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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   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
Tidal wetlands play an important role with respect to climate change because of both their sensitivity to sea-level rise and their ability to sequester carbon dioxide from the atmosphere. Policy-based interest in carbon sequestration has increased recently, and wetland restoration projects have potential for carbon credits through soil carbon sequestration. We measured sediment accretion, mineral and organic matter accumulation, and carbon sequestration rates using 137Cs and 210Pb downcore distributions at six natural tidal wetlands in the San Francisco Bay Estuary. The accretion rates were, in general, 0.2?C0.5?cm?year?1, indicating that local wetlands are keeping pace with recent rates of sea-level rise. Mineral accumulation rates were higher in salt marshes and at low-marsh stations within individual sites. The average carbon sequestration rate based on 210Pb dating was 79?g?C?m?2?year?1, with slightly higher rates based on 137Cs dating. There was little difference in the sequestration rates among sites or across stations within sites, indicating that a single carbon sequestration rate could be used for crediting tidal wetland restoration projects within the Estuary.  相似文献   

8.
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   

9.
Marsh sediment accumulation is predominately a combination of in situ organic accumulation and mineral sediment input during inundation. Within the Pamlico River Estuary (PRE), marsh inundation is dependent upon event (e.g., storms) and seasonal wind patterns due to minimal astronomical tides (<10 cm). A better understanding of the processes controlling sediment deposition and, ultimately, marsh accretion is needed to forecast marsh sustainability with changing land usage, climate, and sea level rise. This study examines marsh topography, inundation depth, duration of inundation, and wind velocity to identify relationships between short-term deposition (tile-based) and long-term accumulation (210Pb and 137Cs) recorded within and adjacent to the PRE. The results of this study indicate (1) similar sedimentation patterns between the interior marsh and shore-side marsh at different sites regardless of elevation, (2) increased sedimentation (one to two orders of magnitude, 0.04–4.54 g m?2 day?1) within the interior marsh when the water levels exceeded the adjacent topography (e.g., storm berm), and (3) that short-term sea level changes can have direct effects on sediment delivery to interior marshes in wind-driven estuarine systems.  相似文献   

10.
In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,210Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr?1 (0.25 to 0.60 cm yr?1) based on the constant flux model, 0.40±0.15 cm yr?1 (0.15 to 0.58 cm yr?1) based on the constant activity model, and 0.44±0.11 cm yr?1 (0.30 to 0.59 cm yr?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess210Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.  相似文献   

11.
Natural radionuclides in the uranium and thorium series were measured in solid tidal phases (suspended particles, bottom sediment, surface microlayer colloids) of a salt marsh in lower Delaware. The purpose was to identify potential processes responsible for trace element cycling (sources, redistribution and exchange) in salt water marshes and with their coastal waters. Generally, concentrations of U, Th,210Pb, and210Po on the tidal solid phases suggest a general mechanism by which tidal marshes appear to be trapping the nuclides into their interiors. The processes may include transport of enriched fine particles into the marsh, capture by salt marsh grass and chemical fixation by redox processes at the sediment surface. Specifically, the uranium contents of most of the samples are similar with activity ratios234U238U≧1, indicating a mixture of detrital and nondetrital (authigenic) uranium inputs such as seawater or ground water. Since the230Th daughter is generally deficient by about 50%, the authigenic enrichment process appears to favor uranium and is potentially linked to the extensive diagenetic sulfur redox cycle of salt marsh sediments. The210Po/210Pb activity ratio is less than one on Spartina adsorbed solids, and could suggest a general process in salt marshes which favors210Pb enrichment by atmospheric fallout over enrichment of210Po on time scales of weeks which correspond to complete tide marsh exchange. A228Th/232Th activity ratio of less than unity on the solids adsorbed onto marsh grass suggests a net process whereby diffusive loss of the intermediate daughter228Ra from the adsorbed solids to tidal waters dominates over potential228Th scavenging by suspended sediment.  相似文献   

12.
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.
Single cores from two salt marshes in the United Kingdom located near different nuclear facilities were investigated to compare chronostratigraphic estimates derived from the natural radionuclide 210Pbexcess with estimates from the known times of introduction of artificial radionuclides to the environment. Both cores selected had clear visual indications of redox zonation, and evidence for diagenetic redox perturbation of the radionuclide records was also sought. In the core from Beaulieu Marsh on the south coast of England, the redox zonation was revealed by the profiles of the redox-sensitive elements Mn + I, Fe + P + As, and S, and the records of nuclear discharges were entirely contained within oxidized conditions in the upper 40 cm. The constant flux/constant sedimentation 210Pbexcess accumulation estimate was 76% of that derived from the 1963 fallout 137Cs level (0.35 vs. 0.46 g cm−2 yr−1 dry mass), but the constant flux 210Pbexcess method indicated that accumulation rates were lower at Beaulieu before ∼1950. On any timescale, 137Cs appears earlier in the sediment record than its introduction to the environment, but although downward diffusion of 137Cs relative to 241Am has clearly occurred, the 137Cs peak still appeared in place and there was negligible penetration of 137Cs into underlying reduced conditions. This core also contained a peak of the 60Co discharges from either or both the Winfrith and La Hague nuclear plants that peaked in 1980 and 1984, respectively. The sediments in the core from Wyre Marsh on the eastern coast of the Irish Sea had accumulated more rapidly than those at Beaulieu, and in this case the redox zonation could be established only from Mn and S profiles. Here, the constant initial activity 210Pbexcess accumulation rate estimate was 125% of that derived from the 137Cs peak correlated with the 1975 Sellafield discharge maximum (0.79 vs. 0.64 g cm−2 yr−1). Sellafield discharge 137Cs swamps fallout or Chernobyl 137Cs signals in this core, but the 137Cs and 241Am sediment records are well separated and remain consistent with the slightly different discharge patterns over time. This is so despite the fact that the maximum activity levels of both isotopes are now located well into reducing conditions out of which Mn must have migrated. The 210Pb profile appeared similarly unaffected by the oxidized/reduced boundary in this case. This core was too short to define the limits of any downward 137Cs migration. 210Pbexcess accumulation rate estimates for salt marshes should be viewed with some caution because of the steady-state assumptions inherent in all 210Pbexcess methods and the potential for fluctuating accumulation conditions and open system behavior in salt marshes.  相似文献   

14.
One year’s measurements of surficial sedimentation rates (1986–1987) for 26 Maine marsh sites were made over marker horizons of brick dust. Observed sediment accumulation rates, from 0 to 13 mm yr?1, were compared with marsh morphology, local relative sea-level rise rate, mean tidal range, and ice rafting activity. Marshes with four different morphologies (back-barrier, fluvial, bluff-toe, and transitional) showed distinctly different sediment accumulation rates. In general, back-barrier marshes had the highest accumulation rates and blufftoe marshes had the lowest rates, with intermediate values for transitional and fluvial marshes. No causal relationship between modern marsh sediment accumulation rate and relative sea-level rise rate (from tide gauge records) was observed. Marsh accretionary balance (sediment accumulation rate minus relative sea-level rise rate) did not correlate with mean tidal range for this meso- to macro-tidal area. Estimates of ice-rafted debris on marsh sites ranged from 0% to >100% of measured surficial sedimentation rates, indicating that ice transport of sediment may make a significant contribution to surficial sedimentation on Maine salt marshes.  相似文献   

15.
Vertical accretion of impounded marsh and adjacent natural marsh at four sites in southwestern Louisiana was estimated in 1994 by determining the depth of a stratum containing137Cs deposited in 1963. With relative marsh elevation, soil bulk density, organic matter content, and organic and mineral matter accumulation rates were used to describe soil formation. Three sites were impounded in 1956 and one site in 1951. Impounded marshes had lower marsh surface elevation than natural marshes because of hydrologic isolation from tidal sediment subsidies and substrate oxidation during forced drying. The elevation of natural marshes ranged from 12 cm to 42 cm higher than the elevation of the impounded marshes in 1963 and from 20 cm to 32 cm higher in 1994. Vertical accretion between 1963 and 1994 ranged from 9 cm to 28 cm in impounded marsh and from 15 cm to 21.5 cm in natural marsh. Only in impounded marsh that remained permanently flooded was accretion greater than in natural marsh.  相似文献   

16.
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   

17.
现代沉积的210Pb计年   总被引：74，自引：10，他引：74       下载免费PDF全文
万国江 《第四纪研究》1997,17(3):230-239
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   

18.
长江口-杭州湾毗连海区的现代沉积速率   总被引：30，自引：9，他引：21  
运用沉积地层同位素210 Pb、137Cs测年技术,估算长江口 杭州湾毗连海区的现代沉积速率。研究结果表明,以泗礁水道为界,东侧的马鞍列岛区受外海高盐水的顶托,长江来沙少,淤积缓慢;西侧至长江口、杭州湾海岸交汇点 (南汇咀 )之间水域,是长江泥沙扩散南下的主要通道。其中,受长江冲淡水次级锋和杭州湾锋面输沙影响的湾口浅滩区,沉积速率约 3cm/a;洋山港区,水深大于 20m的峡道区,受强动力作用而处于侵蚀状态,峡道两端出口的浅水区,沉积速率为0.3～ 1.6cm/a;马迹山港区,位于典型的峡道沿岸淤泥质陡坡,其沉积过程不稳定,正常沉积速率为 0.7～ 3.4cm/a,但常有快速淤积和地层滑塌流失现象发生.  相似文献   

19.
During the mid-late Holocene large sections of the Scottish coastline have been characterized by falling relative sea-levels resulting from differential glacio-isostatic uplift of this region of northern Britain. The complex interplay between crustal and sea-level movements continues to influence the morphological development of the Scottish coast. A number of geophysical models predict ongoing uplift of the Scottish landmass. However, a number of recent studies based upon the analysis of satellite altimetry data indicate a late 20th Century acceleration in the rate of eustatic sea-level rise.Detailed geochemistry, radiometric dating, and diatom analysis on selected sediment cores from four mature coastal marsh environments in Argyll, western Scotland, provides an opportunity to investigate the linkages between Twentieth century crustal movements, eustatic sea-level rise and recent rates of sedimentation recorded within marsh sediments across the proposed Scottish glacio-isostatic uplift dome.Solid-phase major and trace element geochemistry has been used to examine the extent to which post-depositional physical disturbance and/or chemical reactions may have influenced the reliability of the radiometric dating methods. Geochemical data indicate that the evolution of these marsh environments has not been significantly influenced by physical disturbance and overall the supply of minerogenic material to the marshes has been quite uniform.Vertical distributions of 210Pbexcess and 137Cs activity have been measured and used to develop models of recent marsh vertical accretion. Dating of the cores reveals subtle variations in the rates of sediment accumulation over the last c. 70 years between sites. For much of the last hundred years or so, sedimentation rates have been in good overall agreement with various estimations for sea-level rise, although at the more easterly sites these estimates are generally exceeded. However, quasi-equilibrium between marsh sedimentation and sea-level rise for much of the Twentieth Century is indicated from the Diatom analysis.Over the most recent period of marsh development (<10 years), a significant increase in the rate of surface sedimentation is recorded at all sites across the study area. Diatom analysis of these surface layers reveals an increase in the relative abundance of marine (polyhalobous) taxa in the near-surface sediments. This signifies a very recent increase in the rate of regional relative sea-level rise indicating that a regional threshold in coastal forcing has now been exceeded.These findings provide clear evidence that recent relative sea-level rise is now outpacing estimated rates of glacio-isostatic adjustment (GIA) across the proposed Scottish uplift dome.  相似文献   

20.
Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (210Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.  相似文献   

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