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1.
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   

2.
Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in 210Pb dated and pre-210Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal.  相似文献   

3.
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   

4.
A multiproxy paleolimnological study of Douglas Lake, Michigan, was undertaken to elucidate the history of productivity and oxygen depletion in three basins of this multi-depression lake. Indicators investigated in three dated cores included chlorophyll a, Fe and Mn stratigraphy, and fossil chironomid assemblages. The coring sites were chosen to correspond to modern studies of oxygen depletion rates, and to determine if conclusions reached in these studies were supported by paleolimnological evidence. Stratigraphies of chlorophyll a, Fe and Mn indicate that two of the basins, South Fishtail Bay and Fairy Island, have been eutrophic and anoxic for a long period of time, predating European settlement. The third basin, Grapevine Point, has been consistently less productive, and had less severe oxygen depletion. Results of the chironomid analysis agree with these conclusions, including a change from mesotrophic to eutrophic indicator taxa in the Grapevine Point basin. All three cores show evidence of increasing trophic state in the most recent sediments, supporting some of the conclusions reached in the modern studies. It is also demonstrated that deforestation of the watershed had profound effects on littoral chironomid assemblages. Paleolimnological investigations also demonstrated the individual nature of the separate basins in Douglas Lake.  相似文献   

5.
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   

6.
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   

7.
Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using 210Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM210 suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core.  相似文献   

8.
The magnetic properties of recent sediments from five short cores from Brothers Water in the English Lake District are compared with those from a set of catchment soil samples. From the comparison, it is concluded that bacterial magnetosomes have made a significant contribution to the magnetic properties of the sediments of the last five decades. This contribution is superimposed on a continuing erosive input of haematite-rich sediment from the parent materials of the catchment and may obscure any possible increase in surface soil input during recent times. The results highlight the difficulty of making sediment-source linkages where biogenic contributions to the magnetic properties of the sediment are important relative to the input of magnetic minerals from catchment erosion.  相似文献   

9.
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   

10.
Lago de Zirahuén (19° 26 N, 101° 44 W) lies within a montane basin in highland Michoacán, Mexico at 2075 m a.s.l. The lake basin has high recreational value and is promoted as a tourist attraction. Four short (<1 m), sediment–water interface cores were used to investigate Late Holocene environmental change. The cores span approximately the last 1000 years, based on 210Pb dating, AMS 14C dating and tephrochronology. Cesium-137, Americium-241 and tephra layers from Volcan Paricutín (AD 1943–1952) and Volcan Jorullo (AD 1759–1774) provide independent age markers. Cores were analysed for diatoms, magnetic susceptibility and heavy metals (iron, lead, manganese, copper and zinc). Drier climate around 1000 years BP is tentatively inferred from the diatom assemblage and is concordant with other regional records. Increased soil erosion between ca. AD 1100 and AD 1550 may be associated with Pre-Hispanic settlement in the basin, but there is no archaeological evidence to support this. A period of catchment stability occurred during the early Colonial Period, ca. AD 1550–1750. The dramatic decline in the indigenous population following the Spanish Conquest in 1521 may have promoted vegetation recovery on the basin slopes. After the mid-18th century, a significant change in diatom species composition is observed along with an increase in soil erosion, as inferred from the magnetic susceptibility profile. Concentrations of copper and lead also increased. These changes are associated with the establishment of a copper smelting industry in the basin and increased Colonial agricultural development. Recently, tourist developments and commercial agriculture have impacted on the basin. The diatom flora has changed dramatically in the last 20 years, apparently in response to the onset of cultural eutrophication. The palaeoenvironmental evidence suggests that the lake is responding rapidly to land use intensification in the basin, which may have implications for future developments.  相似文献   

11.
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   

12.
In this paper we test the utility of the mud deposition boundary depth (mud DBD) theory (Rowanet al. 1992) as a means of maximizing sampling efficiency in paleolimnological investigations, particularly those that apply to210Pb dating. The mud DBD is defined by the relationship between near bottom wave velocity and particle threshold velocity, with wave and particle threshold theory simplified to terms of exposure and depth. Mud DBD theory can be used to define the depositional zone in lakes, and within the depositional zone defined by the mud DBD: 1) there is a high probability of obtaining a representative core, 2) variation in mass sediment accumulation rate (MSAR) is not correlated with water depth, and 3) variation in MSAR is considerably reduced from the whole lake average. This suggests that mud DBD theory can account for the effects of sediment focusing, and that the mud DBD defined depositional zone is the zone to which fine-grained sediments are focused. Finally, we have shown that to optimize sampling effort, 5 to 10 cores within the depositional zone are necessary for a reasonably precise estimate of the mean mass sediment accumulation rate. In addition, the use of mud DBD theory prior to sampling can dramatically reduce the cost associated with analyzing large numbers of cores for210Pb.This is the fifth of a series of papers to be published by this journal following the 20 th anniversary of the first application of210Pb dating of take sediments. Dr P. G. Appleby is guest editing this series.  相似文献   

13.
Devils Lake is a closed, saline lake in North Dakota; it is typical of lake basins in the Great Plains Region, where windy conditions and fluctuating water-levels disturb sediment and confound chronostratigraphy. Pollen analysis and 210Pb dating of two cores collected from bathymetrically contrasting embayments demonstrate (1) how certain agriculture-related pollen types differ in their value as chronostratigraphic markers, (2) how pollen and 210Pb stratigraphies can be reconciled to determine the approximate depth of sediment mixing, and (3) the importance of coring-site selection, especially in lakes with unstable sedimentary conditions.  相似文献   

14.
Post-depositional mobility of137Cs,239+240Pu and210Pb was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the137Cs and239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of210Pb, because 1) mass accumulation rates based on210Pb agree with those based on137Cs and239+240Pu peak depths and with those based on varve counts, and 2)210Pb ages agree with varve ages. Significant mobility of137Cs is evident from the penetration of137Cs to depths 15–20 cm deeper than239+240Pu. Deep penetration of137Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that137Cs is present in two distinct forms in these sediments, 67–82% as an immobile form and 18–33% reversibly adsorbed with a K d of approximately 5000. The profiles can be interpreted equally well assuming a small portionof the total137Cs was present as an extremely mobile phase (K d 5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH 4 + concentrations in porewaters may enhance diffusion of the mobile form of137Cs, but not of the immobile form of137Cs that defines the sharp gradient. Mobility of137Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of137Cs in the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons).239+240Pu is a more reliable sediment chronometer than137Cs because it is significantly less mobile.This is the sixth 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   

15.
A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. 210Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawacs southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the 210Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change.  相似文献   

16.
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   

17.
The formation processes of the late Neogene sedimentary basins in Northern Hokkaido have been investigated on the basis of rock magnetism, structural geology and numerical modelling. Untilted site‐mean directions of remanent magnetization of the Wakkanai Formation, obtained from oriented core samples in Horonobe, suggest remarkable counterclockwise block rotation (ca. 70°) since the late Neogene. Uniform microscopic fabric of the siliceous sediments was inferred from the alignment of the principal axes of the anisotropy of magnetic susceptibility (AMS). After correction for tectonic rotation, the maximum axis of AMS, which reflects the sedimentary fabric of the dominant paramagnetic minerals, is in an E‐W direction, which is concordant with the influx direction of diatomaceous particles into the N‐S elongate sedimentary basins. The difference in the bulk initial magnetic susceptibility of the siliceous sediments of the Wakkanai Formation between the depocenter of the basin and its peripheral part implies that terrigenous non‐magnetic fraction has been sorted out during transportation of the detrital grains as gravity flows. As for the development mechanism of the N‐S elongate late Neogene basins in Northern Hokkaido, their depocenter arrangement and subsidence pattern indicates dextral motions upon a longitudinal fault zone along the Eurasian convergent margin. Dislocation modelling was adopted to explain vertical displacement and rotational motion around the study area and successfully restored the deformation pattern based on the assumption of dextral slip at a left‐stepping part of a strand of the transcurrent fault.  相似文献   

18.
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   

19.
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   

20.
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   

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