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1.
Spheroidal carbonaceous particles (SCPs) from the high temperature combustion of fossil-fuels are stored in lake sediments and provide an unambiguous record of industrially derived atmospheric contamination. It has been assumed that a single sediment core provides an accurate representation of SCP accumulation but to date there have been no studies to determine within-lake variability of the SCP sediment record. This paper describes the SCP profiles of ten sediment cores taken from the deep water area of Loch Coire nan Arr, a remote lake in north-west Scotland, UK. Although each core shows the basic SCP profile used for sediment dating in the UK there is considerable variation between cores. The conversion of SCP concentrations to cumulative percentages resolves a great deal of this variation with the result that more accurate cross-correlation and hence date allocation is possible, especially in the post-1945 section of the cores. However, significant departures from the usual SCP profile still reduce the effectiveness of this approach. It is concluded that: (i) SCP profiles from single cores from the deepest areas of the lake usually provide an accurate representation of the historical record of atmospherically deposited pollutants and that inter-core variability or noise does not affect the temporal interpretation of that profile, and (ii) comparisons of total SCP inventories are a better way of comparing historical deposition between sites as the impact of temporary variability is considerably reduced.  相似文献   

2.
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   

3.
We present 137Cs profiles for three low lying coastal lagoons in Southwest England that show a decline in activity with sediment depth. 137Cs inventories are lower than expected by comparison with local reference inventories despite the fact that sampling was undertaken in the deep-water zone of each lake where sediment and 137Cs focusing would be expected. At all three locations, lake sediment 7Be and unsupported 210Pb (210Pbun) inventories are not significantly lower than the local reference inventory. 137Cs inventories in the study cores range from 38 to 95% of local reference inventories. The standing water level and mud: water interface at two sites are below maximum tide level and, at all three sites, salinity increases significantly in the water columns between low and high tide and in the pore waters of the underlying sediments. We suggest that the difference in hydrostatic pressure between sea level and standing water levels in the lagoons forces salt water up through the sediment column and that monovalent cations (especially Na+ and K+) replace 137Cs on exchange sites leading to the upward migration and loss of 137Cs. Rising sea levels may therefore contribute to remobilisation and release of 137Cs to the aquatic environment from the sediments of coastal lagoons.  相似文献   

4.
Lead-210 assay and dating are subject to several sources of error, including natural variation, the statistical nature of measuring radioactivity, and estimation of the supported fraction. These measurable errors are considered in calculating confidence intervals for 210Pb dates. Several sources of error, including the effect of blunders or misapplication of the mathematical model, are not included in the quantitative analysis. First-order error analysis and Monte Carlo simulation (of cores from Florida PIRLA lakes) are used as independent estimates of dating uncertainty. CRS-model dates average less than 1% older than Monte Carlo median dates, but the difference increases non-linearly with age to a maximum of 11% at 160 years. First-order errors increase exponentially with calculated CRS-model dates, with the largest 95% confidence interval in the bottommost datable section being 155±90 years, and the smallest being 128±8 years. Monte Carlo intervals also increase exponentially with age, but the largest 95% occurrence interval is 152±44 years. Confidence intervals calculated by first-order methods and ranges of Monte Carlo dates agree fairly well until the 210Pb date is about 130 years old. Older dates are unreliable because of this divergence. Ninety-five per cent confidence intervals range from about 1–2 years at 10 years of age, 10–20 at 100 years, and 8–90 at 150 years old.This is the third 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   

5.
Short core stratigraphy of total N, P, Zn, Hg and diatoms were investigated in the varved sediment of Lake Heinälampi, a small 8 m deep pond in East Finland. The sediment has recently changed from organic mud to minerogenic silt. The presence of varved sediment offers independent control for 210Pb and 137Cs dates and these are in good agreement: the sediment quality initially changed around the year 1930 due to increaed field erosion, and the inflow of mineral matter was further increased by ditching of paludified forests in 1980.Diatom analysis reveals that eutrophication has occurred before and concurrent with the sedimentary change. The causes of eutrophication and the increased levels of Zn and Hg are discussed.  相似文献   

6.
We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to immobilize Cs.This is the ninth of a series of papers to be published by this journal following the 20 th anniversary of the first application of210Pb dating of lake sediments, Dr. P. G. Appleby is guest editing this series.  相似文献   

7.
There is an important volume of published information on Lac Dufault (Québec) which describes the history of metal inputs over 70 years and the changes that occurred in the lake as a result of this contamination. We used this abundant source of chronological markers to test the hypothesis that lake sediments can provide true historical records of trace metal loading from metal mining. Sediment cores were obtained from the deepest zone of the lake (19 m). The sediments were dated using 210Pb and 137Cs and they were analyzed for total elemental concentrations (Ca, Cd, Cu, Fe, P, Pb, S, Zn). Metal profiles in the sediment core preserved the distinct signatures of different mine exploitations documented in the lake watershed. In particular, the core recorded: the beginning of industrialization in 1926; increasing sedimentation rates associated with perturbations in the lake watershed; the maximum of sediment [Cu] and the contemporary exploitation of ore bodies rich in Cu; the maximum of sediment [Fe] and the contemporary production of pyrite by a mining operation; the low sediment [Cu] and [Zn] in the 1950s and the low contemporary production of these metals by mining operations; the maximum of sediment [Cd] and the contemporary production of Cd by a subsequent mining operation. Anomalies in the distribution of 214Pb activities in sediments reflected the intensity of acid mine drainage (AMD). There is good evidence that the lake resisted acidification from this AMD for the last thirty years. Overall, our results support the thesis that profiles of sedimentary Cd, Cu, Zn, Pb and Fe levels reflect the past history of metal input to Lac Dufault.  相似文献   

8.
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   

9.
This paper reports results and analysis of210Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for210Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.210Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific210Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported210Pb inventories in sediments, but the relationship does not occur in seepage lakes.210Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.210Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of210Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported210Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.This is the third of a series of papers to be published by this journal following the 20th anniversary of the first application of210Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.  相似文献   

10.
Shallow basins in the savannas of Peten, Guatemala filled with water after 305±55 BP (calibrated age+1430–1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low 210Pb flux rates to Chimaj (0.085 pCi cm-2 yr-1) and Chilonche (0.134 pCi cm-2 yr-1) are attributed to low 222Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm-2 yr-1 and 0.047 g cm-2 yr-1 respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Peten's vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.  相似文献   

11.
We have measured the distribution coefficient (Kd) of210Po and210Pb in laboratory systems and in natural freshwater systems. In the laboratory systems, an inverse relationship was observed between the particle concentration of sand or lake sediment, and the distribution coefficients of210Po and210Pb. The slope of the log-linearK d vs particle concentration relation is consistent with existingK d-particle concentration theories. These laboratory observations are consistent with similar measurements in two lakes. TheK d values of Po and Pb for the bottom sediment-pore water system with a high particle concentration were 10 to 100 times lower than those for dilute concentrations of particles suspended in the lake water. TheK d of210Pb in the sediments was >104 so that the diffusive transport of210Pb has only a small influence on the interpretation of210Pb concentration-depth profiles and the210Pb dating of these sediments.This is the second of a series of papers to be published by this journal following the 20th anniversary of the first application of210Pb dating of lake sediments. Dr P. G. Appleby is guest editing.  相似文献   

12.
The Portil lagoon is a natural freshwater reservoir located at the southwest of Spain, near the coast. In its surroundings an important tourist complex has been developed since the earlier 1970s. This has resulted in increased loads of nutrients, specially during summer months. In order to evaluate the impact from humans on the lagoon caused by anthropogenic activities, we have determined vertical profiles of excess 210Pb and 137Cs in a sediment core taken from its bottom. Vertical profile of excess 210Pb seems to indicate that the sediment core was affected by large-scale mixing processes. Nevertheless this possibility was ruled out based on the shape of the 137Cs vertical distribution after modelling the 137Cs profile considering large-scale mixing. The chronology developed from 210Pb and 137Cs has allowed us to calculate the temporal evolution of the mass sedimentation rate during the last century. This mass sedimentation rate increased in 1973: from 0.08(2) g cm-2 a-1 in the period 1900-1973 to 0.17(5) g cm-2 a-1 in the period 1973-1995. This increase could be related to land-movements in the drainage area and to permitted sewage inputs into the lagoon, both with its origin in the growing/running of the surrounding tourist complex.  相似文献   

13.
A combination of tephrochronology and 14C, 210Pb, and 137Cs measurements provides a robust chronology for sedimentation in Upper Klamath Lake during the last 45 000 years. Mixing of surficial sediments and possible mobility of the radio-isotopes limit the usefulness of the 137Cs and 210Pb data, but 210Pb profiles provide reasonable average sediment accumulation rates for the last 100–150 years. Radiocarbon ages near the top of the core are somewhat erratic and are too old, probably as a result of detrital organic carbon, which may have become a more common component in recent times as surrounding marshes were drained. Below the tops of the cores, radiocarbon ages in the center of the basin appear to be about 400 years too old, while those on the margin appear to be accurate, based on comparisons with tephra layers of known age.Taken together, the data can be combined into reasonable age models for each site. Sediments have accumulated at site K1, near the center of the basin, about 2 times faster than at site CM2, on the margin of the lake. The rates are about 0.10 and 0.05 cm/yr, respectively. The chronological data also indicate that accumulation rates were slower during the early to middle Holocene than during the late Holocene, consistent with increasing wetness in the late Holocene.  相似文献   

14.
210Pb and 137Cs dating methods in lakes: A retrospective study   总被引：1，自引：0，他引：1  
210Pb has been used for more than two decades to provide the geochronology of annually deposited sediments and to construct pollution histories. Evidence from some lakes suggests that this radionuclide may be adequately mobile to compromise dating reliability. This study provides one test of that possibility by comparing recent measurements of 210Pb and trace metals to ones carried out more than 20 yrs in the past. 137Cs dating is used to confirm sediment accumulation rates in the recent cores. In the three Connecticut, USA, lakes studied, sediment accumulation rates changed abruptly to higher values between 40-50 yrs ago (increasing by factors of 2.2, 2.9, and 3.0). In all three lakes, rates calculated from 210Pb distributions both above and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yrs of burial, the changes in slope in 210Pb distributions occurred at the same depth in each pair of cores. The depth of sharp peaks in concentrations of trace metals also matched. In general, this evidence supports the idea that sediments in these lakes have simply been buried, without significant diagenetic remobilization of 210Pb and trace metals . Nevertheless, some important differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during the past 33 yrs as calculated from 137Cs and 210Pb in the recent cores. Most potential causes for this difference can be ruled out, and it appears that one of the two nuclides is remobilized compared to the other. There were also significant differences in the total inventories of both 210Pb and trace metals (both up to 2 ×) between recent and older cores in some cases. This may be due to dissimilar sediment focusing, since it is not known for certain whether the new cores were collected at exactly the same sites as in the past.  相似文献   

15.
210Pb geochronologies should be validated with independent tracers such as 137Cs. In the cases with constant 210Pb activity in the topmost sediments, the presence of a distinct 137Cs peak within the 210Pb plateau has been used as a definitive demonstration of acceleration (increase in the sedimentation rate in recent years) versus fast mixing. Nevertheless, some limitations can be identified in the use of semiquantitative arguments, and a global understanding of the whole 137Cs activity profile is then required. Particularly, the incomplete mixing within the top sediment zone (described through the Incomplete Mixing Zone model) can explain quantitatively and simultaneously the 137Cs peak and the flattening in the 210Pb activity profile. This is demonstrated using selected examples from literature data. Thus, measured constant 210Pb activities in the top 6 cm of a sediment core from Lake Zürich. Nevertheless, they found 7Be only in the uppermost layer, the distinct 137Cs maximum at 6 cm depth, and undisturbed varves. The fast mixing seemed then opposed to common sense. The constant rate of supply model shows acceleration and it adequately matches the position of the 1963 137Cs peak. Nevertheless it fails to explain the whole 137Cs profile when handling time series of 137Cs atmospheric deposition. Finally, it is shown how the incomplete mixing of the activity (through the pore water) over a certain mass depth at the top sediment, with a finite value of the mixing coefficient, can quantitatively explain the whole activity versus mass depth profiles of 137Cs and 210Pb, and the presence of 7Be only in the uppermost sediment layer. A further validation of these ideas is presented from other literature data.  相似文献   

16.
Low-background gamma counting: applications for210Pb dating of sediments   总被引：1，自引：0，他引：1  
Sediment cores from three lakes were dated with210Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of210Pb,226Ra, and137Cs in sediment samples because sample preparation is simple and non-destructive,226Ra activity provides a direct measure of supported210Pb activity for each sample analyzed, and137Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported210Pb activity was estimated equally well from226Ra activity of each sampling interval or from the mean total210Pb activity of constant activity samples at depth. Supported210Pb activity was constant with depth in this core. In a short freeze core, determining226Ra activity of every sample proved advantageous in estimating supported210Pb activity because supported210Pb activity could be estimated from210Pb measurements only at the deepest sampling interval. Supported210Pb activity estimated from226Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core226Ra activity exceeded210Pb activity at the top of the core and varied 20 fold with depth. This high input of226Ra in disequilibrium with210Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported210Pb activity is constant in sediment cores and can be estimated from the mean total210Pb activity at depths where210Pb activity is constant. We recommend using gamma counting or another independent assay of226Ra to validate the assumption of constant supported210Pb activity in sediment cores if there is reason to expect that226Ra activity varies with depth.This is the fourth of a series of papers to be published by this journal following the 20 th anniversary of the first application of210Pb dating of lake sediments. Dr. P.G. Appleby is guest editing this series.  相似文献   

17.
Providence Canyon, one of a series of large gullies in the upper Coastal Plain of the southeastern United States, formed as a result of deforestation and agricultural development in the early 1800's. Sediment eroded from the canyon aggraded the floodplain downstream, dammed tributary valleys, and formed North and South Glory Hole lakes (NGH, 4.8 ha and SGH, 2.5 ha). Sedimentary sequences in these lakes include a basal unit (I) of layered sand and clayey-sand overlain by 0.05-0.1 m of mud, fine sand and organic matter, with large fragments of wood (Unit IIa). An upper unit (IIb) 0.29-1.61 m thick consists of silt and clay containing discrete layers of sand. We interpret Unit I as floodplain alluvium deposited before the lakes were dammed, Unit IIa as sediment deposited during the early phase of the lakes when detritus from trees killed by flooding was abundant, and Unit IIb as lacustrine mud deposited after lake levels stabilized, with periodic pulses of sand eroded from Providence Canyon introduced to the lake by backflooding events. Basal dates extrapolated from a 210Pb chronology for the upper part of SGH core suggests that development of the canyon and formation of the lakes began in the 1840's, and that lake levels stabilized by about 1880. Although consistent with historical accounts of the age of Providence Canyon, these dates must be considered as approximate because of uncertainty in extrapolating dates to the bottom of the core.In contrast, the 210Pb chronology for the upper portion of the core (post 1930) is validated independently using historical climatic records, and indicates that lacustrine sedimentation faithfully records recent land-use change visible in historical aerial photography. An 8-fold increase in lacustrine sedimentation occurred after the clearing of forest near SGH in the 1930's and 1940's, and a 1.5-fold increase occurred because of road construction in the l950's. Individual sand layers deposited between 1830 and 1957 correlate with erosion at Providence Canyon during major storm events. Since then, downcutting and headward incision by the stream draining Providence Canyon have reduced backflooding to the Glory Hole lakes. As a result, the thickness of individual sand layers decreased, although increases in mass sedimentation rates for mud correspond more directly with large precipitation events after channel incision cut off the major source of sediment from Providence Canyon. The results of this study illustrate the value of the lacustrine sedimentary record in assessing geomorphic, climatic, and human-induced environmental change in heavily disturbed landscapes.  相似文献   

18.
Environmental degradation, including shallowing, deterioration of aquatic habitat and water pollution, has arisen from the inflow of fine sediment to Lake Takkobu in northern Japan. The lake has experienced gradual environmental degradation due to agricultural development, which has introduced both fine sediment and sediment-associated nutrients into the lake. We have reconstructed the history of sediment yield to Lake Takkobu in Kushiro Mire over the last 300 years and have examined trends with reference to land-use development. Fifteen lake sediment core samples were obtained, and various physical variables of lake sediments were analyzed and dated using 137Cs and tephrochronology. The physical variables showed that all points contained mainly silt, except for two points close to the river mouths, where the mean diameter was < 35 μm. The peaks were defined as a “signal” when the physical variables were synchronous in a profile. These were created by floods and engineering works constructing drainage systems. The signal of canal construction in 1898 was detected in all core points. Lake Takkobu core samples contained two tephra layers. From the refractive indices of dehydrated glasses, the lower tephra layer was identified as Ko-c2 (1694) and the upper tephra layer as Ta-a (1739). A clear peak in the 137Cs concentration was detected at all the sampling points, except for the site close to the Takkobu River. This site showed two peaks in the 137Cs concentration, which was attributed to perturbation from flood events and a drainage project. The maximum 137Cs concentration was identified as the sediment surface from 1963, enveloped by the 1962 and 1964 signals. The sediment yield averaged over the last 300 years for Lake Takkobu was reconstructed for four periods using the signal, tephra and 137Cs as marker layers. The sediment yield under the natural erosion condition for the first two periods was 226 tons/year from 1694 to 1739 and 196 tons/year from 1739 to 1898. The development of the Takkobu watershed started in 1880s with partial deforestation and channelization in 1898, 1959, and 1962 leading to an increased sedimentation yield of 1016 tons/year from 1898 to 1963. Continued deforestation, channelization works in 1964, road construction in 1980–1990s, as well as agriculture development caused a further increase to 1354 tons/year from 1963 to 2004. Compared to the averaged natural sedimentation yield of 206 tons/year until 1898, initial land-use development in a catchment accelerated lake sedimentation, indicated by the 5-fold sediment yield. With increasing agricultural development since 1960s, sedimentation yields were highest for 1963–2004; a 7-fold increase compared with pre-impact conditions. To reduce sediment yield, riparian buffers along the rivers should be preserved or rebuilt, and sluices may function effectively during short-term periods of flooding. Environmental management policy and laws restricting uncontrolled and inappropriate land-use might help in addition to ensure longer-term environmental health by reducing the sedimentation rate.  相似文献   

19.
Spatial variability of sediment and diatom deposition was assessed in a small monomictic, eutrophic lake in Northern Ireland (Lough Augher, Co. Tyrone) using measurements from 17 sediment cores. Loss-onignition profiles in water depths >6 m showed good repeatability, while littoral cores were more variable with localised profiles. Dry mass accumulation rates, derived by biostratigraphic correlation to a 210Pb dated master core, were variable and not correlated with water depth. Basin mean dry mass accumulation rate was 0.068 g cm-2 yr-1 (range 0.036–0.09) prior to 1900, and 0.19 g cm-2 yr-1 (range 0.11–0.3) after 1974. Post-1940 cumulative fluxes were estimated for dry mass (range 3.49–916 g cm-2) and diatoms (range 16.9–113.8×107 frustules cm-2). Cumulative dry mass was inversely correlated (r=–0.64) with distance from the inflow, indicating its localised influence. No variable was correlated with water depth except frustules of planktonic diatoms (r=0.66). However, high cumulative fluxes of diatoms and dry mass away from the inflow suggest that the expansion of the littoral macrophyte community may be responsible for decreased resuspension in shallow water, and together with increased sediment trapping, has enhanced sediment accumulation in shallow water over recent time periods.  相似文献   

20.
The water quality of Dianchi Lake declines quickly and the eutrophication is getting serious. To identify the internal pollution load of Dianchi Lake it is necessary to evaluate its sediment accumulation. Sedimentation rates of Dianchi Lake are determined by 137Cs dating. However, 137Cs vertical distribution in sediment cores of Dianchi Lake has special characteristics because Dianchi Lake is located on the southeast of the Qinghai-Tibet Plateau, the Kunming quasi-stationary front is over the borders of Yunnan and Guizhou where the specific precipitation is distributed. Besides 1954, 1963 and 1986 137Cs marks can be determined in sediment cores, a 137Cs mark of 1976 representing the major period of 137Cs released from China unclear test can be determined and used for an auxiliary dating mark. Meanwhile Dianchi Lake is divided into seven sections based on the water depth, basin topography, hydrological features and supplies of silt and the lakebed area of each section is calculated. The mean annual sedimentation rates for seven sections are 0.0810, 0.1352, 0.1457, 0.1333, 0.0904, 0.1267 and 0.1023 g/cm2a in 1963–2003, respectively. The gross sediment accumulation of the lake is 26.18×104 t/a in recent 17 years and 39.86×104 t/a in recent 50 years. Foundation: National Natural Science Foundation of China, No.40771186; The Key Project of the State Key Laboratory of Soil Sustainable Agriculture, Nanjing Institute of Soil Sciences, Chinese Academy of Sciences, No.5022505 Author: Zhang Yan (1962–), Ph.D and Associate Professor, specialized in environmental change.  相似文献   

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