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1.
Most 210Pb dating models assume that atmospheric flux of excess 210Pb (210Pbexc) to the sediment–water interface remains constant over time. We revisited this assumption using statistical analysis of a database of laminated sediments and evaluated the implications for radiometric dating of recent deposits. A bibliographic survey enabled us to create a database with 10 annually laminated sediment cores from a variety of aquatic systems. The database has records of 210Pbexc flux, initial 210Pbexc activity, and sediment accumulation rate (SAR). 210Pbexc flux to sediments varied with time, and 1/3 of the data had relative deviations from the mean value >25 %. There was no statistically significant correlation between activities at the core top and SAR, whereas a statistically significant (p < 0.01) linear regression between 210Pbexc flux and SAR was found for nine of the ten cores. Thus, in most of the studied aquatic systems, 210Pbexc flux to the sediment was governed primarily by flux of matter, rather than by direct atmospheric 210Pbexc deposition. Errors in chronology and SAR, attributable to varying 210Pbexc flux and estimated by the constant rate of supply (CRS) model, were evaluated from its analytical solutions, and tested against SAR values from this database that were derived independently from varves. We identified several constraints for general application of the CRS model, which must be taken into account to avoid its misuse. 相似文献
2.
Application of210Pb in soils 总被引:2,自引:0,他引:2
Helmut Dörr 《Journal of Paleolimnology》1995,13(2):157-168
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
3.
Jeroen E. Sonke William C. Burnett Jurian A. Hoogewerff Sieger R. van der Laan Jaco Vangronsveld D. Reide Corbett 《Journal of Paleolimnology》2003,29(1):95-107
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
4.
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
5.
Trace metals and polycyclic aromatic hydrocarbons (PAH) were analyzed in sediment cores from three central Alberta lakes to
determine the contributions of local coal-fired power plants to contaminant loadings. In Wabamun Lake, with four power plants
built since 1950 within a 35-km radius, sediment concentrations of mercury, copper, lead, arsenic and selenium have increased
by 1.2- to 4-fold. Trace metal enrichments were less pronounced in Lac Ste. Anne and Pigeon Lake, situated 20 km north and
70 km south of Wabamun Lake, respectively. Total Hg flux to Wabamun Lake sediments (21–32μg m−2 yr−1) has increased 6-fold since 1950, compared to 2- and 1.5-fold increases in Lac Ste. Anne and Pigeon Lake, respectively, since
circa 1900. Total PAH flux to surface sediments was 730–1100μg m−2 yr−1 in Wabamun Lake, 290–420μg m−2 yr−1 in Lac Ste. Anne, and 140–240μg m−2 yr−1 in Pigeon Lake. Without adoption of pollution-abatement technology that compensates for increases in generating capacity,
continued expansion of coal-burning industry in Alberta will result in increased contaminant deposition, primarily from local
sources. 相似文献
6.
7.
Reservoir sediments are rarely used as environmental archives because of the potential for sediment disturbance by fluctuating
water levels. However, rapid rates of sedimentation, proximity to urban centres and often the existence of management records,
may make them potentially important resources for reconstructing recent, anthropogenically-derived environmental change. This
project assesses the potential of reservoir sedimentary records for reconstructing past atmospheric and drainage basin fluxes
of heavy metals (manganese, iron, nickel, copper, zinc and lead) in the southern Pennines, UK. Five reservoirs were selected
on the basis of management history and drainage basin characteristics. Multi-parameter analysis showed sediments to be replicable
across the ȁ8accumulating zone’ with reasonably consistent rates of sedimentation. Water level fluctuations were not found
to detrimentally affect sediment records in the deepwater area of the reservoirs. In fact, spheroidal carbonaceous particle
(SCP) profiles show trends in inputs that closely reflect major changes in industrialisation, indicating the reservoir sediments
to be excellent records of particulate inputs. Only lead (Pb) and zinc (Zn) were significantly enriched in the reservoir sediment
in comparison to background levels. Manganese (Mn), iron (Fe) and to a limited degree, copper (Cu), appeared to be affected
by post-deposition mobility. Preliminary calculations of Pb fluxes indicate that over 80% of the current Pb input to the reservoirs
is from Pb deposited onto drainage basin soils in the past, rather than from direct atmospheric deposition or natural background
inputs. In Howden reservoir, for example, the total Pb flux to deepwater sediment cores in 2000 was 119 mg m−2 a−1. Of this, an estimated 99 mg m−2 a−1 was from anthropogenically-derived Pb, initially deposited onto drainage basin soils and subsequently entering the reservoir
via erosion and leaching processes. There is, therefore, no indication that the flux of Pb to the aquatic system is declining
in response to reductions in Pb deposition. The ecotoxicological effects of the high and continuing Pb flux to these reservoirs,
despite recent decreases in atmospheric deposition, is an area requiring further investigation. 相似文献
8.
Biogeochemical records of paleoenvironmental changes in Nainital Lake,Kumaun Himalayas,India 总被引:1,自引:0,他引:1
Preetam Choudhary Joyanto Routh Govind J. Chakrapani Bhishm Kumar 《Journal of Paleolimnology》2009,42(4):571-586
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns
about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield
a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC
flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain
hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of
the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the
lake has remained largely unchanged, and the perceived human-induced impacts are limited. 相似文献
9.
François Michaud Hermes U. Ramírez-Sánchez Claude Parron Pedro F. Zárate-del Valle François Fernex Geneviève Barci-Funel 《Journal of Paleolimnology》2006,35(4):819-836
Lake Chapala, located 120 km northeast of Colima Volcano, lies at the north and northeast of the Citala rift in the Trans-Mexican
Volcanic Belt. It belongs to the watershed of the Lerma River, which originates from the Mexico City area, 450 km to the east.
Sediment cores, 0.5–2 m long, were collected from the lake. Magnetic susceptibility of the lake sediments generally ranges
from 400 to 800 × 10−9 m3 kg−1; but in some layers it exceeds 1000 or 1500 × 10−9 m3 kg−1. The magnetic susceptibility vertical profiles display a thin peak (2–5 cm) or a double peak with magnetic susceptibility
> 2000 × 10−9 m3 kg−1. Scanning electron microscope analysis shows that the main mineral responsible for the magnetic susceptibility is titano-magnetite,
which is relatively abundant in the magnetic layers. In most of the cores, the layer with magnetic susceptibility > 2000 ×
10−9 m3 kg−1 is coarser grained than the directly under and overlying sediments, which are composed of fine silt. But not all coarser
levels are magnetic nor are all magnetic levels bound to coarser layers. The high titano-magnetic concentration probably originates
from volcanic ash falls. Sedimentation rates, evaluated for several sites by means of the 210Pb and 137Cs methods, allow a date approximation (between 1535 and 1670) for the strong magnetic layer deposition. Since important eruptions
of Colima Volcano, with ash fall, were reported from 1576 to 1623, the main peak of magnetic susceptibility in the sediment
profiles is assumed to be related to these events. The main magnetic layer extends to greater depth in the profiles of the
eastern part of the lake plain than at the west. It can, therefore, be suggested that a relative subsidence occurs in the
eastern part of the lake; the axis of the eastern plain overlaps the area displaying the maximum subsidence rate and seems
therefore to correspond to the prolongation of Citala rift. 相似文献
10.
The lead pollution history, based on the accumulation rate of total Pb and ratio of stable isotopes (206Pb/207Pb), was studied in the annually laminated sediment of a small lake in Finland (62°20′ N; 25°41′ E). The sediment chronology
based on varve counting provided a unique opportunity to explore and date signals of Pb emissions, including the ancient metallurgical
activities of the Roman Empire at the beginning of the Current Era. Changes in the ratio of stable isotopes gave a pronounced
signal of the atmospheric Pb fallout in AD 32–392, although this was not distinguishable in the accumulation rate of total
Pb, as it was observed in previous work. Calculated accumulations of the ancient pollution Pb were low, the highest values
being 0.2–0.3 mg m2 a−1 in AD 144–392, corresponding 14–21% of the accumulation of total Pb. The accumulation of pollution Pb collapsed in the fifth
century and remained at or close to the background level up to the eleventh century. After this, the accumulation rate of
pollution Pb began to increase and reached 1.2 mg m2 a−1 in AD 1420–1439, corresponding to 44% of the total Pb accumulation. During five centuries, from AD 1420–1895, the average
accumulation of pollution Pb was 2.6 mg m2 a−1, the variation being from 0.8 to 4.8 mg m2 a−1. The accumulation of Pb started to increase exponentially in the early twentieth century, and the highest accumulations of
pollution Pb (11–22 mg m2 a−1, corresponding 50–76% of the annual accumulation of total Pb) were dated to AD 1926–1985. The banning of the use of leaded
fuel has led to a pronounced decrease in the accumulation of pollution Pb since the 1980s, and the present accumulation rate
represents the level that prevailed 80–120 years ago. 相似文献
11.
Neil L. Rose David Morley Peter G. Appleby Richard W. Battarbee Tiiu Alliksaar Piero Guilizzoni Erik Jeppesen Atte Korhola Jaan-Mati Punning 《Journal of Paleolimnology》2011,45(4):447-468
Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many
over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment
soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment
of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day
conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions.
Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve
our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from 210Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different
types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates,
11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity
(3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake
classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior
to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general
acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six
lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm−2 yr−1) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm−2 yr−1). SARs for other lake-types ranged between 0.012 and 0.024 g cm−2 yr−1. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for
lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed
exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found
in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation
of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future. 相似文献
12.
Margarita Caballero Gabriela Vázquez Socorro Lozano-García Alejandro Rodríguez Susana Sosa-Nájera Ana Carolina Ruiz-Fernández Beatriz Ortega 《Journal of Paleolimnology》2006,35(1):83-97
The Sierra de Los Tuxtlas is a recently active volcanic field, with eruptions in 1664 and 1792. It holds one of the reserves
of tropical evergreen forest in Mexico, as well as several maar lakes. One of them, Lago Verde, was chosen for a three-fold
study (1) on its present limnological conditions, (2) on the algal community living in the water column and preserved in the
surface sediments; and (3) on its recent history (ca. 340 yr). The palaeolimnological study was based on multiproxy analyses
on core material dated by 210Pb, 137Cs and 14C. Lago Verde is a small, shallow lake with dilute, slightly alkaline water (CO32− + HCO3− > Cl− > SO42−, Na+ + K+ > Ca2+ > Mg2+). It is turbid, eutrophic, with high phosphorus levels. It is a warm polymictic lake, with thermal and oxygen stratification
establishing by midday during the warm months. The lake does not stratify in winter. Diatoms dominate the phytoplankton community
in the cold ‘nortes’ season, Cyanobacteria in summer, and Chlorophyta in autumn. Cyanobacteria (Chroococcales) are not well
preserved in the surface sediments while Chlorophyta are better preserved. Sedimentary diatoms are well preserved, dominated
by the three most abundant species in the water column: Achnanthidium minutissimum, Fragilaria capucina and Aulacoseira granulata. The base of the studied sequences is constrained by the historic eruption of 1664. The period from 1664 to 1963 is characterised
by a meso-eutrophic lake. Tropical Forest vegetation reaches maximum values between ca. 1800 and 1963. Between ca. 1785 and
1885 the lake was slightly shallower than in the rest of the 1664–1963 period, probably recording climatic variability. An
early pulse of anthropogenic disturbance was recorded by ca. 1921 and after ca. 1963 intense forest clearance and high erosion
rates led to a more turbid, more productive, nutrient-rich lake. The highest anthropogenic impact was reached by ca. 1988;
afterwards the lake and its basin reached a new balance, with the establishment of the present modern conditions. 相似文献
13.
I. D. L. Foster T. M. Mighall H. Proffitt D. E. Walling P. N. Owens 《Journal of Paleolimnology》2006,35(4):881-895
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
14.
Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment
profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating
methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the 210Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, 137Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by 210Pb for three cores from the central mud zone of the lake. Unsupported 210Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely
mixed for at least the last 75 years. Onset and maximum activity of the radioisotope 137Cs in the cores coincides with the 210Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of 137Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically
deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation
in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted 210Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to
be about 1950 by 210Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured
in one core, and the observed peak corresponds to a 210Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines
in native types correspond generally with the expected dates assigned by 210Pb dating. Conformity between the 210Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and
reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions. 相似文献
15.
Lead-210 methods have been used to establish a chronology of sedimentation extending back almost 80 years in Black Mountain Lagoon near Guyra on the New England Tablelands of north-eastern New South Wales, Australia. Estimates of the direct atmospheric fallout of unsupported 210Pb and historical records of the pattern of phosphorus input to the lake provide support for the 210Pb chronology. The sediments in the lake record an episode of disturbance which took place prior to c .1916, but after c .1790-1860. Since then, however, the site-specific rate of minerogenic sedimentation has been maintained at a relatively low and constant level of 0.93 kg m-2 a-1. [Note: the following symbols are used in this paper: a = year (annum), and d min-1 g-1 = radioactivity (spontaneous nuclear disintegrations per minute per gram of material).] This is despite major shifts in land use and intensification of agriculture, despite the occurrence of significant floods and droughts, and in the absence until recent times of the application of soil conservation practices. These rates may be contrasted with rates of 0.023 kg m-2 a-1 from the period 12.3-5.6 ka. These are at least an order of magnitude lower than those of the twentieth century. The lake sediments preserve evidence of the use of agricultural chemicals on the catchment during the twentieth century. These include phosphorus, zinc, arsenic and lead. Soils and sediments may act as long-term toxic stores for such chemicals, a problem often overlooked in rural areas. 相似文献
16.
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
17.
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
18.
James P. Terry Rajeev Lal Sitaram Garimella 《Singapore journal of tropical geography》2011,32(1):102-114
Low‐energy gamma ray spectroscopy has been employed to estimate floodplain sedimentation rates using measurements of 210Pb in floodplain alluvium. The utility of the technique is assessed through the analysis of excess (unsupported) 210Pb profiles in three sediment cores taken from the floodplain of the Labasa River on Vanua Levu in northern Fiji. A low‐energy germanium spectrometer (LEGe) was used for the nondestructive determination of excess 210Pb in a region cultivated intensively with sugarcane. Measured average historical (c. 25 years) vertical accretion rates are between 2.2 and 4.4 cm yr?1. The findings are broadly comparable with published sedimentation rates from analyses of radionuclide profiles elsewhere in the tropical South Pacific Islands, but the rates are higher than those measured previously at the same Labasa River sites using 137Cs profiles. Accelerated soil erosion owing to cane burning and land tillage seems to be largely responsible for sediment production, although flood‐related effects such as channel accretion by coarse bedload and the emplacement of large organic debris also influence floodplain sedimentation. However, application of the 210Pb technique in Fiji (and perhaps neighbouring island countries) is found to have serious drawbacks compared to the more robust 137Cs method, owing principally to the low 210Pb concentrations in the sandy alluvial sediment tested. 相似文献
19.
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
20.
A. Laissaoui M. Benmansour N. Ziad M. Ibn Majah J. M. Abril S. Mulsow 《Journal of Paleolimnology》2008,40(3):823-833
Global fallout is the main source of anthropogenic radionuclides in the Mediterranean Sea. This work presents 137Cs, 239+240Pu and 241Am concentrations in the water column in the southwest Alboran Sea, which was sampled in December 1999. A sediment core was
taken at 800 m depth in the area (35°47′ N, 04°48′ W). 210Pb, 226Ra, 137Cs and 239+240Pu specific activities were measured at multiple depths in the core for dating purposes. 137Cs and 239+240Pu profiles did not show defined peaks that could be used as time markers, and they extended up to depths for which the 210Pb-based constant rate of supply (CRS) dating model provided inconsistent dates. These profiles can be useful to test dating
models, understood as particular solutions of a general advection–diffusion problem, if the time series of radionuclide inputs
into the sediment is provided. Thus, historical records of depth-averaged 137Cs and 239+240Pu concentrations in water, and their corresponding fluxes into the sediment, were reconstructed. A simple water-column model
was used for this purpose, involving atmospheric fallout, measured distribution coefficient (k
d) values, and a first-estimate of sedimentation rates. A dating model of constant mixing with constant sedimentation rate
was applied successfully to three independent records (unsupported 210Pb, 137Cs and 239+240Pu), and provided the objective determination of mixing parameters and mass sedimentation rate. These results provide some
insight into the fate of atmospheric inputs to this marine environment and, particularly, into the contribution from the Chernobyl
accident. 相似文献