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The 13C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to 13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of 18O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the 18O variability.  相似文献   

15.
Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Noril'sk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to 18O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water 18O, providing a signal that is inherently more representative of average thaw season lake water 18O than the measure of instantaneous 18O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from calibration samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.  相似文献   

16.
We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 14C yr BP), pollen data suggest moist conditions, but high 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in 18O values and associated change in ostracod species at 6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 14C yr BP may indicate early human disturbance. By 2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.  相似文献   

17.
Wallywash Great Pond (17° 57 N, 77° 48 W, 7 m a.s.l.) is the largest perennial lake in Jamaica. It occupies a fault trough within the karstic White Limestone. The Great Pond is a hardwater lake with a pH of 8.2–8.6 and an alkalinity of 3.6–3.9 meq 1–1. Its chemistry is strongly influenced by the spring discharge from the limestone. The lake water is subject to degassing, evaporation and bicarbonate assimilation by submerged plants and algae, resulting in marl precipitation. A 9.23 m core (WGP2), taken from a water depth of 2.8 m, was analysed for magnetic susceptibility, loss-on-ignition, carbonate content, mole % MgCO3 in calcite, and stable isotopes in the fine carbonate fraction. The chronology is based on ten14C and four U/Th dates. Four main sediment types alternate in the core: marl; organic, calcareous mud; organic mud or peat; and earthy, brown, calcareous mud. The marls represent periods of wet/warm climate during sea-level highstands and the organic deposits, shallower, swampy conditions. In contrast, the brown, calcareous muds were laid down when the lake was dry or ephemeral. The last interglacial (120 000- 106 000 yr BP) is represented by three distinct marl units. After a dry interval, stable, wet/warm conditions set in from 106 000 to 93 000 yr BP. A dry/cool climate prevailed between 93 000 and at least 9500 yr BP. Three subsequent cycles of alternating wet and dry conditions culminated in flooding of the basin by the Black River during the late Holocene. These recent events cannot be accurately dated by14C due to significant and temporally-variable inputs of dead carbon from the springs.  相似文献   

18.
With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.  相似文献   

19.
We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (18Op) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (18Oc). Comparison of the calcite oxygen isotope proxy for 18Op in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This 18Oc/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite 18O values since the 1960s. Possible explanations for this recent 18Oc shift, as it is not related to mean annual air temperature, include changes in 18Op due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in 18Oc data.  相似文献   

20.
A portion of a sedimentary basin is subdivided conceptually into hexagons of equal area. The area of each hexagon is equal to the minimum area an oil field should have to be commercial. Hexagons can be full of oil or empty. A field size 1 consists of a cell with oil surrounded by six empty cells; a field size 2 consists of two adjacent cells with oil surrounded by eight empty cells, etc. Principles of Percolation Theory are used to determine the probability distribution of the areas of the oil fields existing in this portion of the basin. The only piece of information necessary to determine this probability distribution is the Success Ratio (number of successful exploration wells/total number of exploration wells drilled in this portion of the basin). This approach has several practical applications.A probabilistic model is introduced to predict to which extent potential oil traps are filled with oil. The model assumes that the probability that an oil unit will end up in a particular trap, is proportional to the surface area of the trap. The model predicts that independently of the distribution of the trap volumes, there will be a critical trap volume. All the traps having a volume less than this critical volume, will be filled to spill point. An equation is deduced to predict, for all traps having a volume greater than the critical, the volume of oil that can be encountered in the trap, provided the volume of the trap is known.  相似文献   

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