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1.
We present a historical overview of applications of210Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of210Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several210Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by137Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm–2 y–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the210Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm–2 y–1. In other parts of the lake a significant deficit in the inventory of210Pb was found in the sediments. This could be due to a chemical redissolution of210Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of210Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of210Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved210Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and210Pb in sediments of Lake Zurich was found.This is the first 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   

2.
A sedimentary record of human disturbance from Lake Miragoane,Haiti   总被引：1，自引：0，他引：1  
Lake Miragoane, Haiti is one of the largest, natural freshwater lakes in the Caribbean (A=7.06 km2, zmax=41.0 m, conductivity = 350 S cm–1). Lake waters are dominated by calcium and bicarbonate ions. The lake was thermally stratified, and oxygen profiles were clinograde during summer visits in 1983 and 1985. A 72-cm mud-water interface core was taken near the center of the lake and dated with 210Pb. The local 210Pb fallout rate is low (0.09 pCi cm–2 yr–1), about 20% of the global average. Bulk sedimentation rates ranged from 0.008 to 0.030 g cm–2 yr–1 during the past 130 years (0–8 cm depth). Sediment geochemistry and pollen have been analyzed in the topmost 58 cm of the section. Tentative ages were assigned to the core by extrapolation of 210Pb dates. According to this preliminary chronology, the bottom part of the core (58–30 cm) records pre-Columbian sedimentation (1000–500 B.P.) and contains pollen evidence of intact, dry and mesic forest. Pre-Columbian deposits are rich in organic matter (x = 30%) and relatively poor in carbonates (x = 15% as CO2). The top 30 cm of the core preserve the record since European contact (500 B.P. to present). Pollen data reveal two episodes of deforestation following European arrival. Consequent soil erosion is documented by a decrease in organic matter content (x = 15%) and an increase in carbonates (x = 27% as CO2). Surficial sediments reflect the widespread deforestation and soil loss that characterize the watershed today.  相似文献   

3.
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   

4.
Application of210Pb in soils   总被引：2，自引：0，他引：2  
The spectroscopic measurement of soil samples is described.210Pb data from a soil survey in western-Europe are briefly reviewed. The average210Pb flux from the atmosphere, as determined from210Pb exc soil inventories, is 90 Bq m–2a–1. A simple one-dimensional box-chain model is described. The model simulates the vertical, post depositional transport of natural and fall-out radionuclides in the soil. Simulation of measured210Pb exc ,134Cs,137Cs, and241Am soil profiles shows that mixing (bioturbation) is a very efficient transport mechanism. Lead seems to be strongly fixed to organic and clay particles. It is transported by the displacement of the organic and clay carrier substances. The mean residence time of lead, caesium, plutonium and americium in organic rich forest soils is in the order of 250–1000 years. An applicability study in investigate the use of210Pb in erosion problems showed erosion rates from 60–180 g m–2a–1 on organic rich forest and meadow sites with 10°–25° slopes.This is the eighth 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   

5.
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   

6.
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   

7.
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   

8.
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   

9.
The structure and function of many Korean ecosystems have been rapidly modified since the 1960s when industrialization of the nation began. Ulsan City was the first in Korea to develop into a major industrial complex. To assess anthropogenic impacts on ecosystems surrounding Ulsan, sediment cores were collected from Mujechi-neup (bog) and Sanggae reservoir of Ulsan, and these cores were 210Pb dated using the CRS model. Physical and chemical characteristics and pollen were analyzed, and the rates of sediment accumulation were calculated. Unsupported 210Pb inventories in Mujechi-neup and the Sanggae reservoir were 18.04 and 16.53 pCi cm–2, and the corresponding 210Pb fluxes were 0.56 and 0.52 pCi cm–2 yr–1, respectively. The overall accumulation rate of dry matter was 0.26 kg m–2 yr–1 since 1852 (14 cm in depth) in Mujechi-neup. In the Sanggae reservoir, the accumulation rates of dry matter were increased from 2.1 in 1965 to 6.0 kg m–2 yr–1 in 1999. Pollen analysis revealed that three pollen zones existed in Mujechi-neup; a Pinus pollen-dominated zone from 0 to 5 cm in depth (1974 year), an Alnus pollen-dominated zone from 5 to 15 cm in depth (1827 year), and a Quercus pollen-dominated zone below 15 cm in depth. The shift from an Alnus dominated zone to a Pinus dominated zone was related to the Korean War between 1951 and 1953 and reforestation activities in the 1970s. In the Sanggae reservoir, there was an increase of Humulus pollen since 1996, an increase of Ambrosia pollen since the 1960s and the decrease of Graminae pollen since 1993 with the expansion of the industrial area. Similar to Mujechi-neup, the Sanggae reservoir also consists of three pollen zones: a Pinus and Typha pollen-dominated zone from 0 to 10 cm in depth (since 1993), a Graminae pollen-dominated zone from 10 to 22 cm in depth (between 1947 and 1993), and a Pinus pollen-dominated zone below 22 cm in depth (before 1947). The increase of Typha and Humulus in the 1990s indicates an increase in the inflow of nutrients into the wetlands. Also, pollen analysis revealed that Ambrosia was introduced in the 1960s during industrialization of the area. The sediment was composed of humic peat in Mujechi-neup and was composed of clay in the Sanggae reservoir. High LOI, P, and low C/N ratio between the depths of 5 to 7 cm in the Sanggae reservoir indicates an increased input of P and N. As a result, the growth of Typha was at its maximum. The total Pb content in Mujechi-neup has been increasing since the 1870s, and its accumulation in the Sanggae reservoir has been increasing since the 1960s. Thus, the history of local-scale disturbances and human activities in the watershed was reconstructed through paleoecological studies in Ulsan.  相似文献   

10.
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   

11.
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   

12.
Precipitation rates in the Atacama Altiplano 22–24°S were 400–500 mm yr–1 during late glacial and early Holocene times as opposed to 200 mm yr–1 today. This humid phase (Tauca phase) was likely due to strengthened tropical (monsoonal) circulation, which brought continental moisture to the Atacama Altiplano. The lake level of Laguna Lejía (23°30S, 4350 m) at that time was up to 25 m higher than it is today. Mg/Ca and Sr/Ca data from lake sediments show that, what today is a highly saline lake was a freshwater lake at that time. Seasonally-laminated calcareous sediments were deposited between 13 500 and <10 400 yr B.P. indicating the maximum of the humid phase. Climatic changes in the past are important for current groundwater resources.14C and3H data from lake-, ground- and well water suggest that modern groundwater formation (i.e. water <40 years) in the Altiplano is very limited under current arid conditions. We conclude that significant amounts of the water resources in this area originated during the time of the late-glacial and early Holocene humid climate. Tritium data from snow samples show that the moisture in the Altiplano at 22–24°S is mainly of continental origin, whereas precipitation from the westerlies hardly contributes to the water supply in this area. This precipitation pattern matches the paleodata, and we suggest that current precipitation formation may provide an analogue framework for late-glacial circulation in this area.This is the 3rd in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.  相似文献   

13.
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   

14.
Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km2 of catchment area and 40 km2 of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. 210Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr–1, which also was confirmed by 137Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (LF), susceptibility of anhysteretic remanent magnetism (ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/LF) had much more information to reveal environmental changes. The ARM/LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.  相似文献   

15.
Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.  相似文献   

16.
The Upper Río Lerma valley, Estado de México, is a high-altitude (2575 m a.s.l.) basin floored by Quaternary alluvial, lacustrine and pyroclastic deposits. Two pits were dug in the swampy bed of the recently drained L. Chiconahuapan. Ten 14C dates have been obtained from these profiles, which consist of diatomaceous organic lake muds and peats with intercalated tephras. The oldest unit is the Upper Toluca Pumice (Tripartite Ash), dated 11 580±70 yr BP. Analyses of sediment chemistry, loss-onignition, mineral-magnetic variations and subfossil diatom assemblages provide evidence of environmental changes since this date. Alkaline ponds or freshwater lakes developed during the intervals 9000–6000, 6000–5500, 3600–1400 and 800–0 yr BP, and acidic marshes or bogs during the intervening dry episodes. An important phase of accelerated erosion, beginning around 3100 yr BP and culminating around 1400–700 yr BP, appears to have been associated with human disturbance of the basin soils.  相似文献   

17.
Pb-210 dating of two metal-polluted organic sedimentcores obtained near a former pyrometallurgical zinc smelter in Lommel, Belgiumhave been used to reconstruct atmospheric lead deposition rates during the20th century. Independent knowledge concerning historical pollutionevents and 137Cs fall-out profiles has allowed a criticalevaluation of the CRS, CIC and CF-CS models for the 210Pb ageinterpretation. Resulting ages for the three models suggest that, in this case,the CIC model gives the most accurate interpretation of historical pollutionevents and atmospheric lead fall-out. The 210Pbwater-sediment flux was estimated at 141–1158Bq·m–2·yr–1 for one site and62–106 Bq·m–2·yr–1 at theother site, during the last century. The large difference illustrates thatsediment focusing was important on a small spatial scale (10 m).The direction of focusing correlates with the predominant wind direction.Maximum atmospheric lead deposition rates were found to be 1.63 ± 0.59g·m–2·yr–1 around 1968 AD,which is 2 orders of magnitude larger than the Belgian average in 1980 AD, and5 orders larger than Holocene atmospheric lead deposition.  相似文献   

18.
Sediment focusing in six small lake basins is inferred from sedimentary radionuclide profiles. The activities of137Cs and9+240Pu do not decrease to zero above the sub-surface maximum but tail off gradually. Similar behavior is observed in the sediments of three lakes for134Cs and60Co, which were added only to the lake. This upward tailing is not caused by bioturbation or erosion from the watershed. Sediment focusing is the likely cause, further substantiated by inventories in deep sediments of9+240Pu and210Pb that are twice as high as expected from atmospheric fallout. Inventories in deep sediments of137Cs are in several cases lower than expected from atmospheric fallout, and cannot be explained by hydraulic flushing from the lakes, as this would require unreasonable flushing rates. The low137Cs inventories and the sedimentary profiles of the137Cs/9+240Pu ratio are consistent with137Cs removal from the water column by boundary scavenging to shallow sediments. The profiles and inventories of all three nuclides could be reproduced with a simple three box model, assuming focusing of shallow sediments (50–60% of total sediment area) to deep sediments with a rate constant of 5–10% yr–1.This is the seventh of a series of papers to be published by this journal following the 20 th anniversary of the first application of210Pb dating of lake sediment. Dr P. G. Appleby is guest editing this series.  相似文献   

19.
During recent years, numerous studies dealing with Holocene lake level fluctuations have been conducted in Finnish Lapland. However, no quantification of lake level variations exists to date. Here, we applied a recently developed modern cladocera – lake depth transfer model to subfossil cladocerans analysed from three small and shallow (< 6 m) kettle-hole lakes in northwestern Finnish Lapland to provide estimates of the amplitudes of long-term lake-level changes in the region. The quantitative inferences were compared to pollen, charcoal and geochemical records from one of the study sites. The lake levels were inferred to be high during the early Holocene; they faced marked reduction up to 4–6 m in the mid-Holocene (≈7000–4000 cal yr BP), and rose again during the latter part of the Holocene. There is some indication of lowered lake levels around 1500 cal yr BP, but interpretation of such small-scale changes is hazardous due to large prediction errors in the initial cladoceran model. The overall pattern of the Holocene lake level variation generally followed the regional changes in climate humidity as reconstructed in previous studies by means of other sedimentary proxy indicators, such as pollen and oxygen isotopic compositions. We postulate that changes in winter precipitation may have had a greater influence on lake-levels than variations in summer precipitation or evaporation.  相似文献   

20.
A pollen record from Puyehue area (40°S; 72°W) in the southern Lake District, Chile, indicates that prior to 13,410 14C yr BP (ca. 16,500–15,200 cal yr BP), cold resistant and hygrophilous vegetation, particularly Nothofagus forest and myricaceous vegetation, covered the area. From ca. 15,000 cal yr BP onward, the forest became increasingly dense. Between 10,010 and 7450 14C yr BP (ca. 11,000–8000 cal yr BP), the expansion of Nothofagus obliqua and the spread of grasses suggests the climate became warmer and semi-arid. Lowland deciduous forest (Nothofagus obliqua, Aextoxicon punctatum, Laurelia sempervirens) and Valdivian rainforest (Nothofagus dombeyi, Eucryphia cordifolia, Caldcluvia paniculata, Aextoxicon punctatum, Laureliopsis philippiana) were abundant. During the next two thousand years, stable warm climatic conditions prevailed, and the diversity of the vegetation increased. From 5760 to 1040 14C yr BP (ca. 6500–900 cal yr BP), the North Patagonian rainforest expanded. The presence of Pilgerodendron/Fitzroya, together with Nothofagus forest, suggests that humid conditions prevailed. During the last millennium, human impact intensified and regional vegetation was disturbed, particularly the lowland deciduous forest and Valdivian rainforest. North-Patagonian and subantartic taxa, such as Podocarpus nubigena, Pilgerodendron/Fitzroya, Nothofagus dombeyi type, Austrocedrus chilensis and Drimys winteri, occupied the low and high-altitude parts of the Cordillera. Five hundred years ago, shrub and grasses expanded in the Nothofagus forest, suggesting that forest became more open under cool–cold, and humid climatic conditions. These conditions prevail to the present day. This is the fourth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.  相似文献   

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