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1.
John Van Sickle Walter C. Weimer David P. Larsen 《Geochimica et cosmochimica acta》1983,47(12):2189-2197
Rates of surficial sediment mixing and sediment burial are measured in Shagawa Lake, Minnesota, using radionuclide tracers. Based on 106Ru profiles in 16 cores, mixing rates average 13 cm2/yr within the upper 9 cm of sediment. Two other nuclides, 210Pb and 137Cs, provide further evidence for calculation of mixing rates. In addition, 210Pb profiles estimate sediment burial rates (about 0.4 cm/yr). Mixing estimates are shown to be fairly insensitive both to uncertainties in sediment burial rates and to temporal patterns of 106Ru deposition. 相似文献
2.
J.Kirk Cochran 《Geochimica et cosmochimica acta》1985,49(5):1195-1210
Particle mixing rates (DB) calculated from excess 210Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm2/y, with variation of a factor of 3–4 at a single site. Diffusion of the 236Ra daughter 222Rn may affect 210Pb distributions under conditions of slow mixing and low 210Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides 239,240Pu and 137Cs but no excess 210Pb (relative to 226Ra). There is no clear relationship between 210Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of 210Pb mixing rates with those calculated from 239,240Pu and 137Cs distributions reveals better agreement for a pulse input of the fallout radionuclides (DB = 0.03?0.4 cm2/y) than for continuous input at a constant rate (DB = 0.1?1.6 cm2/y), although the Pu and 137Cs data are better fit by the latter model. The agreement may be fortuitous because 239,240Pu and 137Cs appear significantly deeper than 210Pb in at least one core. Tracer separation could be caused by particle size-selective mixing by the benthic fauna or by chemical mobilization. If the fallout radionuclides are scavenged from surface waters by large, organic-rich particles such as fecal pellets, their release and migration may result from decomposition of the carrier in surface sediments. Either a relatively unreactive form of Pu (e.g. oxidized Pu) has been released by this process or a one-dimensional model is inadequate to explain its observed penetration into the sediments. Activity ratios of in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of 239,240Pu near the weapons test site at Christmas Island (2°N). The ratios and fluxes to the sediment (assuming constant input) at the siliceous ooze-red clay site are consistent with published sediment trap data from a nearby site. Thus if fallout radionuclide fluxes to the sea floor were higher in the past, both 239,240Pu and 137Cs have been released from sinking particles. 相似文献
3.
Comparisons of sedimentation rates obtained by 210Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods. Most of the discrepancy between the methods can be explained by the analytical precision of the 210Pb method and by the exactness with which time horizons can be identified and dated for the pollen method. X-radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and/or mixing of sediments. Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., 210Pb trends and/or “possible” horizon assignments). Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth. The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments. Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon. 相似文献
4.
210Pb具有百年时间尺度沉积计年的重要价值。210Pbex计年假设：沉积物是封闭系统；进入水体的210Pb能有效地转移到沉积物中并不发生沉积后迁移；非过剩210Pb与其母体226Ra保持平衡。210Pbex计年可用稳定输入通量-稳定沉积物堆积速率模式、常量初始浓度模式或恒定补给速率模式。沉积物柱芯必须保持原态并以0．5～1cm间隔分截；用相应层节226Ra校正。沉积物表层混合作用及222Rn的丢失可能导致顶部210Pbex异常。季节性缺氧湖泊沉积物顶部可能存在210Pb及210Po的再迁移。210Pb与137Cs两种计年方法原理上具有根本差别。210Pb，137Cs与沉积纹理方法对比是准确计年的重要保证。 相似文献
5.
This paper describes the use of 210Pb and 137Cs radioactivity measurements to determine the rates of sedimentation in the Great Lakes. Cores from eight locations in Lake Michigan were chosen for examination to cover as wide as possible a range of sedimentation rates and representative sedimentary environments. The surficial 210Pb activity in the sediments varies between 7 and 23 pCi/g dry wt and its profile in each core shows the expected exponential decrease with depth consistent with the assumption of uniform sedimentation rate over the last hundred years and secular equilibrium between supported 210Pb and 226Ra (0.5-1.0 pCi/g dry wt). Companion measurements of 137Cs indicate that the coring technique satisfactorily recovered the uppermost levels of the deposit and that the mobility of both radionuclides within the sediment is probably small.Based on the limited number of cores analyzed to date, it appears that modern sedimentation rates are not very different from average rates for the last 7000 yr. The excess 210Pb appears to originate primarily from atmospheric fallout, but a further inventory of the 210Pb distribution over the lake bottom must be made to properly assess the significance of other sources. The spatial distributions of both 137Cs and 210Pb at certain stations suggest that the mode of transport of these radionuclides are comparable and involve attachment to settling particles. A mathematical model is developed which accounts for the observed limited mobility of both 210Pb and 137Cs in several of the cores in terms of post-depositional redistribution by physical or biological mixing processes. 相似文献
6.
Susan F. Sugai 《Estuaries and Coasts》1990,13(4):380-392
Sediment profiles of210Pb and137Cs in cores collected at increasing distances from the heads of Smeaton Bay and Boca de Quadra fjords indicate that watersheds influence the inventories of radioisotopes present and that the steep topographies of the fjords enhance sediment redistribution. Episodic deposition of terrestrially derived sediment was responsible for roughly 50% of the137Cs and 45% of the210Pb inventories in shallower (less than 180 m) locations in Wilson and Bakewell arms of Smeaton Bay.210Pb sedimentation rates at shallower sites when corrected for episodic deposition were less than sedimentation rates obtained in the deep basins of the fjords where sediment focusing and increased primary productivity in the overlying water column occur. Higher fluxes of dissolved Mn from surficial sediments and subsequent reoxidation in the overlying water may have enhanced scavenging of210Pb in basin locations, resulting in higher inventories. Episodic events have occurred frequently in Smeaton Bay and Boca de Quadra suggesting that steady-state conditions with respect to sedimenting particles can be achieved only when averaged over long time periods approaching the time over which137Cs and210Pb are useful. 相似文献
7.
C. Tsabaris V. Kapsimalis G. Eleftheriou M. Laubenstein H. Kaberi W. Plastino 《Environmental Earth Sciences》2012,67(3):833-843
Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of 137Cs and 210Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of 137Cs and excess 210Pb in the sediment cores. The spatial distribution of 137Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for 137Cs and gross alpha counting using alpha counter for 210Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18?±?0.02 to 0.22?±?0.03?cm?year?1. The 137Cs inventories varied from 205?±?15 to 602?±?39?Bq?m?2, while the 210Pb inventories of the studied cores exhibited average value of 140?±?9?Bq?m?2. Elevated 137Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea. 相似文献
8.
对东平湖沉积物柱状岩芯放射性年代学研究表明： 12cm处的 137Cs比活度峰值对应于1963年人工核试验高峰,9cm和6cm处的 137Cs比活度蓄积峰可能分别与1974年的全球核素散落和1986年前苏联切尔诺贝利核电站核泄漏有关; 210Pb比活度随深度呈指数衰减,利用 137Cs核素1963年对应的蓄积峰进行校正,采用 210Pb计年的CRS模式建立了1889年以来东平湖现代沉积年代序列。根据 210Pb年代序列,计算了东平湖现代沉积速率,发现近百年来东平湖地区沉积速率有明显变化： 1889～1945年,沉积速率较高,平均达0.297g/cm2•a; 1945～1963年,沉积速率整体处于下降趋势; 1963～2000年,沉积速率比较稳定,平均约0.141g/cm2•a; 约2000年以来,沉积速率有增高趋势。通过分析认为,1945年以前东平湖较高的沉积速率可能与东平湖作为黄河水的自然滞洪区有关; 1950年代国家对东平湖的治理改造,控制了黄河水的自然倒灌,导致了东平湖沉积速率的迅速下降; 1963年以后,由于黄河上中游大型水利工程设施的修建,抑制了东平湖调洪功能的发挥,导致东平湖沉积速率低而稳定; 近年来,东平湖沉积速率有增大趋势,这可能与湖区人民发展围网养殖带入较多的沉积物有关。 相似文献
9.
对东北四海龙湾玛珥湖SHLF6孔纹层沉积物的137Cs放射性测量表明:137Cs比活度的最大值出现在55cm处,对应于1963年世界原子弹试爆高峰期。纹层计年表明0～6cm共有35个纹层层偶。从75cm到65cm,137Cs比活度从256±009dpm/g急剧增加到1868±017dpm/g,可能65cm对应于1954年。通过测量226Ra子核214Pb和214Bi（能量为295keV,352keV和609keV）放射的光子数获得226Ra比活度数据,然后求得过剩210Pb比活度（210Pbuns）。210Pbuns比活度随深度增加而呈指数衰减,其异常波动可能与人类活动以及沉积速率变化有关,例如55cm处210Pb比活度较高,与137Cs的峰值对应,这可能与1963年前后人工核实验的高峰有关,因为核试验不仅产生137Cs,而且可以产生208Pb和210Pb;45cm处210Pb比活度较低,而226Ra较高,可能与人类活动加剧,导致沉积速率增加有关。根据210PbunsCRS模式,SHLF6孔0～19cm的平均沉积速率为20mg/cm2·a,或约为011cm/a。210Pb测年数据与137Cs时标及纹层计年均有很好的一致性。四海龙湾玛珥湖发育的纹层为年纹层,可以建立高分辨率时间序列。 相似文献
10.
Continental shelf sediments from nine locations off Washington and Oregon have 239,240Pu inventories which average 8.0 ± 2.6 mCi/km2. The Columbia River and seawaters advecting over the shelf supply Pu which is removed to underlying sediments, principally through scavenging by inorganic paniculate matter. Mass balance calculations argue that less than 20 percent of the advected Pu need be scavenged from the water column to balance river input and total shelf sediment inventories. The percentage of the Pu removed through scavenging is consistent with observed participate concentrations in shelf waters and published sediment/water distribution coefficients.No marked separation of Pu from 137Cs is observed with depth in Pacific shelf sediments as has been reported in Atlantic coastal sediments. This interocean distinctness can be explained by differences in particle mixing and downward diffusion of Cs in sediments of varying porosities. The transuranic inventories and Pu/Cs ratios in the Pacific sediments do not support the hypothesis of Livingston and Bowen that Pu is remobilized within the sediment column by ‘complexone’ formation with (principally) organic substances.Excess 210Pb/239,240Pu inventory ratios in eight representative cores from the Washington shelf average 100 ± 19, even though absolute values of both inventories vary by much larger factors. This reasonably constant ratio, for a given water depth, permits estimation of total Pu inventories and prediction of sites of unusual Pu accumulation from data on the more easily measured natural radionuclide. 相似文献
11.
放射性同位素定年技术在崇明岛潮滩沉积速率上的应用研究 总被引：1，自引：0，他引：1
选取崇明岛北侧由东至西4个潮滩沉积柱,分析137Cs、226Ra和210Pb放射性比活度,应用210Pb和137Cs放射性同位素定年方法计算崇明岛潮滩沉积物沉积速率。对于东西两端,210Pb方法测得速率分别为3.08cm/a和2.34 cm/a,而137Cs测得速率分别为6.19 cm/a和2.06 cm/a。结果表明,137Cs定年方法计算出的潮滩沉积速率普遍大于210Pb方法结果;但两者反映了相同的速率规律。崇明岛主力生长方向为东和东北,西侧沉积作用相对较弱,表现出"东快西慢"的特点。沉积纵向上,1954年以来,自下而上沉积速率逐渐减缓。 相似文献
12.
Determination of recent deposition rates in Lake Constance with radioisotopic methods 总被引：3，自引：0，他引：3
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
13.
Sources and history of heavy metal contamination and sediment deposition in Tivoli South Bay, Hudson River, New York 总被引：1，自引：0，他引：1
Gaboury Benoit Edward X. Wang William C. Nieder Michael Levandowsky Vincent T. Breslin 《Estuaries and Coasts》1999,22(2):167-178
Persistent inorganic constitutents preserved in sediments of aquatic ecosystems record temporal variability of biogeochemical
functioning and anthropogenic impacts.210Pb and137Cs dating techniques were used to study the past variations of heavy metals (Pb, Cu, and Zn) and accumulation rates of sediments
for Tivoli South Bay, in the Hudson River National Estuarine Research Reserve ecosystem. South Bay, a tidal freshwater embayment
of the Hudson, may play an important role in the sediment dynamics of this important river. The measured sedimentation rate
range of 0.59 to 2.92 cm yr−1 suggests that rapid accumulation occurred during the time period represented by the length of the cores (approximately the
past 50 yr). Direct measurements of sediment exchange with the Hudson River reveal high variability in the sediment flux from
one tidal cycle to the next. Net exchange does not seem to be adequate to explain sediment accumulation rates in the bay as
measured by210Pb and137Cs. The difference may be supplied from upland streams or the Hudson River during storm events. Concentrations of the metals
Pb, Cu and Zn were found to be well correlated with each other within individual cores at five of six sites tested. This suggests
a common proximate source for the three metals at a specific site. The evidence is consistent with mixing in some environmental
compartment before delivery to the bay. While metals self-correlate within individual cores, absolute concentrations, depth
distribution patterns, and ratios of the metals to each other vary among the cores collected at different locations within
the bay. Organic matter, Fe content, and particle size distribution of sediments do not account for the intercore variations
in metal concentration. It is likely that cores collected from different sites may have derived metals from different sources,
such as watershed streams and tidal exchange with the Hudson River. 相似文献
14.
1964,1975及1986年三个137CS时标计算出红枫湖和洱海沉积物平均堆积速率完全一致,说明1975年和1986年次级蓄积峰作为计年时标的可靠性。洱海沉积物平均堆积速率为0．047±0．002g／(cm2·a),与210Pb方法所获得的结果一致;红枫湖为0．17±0．01g／(cm2·a),与其建湖以来沉积物实际堆积的观察结果吻合。洱海1964年沉积物中137CS的比活度(校正到沉积年代)仅46．4Bq／kg,而红枫湖达145Bq／kg。由于红枫湖沉积物堆积速率约为洱海的3．7倍,所以其1964年层节沉积物中137CS蓄积量比洱海大11.5倍。红枫湖沉积物中137CS累计值的99．4％分配于1985年以前,而洱海的19．4％分配于1986年以后,显示出137CS在洱海沉积物中的较大扩散能力。 相似文献
15.
Steven L. Huntley Richard J. Wenning Steave H. Su Nancy L. Bonnevie Dennis J. Paustenbach 《Estuaries and Coasts》1995,18(2):351-361
In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for 210Pb activity at predetermined depths and the rate of sediment accretion (cm yr?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from 210Pb measurements (RPb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent 137Cs analyses were focused on this depth interval. Sediment accretion rates derived from 137Cs measurements (RCs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr?1 to 10.2 cm yr?1 and averaged 6.8 cm yr?1. The RCs estimates for cores from this area ranged from 3.8 cm yr?1 to 8.9 cm yr?1 and averaged 6.6 cm yr?1. The RCs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr?1 and 0.66 cm yr?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary. 相似文献
16.
Fu WANG Hong WANG Jianfen LI Yandong PEI Changfu FAN Lizhu TIAN Zhiwen SHANG Meiyu SONG Yan GENG 《Frontiers of Earth Science》2008,2(3):276-282
The representative 210Pb and 137Cs age-depth profiles were selected from more than 100 sites in the Circum Bohai Sea (CBS) coastal region to examine the sedimentation
rates and associated environmental changes. There are three unique 210Pbexc distribution patterns: 1) ideal-decaying type, representing relatively constant sediment supply and quieter environmental
setting; 2) wiggling type, but with approximately equivalent amplitude, representing coarser sediments and associated stronger
flow current environment; and 3) episodic-eventinfluencing type, each representing typhoon storm-influenced depositional environment.
Our results also show that there is a lower sedimentation rate (ca. 0.1 cm/a) in the coastal lowlands, a higher sedimentation
rate (ca. 0.5–3 cm/a) in the intertidal flat and an intermediate rate of ca. 0.58 cm/a in the subtidal zone near the Huanghua
Harbor. The interbedded silt layer occurring in the core sediment reveals lower 210Pb activities, indicating a discontinuous sedimentation mostly due to typhoon events which dwarfs against 210Pb and 137Cs applicability. Overall, the 210Pb and 137Cs measurements of the present study provide physical insight into the evaluation of the coastal-marine ecological environment
and associated management. 相似文献
17.
This study uses radiometric analysis (210Pb and 137Cs) of short sediment cores with high-resolution sampling (1-cm interval) to trace sedimentation rates in the Nile Delta lagoons,
particularly since completion of the Aswan High Dam in 1964. A declining trend in 210Pbex as calculated by the CIC model is clearly identified in about 10 cm of the upper-core sediments from the lagoons of Manzala
and Edku, accompanied by two spikes of 137Cs in cores from the lagoons of Burullus and Edku. These findings illustrate average post-dam sedimentation rates ranging
from 0.22 to 0.27 cm a−1 in the lagoons, in contrast with those found previously based on low-resolution sampling. The lower sedimentation rates in
the lagoons are a consequence of a dramatic reduction in riverine sediment load to the coastal area as a result of the damming.
Although widespread erosion occurs along the open estuarine coast, the lagoon setting remains calmer than before due to coastal
diking and freshwater regulation in the delta plain in the past decades. This provides the possibility of continuously preserved
radiometric records in the less-bioturbated lagoon sediments. Dating individual layers using the CRS model has revealed increasing
sedimentation rates in Manzala and Burullus since the 1980s, which can largely be explained as a consequence of the reduction
in lagoon area due to intensifying reclamation. The post-dam sedimentation in the shrinking lagoons may have some adverse
ecological consequences due to finer sediment’s affinity with pollutants. These findings would shed light on the environmental
conservation and socioeconomic development in the Nile Delta region. 相似文献
18.
Many salt marshes in densely populated areas have been subjected to a reduction in tidal flow. In order to assess the impact of tidal flow restriction on marsh sedimentation processes, sediment cores were collected from flow-restricted restricted salt marshes along the Connecticut coast of Long Island Sound. Cores were also collected from unrestricted reference marshes and from a marsh that had been previously restricted but was restored to fuller tidal flushing in the 1970's. High bulk densities and low C and N concentrations were found at depth in the restricted marsh cores, which we attribute to a period of organic matter oxidation, sediment compaction, and marsh surface subsidence upon installation of flow restrictions (between 100 and 200 years before the present, depending on the marsh). Recent sedimentation rates at the restricted marshes (as determined by137Cs and210Pb dating) were positive and averaged 78% (137Cs) and 50% (210Pb) of reference marsh sedimentation rates. The accumulation of inorganic sediment was similar at the restricted and reference marshes, perhaps because of the seasonal operation of the tide gates, while organic sediment accretion (and pore space) was significantly lower in the restricted marshes, perhaps because of higher decomposition rates. Sedimentation rates at the restored marsh were significantly higher than at the reference marshes. This marsh has responded to the higher water levels resulting from restoration by a rapid increase in marsh surface elevation. 相似文献
19.
Cooper Sherri Rumer McGlothlin Sunghea Kim Madritch Michael Jones Daniel L. 《Estuaries and Coasts》2004,27(4):617-633
Sediment cores were collected from the Neuse and Pamlico River estuaries, North Carolina, at seven different sites, and the
data show strong anthropogenic influence on water quality. The sediments from these cores were dated using210Pb,137Cs,14C, and pollen horizon techniques. Specific parameters investigated include bulk density, sedimentation rates, diatom assemblage
changes, nutrient and trace metal flux, and vegetation changes as recorded in the pollen record. The greatest increases in
sedimentation, nutrient and metal flux and changes in diatom assemblages have occurred in the past 50–60 yr in the Pamlico
and Neuse. Diatom diversity has decreased and small planktonic forms have become dominant over time, most likely due to eutrophication
and increased turbidity and sedimentation. Major changes occur before phytoplankton surveys and monitoring were initiated.
Overall trends are similar to those found in Chesapeake Bay, although the time frame of major changes is more recent. Dominant
small planktonic diatom species differ between Chesapeake Bay and the Neuse and Pamlico. Variance in paleoecological indicators
between these mid-Atlantic estuaries may be due to geomorphology and land use history. 相似文献
20.
Robert W Talbot 《Geochimica et cosmochimica acta》1984,48(10):2053-2063
The vertical distribution of 210Pb and 210Po in the dissolved (<0.4 μm) and the paniculate (>0.4 μm) phases was measured in Crystal Lake, Wisconsin, to examine the spatial and temporal variability during the seasonal cycle of this oligotrophic lake. The concentration of unsupported 210Pb in the water column is maintained principally by atmospheric input. However, most of the 210Po in Crystal Lake is produced in situ from radioactive decay of 210Pb.Mass balance considerations indicated that the removal rates of 210Pb and 210Po from the water column to the sediment varied temporally by nearly an order of magnitude. During transient periods of high biological productivity, a large net flux of these nuclides into the sediment occurred. In addition, 210Pb was rapidly stripped from the water column during fall turnover. It was during these short-lived events that most of the annual net removal of 210Pb and 210Po occurred. The mean removal residence time was estimated to be 0.095 yr for 210Pb and 0.26 yr for 210Po. These residence times suggest that there is a difference between 210Pb and 210Po in the extent of their recycling in the water column. Calculations indicated that there was a cyclic response of the water column 210Po inventory corresponding to successive time periods where there was a net loss or net gain. This cycling is attributed to rapid biological removal and subsequent release from the sediment of freshly deposited 210Po. For 210Pb, replenishment of the water column appeared to occur mainly from atmospheric input. 相似文献