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1.
Samples of surface deposits in the Lake Sugan catchment, as well as surface lake sediments, eolian materials occulted in the lake ice cover, and airborne dust were collected for grain-size analysis. The results show that the coarse fraction of the lake sediments could be transported by ambient winds and to a lesser extent by river flow in the study area. Sediment cores were retrieved from Lake Sugan in December 2000, and 210Pb and 137Cs dating and grain-size analyses were performed on these samples. 210Pb ages and the volume percentage of the fraction of lake sediments >63 μm were used to reconstruct the dust storm history from 1957 to 2000. Observational data for dust storm events collected at a local meteorological station largely agrees with the reconstructed trend for the past 44 years, suggesting that lake sediments can be employed to trace the dust storm history of the northern Qinghai–Tibetan Plateau.  相似文献   

2.
Vast palaeolakes once occupied the Qinghai‐Tibetan Plateau (TP). Analyses of the sedimentary records of these lakes could potentially provide an extensive dating archive. Many previously constructed age‐depth models simulating lacustrine cores have been principally based on radiocarbon (14C) dating. However, such dating could have been hampered by the so‐called ‘lake reservoir effect’ (LRE) and the reworking of lakebed sediments, resulting in inaccurate 14C age‐depth models and limiting interpretations of existing lacustrine palaeoclimatic records. Lake Linggo Co is located on the central TP, in one of the coldest and most arid regions of Tibet. We dated a 9.87‐m‐long lacustrine core extracted from the lakebed at a water depth of ~60 m using a combination of 210Pb, 14C and optically stimulated luminescence (OSL) techniques. Some 14C ages showed significant age reversals; all the 14C ages were much older than the 210Pb and OSL ages for the same depths. This could possibly be attributed to the presence of old, inherited carbon, causing the inherited 14C age to appear unstable during the deposition period. The 210Pb and OSL ages were roughly concordant, and were also consistent with the stratigraphical succession. We therefore suggest that 14C dating may, on its own, be inadequate for accurate dating of lacustrine sediment sites on the TP, and that the OSL method should also be applied in order to evaluate the reliability of any 14C ages. With this approach, we constructed an age‐depth model, revealing sedimentation rates of 1.7, 0.8, 6.8 and 0.6 mm a−1 between 0–1.9, 1.9–4.2, 4.2–4.4 and 4.4–9.4 ka, respectively.  相似文献   

3.
 Bottom-water data and trace metal concentration of Cu, Cr, Ni, Pb, Co, Zn, and organic matter in surficial sediment samples from 13 sampling stations of Lake Chapala in Mexico were studied. The lake is turbid with a great amount of flocculated sediments as a result of wind mixing, sediment re-suspension, and Lerma River discharges. Al distribution pattern in sediments was used as an indicator of the Lerma River discharges into Lake Chapala. The highest values of Cu (33.27 ppm), Cr (81.94 ppm), Pb (99.8 ppm), and Zn (149.7 ppm) were detected in sediments near the lake outlet. The bioavailable metal fraction is low for all metals except Pb, which shows 65–93% of the total metal concentration in bioavailable form. The minimum energy zone in the lake was related to organic matter concentration and was located in the SE part of the lake. An analysis of the studied parameters shows two zones: eastern zone (fluvio-deltaic) and central-western zone (lacustrine). Received: 9 September 1998 · Accepted: 16 November 1998  相似文献   

4.
An iterative least-squares optimization technique is utilized in conjunction with a one-dimensional representation of the mass transport equation to generate theoretical210Pb concentration/depth profiles beneath the water-sediment interface that are best-fit approximations to directly measured210Pb concentration/depth profiles at various locations within the Great Lakes system. The outputs of such an optimization analysis are the diffusion coefficientsD M (molecular) andD B (bioturbation) associated with the transport of210Pb radionuclides in lake bed sediments. For all stations studied, the estimated values ofD B are consistently larger than the estimated values ofD M , emphasizing the importance of accounting for the effects of bioturbation in the modelling of contaminant transport through lake bed sediments.  相似文献   

5.
Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleo- and active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appear as small islands composed of solid, viscous and black bitumen. Aliquots of tar were analyzed by GC-MS and the mixtures are comprised of geologically mature biomarkers and an UCM. PAH and n-alkanes are not detectable. The biomarkers consist mainly of hopanes, gammacerane, tricyclic terpanes, carotane and its cracking products, steranes, and drimanes. The biomarker composition and bulk C isotope composition (δ13C = −21.4%) indicate an organic matter source from bacteria and algae, typical of lacustrine ecosystems. The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala exceeding 40 ka (14C) in age and then forced to the lakebed by tectonic activity. The absence of alkanes and the presence of an UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 °C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered in future energy resource exploration in such regions.  相似文献   

6.
Economic reform in China since 1978 has accelerated economic development nationwide hugely, but has also brought about some environmental pollution. In order to identify the primary Pb source to the atmosphere in the central Guizhou region, Pb isotopic ratios in the acid soluble fraction of sediment from Hongfeng Lake were investigated. Lead isotopes in the lake sediments record the history of regional atmospheric Pb pollution. Before the economic reform in 1978, the 208Pb/206Pb and 206Pb/207Pb ratios in the leachates of lake sediments were constant, with a range of 2.0060 to 2.0117 and of 1.2314 to 1.2355, respectively. In the early period of economic reform (1978 to 1988), with the rapid industrial growth in Guizhou province, the acid soluble Pb isotope ratios in the lake sediments changed sharply: the 208Pb/206Pb ratios increased from 2.0212 to about 2.05, while the 206Pb/207Pb ratios decreased from 1.2251 to 1.2060. Emissions from Pb-ore-related industries are suggested to be the major pollution source of Pb in this period. Due to output from a local power plant since 1988, the isotope ratios of the acid soluble Pb in sediments in 1990s are characterized by a little higher radiogenic Pb (208Pb/206Pb = 2.0340–2.0400; 206Pb/207Pb = 1.2122–1.2158) than for the 1980s.  相似文献   

7.
This study evaluates the accuracy of optically stimulated luminescence to date well-preserved strandline sequences at Manistique/Thompson bay (Lake Michigan), and Tahquamenon and Grand Traverse Bays (Lake Superior) that span the past ∼4500 yr. The single aliquot regeneration (SAR) method is applied to produce absolute ages for littoral and eolian sediments. SAR ages are compared against AMS and conventional 14C ages on swale organics. Modern littoral and eolian sediments yield SAR ages <100 yr indicating near, if not complete, solar resetting of luminescence prior to deposition. Beach ridges that yield SAR ages <2000 yr show general agreement with corresponding 14C ages on swale organics. Significant variability in 14C ages >2000 cal yr B.P. complicates comparison to SAR ages at all sites. However, a SAR age of 4280 ± 390 yr (UIC913) on ridge77 at Tahquamenon Bay is consistent with regional regression from the high lake level of the Nipissing II phase ca. 4500 cal yr B.P. SAR ages indicate a decrease in ridge formation rate after ∼1500 yr ago, likely reflecting separation of Lake Superior from lakes Huron and Michigan. This study shows that SAR is a credible alternative to 14C methods for dating littoral and eolian landforms in Great Lakes and other coastal strandplains where 14C methods prove problematic.  相似文献   

8.
博斯腾湖湖泊沉积物光释光年代测量*   总被引：2，自引：0，他引：2  
使用光释光年代学的单片再生法测量了博斯腾湖沉积剖面中碳酸盐泥及粉砂质泥底部的浅湖相灰色粉细砂和风成沙的年龄,对剖面上部碳酸盐层中陆生植物残体进行了AMS 14 C测年。通过不同测片的等效剂量(De)值的分布状况评价了样品的晒褪程度,选择不随灵敏度校正后的自然释光信号变化的相对集中的等效剂量(De)值计算了样品的埋藏年龄。通过这些年龄结果的对比,发现石英矿物的OSL年龄和AMS 14 C年龄在地层上是一致的,表明尽管在浅湖相细砂中存在不完全晒褪,但根据相对较小而集中的De值计算得到的年龄结果是可靠的。这些年龄结果和地层资料揭示末次冰消期以来至早全新世,博斯腾湖处于无水干盆地向深水湖泊转化的浅水湖泊状态,现代深水博斯腾湖大约形成于距今8ka前后。  相似文献   

9.
Sedimentation rates were determined with the 210Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm?2 y?1 in the central part to 0.13 g cm?2 y?1 in the eastern part of the lake and then increases rapidly towards the Rhine delta. In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the 210Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production. Dating with 137Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones. A positive correlation of 210Pb fluxes and sedimentation rates indicates that 210Pb flux into sediments follows the distribution pattern of solids. 210Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long-range particulate input variations remains to be proved. Sedimentation rates determined with the 210Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.  相似文献   

10.
The distribution of 210Pb, 137Cs, and Ambrosia (ragweed) pollen in two sediment cores from Lake Ontario and in three cores from Lake Erie provides independent estimates of sediment accumulation rates. Geochronology with 210Pb is based on radioactive decay of the isotope following burial in sediments. The method can reveal with precision changes in sedimentation occurring over the past 100 yr or so. Geochronologies with 137Cs and Ambrosia are based on the occurrence of a horizon corresponding, respectively, to the onset of nuclear testing 25 yr ago and to regional forest clearance in the middle 1800s. These methods provide estimates of long-term average sediment accumulation rates. In all but one core, the distributions of 137Cs and 210Pb indicate no physical mixing of near-surface sediments. In two cores, including one from central Lake Erie collected by diver, all three estimates of sedimentation rates are in excellent agreement. In two other cores, rates based on 210Pb are significantly higher than those inferred from Ambrosia pollen profiles. Lower average rates appear to result from occasional massive losses of sediments. Such events, apparent in the distribution of 210Pb but not in pollen records, correlate with the occurrence of major storm surges on the lakes during this century. In one core from western Lake Erie, exponential distributions of both 210Pb and Ambrosia appear to be artifacts which may result from extensive biological or physical reworking of sediments in shallow water (11 m). Previous indications of increased sedimentation in Lake Erie since about 1935 based on Castanea (chestnut) pollen data are not substantiated.  相似文献   

11.
《Applied Geochemistry》2006,21(4):701-712
Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250 °C over brief geological times.  相似文献   

12.
Deer Lake is an impoundment located near Ishpeming, Michigan, USA. Iron mining assay laboratories located in Ishpeming disposed of Hg salts to the city sewer whose outfall was located along an inlet to Deer Lake. An effort to remediate the system in the mid 1980s which consisted of drawing down water in the impoundment in order to volatize Hg from the sediments did not result in recovery of the system. Since the mid 1990s, the remediation strategy has been to allow the continual burial of the contaminated sediments, i.e., natural recovery. The goal of this study was to assess the effectiveness of this strategy. This was accomplished by investigating the state of the system in terms of its recovery and estimating the time frame for recovery. Sediment cores were collected in 2000 to determine historical trends in accumulation rates and concentrations of Hg and other metals. Sedimentation rates and sediment ages were estimated using 210Pb. Event-based dating (e.g., peak of 137Cs in 1963) was used to supplement 210Pb data due to non-monotonic features in the 210Pb profile and activities that were not at supported levels at the base of the core. Selected results are that: (1) drawdown significantly influenced sedimentation patterns causing slopes for 210Pb profiles that reflected the influx of older sediment, (2) periods of Fe production correlate to Hg loading indicating the point source for contamination, a relationship not previously identified, (3) Hg:Al ratios indicate a recent change to a watershed pathway for Hg loading and (4) Hg concentrations had decreased from their peak, remain elevated, and were increasing after 1997. The cause of the recent Hg concentrations may be related to influx of contaminated watershed soils or sediments. Estimating the time frame for recovery is challenging in this system because the process of natural recovery seems to have been arrested and deeper, uncontaminated sediments, were not recovered as a basis for reference. However, a recovery to background conditions is likely not achievable since rates of Hg loading to nearby lakes and the current rate of atmospheric deposition are greater than an estimate of background conditions for Deer Lake. Assuming recovery continued after 2000, estimates of the time required for recovery varied based on the system state used to define it (e.g., recent rates of wet Hg deposition or Hg surface concentrations/fluxes from similar systems), but were less than 12 a. However, the recent increasing values of recovery indicators (e.g., Hg concentrations) suggests that these estimates are conservative and will be longer if recovery remains arrested, which may in part be due to the legacy of Hg contamination on the landscape. This study shows that estimates of recovery of highly disturbed lake systems can be made in the absence of within lake reference conditions by using comparisons to reference systems and challenges of estimating ages from atypical 210Pb activity profiles can be overcome in part using event-based dating techniques.  相似文献   

13.
The age framework of Qarhan Salt Lake in arid western China is still controversial due in part to (1) age discrepancy between conventional 14C and 230Th dating results, and (2) no AMS 14C ages of organic carbon from drilling cores in Qarhan Salt Lake were reported until now. In order to discuss these chronological problems, upper 54.50 m lacustrine sediments from a drilling core (ISL1A) recovered from Qarhan Salt Lake were dated based on 230Th and AMS 14C dating techniques. Results show that (1) AMS 14C ages of total organic carbon (TOC) from 4.65 to 30.29 m are almost in stratigraphic order and consistent with 230Th ages of halite in the corresponding layers; (2) AMS 14C ages of TOC from 30.29 to 54.50 m are younger with increasing depth. This phenomenon was also found in Shell Bar in the study area, suggesting that AMS 14C ages from upper 30.29 m are more reliable while those from lower 24.21 m in ISL1A may be underestimated; (3) 230Th ages of halite from lower 24 m lacustrine sediments are obviously older than AMS 14C ages of TOC in the corresponding layers, which results into different age framework of salt lake sediments in Qarhan Salt Lake; (4) if extrapolating these reliable AMS 14C ages in ISL1A, similar age framework with 230Th ages in this core confirms that 230Th ages are much close to the true ages of these sediments, which suggests that the forming timing of the bottom salt layer is ~50 ka.  相似文献   

14.
The deposits of Glacial Lake Quincy overlie a diamicton associated with the classically defined Illinoian limit in central Indiana. This lake covered at least 180 km2 with a depth of > 20 m and developed when the Illinoian ice sheet retreated 15 km from the maximum limit, causing lake impoundment against Devore Ridge. Overflow from Glacial Lake Quincy eroded across the ridge forming a number of steeped-walled outlets. A section along Mill Creek exposes a sedimentologic sequence associated with Glacial Lake Quincy from a subglacial diamicton to ice-proximal to ice-distal glacial lacustrine sediments. We report new optical ages by multiple aliquot regenerative dose procedure for the fine-grained rhythmically bedded sediments presumed to represent the lowest energy depositional facies, dominated by suspension settling, which maximized sunlight exposure. In turn, optical ages were determined on the fine-grained (4-11 μm) polymineral and quartz fractions under infrared and blue excitation, which yielded statistically similar ages. Optical ages span from ca. 170 to 108 ka, with the average of 16 optical ages indicating deglaciation at ca. 135 ka, generally coincident with Marine Oxygen Isotope Stage 6-to-5 transition and rise in global sea level.  相似文献   

15.
High-time resolution 14C dating of Lake Baikal sediment cores indicates negative and positive anomalies of calculated linear sedimentation rate (LSR; 1.1 and 35.6 cm/ka, respectively) during the period of climate transition from the last glacial to Holocene. The timing of the Lake Baikal apparent LSR anomalies is consistent with that of the changes in the atmospheric radiocarbon concentration (Δ14C) during Younger Dryas rapid cooling event. 14C dating of lipids in the Lake Baikal surface sediments revealed that the sources of sedimentary lipids were different in each basin. In the Northern Basin of Lake Baikal, the 14C age of total lipids from the surface sediment (4.0 14C ka) was found to be older than that of TOC (1.6 14C ka). By contrast, the 14C age of total lipids in the Southern Basin was younger than that of the TOC by ca. 0.7–3.0 ka.In the Lake Hovsgol sediment cores, ages of the main lithologic boundaries during the last glacial–interglacial transition were estimated based on new 14C data sets. TOC concentration in the cores started to rapidly increase at 13.8 ± 0.3 14C ka at the base of the basinwide finely laminated layer deposited during Bølling/Allerød. The base of the layer diatomaceous mud corresponds to the end of Younger Dryas event (10.6 ± 0.1 14C ka).  相似文献   

16.
This article presents the results of a geochemical investigation of sediments from Lake Druzno (northern Poland), a reservoir fed by freshwater from the catchment, with periodic input of brackish water from Vistula Lagoon. This study analyzed the spatial variation in heavy metal content in surface sediments as well as the temporal changes in metal content in two sediment cores dated using the 210Pb method. In the surface sediments, the highest metal concentrations were recorded in the northern part of the lake, with lower concentrations in the central and southern parts. Absolute values of metal concentrations in the cores were low, but normalization with respect to Al showed an increase during the second half of the 20th century. Mean enrichment factors (EF) in sediments from the second half of the 20th century ranged from insignificant (1–1.2) for Fe to (1.55–3.3) for Cu, Cd, Pb and Zn. The sediments deposited before 1950 had lower EF values (>1.5) and had low variability. Results from both the surface sediments and the cores indicate that the main source of lake pollution is brackish water intrusion from the Vistula Lagoon via the contaminated River Elbląg.  相似文献   

17.
210Pb, 137Cs and 14C dated sediments of two late Holocene landslide lakes in the Provincial Park Lagunas de Yala (Laguna Rodeo, Laguna Comedero, 24°06′S, 65°30′W, 2100 m asl, northwestern Argentina) reveal a high-resolution multi-proxy data set of climate change and human impact for the past ca. 2000 years. Comparison of the lake sediment data set for the 20th century (sediment mass accumulation rates MARs, pollen spectra, nutrient and charcoal fluxes) with independent dendroecological data from the catchment (fire scars, tree growth) and long regional precipitation series (from 1934 onwards) show that (1) the lake sediment data set is internally highly consistent and compares well with independent data sets, (2) the chronology of the sediment is reliable, (3) large fires (1940s, 1983/1984–1989) as documented in the local fire scar frequency are recorded in the charcoal flux to the lake sediments and coincide with low wet-season precipitation rates (e.g., 1940s, 1983/1984) and/or high interannual precipitation variability (late 1940s), and (4) the regional increase in precipitation after 1970 is recorded in an increase in the MARs (L. Rodeo from 100 to 390 mg cm−2 yr−1) and in an increase in fern spores reflecting wet vegetation. The most significant change in MARs and nutrient fluxes (Corg and P) of the past 2000 years is observed with the transition from the Inca Empire to the Spanish Conquest around 1600 AD. Compared with the pre-17th century conditions, MARs increased by a factor of ca. 5 to >8 (to 800 +130, −280 mg cm−2 yr−1), PO4 fluxes increased by a factor of 7, and Corg fluxes by a factor of 10.5 for the time between 1640 and 1930 AD. 17th to 19th century MARs and nutrient fluxes also exceed 20th century values. Excess Pb deposition as indicated by a significant increase in Pb/Zr and Pb/Rb ratios in the sediments after the 1950s coincides with a rapid expansion of the regional mining industry. Excess Pb is interpreted as atmospheric deposition and direct human impact due to Pb smelting.  相似文献   

18.
Because of several difficulties with the application of radiocarbon (14C) dating to Arctic Ocean sediments, numeric dating techniques are needed that can complement, supplant and reach beyond the 14C method. However, large age overestimates (often >7 kyr) for near-sediment-water-interface horizons from Arctic Ocean cores have been almost universal when luminescence dating has been applied to multigrain aliquots of fine silt (4–11 μm) quartz and feldspar grains. Here micro-hole quartz-grain photon-stimulated-luminescence (PSL) dating is applied to the 0.5–2.0 cm horizons of multicores from high-sedimentation-rate sites spanning depths from 87 m to 1140 m at the Alaska margin of the Arctic Ocean. Expected near zero ages (0–200 a) result when grains larger than ～11 μm are used, demonstrating that fine-silt age overestimations here and perhaps elsewhere in the Arctic Ocean are a function of grain and aliquot size. At the 87 m site, the micro-hole PSL approach revealed no significant gradient in age estimates over the 1–26 cm horizon range, implying that bioturbation reached to at least 26 cm. Micro-hole PSL dating of 25–62 μm quartz grains from trans-ocean sea-ice sediment also produced expected near zero ages, in contrast to earlier reported long-bleach multigrain PSL results from 4–11 μm fractions of the same samples. The micro-hole PSL approach thus surmounts the age overestimation problem associated with the use of multigrain silt fractions, and overcomes limitations of the 14C method in this region. Finally, results unexpectedly suggest the potential of micro-hole quartz PSL for use in provenance studies of Arctic Ocean sea-ice sediment.  相似文献   

19.
In regions with limited knowledge of the historical volcanic record, like remote areas in the Andean Southern Volcanic Zone, the definition of reliable age-depth models for lake sequences represents a valuable tool for tephra layers dating. In Lake Futalaufquen (42.8°S), Northern Patagonia, a short sedimentary sequence was extracted after the AD 2008 Chaitén eruption with the purpose to analyze the records of volcanic eruptions at these poorly studied latitudes. The sequence was dated by 210Pb, 137Cs, and 14C techniques. Five tephras were identified for the last 1600 years, restricted to the last 5 centuries. Sedimentology, morphology, and geochemical properties allowed the characterization of the tephras and their correlation with tephras recently identified proximal to the sources, mainly from Chaitén and Huequi volcanoes, and Michinmahuida accessory cones, representing the first distal records reported of these tephras. Furthermore, tephras modeled ages obtained by the sequence age-depth model shrink the ages for the volcanic events, like a potential cycle of activity from Michinmauida accessory cones during AD 1530 ± 55, one eruption from Huequi volcano at AD 1695 ± 50, and a possible recent eruption from Chaitén at AD 1775 ± 40. Additionally, the work contributes to improve the regional volcanic records knowledge, basic for volcanic hazard assessment.  相似文献   

20.
Sediments of the Lagoa Vermelha (Red Lake), situated in the Ribeira Valley, southeastern Brazil, are made of a homogeneous, organic-rich, black clay with no visible sedimentary structures. The inorganic geochemical record (Al, As, Ba, Br, Co,Cs, Cr, Fe, Mn, Ni, Rb, Sc, Sb, V, Zn, Hg and Pb) of the lake sediments was analyzed in a core spanning 2430 years. The largest temporal changes in trace metal contents occurred approximately within the last 180 years. Recent sediments were found to be enriched in Pb, Zn, Hg, Ni, Mn, Br and Sb (more than 2-fold increase with respect to the “natural background level”). The enhanced accumulation of Br, Sb, and Mn was attributed to biogeochemical processes and diagenesis. On the other hand, the anomalous concentrations of Pb, Zn, Hg and Ni were attributed to pollution. As Lagoa Vermelha is located in a relatively pristine area, far removed from direct contamination sources, the increased metal contents of surface sediments most likely resulted from atmospheric fallout. Stable Pb isotopes provided additional evidence for anthropogenic contamination. The shift of 206Pb/207Pb ratios toward decreasing values in the increasingly younger sediments is consistent with an increasing contribution of airborne anthropogenic lead. In the uppermost sediments (0-10 cm), the lowest values of the 206Pb/207Pb ratios may reflect the influence of the less radiogenic Pb from the Ribeira Valley District ores (206Pb/207Pb between 1.04 and 1.10), emitted during the last 50 years.  相似文献   

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