Difficulties in interpreting fast mixing in the radiometric dating of sediments using 210Pb and 137Cs

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

Environmental Change and Atmospheric Contamination on Svalbard: Sediment Chronology

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.     

云南抚仙湖近现代沉积速率变化研究

借助GPS定位,采集了云南抚仙湖不同区域的7 个沉积物柱芯。通过对抚仙湖沉积物柱芯样品的放射性核素²¹⁰Pb 和¹³⁷Cs 测试分析,发现所有柱芯中均存在3 个公认的¹³⁷Cs 计年时标(1954 年¹³⁷Cs首次沉降、1963 年全球公认的¹³⁷Cs最大沉降蓄积峰和1986 年前苏联切尔诺贝利核事故产生的¹³⁷Cs 沉降蓄积峰),部分柱芯中存在20 世纪70 年代中期中国大气核试验形成的1975/1976 年次级蓄积峰,分析了该次级定年时标存在的合理性。借助²¹⁰Pb CRS计年模式获得了抚仙湖过去百余年来的沉积年代,与¹³⁷Cs 时标计年结果相比较存在一定偏差,对两种计年结果产生差异的可能原因进行了探讨。基于²¹⁰Pb 和¹³⁷Cs 计年结果,结合历史文献记载得出,自19 世纪中期以来,抚仙湖各个沉积物柱芯(FX6 除外) 的沉积速率变化规律具有相似性,大致可以划分为3 个阶段：A：自然演化阶段,B：人为扰动阶段,C：人为改造阶段。这种不稳定的沉积环境与抚仙湖地区相应历史时期的人类活动有密切关系,表明人类活动是影响短时间尺度下环境变化的主要驱动力。     

Chronology of Sediment Deposition in Upper Klamath Lake, Oregon

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.     

How reliable is the210Pb dating method? Old and new results from Switzerland

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 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

1400-year cold/warm fluctuations reflected by environmental magnetism of a lake sediment core from the Chen Co, southern Tibet, China

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.     

The design and performance of a new box corer for collecting undisturbed samples of soft subaquatic sediments

The Baikal sediment box corer represents an innovation in design of the closing mechanism and involves a single, thin, and flexible stainless steel blade rather than closing jaws. With this light-weight box corer only a relatively small cross-sectional area is offered to the sediment, allowing easier penetration. The closure blade is propelled smoothly across the base of the box in a downwardly convex path by a set of constant force springs. Depending on choice of blade-release mechanisms, the corer can either be triggered immediately as the support cable slackens, after a delay of a few seconds, or immediately upon retrieval. The box corer has been used successfully to sample poorly consolidated sediments in freshwater lakes over a depth range of 8 to 1624 m. Cores of deep-water surficial sediment from Lake Baikal were tested for representativity and replicability by profiling natural and artificial radioisotopes and lithostratigraphic features.     

Sedimentary response of Pagham Harbour, southern England to barrier breaching in AD 1910

This paper examines the evolution of Pagham Harbour, southern England following storm-induced breaching of a protective barrier in AD 1910. Stratigraphic studies of sediments collected from intertidal areas show the presence of a distinct stratigraphic horizon in the northeast of Pagham Harbour at ca. 0 to +1.0 m ordnance datum (OD). Radiometric data indicate that this horizon is a ‘reclamation surface’ formed after land claim in AD 1846. Following marine flooding in AD 1910, sediment has accreted relatively rapidly (at a broadly constant rate of between 4 and 8 mm a⁻¹), wave and/or tidal energy have decreased and extensive marshes have developed. An asymptotic reduction in sediment accretion rate through time, as predicted in various theoretical models of salt marsh accretion, is not observed. Over the entire Pagham Harbour area, the period between AD 1948 and AD 1986 has seen an average marsh loss of 0.0087 km² a⁻¹, which is relatively small in comparison with other more exposed sites in the local area. Historically breached sites such as Pagham Harbour are common around European coasts, and these provide important natural laboratories within which the medium-term (decadal to centennial) coastal response to barrier breaching, and to managed-realignment coastal protection schemes, can be assessed.     

210Pb、137Cs 计年法在现代海岸带沉积速率 研究中的应用述评

²¹⁰Pb、¹³⁷Cs 计年法已被国内外广泛用于湖泊、河流和海洋现代沉积速率的研究中。本文在 回顾了210Pb、137Cs 计年法应用于海岸带沉积速率研究的现状以及存在问题的基础上, 根据海岸带 沉积环境特征分析了²¹⁰Pb、¹³⁷Cs 计年法在应用中应当注意的问题, 包括采样精准性, 样品分离的 分辨率, 数据校正方法的选择及两种方法的相互印证等。从几个方面探讨了提高沉积速率测定精 度的可能性, 同时指出, ²¹⁰Pb、¹³⁷Cs 的扩散混合模型的建立, ¹³⁷Cs 沉积滞后的问题, 以及¹³⁷Cs、²¹⁰Pb 计年法的应用范围等仍有待深入探讨。     

Evaluation of210Pb dating in lake sediments using stable Pb,Ambrosia pollen,and137Cs

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

A vacuum sampler for subsampling freeze-dried laminated sediments with the application toin situ frozen varves of Mondsee,Austria

A method was developed for subsampling freeze-dried laminated sediments that employs a vacuum to draw off sediment from the sample in a precise way. The technique was tested on anin situ frozen sediment core of Mondsee containing varves 2 to 7 mm thick. Material was subsampled for¹³⁷Cs dating.     

210Pb and 137Cs dating methods in lakes: A retrospective study

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

Application of210Pb in soils

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.     

Mixing, sediment accumulation and focusing using 210Pb and 137Cs

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

Post-depositional 137Cs Mobility in the Sediments of Three Shallow Coastal Lagoons, SW England

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 (²¹⁰Pb_un) 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.     

Low-background gamma counting: applications for210Pb dating of sediments

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.     

云南滇池近现代沉积速率及气候干湿变化的粒度记录

通过对云南滇池沉积物柱芯DC1样品的放射性核素137Cs和Pb210测试分析,发现该柱芯Cs137自1954年首次沉降以来存在1963年、1975年和1986年三个较为明显的蓄积峰,获得滇池自Cs137相应的时标年份到2007年的平均沉积速率分别为0.062g/cm2·a-1、0.051cm2·a-1, 0.049cm...     

Sediment accumulation of Dianchi Lake determined by 137Cs dating

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

The Fossil Record of Freshwater Micro-Algae <Emphasis Type="Italic">Pediastrum</Emphasis> Meyen (Chlorophyceae) in Southern South America

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.     

江苏北部潮滩沉积物中~(137)Cs和~(210)pb的分布特征(英文)

Seven cores were collected from different sediment zones of tidal flats at Xin-yanggang in north Jiangsu province in August 2007. Sediment grain-size distribution and radioisotopes of ~(137)Cs and ~(210)Pb analysis were carried out for these cores. Sediment rates of the cores and radioisotopes distribution in surface sediment in different zones of the tidal flat were calculated from the ~(137)Cs and ~(210)Pb activities in sediments cores. The results indicated that each tidal zone had experienced different evolution phases, hydrological dynamics in the tidal flats made the grain-size of the surface sediment change gradually. ~(137)Cs and ~(210)Pb activities on the superficial layer of the cores varied spatially and the reason was discussed. On tidal flats, the fluctuation of ~(137)Cs and ~(210)Pb activities in the cores reflected the special sedimentary characteristics. Vegetation affects the grain-size distribution and the vertical profiles of ~(137)Cs and ~(210)Pb in the upper depths. ~(137)Cs and ~(210)Pb chronology got the comparable average sediment rates on the tidal flat. The characteristics of ~(137)Cs and ~(210)Pb in the cores reflected various depositional dynamical environments in different tidal zones and gave information on the different evolvement phases of the tidal zones. Based on the information of grain-size distribution, texture of the cores, sediment rates and topography, the evolution lines of the tidal flat were reconstructed.