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1.
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   

2.
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   

3.
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   

4.
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   

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.
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   

7.
Activity profiles of excess 234Th, excess 210Pb, 232Th, 230Th, 234U and 238U, and 228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of 228Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and 210Pb in this coastal zone.Comparison of excess 234Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess 234Th have no excess 228Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of 228Th due to loss of soluble 228Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. 232Th and 230Th activities and 230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic 230Th. Steady state 210Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved 210Pb in sea waters adverting into Puget Sound.Excess 234Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess 210Pb profiles. At least six of eight 234Th profiles show that mixing within the 210Pb-defined surface mixed layer is depth dependent. In three profiles, 234Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the 234Th data are consistent with 210Pb profiles determined for the same sediments, strongly suggesting that 234Th and 210Pb are mixed equivalently and in a multilayered manner.  相似文献   

8.
The sedimentation rates and diffusive sediment mixing coefficients at several Lake Ontario locations have been derived from measurements of unsupported210Pb profiles in sediment cores. The values of mixing coefficients obtained in the present study are significantly lower than those obtained previously through an analysis of porosity profiles. The present estimates, however, are consistent with the rather well-preserved pollutant profiles at some of these locations. It is observed that the more realistic value of the mixing coefficient, obtained by inclusion of the sedimentation rate parameter, follows the sign opposite to that for the constant obtained by regression analysis of the porosity data. Further work is required to delineate this apparent relationship between two important physical characteristics of deposited sediments.Analysis of available suspended sediment data shows that Niagara River supplies about 1.8 million tonnes of sediment annually to Lake Ontario. This value is significantly lower than that (4.6 mt/yr) used previously in constructing sediment and pollutant budgets for Lake Ontario. From the presently derived sedimentation rate and suspended solid discharge estimates, an average value of 441 km2 (range 220-938 km2) is obtained for the minimum area of Lake Ontario over which the Niagara River-supplied fine sediment is deposited.  相似文献   

9.
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   

10.
Palaeomagnetic investigation of three sediment cores from the Chukchi and Beaufort Sea margins was performed to better constrain the regional chronostratigraphy and to gain insights into sediment magnetic properties at the North American Arctic margin during the Holocene and the preceding deglaciation. Palaeomagnetic analyses reveal that the sediments under study are characterized by low‐coercivity ferrimagnetic minerals (magnetite), mostly in the pseudo‐single domain grain‐size range, and by a strong, stable, well‐defined remanent magnetization (MAD <5°). Age models for these sediment cores were constrained by comparing their palaeomagnetic secular variations (inclination, declination and relative palaeointensity) with previously published and independently dated sedimentary marine records from the study area. The magnetostratigraphical age models were verified by AMS radiocarbon dating tie points, tephrochronology and 210Pb‐based sedimentation rate estimate. The analysed cores 01JPC, 03PC and 02PC span c. 6000, 10 500 and 13 500 cal. a BP, respectively. The estimated sedimentation rates were stable and relatively high since the deglaciation in cores 01JPC (60 cm ka−1) and 03PC (40–70 cm ka−1). Core 02PC shows much lower Holocene sedimentation rates with a strong decrease after the deglaciation from ~60 to 10–20 cm ka−1. Overall, this study illustrates the usefulness of palaeomagnetism to improve the dating of late Quaternary sedimentary records in the Arctic Ocean.  相似文献   

11.
长江河口水下三角洲210Pb分布特征及其沉积速率   总被引：5，自引：0，他引：5  
通过对长江水下三角洲采集的十个沉积物柱状样中的放射性核素210Pb分析得知,长江水下三角洲表层210Pb放射性比度在2.15~4.22 dpm·g-1之间,210Pb沉降通量在1.50~11.21 dpm·cm-2 yr-1之间,过剩210Pb总量在>48.29~>361.68 dpm·cm-2之间;210Pb沉降通量以及过剩210Pb总量均高于理论值,这表明在沉积物中存在210Pb的净输入和聚集。由210Pb放射性比度剖面所反映的沉积速率可知长江水下三角洲泥质沉积区沉积速率介于1.36~4.11 cm·yr-1之间;总体上呈现近岸沉积速率较低,沿31°N纬线向20 m等深线沉积速率升高的趋势,从122°15′E到122°30′E范围为长江水下三角洲的泥质沉积物堆积中心,其平均沉积速率为3.51 cm·yr-1。  相似文献   

12.
Rates of sedimentation in two Lake Superior deposits were determined by both ragweed pollen and 210Pb geochronologies. The former yields an average rate over the time since the first appearance of enhanced concentrations of the pollen as a consequence of human settlement. Sedimentation rates derived on these two bases can be brought into accord if the first appearance of ragweed pollen in the sediments was around 1830 and if the sedimentation rates have been uniform over the past century.  相似文献   

13.
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   

14.
Sedimentation rates were determined by210Pb in three sediment cores from the main basin of Lake Constance. Rates vary from 0.094 to 0.133 g cm?2y?1, in agreement with previous determinations. A constant radionuclide flux (CR) model reveals long-term, quasi-synchronous fluctuations of sedimentation rate on the basin-wide scale. On average, the rate of sedimentation remains much the same from the beginning of this century indicating little effect of human activity in the watershed on fine-silt sediment supply. The anthropogenic fluxes of Zn, Pb, and Cd in the three cores are compared and the application of210Pb as a heavy metal tracer in Lake Constance is examined. It seems to be a very good tracer for Pb, moderately good for Zn and Cu, and not useful for Cd.  相似文献   

15.
The210Pb dates from a salt-marsh sediment sequence at Tites Point, Severn Estuary, southwest England, were compared with the results of three other independent dating techniques. These methods are inspection of dated topographic maps for the presence of absence of the salt marsh, dating of artifacts in a well-defined horizon within the sequence, and counting annual bands. All four sets of dates showed good agreement to about 30 cm depth from the present marsh surface; thereafter, dates from the210Pb method are earlier than those from the other methods, which are themselves in agreement. It is concluded that a wide range of methods can be successfully used for dating salt-marsh sediments (e.g., radiometric techniques, topographic maps, archaeological evidence, annual banding). Where possible, it is important to confirm dates using at least two independent techniques.  相似文献   

16.
In total 27 short and one long sediment core, and 278 surface sediment samples from the Baltic Sea were analyzed for mercury (Hg), and organic carbon contents. Thirteen short cores and the long core were dated by radionuclide methods (210Pb, 137Cs, AMS14C). The dataset allows discriminating between natural and human induced changes on the Hg levels in Baltic Sea sediments. Preindustrial Holocene background concentrations vary between 20 and 50 μg Hg per kg dry sediment and are positively correlated with organic carbon changes. Strong human induced pollution is recorded for the second half of the past century and caused high Hg concentrations of up to several hundred μg Hg per kg dry sediment even in Baltic Sea basins. Maximum concentrations are found at industrial and war waste dumping sites (local hot spots). An Hg concentration decreasing trend toward the present day is observed at most coring sites, a result of environmental measures undertaken during the last two decades. At sites where it is possible to calculate Hg fluxes, the natural accumulation rates vary between 2.1 and 5.4 μg Hg per m2 per year. Anthropogenically sourced Hg accumulation rates vary in a wide range of 30 and 300 μg Hg per m2 per year for the time span of maximum pollution. In areas characterized by discontinuous sedimentation only “inventories” of human sourced Hg expressed as the total amount of deposited Hg (above the natural background) per m2 can be calculated. The inventories of the investigated cores vary in the range of 1 and 8 mg Hg per m2. Additionally, influences of sediment dynamics on spatial distribution pattern of Hg concentrations in surface and subsurface sediments are discussed.  相似文献   

17.
Polycyclic aromatic hydrocarbon (PAH) compositions were determined in plankton, sediment-trap-collected particulate material and sediment cores from Dabob Bay using a high performance liquid Chromatographie (HPLC)/fluorescence technique. The annual flux of individual PAH measured in a series of sediment traps was compared with the flux of corresponding compounds determined from 210Pb dated bottom sediments. Systematic seasonal variations in the fluxes and concentrations of PAH, Al and organic carbon in the trap-collected particulates and seasonally collected plankton were also investigated to determine whether or not PAH are associated with either terrestrial or marine-derived materials.Concentrations of all PAH studied increased tenfold within the last 80–100 yr of sediment deposition, except for perylene which displayed a reasonably constant concentration profile. This suggests at least two sources contribute to the observed sedimentary PAH compositions in Dabob Bay, i.e., anthropogenic combustion and a natural source. Plankton and sediment trap-collected particulates contained PAH mixtures qualitatively similar to underlying surface sediments. Microscopic examination indicated fecal pellets were the major form of particulate material in the sediment traps. The fecal pellets collected in the sediment trap time series quantitatively account for essentially 100% of the PAH fluxes measured in the 210Pb dated sediments, implying Zooplankton fecal pellets control the removal of PAH to Dabob Bay sediments. These measurements provide clear evidence that the PAH studied are not produced after sediment deposition. The observed seasonal covariations of PAH and Al in both sediment trap and plankton samples further indicate that PAH originate from terrestrially-based sources, are introduced into the marine environment by runoff and erosion or atmospheric deposition and are not produced by marine plankton.  相似文献   

18.
Post-Aswan dam sedimentation rate of lagoons of the Nile Delta,Egypt   总被引：1，自引：0，他引：1  
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   

19.
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   

20.
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   

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