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1.
在广东省东南部惠州市的黄洞水库中取得沉积物柱芯,对其样品做了210Pb 和137Cs 比活度分析、粒度分析和孢粉及炭屑分析,并收集了这一水库邻近气象站的降水数据,计算了1960 年以来的年降水量、夏季（6-9 月） 降水量和24 小时暴雨量。利用上述210Pb和137Cs比活度分析结果建立了沉积物剖面的年代框架,结合孢粉浓度、炭屑浓度与降水量计算结果,对沉积剖面的粒度变化做出了解译,并初步推测1960-2009 年间水库所在流域的土壤侵蚀变化情况及其原因。研究表明,在该亚热带丘陵地区植被覆盖度较高的自然条件下,侵蚀作用对降水量波动和植被（尤其是乔木、灌木等木本植物） 覆盖度变化的响应十分灵敏,水库沉积物的粒度变化在一定程度上可以反映流域内土壤侵蚀相对强度的变化。  相似文献   

2.
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3.
Using the USPED (Unit Stream Power Erosion Deposition) model, three land use scenarios were analysed for an Italian small catchment (15 km2) of high landscape value. The upper Orme stream catchment, located in the Chianti area, 30 km south of Florence, has a long historical agriculture record. Information on land use and soil conservation practices date back to 1821, hence offering an opportunity to model impacts of land use change on erosion and deposition. For this study, a procedure that takes into account soil conservation practices and potential sediment storage is proposed. The approach was to calculate and model the flow accumulation considering rural and logging roads, location of urban areas, drainage ditches, streams, gullies and permanent sediment sinks. This calculation attempts to assess the spatial variability, especially the impact of support practices (P factor). Weather data from 1980–2003 were taken into account to calculate the R factor. However, to consider the intense pluviometric conditions in terms of the erosivity factor, the 0.75th quantile was used, while the lowest erosivity was modelled using the 0.25th quantile. Results of the USPED model simulation show that in 1821 the mean annual net erosion for the watershed was 2.8 Mg ha− 1 y− 1; in 1954 it was 4.2 Mg ha− 1 y− 1; and in 2004 it was 5.3 Mg ha− 1 y− 1. Conservation practices can reduce erosion processes by ≥ 20 Mg ha− 1 y− 1 when the 1821 practices are introduced in the present management. On the other hand, if the support practices are not considered in the model, soil erosion risk is overestimated. Field observation for the present-day simulation confirmed that erosion and associated sediment deposition predicted by the model depend, as expected, on geomorphology and land use. The model shows limitations that are mainly due to the input data. A high resolution DEM is essential for the delineation of reliable topographic potential to predict erosion and deposition especially in vineyards.  相似文献   

4.
Reservoir sediments are rarely used as environmental archives because of the potential for sediment disturbance by fluctuating water levels. However, rapid rates of sedimentation, proximity to urban centres and often the existence of management records, may make them potentially important resources for reconstructing recent, anthropogenically-derived environmental change. This project assesses the potential of reservoir sedimentary records for reconstructing past atmospheric and drainage basin fluxes of heavy metals (manganese, iron, nickel, copper, zinc and lead) in the southern Pennines, UK. Five reservoirs were selected on the basis of management history and drainage basin characteristics. Multi-parameter analysis showed sediments to be replicable across the ȁ8accumulating zone’ with reasonably consistent rates of sedimentation. Water level fluctuations were not found to detrimentally affect sediment records in the deepwater area of the reservoirs. In fact, spheroidal carbonaceous particle (SCP) profiles show trends in inputs that closely reflect major changes in industrialisation, indicating the reservoir sediments to be excellent records of particulate inputs. Only lead (Pb) and zinc (Zn) were significantly enriched in the reservoir sediment in comparison to background levels. Manganese (Mn), iron (Fe) and to a limited degree, copper (Cu), appeared to be affected by post-deposition mobility. Preliminary calculations of Pb fluxes indicate that over 80% of the current Pb input to the reservoirs is from Pb deposited onto drainage basin soils in the past, rather than from direct atmospheric deposition or natural background inputs. In Howden reservoir, for example, the total Pb flux to deepwater sediment cores in 2000 was 119 mg m−2 a−1. Of this, an estimated 99 mg m−2 a−1 was from anthropogenically-derived Pb, initially deposited onto drainage basin soils and subsequently entering the reservoir via erosion and leaching processes. There is, therefore, no indication that the flux of Pb to the aquatic system is declining in response to reductions in Pb deposition. The ecotoxicological effects of the high and continuing Pb flux to these reservoirs, despite recent decreases in atmospheric deposition, is an area requiring further investigation.  相似文献   

5.
Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the 137Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year−1 for the period from 1964 to 2002 with an average of 0.85±0.41 cm year−1. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year−1 and an average of 1.29±1.04 cm year−1. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain.  相似文献   

6.
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   

7.
A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km− 2 year− 1. Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY = a × e−b C) which was found between vegetation cover and sediment yield at the catchment scale (103–109 m2), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–103 m2). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates.  相似文献   

8.
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   

9.
Reservoirs provide important water resources and require careful management, through ecological monitoring, to identify and mitigate changes in water quality. Long-term data on vegetation changes and the impacts of human activities on reservoir water chemistry, however, are often limited. Setting restoration targets can therefore be problematic. Palaeoenvironmental research has made little use of reservoir sediments and there is great potential for palaeoecological data to be incorporated into management planning. Diatoms and pollen were analysed in sediment cores from Venford Reservoir, southwest England, to infer pH and land-use changes, respectively, over the last century. Diatom-inferred (DI) pH indicates that reservoir pH declined from ~pH 6.0 in the early part of the record and reached a low between AD 1920 and 1940 (~pH 5.6), which was likely associated with fossil fuel combustion and acid deposition. DI-pH then increased, but values remained relatively low, even in the most recent sediments (~pH 5.7), and the magnitude of inferred pH change over time was small. Land-use changes, such as increased grazing intensity and erosion, and establishment of pine plantations, also likely influenced reservoir water chemistry changes over time. Understanding the impacts of such factors on water chemistry has implications for future catchment land-use planning, which is essential for managing water resources. The pollen record indicates a shift from heather-dominated to grass-dominated vegetation since ~AD 1935–1950, which could be related to increased grazing intensity. The palaeoecological dataset is valuable as a long-term record against which short-term monitoring datasets and future changes can be assessed.  相似文献   

10.
Sediment cores collected from eight lakes along the western coast of Svalbard as part of a project investigating atmospheric pollution and environmental change in Arctic regions were dated radiometrically using natural (210Pb) and artificial (137Cs and 241Am) fallout radionuclides. At four sites the sedimentation rates were relatively uniform, and in consequence the 210Pb dates were relatively unambiguous. At the remaining sites there were irregularities in the 210Pb activity versus depth profiles, indicating significant variations in the net sediment accumulation rate during the past 120 years. At these sites, there were significant differences between 210Pb dates calculated using the two standard simple dating models, constant rate of supply (CRS) and constant initial concentration (CIC). In most cases, stratigraphic dates based on the 137Cs and 241Am records supported use of the CRS model, though at one site (Ossian Sarsfjellet) the CIC model appeared more appropriate. The irregularities in the 210Pb records were mainly caused by episodes of accelerated sedimentation due, for example, to inwash or slump events, though at some sites there appears to have been a systematic increase in sediment accumulation rates in recent decades. Sediment accumulation rates were generally lower at the northern sites, and higher at the more southerly locations. Mean sediment accumulation rates varied by an order of magnitude, ranging from 0.002 to 0.050 g cm−2 y−1 (0.02–0.10 cm y−1). 210Pb fluxes measured from the core inventories were mostly in the range 34–80 Bq m−2 y−1 typical of Arctic sites. Much higher values, recorded at two sites (Birgervatnet and Daltjørna), may be due to significant inputs from the catchments during spring thaw.  相似文献   

11.
Pollen and spores with resistant exines are preferentially preserved in soils, and during periods of soil erosion they can become incorporated into lake sediments. As a result, the contemporary vegetation may be poorly represented by the palynomorphs in the lake sediments because of the reworked component of inwashed pollen and spores. We record the proportion of palynomorphs with corroded exines in sediment cores from four lakes in the eastern North Island of New Zealand to document changing sources of palynomorphs over the last 2000 years. During this period, the catchments experienced major vegetation disturbances, both natural (from volcanism and fire) and anthropogenic including deforestation ca. 600 years ago, and the European conversion of fern-scrubland to pasture in the 19th century. Corroded palynomorphs are more abundant in inwashed sediments than authigenic sediments. Catchment soil disturbance was minor during the forested period, and characterised by small, inwashed, sediment pulses after storms, and a relatively low percentage of corroded palynomorphs. Although initial Maori forest clearance by fire led to a temporary increase in erosion in one lake catchment, rapid replacement of forest by a dense bracken fern cover helped to minimise soil erosion and reworking of palynomorphs in this period. European pastoralists replaced the bracken fern with shallow-rooted pasture grasses about 150 years ago. In erosion prone lake catchments, this led to a rapid increase of inwashed eroded soils and littoral sediments, and their component of resistant palynomorphs, reaching the lake sediments. As a result, the palynological records from these catchments during the European period are distorted by reworking. By contrast, over the same period, the palynological record from a lake with no inflowing streams and stable catchment soils more faithfully represented the contemporary vegetation cover. Exine corrosion has been used to help identify periods of reworking in the lake sediments and to allow for a correction of distortion caused by reworking.  相似文献   

12.
This study contrasts the effects of human disturbances on two very different lake/catchment systems: Hatch Pond and Beaver Lake, New Hampshire. Hatch is in a steep mountain catchment remote from urban/industrial centers, and is a small, relatively deep, primarily seepage lake with moderate flushing rate. The catchment has been continuously forested, but disturbed by partial loggings and a forest fire. Beaver’s more extensive and gently sloping catchment is much closer to urban/industrial centers, and has been agricultural and recently suburban. Beaver is a larger but shallower drainage lake with much more rapid flushing than Hatch. We compared the sedimentary records and inferred limnological responses to catchment land uses and air pollution inputs of these mesotrophic lakes, and interpreted differing responses as a function of basic differences between the lakes and their catchments. Some chemical and biological variables in the sediment of both lakes respond sensitively to the first disturbances of the catchments by Euro-American settlers. Diatom-inferred Cl at both lakes indicates a modest increase of salinity at this time. Hatch sediment contains a record of soil erosion starting 1810 with first logging and fire. For ~125 years, erosion was largely of upper eluviated soil, but by 1935 it reached deeper into the illuviated (enriched) horizon. Maximal lake trophic state based on diatom-inferred limnological variables occurred in the mid-1900’s period of maximum sedimentation of illuviated soil. The sediment record since 1964 reflects forest maturation, and soil stabilization and acidification, but not lake acidification. At Beaver, the limnological effects of forest clearance and agriculture starting ~1700 were relatively mild due to gentleness of slopes and soil characteristics favoring stability. A near doubling of earth elements, and major increases in trace metal pollutants and coal combustion particles (CCP) occurred in sediment dating around 1900. Landscape analysis as well as historical and sedimentary records preclude the catchment as the major source of these increases. The most likely source is atmospheric fallout of CCP with its metal load. Despite controls on CCP and other emissions starting 1960, concentrations of earth elements and trace metals in Beaver sediment remain high due to soil mobilization by residential development of the catchment. The trophic state of Beaver Lake has increased, but the relative increase has been smaller than at Hatch, despite more intensive land uses and greater aerial inputs at Beaver. We propose that this lesser eutrophication is due to rapid flushing of Beaver Lake with stream water from relatively undeveloped parts of the extensive catchment, and lower sensitivity to nutrient inputs of this naturally more eutrophic lake. A major shift from unicellular to colonial chrysophytes at Hatch starts in the late-1800’s, and at Beaver in the early 1900’s. Colonial taxa in lesser quantities appear at the time of first settlement of the Beaver catchment. At both lakes the shifts are correlated with increasing metal fluxes from the opening of catchment biogeochemical cycles, as with the aforementioned erosional sequence at Hatch. But at Hatch aerial inputs of trace metals, and at Beaver aerial inputs major and trace metals associated with CCP are also correlated with the major chrysophyte shift. It appears that the chrysophytes have responded to stressors associated with both catchment disturbance and regional atmospheric inputs.  相似文献   

13.
Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km2 gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using 137Cs and 210Pbex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.  相似文献   

14.
Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide 210Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout 210Pb, 137Cs and 239+240Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of 210Pb in the sediments derives from erosive inputs from the catchment. For 239+240Pu the figure rises to 61%. Reduced amounts of 137Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for 210Pb and 239+240Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of 239+240Pu from the catchment to the lake.  相似文献   

15.
Lake sediments can be significantly impacted by industrial activities. These impacts vary among sites and include both local point sources and atmospherically-derived pollution. Here we present results of a lake sediment pollution record from Ullswater, UK, where lead mining activities have taken place within the catchment since 1690, although large-scale mining did not begin until 1840. Metal concentration data from 12 cores taken along a lake bed transect illustrate that lead mining at Greenside Mine had a significant impact on the lake sediments. High Pb concentrations were identified throughout Ullswater, and exhibited a spatial gradient from south to north, with concentrations decreasing with greater distance from the main source of input at Glenridding. Furthermore, inter-element correlations exhibit spatial variation that reflects the processes by which they are incorporated into the lake sediment record. Together, these observations illustrate potential shortcomings in palaeoenvironmental reconstructions and pollution studies based on single cores from large and morphologically variable lakes. Sedimentation rates were estimated by matching this pollution record with the historical record of mining activities. Within Ullswater, sedimentation rates from 1840 to the present varied from 0.67 to 2.33 mm year−1 with a mean of 1.4 mm year−1; highest sedimentation rates were observed in the deepest section of the lake and close to the main inputs, and lowest sedimentation rates were observed in the northern part of the lake, furthest from the main input. Despite the considerable changes in mining techniques and production, there is little evidence to suggest significant changes in sedimentation rates over time.  相似文献   

16.
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   

17.
《Geomorphology》2007,83(1-2):183-192
Wind-splash is a process in which wind and rain combine to cause soil erosion. In upland Britain, the conditions necessary for wind-splash erosion are relatively common and frequently occur in locations where blanket peat is an important land cover. A typical location is Moss Flats (North Pennines, northern England, UK). Wind-splash processes were monitored intensively at this site over 3 months using a circular configuration of mass flux sediment samplers, and meteorological data logged from an on-site automatic weather station. Maximum peat flux rates were measured between south-southwest and west-northwest directions in association with relatively moderate intensity, frontal rainfall, typically 4–6 mm h− 1. Wind-splash processes operate in any direction due to changeable synoptic weather patterns. Windward peat fluxes were typically 2–13 times greater than those recorded at leeward orientations. Spatial patterns of erosion are reflected in the wider landscape through the development of small-scale, erosional landforms (peat hags), which frequently display preferred orientations within the range of maximum peat flux. It is suggested that wind-splash may be a more important process of peat erosion than hitherto reported in UK upland areas.  相似文献   

18.
David J. Milan 《Geomorphology》2012,138(1):319-328
Quantitative assessments of the impacts of extreme floods on channel morphology are rare. Real Time Kinematic (RTK) GPS surveys of a 500-m reach of the Thinhope Burn, an upland gravel-bed stream in the UK, taken in 2003 and 2004 permitted an assessment of geomorphic work whilst the channel was at steady-state. A large flood that occurred on 17 July 2007 resulted in a catastrophic impact to the Thinhope Burn valley floor. The reach was re-surveyed after the event in 2007, and again in 2008 and in 2011. Digital elevation models were produced from the survey data, which allowed the spatial patterns of erosion and deposition and volumetric changes between surveys to be established. A total of 5202 m3 of deposition and 2125 m3 of erosion was recorded in the reach following the flood event. Field walking of the catchment and comparison of aerial photographs for 2003 and 2007 suggested that most of the material mobilised had originated from existing sediment held in terraces and paleoberms on the valley floor. Although slope failures were evident, including peat slides in the headwaters, delivery of sediment from coupling zones to the channel was thought to play a secondary role in the geomorphic response shown by the channel. Similarly, large volumes of erosion and deposition were found after resurveys in 2008 and 2011, suggesting that the system was still in its relaxation phase. The results obtained in this investigation coupled with historical information on the flood history of Thinhope Burn dating back to 1766 suggested that rare large floods are the geomorphically effective flows in the catchment.  相似文献   

19.
Geochemical analysis of six radiometrically dated short cores of recent sediment from Lake Baikal shows clear evidence of enhanced Pb supply. However, the sediment concentration increases are very small; the average Pb concentration rises from a baseline value of 10.9 g g-1 to a peak value of only 14.8 g g-1. In contrast to the more polluted lakes commonly studied in Europe and North America, variation in Pb concentration is far more strongly influenced by natural variation than by pollution. In sediment deposited over the last 150–200 years 73% of the variance in the sediment Pb concentration can be accounted for by variation in bulk composition of the sediment, and by atmospheric pollution. Factors influencing Pb concentrations over this time period are, in order of decreasing average importance (fraction of total variance explained), catchment supply (indicated by 226Ra activity variation) (43%), anthropogenic Pb emissions (24%), and dilution by ferromanganese hydroxides (6%). On longer (1000s of years) time scales dilution by biogenic silica is probably more important.The recent enhanced supply of catchment Pb correlates with accelerating accumulation rates, indicating a link with enhanced erosion. Anthropogenic sources dominate only in the southern basin, where local fossil-fuel burning industry is situated. The evidence for a local industrial source for the Pb pollution is strengthened by the high correlation between the inventories for Pb and for spheroidal carbonaceous particles. The absence of detectable anthropogenic Pb enrichment in the northern part of the lake suggests that long-distance Pb pollution is small compared with the local natural supply.  相似文献   

20.
Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the past c. 190 years. A reliable chronology is available from AMS 14C and 210Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent nuisance algal blooms in 1933, 1934 and 1939 using CuSO4 addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the 210Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years. The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually allowed unambiguous identification of the signatures of change and their causes.  相似文献   

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