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1.
An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.  相似文献   

2.
Pollen spectra from seven short cores taken from deltaic sites in the central and northern parts of Lake Tanganyika provide information about vegetation changes around the lake during the last 5000 years. Pollen analysis was undertaken to understand the history and timing of catchment deforestation and its causal linkage to excess sedimentation and ecosystem change in Lake Tanganyika. The spectra are dominated by grass pollen at all levels in every core. Grass pollen percentage values range between 40 and 80%. Low values of arboreal pollen taxa (1–20%) were documented from most cores except core LT-98-2M. Core LT-98-2M represents the longest duration vegetation record of this study (close to 5000 years BP), and records the onset of increasingly arid conditions in the Late Holocene, especially after 500 A.D., with the probable replacement of forest by open grassland in the Mahale Mountains region. The pollen/spore content for other cores showed a consistent trend of a decrease in grass pollen and an increase in pteridophyte and forest indicator pollen taxa during the last few centuries, contemporaneous with other indications of increased watershed disturbance from forest clearing (especially isotopes and lake faunal change). The timing or strength of this trend is not tied to specific levels of watershed disturbance. However, increasing fern spore abundance does occur progressively later towards the south, where modern human population densities are lower. Although increasing fern spore abundance is consistent with a land-clearing hypothesis, the rising arboreal pollen percentages are seemingly counterintuitive. One possible explanation is that increasing arboreal pollen proportions reflects the recycling of abundant pollen of this type from rapidly eroding soils. Another likely explanation for this finding is that land clearing involved the replacement of miombo woodland, with its mixture of trees producing little pollen and understory grasses producing large amounts of pollen, by the present day cassava, banana, and legume agricultural systems, all of which are poor pollen producers. This shift in catchment vegetation would increase the relative contribution of Afromontane pollen transported long distances from the surrounding highland regions. This hypothesis is consistent with both the lack of correlation of palynological history with specific watershed deforestation attributes, as well as the broader historical record of human habitation in the Lake Tanganyika region. This study also highlights the need for both modern pollen transect data from the region and comparative cores from low elevation swamps or ponds (wetlands) in the region with smaller catchment areas.  相似文献   

3.
Pollen, geochemical and sedimentological data from Sidi Ali, a montane Moroccan lake, provide a 7000 yr record of changes in climate, catchment vegetation and soil erosion intensity. Diatoms, non-silicious algae, macrophyte fossils and ostracods from the same core record the dynamics of the lake ecosystem. Oxygen isotope and trace-element ratios of benthic ostracods appear to be relatively insensitive to climatic variation in this open lake with low water-residence time, but diatom plankton / periphyton (P/L) ratios show lake-level variations that are probably climate controlled. At least two superimposed processes are recorded, but at different timescales: catchment vegetation and soils show long-term changes due to climate and human impact, whereas P/L ratios suggest century-scale oscillations in lake depth. The timing of changes in algal and macrophyte productivity and carbon cycling within the lake broadly corresponds to changes in terrestrial vegetation, suggesting either that lake nutrient status is linked to catchment vegetation and soils, or that both were influenced by climate. The lack of a sensitive and independent (non-biological) climate proxy makes it more difficult to assess the lake's ecological response to short-term climate variation. Overall, the lake's evolution has been influenced both by catchment-mediated nutrient flux and by changes in water balance, thus having characteristics in common with both temperate and arid zone lakes.  相似文献   

4.
We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area.  相似文献   

5.
A palynological investigation was conducted on two cores with Holocene sediments collected from the northeastern littoral part of the border Lake Doirani in northern Greece. The radiocarbon dates indicated that the analyzed sediments accumulated during the last 5000 yrs. The pollen-stratigraphic record revealed the environmental changes in the catchment area, starting from a natural undisturbed landscape to one modified by increasing anthropogenic influences. The tree vegetation dominated by Quercus woods in the lowlands and byPinus, Abies, and Fagus at higher altitudes, lasted for the period 2900 - 830 cal. B.C. Subsequently it was replaced by xerothermic herb and tree vegetation as a result of intensive human activity - and farming and stock-breeding. The accumulation of sediments with more sand and gravel in historical time was the result of increased erosion.  相似文献   

6.
贵州麦岗水库沉积物的矿物磁性特征及其土壤侵蚀意义   总被引：2，自引：0，他引：2  
李春梅  汪美华  王红亚 《地理研究》2010,29(11):1971-1980
对取自贵州西南部紫云县麦岗水库的沉积物柱芯MG1-1进行了包括SIRM、ARM、χlf、χfd、IRM-20mT和IRM-100mT等磁性参数的矿物磁性测量,并计算了ARM/SIRM、HIRM和IRM-100mT/SIRM。根据沉积物的这些矿物磁性特征,并结合137Cs、粒度、TOC、C/N分析结果,推测了麦岗水库流域在1960～2007年(47年)间的土壤侵蚀变化情况。研究表明:麦岗水库流域的土壤侵蚀强度变化虽然存在一些波动,但整体上呈现由强到弱到强再到弱的变化过程。结合降水资料和土地利用/土地覆被变化情况,探讨了影响土壤侵蚀变化的主要原因。结果表明,降水量对流域土壤侵蚀变化的影响不大,而人类活动,特别是土地利用的变化可能是引起流域土壤侵蚀变化的主要原因。  相似文献   

7.
The Holocene magnetic signature due to environmental change has been investigated in sediments from Sägistalsee, a small alpine lake in the Bernese Alps, Switzerland. The environmental signal in the mineral magnetic parameters of the sediments was compared with palynological and geochemical data. The types of magnetic minerals and their grain size reflect changes in the lake catchment, vegetation, and degree of erosional input. The concentration of the magnetic minerals, as expressed by isothermal and anhysteretic remanent magnetizations also reflect changes in vegetation, but may also be related to redox conditions during sedimentation. Climate influence on the mineral magnetic record has been recognized and interpreted as a consequence of the production of an authigenic mineral with particularly uniform magnetic properties during warmer stages and the influx of heterogeneous detrital magnetic mineral during cooler stages and under increased human activity.  相似文献   

8.
Paleolimnological techniques for assessing recent drainage basin disturbance are evaluated in three Moroccan lakes with catchments contrasted in terms of land-use and vegetation. Rates of sediment accumulation in the two lakes with agricultural catchments were relatively high (>1.6 cm yr-1) in the most recent past. Dilution effects prevented core dating by the 210Pb method alone and post-1953 chronologies were constructed by combining 210Pb and 137Cs data. The recent sediment accumulation rate at the currently least disturbed site, where natural Cedrus forest is still abundant, was relatively low (<0.4 cm yr-1) but has increased since the mid-19th century.Magnetic, geochemical, pollen, and diatom studies of all three lake sediment cores linked with modern field survey data show that soil erosion in the most vegetationally disturbed catchment (Dayat-er-Roumi) has been high throughout the recent past and that intensity peaks are probably associated with wetland drainage operations beginning in the 1940's. At the partially forested site (Dayat Affougah), pre-1950's woodland clearance and other land-use changes are the likely cause of past major soil erosion episodes. The site currently dominated by natural Cedrus forest (Lac Azigza) shows only minor disturbance during the past c. 150 years although a major soil erosion episode occrred in the 17th century.Paleolimnological analysis has clearly demonstrated that major landscape change has occurred at all three sites. However, only at the two sites with catchment cultivation do previously accelerated soil erosion and lake sediment accumulation rates persist to the present. Information essential for formulation of appropriate management plans is presented and the importance of paleolimnology in assessing man-induced lake-catchment disturbance is stressed.  相似文献   

9.
Three sediment cores from two lakes, Fish Lake and Phalarope Lake, in Truelove Lowland, Devon Island, N.W.T. were analyzed for diatoms and chemical composition. Multivariate statistical techniques using a range of chemical variables successfully isolated three sediment groupings in the cores. Allochthonous and autochthonous chemical components in the sediments have been used to reconstruct paleoenvironmental conditions. The two lakes began approximately 10600 years ago as shallow marine lagoons that were isolated from the sea as a result of glacio-isostatic rebound. Based on the presence of distinctive diatom assemblages, the three stratigraphic zones are identified as a basal marine zone, an intermediate and transitional brackish/marine zone and an upper freshwater zone. Following isolation from the sea, the lakes were flushed with freshwater produced by snow and ice melt. In Fish Lake, the period of transition from marine to freshwater, which began approximately 7000 years ago, lasted approximately 800 years. In Phalarope Lake, which was isolated from the sea approximately 5000 years ago, flushing by fresh water was completed only within the last 300 years. Fe, Cr, and Mo in the sediments are associated with the isolation phase when lake sedimentation is sensitive to the presence of brackish water and erosion within the lake catchments. In particular, the precipitation of Mo as MoS2 reflects the presence of hypolimnetic anoxia associated with lake isolation. During the early post-isolation phase the response of lake biota to an influx of nutrients is reflected in an increase in biological silica and organic carbon in the lake sediments. On the other hand, the generally low organic content of the sediments indicates that sedimentation in these lakes has been largely determined by variations in non-biogenic factors through time. During the mid Holocene the progressive stabilization of surface materials within the lake catchments is marked by decreasing Cr, As and Na in the sediments. At the same time, an increase in allochthonous Mn and Fe is attributed to progressive soil development. During the last 2500 years the catchments have experienced decreased erosion resulting in a decrease in both allochthonous clastic input and lake productivity.  相似文献   

10.
Diatoms, pollen, physical and magnetic analyses of the sediments have been used to reconstruct the development over the last 6000 years of Lake Bussjösjön, a small lake in southern Sweden. Stratigraphic variations in a core of more than 15 m reveal changes in diatom assemblages, which correspond closely to changes in pollen, loss-on-ignition, and magnetic measurements that are related to land use and vegetation changes in the catchment. From ca 6000 BP to 2700 BP, a forest surrounded what was then a slightly eutrophic lake. The sudden appearance of Cyclostephanos dubius (Fricke) Round and several epiphytic/epipsammic diatoms at 2700 BP coincides with deforestation of the catchment (2700 BP to 2500 BP). A change in land use from predominantly pasture to arable land from 1300 BP to 1100 BP caused a high level of soil erosion with a decrease of C. dubius and the increase of Stephanodiscus species. An increase of epiphytic/epipsammic species coincides with increased arable farming and the change from a field-rotation to a crop-rotation system, and shows not only an increase in eutrophication but also changes in water depth. The influence of the catchment through time resulted in a smaller, shallower and eutrophic to hypertrophic lake.  相似文献   

11.
On upland Triassic sandstone slopes of the western Blue Mountains, nonswamp, sclerophyllous heath (shrub-dominated vegetation) on shallow soils is commonly found downslope and adjacent to sclerophyllous forest on deeper soils. Some consider heath—and thus shallow soils—as favouring west-facing slopes, which are expected to experience drier microclimates due to insolation, strong and desiccating winds, and severe summer fires. However, our analysis of extensive areas with heath on shallow soils, based on vegetation and topographic maps, and fieldwork of uplands with various degrees of dissection, suggests that aspect is a poor predictor of shallow soils. Rather, shallow soils and heath are found on short slopes and the lower segments of longer slopes with the latter significantly steeper than forested segments.The shallow–deep soil boundary, marked by contrasting modern vegetation structures, does not signify a catchment area threshold, and correspondingly, the vegetation patterns are not in balance with distributary catchment processes, as short slopes are mantled exclusively by shallow soils. Instead, the soil depth boundary represents the propagation of base-level lowering signals, which takes place not only by the headward retreat of knickpoints but also via increased lowering of slope segments adjacent to drainage lines. This leads to steep slopes immediately adjacent to canyons, narrow gorges, and small steep valleys, that are mantled by shallow, discontinuous soils undergoing rapid erosion. These steep slopes persist in the landscape for ≥ 10 My after upland stream rejuvenation until incision of more weatherable Permian sediments, underlying the Triassic cliff-forming sandstones, triggers rapid lateral expansion of gorges. Once shallowly mantled and steeper slopes adjacent to streams are consumed by gorge widening, slopes adjacent to wide gorge clifflines reflect former upland drainage patterns rather than the redirected flow to rapidly widening gorges. Hence, modern vegetation patterns reflect a significant phase of landform development, perhaps combined with enhanced erosion during the Last Glacial Period that is compounded by a humped soil production function on bedrock.  相似文献   

12.
Natural regeneration of abandoned farmland in several areas within the catchments of the Sydney Water Board has been very slow. Several hypotheses have been put forward to explain the fact that natural reforestation has not occurred, even after up to 50 years of abandonment. This study examines regeneration in relation to substrate, soil types and soil chemistry. The abandoned farmlands occur on four main soil types: (i) eutrophic krasnozem, now occupied mainly by bracken fern (Pteridium esculentum) associated with young seedlings of Eucalyptus and Acacia; (ii) mesotro‐phic yellow earth, now occupied by blady grass (Imperata cylindrica), with few tree seedlings and scattered, dead Acacia trees; and (iii) mesotrophic alluvial soil, and (iv) oligotrophic lithosol, which both support stands of Acacia. The soil properties of the krasnozem and the yellow earth appear to be suitable for tree establishment and growth, and do not differ from similar soils in nearby areas currently under cultivation or supporting undisturbed forest. Poor forest establishment in these soils might be the result of frequent fires and biotic factors such as competition from the bracken and blady grass sward. The lack of eucalypt reforestation of the poorer soils might be explained by their poorer fertility, perhaps tolerated better by the Acacia species.  相似文献   

13.
The Holocene diatom and pollen records from Kelly’s Lough have been analysed to determine the timing and extent of the acidification in this upland lake. The pollen data during the early Holocene reflect the typical vegetation changes that occur in sediments throughout Ireland during this period. The diatom record begins by being dominated by circumneutral and acidophilous benthic forms. Later tychoplanktonic Aulacoseira species begin to expand and dominate indicating increased water transparency following the stabilization of catchment soils. Peatland development in the catchment is evident from approximately 6,450 cal year BP. The main change in the diatom assemblages at this time is the decline of Aulacoseira species and expansion of periphytic species. At around 1,450 cal year BP, loss-on-ignition (LOI) values, Calluna pollen and microscopic charcoal all increase suggesting the initiation of a major phase of peat erosion and an increased inwash of organic matter to the lake. Lake acidity changed significantly although the initial acidification is very subtle as indicated by the diatom-inferred pH record. Changes in the diatom assemblages might be largely the result of increasing erosion and inwash of organic matter from the catchment to the lake leading to reduced water transparency and more acidic conditions. The diatom flora remains relatively stable until the mid-twentieth century when more acidibiontic species increase. These diatom changes result in the reconstructed pH curve showing a moderate recent acidification from pH 5.7 to 5.1. About half of the total change in pH took place by around the late 1960s. The lowest diatom-inferred pH value occurs in the late 1970s, and parallels the peak in SO2 emissions in Ireland. Acidic conditions seem to have prevailed in Kelly’s Lough throughout its entire history and alkalinity has been low or absent for much of the time. However, soil acidification and inwash of organic acids from peatlands are not a sufficiently effective mechanism to explain the low pH levels found today in Kelly’s Lough. The effect of acid deposition on the waters of Kelly’s Lough is clear and it has probably caused these already naturally acid waters to acidify further.  相似文献   

14.
Environmental magnetic studies were conducted on a 9.42-m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal year BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000–11,500 cal year BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility (χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500–1,000 cal year BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply (and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1,000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. The results also suggest that different interpretational models may need to be applied to lake sediment magnetic mineral assemblages corresponding to different stages of environmental evolution.  相似文献   

15.
Sediment diatom and chemical analyses of cores from three poorly buffered extra-glacial lakes on the northeastern margin of the Canadian Shield (Cumberland Peninsula, Baffin Island) record interactions between aquatic and terrestrial spheres that were influenced by late Quaternary climatic conditions. Although differences exist between each of the lakes, notably with regards to the intensity of pre-Holocene catchment erosion and the timing of the onset of organic sedimentation, an underlying pattern of lake ontogeny, common to all three lakes, is identified. Although intensified watershed erosion characterized the Late Wisconsinan and Neoglacial cold periods, the lakes nonetheless remained viable ecosystems at these times. Sudden catchment stabilization during the late-glacial to earliest Holocene is associated with incipient organic sedimentation. Lake-water pH increased at this time, likely in response both longer base cation residence times as lake flushing rates decreased, and enhanced alkalinity production from sediment biogeochemical reactions. Subsequently, as the catchments remained stable during the productive early Holocene (c.9–7 ka BP), then gradually received a renewed increase of minerogenic sedimentation, the breakdown of sources of lake alkalinity resulted in natural acidification. Burial of cation-rich mineral sediments and the loss of permanent sedimentary sinks for the products of microbial reduction likely impeded within-lake alkalinity production, and catchment-derived base cations appeared ineffective in curtailing pH declines. The general nature of the Holocene development of these lakes is similar to that observed elsewhere on crystalline terrains, following deglaciation. Our data therefore suggest that catchment glaciation is not a necessary precursor for models of lake development characterized by initial base cation enrichment and subsequent gradual acidification.  相似文献   

16.
Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen assemblages indicate open savannah vegetation (campo cerrado) and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic silica and δ13Corg variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns.  相似文献   

17.
Recent sediments of eight small lakes in the northern winter range of Yellowstone National Park were cored to examine stratigraphic records of past changes in limnology and local environment that might be attributed to grazing and other activities of elk, bison, and other large ungulates. Cores of undisturbed sediment were analyzed at close intervals to depths covering the last 100–150 years according to chronologies established by lead-210 dating. Pollen analyses were made to show change in regional vegetation, and diatom and geochemical analyses were made to reveal possible limnological changes resulting from soil erosion and nutrient input from the lake catchments.Variations in sedimentary components prior to establishment of the Park in 1872 indicate some natural variability in environmental factors e.g., erosional inputs in landslide areas west of Gardiner. All lakes had abundant nutrient inputs.After the Park was founded, fire suppression may have been responsible for small increases in pollen percentages of various conifers and Artemisia tridentata (big sagebrush) at different times in different lakes. Perceptible decreases in pollen of willow, aspen, alder, and birch at different times may reflect local ungulate browsing, although drier climatic conditions may have been a factor as well.The most striking manifestation of accelerated erosion in a catchment was found at a lake located beside a road constructed in the 1930s. In contrast to changes at this site, the record of erosion at other lakes is hardly perceptible. Changes in sediment-accumulation rates seen at most sites result from redistribution of sediment within the lake after initial deposition.In the century following Park establishment, the abundance of planktonic diatoms relative to benthic taxa varies among lakes and may reflect differential nutrient inputs or changes in lake level. Four of the five lakes analyzed for diatoms show in the last few decades an increase in planktonic relative to benthic species, implying elevated nutrient inputs. The recent flora, however, is similar to that in pre-Park levels which suggests that these lakes have not been perturbed outside their normal range. Increased nutrient supply in recent decades for at least two of the lakes is supported by the geochemical data, which show an increase in biogenic silica and in organic matter.As a whole, our investigation of the sedimentary record does not support the hypothesis that ungulate grazing has had a strong direct or indirect effect on the vegetation and soil stability in the lake catchments or on the water quality of the lakes.  相似文献   

18.
The quantity and quality of lignin phenols (Σ8, Λ, S/V, C/V and (Ad/Al)v) in the sediments of three small Lobelia lakes with neither inlets nor outlets were investigated and compared with palynological data and lithology of the profiles. The method of alkaline CuO oxidation was used. Substantial differences with respect to sedimentary lignin concentration and provenance recorded along the profiles and among the sites are in good agreement with pollen data describing the dominant type of vegetation and with indices of soil erosion in the lakes’ catchments. This relation shows that the compositions of lignin degradation products in these lakes are closely related to the local environmental conditions. In all the lakes gymnosperm wood is the main source of lignin products; however, each lake is characterized by different S/V index values. Especially good correlations were obtained between pine pollen proportions in sediments and the S/V index. The correlation between the C/V and (Ad/Al)v indices indicates a higher degradation of organic matter when non-woody tissue is more abundant. This pilot study illustrates the usefulness and potential for a wider application of lignin oxidation products in palaeoecological reconstructions. This kind of data would be of special importance when investigating local presence/absence of woody plants and the role of angiosperms/gymnosperms in local vegetation. Estimates of lignin biodegradation levels, as well as the data on diagenetic processes, may afford supplementary information on possible disturbances in sedimentation.  相似文献   

19.
青海湖流域土壤可蚀性K值研究   总被引：13，自引：1，他引：12  
以土壤亚类为基础,依据青海省第二次土壤普查资料建立了青海湖流域土壤亚类理化性质数据库.使用Wischmeier建立的通用方程计算土壤可蚀性K值,利用三次样条函数插值方法转换不同粒径标准的土壤质地;分别使用土壤有机质含量和土壤最小饱和水力传导率来确定土壤的结构和渗透级别.根据计算结果,建立了青海湖流域土壤可蚀性的分级指标.结果显示,中等和高等可蚀性K值的土壤面积分别占流域土壤总面积的72.1%和15.5%,土壤易于侵蚀.使用Arc/Info对青海湖流域土壤地图进行数字化得到值的K空间分布图,分析了流域可蚀性K值的分布规律及其土壤沙化原因,这对青海湖流域水土保持及流域长期规划具有重要意义.  相似文献   

20.
As in other regions colonised from Europe within the last few centuries, Australia's vegetation and soils have been dramatically changed by clearing, cropping and grazing. In southeastern Australia, particularly on the Southern Tablelands, the impacts of European settlement are clearly manifested by channel incision. By using stratigraphic and documentary evidence, in conjunction with aerial photographs, sediment budgets for the post-settlement period have been constructed to define the fluxes and stores of sediment for each of the major geomorphic components of the 136 km2 catchment of Jerrabomberra Creek near Canberra. Using these budgets, and some plausible assumptions, it has been possible to approximate the history of both the sediment delivery ratio and sediment yield for this catchment. While the quantities estimated in this analysis are approximations, the trends through time are credible. Sediment yield increased rapidly to a peak after European settlement, and has returned to a level between the peak and the pre-European value. The delivery ratio has followed a similar trend. The most general conclusion to emerge is that in this landscape both the total sediment flux and the sediment yield of the catchment have been dominated by channel erosion. This result is contrary to the findings in many parts of the world where sheet and rill erosion dominates the fluxes. The soil conservation implication of these results is clear: to control off-site effects of erosion, the focus must be on the channels.  相似文献   

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