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1.
210Pb is widely used for dating recent sediments in the aquatic environment; however, our experiences working in shallow coastal environments in the Pacific coast of Mexico have demonstrated that the potential of 210Pb for reliable historical reconstructions might be limited by the low 210Pb atmospheric fallout, sediment mixing, abundance of coarse sediments and the lack of 137Cs signal for 210Pb corroboration. This work discusses the difficulties in obtaining adequate sedimentary records for geochronological reconstruction in such active and complex settings, including examples of 210Pb geochronologies based on sediment profiles collected in two contrasting areas coastal areas (mudflats associated to coastal lagoons of Sinaloa State and the continental shelf of the Gulf of Tehuantepec), in which geochemical data was used to support the temporal frame established and the changes in sediment supply recorded in the sediment cores which were related to the development of land-based activities during the last century.  相似文献   

2.
Use of fallout radionuclides as indicators of erosion processes   总被引：1，自引：0，他引：1  
The different depth penetration characteristics of 137Cs, 7Be and 210Pb excess in undisturbed soils can be used to identify erosion processes by analysis of sediments derived from surface erosion. Caesium-137 concentrations (half-life 30 years) typically decrease to half the surface value at between 30 and 50 mm. Beryllium-7 (half-life 53 days) has half-penetration depths of between 0.7 and 10 mm, whereas 210Pb excess (half-life 20.2 years) has half-penetration depths between 10 and 30 mm. Experiments designed to determine the applicability of these depth penetration characteristics to soil erosion studies are reported. Surface runoff was artificially generated at two locations in a grazed paddock using a rainfall simulator. Suspended sediment was extracted from runoff and analysed for natural and artificial gamma emitting radio-nuclides. Suspended sediment derived from sheet flow contained initially high values of 137Cs, 7Be and 210Pb excess. As the experiment continued 137Cs concentrations remained high, but 7Be and 210Pb excess value decreased with time. This is interpreted as indicating a change from sheet dominated erosion to rill dominated erosion. During a second experiment artificial rain was allowed to fall onto an eroded gully wall. The derived suspended sediment contained no detectable 137Cs, 7Be or 210Pb excess. Overland flow from above the gully wall was then allowed to run down the gully face and mix with the water falling directly onto the gully wall. There was no detectable change in the radionuclide signature, showing that the gully wall was the predominant source of sediment. This was tested independently by mass balance and 226Ra to 232Th ratios. The good correlation between 210Pb excess and 7Be at this site suggests that the differential technique described here may be applicable over time-scales longer than are possible with 7Be. It may therefore be practical to examine catchment erosion history through analysis of 210Pb excess and 137Cs in sediment cores.  相似文献   

3.
210Pb- and137Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with210Pb/210Po and revealed sediment accumulation rates of (0.055±0.015) g·cm?2·y?1 and (0.09±0.03) g·cm?2·y?1, respectively. A comparable rate of (0.07±0.01) g·cm?2·y?1 has been obtained from137Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm?2·y?1. Constant210Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct137Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of7Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant210Pb activities. Remobilization may also be the reason for an incomplete210Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the210Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.  相似文献   

4.
At two locations in southern Lake Huron (U.S.A.), twelve 35.5-cm diameter cores of fine-grained sediments were taken for comparison of the vertical distributions of210Pb and falllout137Cs with the distributions of benthic macroinvertebrates, mainly oligochaete worms (Tubificidae) and the amphipod,Pontoporeia affinis. Locations were selected on the basis of210Pb distributions measured a year earlier which indicated contrasting depths of mixing of surface sediments. At one location the activity of210Pb is uniform down to about 6 cm and 95% of total invertebrates occur within this zone; at the other location the zone of constant activity is only 3 cm deep but 90% of the invertebrates occur within it. Comparison of published tubificid defecation rates with sediment accumulation rates based on210Pb shows that oligochaetes alone can account for mixing in one case while the effects of amphipods may be required in the case of shallower mixing. If mixing is represented as a diffusional process, eddy diffusion coefficients are at least 5.8 and 3.3 cm2 yr?1 at respective locations. In comparison with bioturbation, molecular diffusion is of minor importance in the post-depositional mobility of137Cs. The necessity for introducing a diffusion coefficient varying continuously with depth is indicated by characteristics of the distribution of137Cs. Biological reworking of near-surface sediments is an important process affecting radioactivity and chemical profiles in profundal deposits of this and probably other Great Lakes.  相似文献   

5.
湖泊沉积记录研究对了解流域侵蚀、揭示区域气候环境变化及人类活动影响、建立外生地球化学循环模式等均具有重要意义.放射性同位素~(210)Pb和~(137)Cs组合定年法是百年尺度湖泊沉积记录研究的首选方法,在过去几十年中得到了广泛应用,大力推动了湖泊沉积记录的高分辨率气候环境变化研究的开展.然而,如何理解和选择适用的计算模式来尽可能地提高定年准确度目前仍存在争议,这在一定程度上限制了该方法的应用.鉴于上述问题,本文基于本实验室近年来从青藏高原获取的若干支代表性湖芯的定年结果,从样品前处理、仪器及测试、数据分析及计算等全过程对~(210)Pb、~(137)Cs定年原理和方法进行了较为系统的阐述;在此基础上,提出了~(210)Pb定年样品封装原则,探讨了高纯锗γ谱仪在放射性活度测量中的影响因素,并建立了准确有效的比活度校正方法;明确指出了恒定初始浓度模式(constant initial concentration,CIC)可用于计算沉积物每层的年龄,而不是部分研究中认为的平均沉积速率,这对于深刻理解该定年模式并提供准确的定年结果具有重要的参考意义.同时,通过对比两种放射性核素的分布特征发现,相对于~(210)Pb而言,~(137)Cs更容易发生垂向迁移进而影响其时标准确性,这需引起研究者的充分重视,因此,当二者定年结果出现明显差异时,建议以~(210)Pb定年结果为准.  相似文献   

6.
Fallout radionuclides (FRNs) 137Cs and 210Pb are well established as tracers of surface and sub‐surface soil erosion contributing sediment to river systems. However, without additional information, it has not been possible to distinguish sub‐surface soil erosion sources. Here, we use the FRN 7Be (half‐life 53 days) in combination with 137Cs and excess 210Pb to trace the form of erosion contributing sediment in three large river catchments in eastern Australia; the Logan River (area 3700 km2), Bowen River (9400 km2) and Mitchell River (4700 km2). We show that the combination of 137Cs, excess 210Pb and 7Be can discriminate horizontally aligned sub‐surface erosion sources (rilled and scalded hillslopes and the floors of incised drainage lines and gully ‘badland’ areas) from vertical erosion sources (channel banks and gully walls). Specifically, sub‐surface sources of sediment eroded during high rainfall and high river flow events have been distinguished by the ability of rainfall‐derived 7Be to label horizontal soil surfaces, but not vertical. Our results indicate that in the two northern catchments, erosion of horizontal sub‐surface soil sources contributed almost as much fine river sediment as vertical channel banks, and several times the contribution of hillslope topsoils. This result improves on source discrimination provided previously and indicates that in some areas erosion of hillslope soils may contribute significantly to sediment yield, but not as topsoil loss. We find that in north‐eastern Australia, scalded areas on hillslopes and incising drainage lines may be sediment sources of comparable importance to vertical channel banks. Previous studies have used the combination of 137Cs, excess 210Pb and 7Be to estimate soils losses at the hillslope scale. Here, we show that with timely and judicious sampling of soil and sediment during and immediately after high flow events 7Be measurements can augment fallout 137Cs and 210Pb to provide important erosion source information over large catchments. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
The fundamental assumption of 210Pb sediment dating is the stable flux of 210Pbex, which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, the 210Pbex activity in sediments will be exponentially reduced with sedimentation age. 210Pbex in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water is suddenly increased, 210Pbex will be significantly deposited and then transferred into sediments. On the one hand such sudden purification effect is obviously unfit for the fundamental assumption of 210Pb dating; on the other hand, the sudden enhancement of 210Pbex flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of 210Pbex in the vertical profile of recent sediments of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of Lake Chenghai in June 1997. The vertical profile of 137Cs activity is characterized by a tree-peak pattern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of 210Pbex activity displays a specific distribution of peaks, and is similar to the vertical profile of Corg. This phenomenon seems to be related to the mechanism of constraining the transfer of 210Pbex into lake sediments. The average atomic ratios of Horg/Corg and Corg/Norg in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, i.e., “deposition-de-composition-accumulation”, the sedimentation fluxes (F(Corg)) of organic carbon (Corg) since 1970 were calculated by modeling. The sedimentation fluxes of 210Pbex (F(210Pbex)) in different years display good synchronous relations with the sedimentation flux of organic carbon (F(Corg)), especially in the years of 1972–1974 and 1986–1989. The variation of F(Corg) led to the variation of F(210Pbex); the variation of F(210Pbex) reflects, to some extent, the historical variation of lake productivity.  相似文献   

8.
An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:210Pb and226Ra;239, 240Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess210Pb (relative to226Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.239, 240Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess210Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess210Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments.The bioturbation rate in the top several centimeters at this station has been determined previously using234Th (24-day half-life). The distribution of239, 240Pu can be used to estimate a bioturbation rate for the underlying layer (to ～10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess210Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.  相似文献   

9.
210Po and 210Pb in mussel (Mytilus galloprovincialis) and sediment samples collected at Candarl? Gulf during the period of 2010–2012 are presented and discussed. The activity concentrations of 210Po and 210Pb were measured by means of alpha spectrometry. Activity concentrations of 210Po and 210Pb in mussels are in the ranged of 332 ± 17–776 ± 23 Bq kg−1 dw and 14 ± 1–40 ± 5 Bq kg−1 dw, for sediments the ranges for 52 ± 5–109 ± 8 Bq kg−1 dw and 38 ± 5–92 ± 9 Bq kg−1 dw, respectively. The estimated consequent annual effective ingestion dose due to 210Po and 210Pb from mussel consumption in Candarl? Gulf coastal region were calculated. The highest dose due to 210Po and 210Po were calculated to be 4232 ± 126 μSv and 126 ± 16 μSv, respectively.  相似文献   

10.
Lake sediments, in which abundant, continuous and high resolved signals are preserved, have the potential to recover the history of terrestrial environmental variation. The value of such paleo-environment signals depends firstly on tying them to accurate …  相似文献   

11.
利用210Pb、137Cs和241Am计年法测算云南抚仙湖现代沉积速率   总被引：6，自引：0，他引：6  
通过对云南抚仙湖沉积物柱芯样品的210Pb、137Cs和241Am测试表明,柱芯剖面上有明显的1963年和1986年137Cs蓄积峰,验证了1975年次级蓄积峰存在的可能性,这些峰形完好的蓄积峰对抚仙湖的现代沉积环境有明显的时标意义。利用137Cs计年法得到抚仙湖沉积物自1963年、1975年及1986年以来到2007年的平均沉积速率分别为0.063g/(cm2·a)、0.052g/(cm2·a)和0.039g/(cm2·a),说明了过去近五十年抚仙湖沉积速率整体上经历了一个由快到慢的过程。借助于241Am 的1963年蓄积峰可以提高137Cs计年的准确性。根据210PbCRS计年模式,计算出每个样品深度所对应的年代,与137Cs计年法比较存在一定的偏差,分析了两种计年方法存在差异性的原因。通过质量深度和年代分析,抚仙湖的沉积速率变化幅度比较大,表明抚仙湖近129年来的沉积环境不稳定,可能与相应历史时期的人类活动有密切的关系。  相似文献   

12.
A mooring of three conical time-series sediment traps was deployed at two sites in the western Northwest Pacific Ocean for 9 months. Total mass fluxes and activities of 210Pb and 230Th were determined for the settling particles to elucidate their scavenging and transport processes. Sediment samples also were analyzed for 210Pb activities. Total mass fluxes, 210Pb fluxes and 230Th fluxes showed large seasonal variations and their weighted mean fluxes tended to increase with depth, with an especially large increase near-bottom. The ratios of the observed 210Pb fluxes to the 210Pb deficiency fluxes in the upper traps at the two sites were only 0.02 and 0.12, and were attributable to advective export of 210Pb from the surface waters. Those ratios in the near-bottom traps ranged between 1.22 and 2.63. This suggests that these high ratios are due to effective particle scavenging, large lateral 210Pb import and input of resuspended particles that have not become incorporated into the sediments. The mean total 230Th fluxes at the near-bottom traps were 4.2–6.7 times higher than that expected from production in the overlying water column. The 210Pb activities in the surficial sediments were much lower than those in the near-bottom traps. The 210Pb accumulation rates estimated from the excess 210Pb inventory in the sediment column were 40–70% higher than the mean 210Pb fluxes at the near-bottom traps. The ratios of the 210Pb accumulation rates to the 210Pb deficiency fluxes at the near-bottom traps ranged between 2.0 and 3.7. The high fluxes of particulate 210Pb and 230Th at the near-bottom traps reflected a combination of enhanced scavenging of dissolved nuclides and the lateral redistribution of particulate matter by downslope and alongshore transports. However, it was not possible to discriminate among the various processes contributing to high nuclide fluxes.  相似文献   

13.
On two short sediment cores from Lake Łazduny (northeastern Poland) different methods for age estimation were applied including varve counting, 210Pb, 137Cs and OSL dating. The investigated sediment consists of finely laminated, organic-rich calcareous gyttja interrupted by a sand layer. For 210Pb we tested the CFCS, CIC, CRS and SIT models and compared the results with the established varve chronology and the 137Cs distribution. The total 210Pb activity distribution is consistent between cores and shows a regular decrease with depth. However, the total inventory of unsupported 210Pb was significantly different and indicates varying rates of sediment focusing. Our results show that all 210Pb dating models used in this study produced different results, while the SIT model performed best in comparison with varve counts and 137Cs peaks. The CFCS model produced a realistic age-depth trend but obtained ages were too old compared to the varve time scale. The age estimates provided by the CRS model were too old as well with an increasing offset downcore and the CIC model failed completely by producing an unrealistic chronology with age inversions. This confirms that only systematic testing of different 210Pb dating models and routine validation using independent time markers produces reliable 210Pb chronologies. OSL dating of two samples from the sand layer provided much too old results which indicate incomplete bleaching during deposition of the analyzed quartz grains. This multiple dating approach demonstrates the considerable potential but also potential pitfalls of dating such young sediments.  相似文献   

14.
Vertical profiles of the activities of 137Cs and 210Pb were measured on floodplain sediment cores and upland soil cores along the Soda Butte Creek and the Yellowstone River to determine floodplain sedimentation rates. The position of mine tailings from a 1950 impoundment failure was used as a stratigraphic marker to estimate the sedimentation rates and to make comparisons with rates provided by radionuclide‐based methods. Mass accumulation (sedimentation) rates calculated from the position of the mine tailings ranged from 0·00 to 0·17 g cm?2 yr?1 and were in good agreement with sedimentation rates calculated from the inventories of 137Cs and 210Pb. Sedimentation rates calculated from the position of the 137Cs peak generally overestimated the sedimentation rates, probably because of increased downward migration of 137Cs caused by the low pH of water moving through the mine tailings or the high permeability of floodplain sediments relative to upland reference soils. This study demonstrates that the 137Cs and 210Pb inventory methods for determining sedimentation rates can be applied to an alpine floodplain where sedimentation events are episodic and where orographic effects on precipitation generate strong downstream gradients in the delivery of atmospheric radionuclides. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

15.
137Cs湖泊沉积年代学方法应用的局限——以Crawford湖为例   总被引：17，自引：5，他引：12  
项亮 《湖泊科学》1995,7(4):307-313
＾１３７Ｃｓ湖泊沉积年代学方法是测定现代湖泊沉积物沉积年代和沉积速率的重要同位素年代学方法之一。对加拿大Ｃｒａｗｆｏｒｄ湖采集的沉积孔柱内＾１３７Ｃｓ垂直分布的研究发现,该方法给出时的标有明显偏差。比较＾２１０Ｐｂ和纹层等年代方法的结果,其１９６３年时标蓄积峰值所在位置明显移向表层,而作为１９５４年时标的该核素出现蓄积的层位则远早于该年沉积物蓄积层位。研究还表明,孔柱中较高的间隙水含量、缺少足以吸  相似文献   

16.
Sediment cores from central Lake Constance were dated with210Pb and137Cs. A sedimentation rate of (0.11±0.02) g·cm−2·y−1 was determined with the210Pb method.137Cs measurements revealed sedimentation rates of (0.11±0.01) g·cm−2·y−1 and (0.08±0.01) g·cm−2·y−1 respectively for two different cores sampled at the same location. The lower Cs-dated value indicates incomplete core recovery and demonstrates the sensitivity of this simple dating method to small losses of material at the water/sediment interface. An unambiguous application of the137Cs method is, therefore, only possible if complete core recovery is ensured. Sedimentation rates based on particulate matter, collected in sediment traps at various water depths, agree with the results of the radioisotope methods. Estimates of 30–125 days residence times for suspended particulate matter were calculated from7Be measurements.  相似文献   

17.
The deposition record of 137Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of 137Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the 137Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ 137Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The 137Cs-derived dating showed that maximum deposition of particulate bound 137Cs in sediments adjacent to the Coruh river mouth was delayed for 14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The 137Cs profile in Coruh sediments showed penetration of 137Cs to much greater depth than would be expected from 137Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm2 yr−1. This value was used to evaluate deposition chronology of 137Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the 137Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.  相似文献   

18.
Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from 210Pb and 137Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm−2 yr−1 (Axios and Aliakmon Rivers) and 0.414 g cm−2 yr−1 (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess 210Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.  相似文献   

19.
Analysis of water samples from the New York Bight area and Narragansett Bay reveals that a small fraction of the total Pu (probably Pu (III + IV) species) is continuously removed to the sediments at a rate similar to that of the particle-reactive isotope228Th. A more “soluble” Pu species appears to be released at times from the sediments to the water column in these nearshore regions. Sediments in shallow areas of the New York Bight south of Rhode Island and Narragansett Bay have high Pu inventories and relatively deep penetration of this element, although the net sediment accumulation rate is generally low (<0.03 g/cm2 yr). The high Pu inventories can be explained if both sediment resuspension and sediment mixing are assumed to be the major controlling factors for the effective transfer of Pu from the water column to the sediments. By simultaneous modelling of the depth distribution of three tracers which operate on vastly different time scales:234Th (half-life 24 days),210Pb (half-life 22 years) and239,240Pu (introduced into the environment during the past 30 years), bioturbation rates ranging from 4 to 32 cm2/yr in the surface mixed layer (5–10 cm thick) and from 0.3 to 2.5 cm2/yr in the layer below (up to 40 cm thick) and net sediment accumulation rates of approximately zero to 0.14 g/cm2 yr were calculated for these areas.  相似文献   

20.
Mercury deposition histories have been scarcely documented in the southern hemisphere. A sediment core was collected from the ecologically important estuarine floodplain of the Berg River (South Africa). We establish the concentration of Hg in this 210Pb-dated sediment core at <50 ng g−1 HgT throughout the core, but with 1.3 ng g−1 methylmercury in surface sediments. The 210Pb dating of the core provides a first record of mercury deposition to the site and reveals the onset of enhanced mercury deposition in 1970. The ratio of methylmercury to total mercury is relatively high in these sediments when compared to other wetlands.  相似文献   

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