Role of Surface Vegetation in 210Pb-Dating of Peat Cores

Environ. Sci. Technol. 2008, 42, 8858–8864

Role of Surface Vegetation in 210Pb-Dating of Peat Cores C A R O L I N A O L I D , * ,† J O R D I G A R C I A - O R E L L A N A , †,‡ A N T O N I O M A R T Í N E Z - C O R T I Z A S , § ´ , †,‡ E V A P E I T E A D O , § A N D PERE MASQUE J O A N - A L B E R T S A N C H E Z - C A B E Z A †,‡,| Departament de Física and Institut de Cie`ncia i Tecnologia Ambientals (ICTA), Universitat Auto`noma de Barcelona, E-08193 Bellaterra, Spain, Departamento de Edafoloxía e Química Agrícola, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain, and Marine Environment Laboratories, International Atomic Energy Agency, MC-98000 Monaco

Received June 05, 2008. Revised manuscript received September 14, 2008. Accepted September 25, 2008.

210Pb-dated

ombrotrophic peat cores have been widely used to reconstruct the atmospheric fluxes of heavy metals for the past century. Many of these studies rarely include the overlying vegetation compartment (i.e., the aerial part of vegetation and decayed plant remains) in the analysis although it represents the first layer capturing atmospheric deposition. The aim of this study was to evaluate the radionuclide and Pb content of this biologically active layer in bogs and to assess its implications on the total inventories and the 210Pb-derived chronology. We analyzed two short ombrotrophic peat cores from the same bog (Chao de Lamoso, Galicia, Spain) for 210Pb, artificial radionuclides (137Cs and 241Am), and Pb. The total Pb inventory was underestimated by about 12% when the plant material was not included in the record. The atmospheric origin of 210Pb and the uptake of 137Cs by roots led to significant activities of these radionuclides in the upper layers. Therefore, removing them from the peat record would imply even larger underestimations of the total inventories, ranging from 25% to 36% for 137Cs and from 39% to 49% for 210Pb. In contrast to the chronologies inferred from the constant rate of supply (CRS) model when only peat layers are considered, the 210Pb chronology agreed well with artificial radionuclide dating when surface vegetation was included. These results suggest that an accurate peat chronology requires an initial evaluation of the relevance of plant inventories and emphasizes the need of considering the biologically active layer when atmospheric fluxes of heavy metals and other pollutants are reconstructed.

Introduction Natural deposits such as polar ice, lake sediments, and peats are archives that allow the reconstruction of changes in atmospheric heavy metal deposition through time (e.g. refs 1-3). In particular, ombrotrophic mires are well suited for this purpose since they are hydrologically isolated from * Corresponding author phone: +34-935811191; fax: +34935812155; e-mail: [email protected]. † Departament de Fı´sica, Universitat Auto`noma de Barcelona. ‡ ICTA, Universitat Auto`noma de Barcelona. § Universidade de Santiago de Compostela. | International Atomic Energy Agency. 8858

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groundwater, they receive nutrients and pollutants exclusively from the atmosphere, and peat organic matter has a high ion-exchange and metal-binding capacity. These properties, together with their wide geographic distribution and temporal development, cause bogs to be considered reliable archives of atmospheric heavy metal deposition, spanning the past thousands of years (e.g., refs 4-6). The inference and interpretation of the temporal trends in atmospheric heavy metal deposition from peat records requires a reliable peat profile chronology. Due to the increasingly anthropogenic metal emissions occurring since the beginning of the Industrial Revolution, most studies have focused on the reconstruction of atmospheric heavy metal fluxes during the past century. The half-life of the natural radionuclide 210Pb (T1/2 ) 22.3 years) and its chemical properties make it suitable for establishing the chronology of archives accumulated over approximately the past 150 years (7, 8). 210Pb is produced in the atmosphere by decay of 222Rn (T1/2 ) 3.8 days) exhaled from the continental crust. The atmospheric 210Pb (namely, excess 210Pb) is attached to aerosols and deposited onto the earth surface by wet deposition or dry fallout. The excess 210Pb activity profile is used to determine the chronology of environmental records. 210 Pb-dating was first proposed by Goldberg (9) and successfully applied in lake sediments. Later, the same technique was applied in peat deposits (10, 11). 210Pb-dating models assume a constant atmospheric flux of 210Pb and no significant migration of the radionuclide within the record. The mean accumulation rate of environmental records can be inferred from the mean slope of the excess 210Pb activity profile (CF:CS model) (7). For environments with changing accumulation rates, dates can be calculated assuming a constant rate of supply of 210Pb (CRS model) (12) or constant initial activity of the radionuclide (CIC model) (13). Since processes such as compaction and organic matter decay alter peat accumulation rates and surface concentrations, the CRS is the 210Pb-dating model most widely used to establish the chronology of recent ombrotrophic peat deposits. Since natural processes could alter the excess 210Pb activity record, 210Pb geochronology should be validated using independent dating techniques. The introduction into the atmosphere of artificial radionuclides (i.e., 137Cs, 239,240Pu, and 241Am) during the nuclear weapons testing period (1954-1963) and, more recently and mainly in Europe, due to the Chernobyl accident (1986) produced well-defined peaks in their activity profiles which can be used as chronological markers (13-15). Nevertheless, the usefulness of artificial radionuclides to provide an independent dating relies on their immobility into the record. For instance, mobility of 137Cs observed in peat (16, 17) restricts its use as a dating tool. The vertical structure of the peat deposit can be divided in a number of horizons depending on the physical and chemical properties (18). The surface layer, where nutrient and pollutant capture processes take place, consists of a vegetation cover adapted to water excess and acidic and oligotrophic conditions. A second layer, below the surface vegetation, consists of partially decomposed plant remains. At depth, vegetation remains may not be recognizable and, strictly speaking, peat layers appear. Therefore, the peat deposit is built up by vegetation growth in the surface and continuous accumulation of plant remains below, being a complex dynamic system with changing physical and chemical conditions with depth. The interpretation of the peat 10.1021/es801552v CCC: $40.75

 2008 American Chemical Society

Published on Web 10/31/2008

FIGURE 1. Location of Chao de Lamoso (Galicia, Spain). record requires a detailed understanding of the different processes taking place during peat accretion and that could influence the retention and mobility of the elements of interest (5, 19). Recent studies carried out in peat cores were addressed to evaluate their use as archives of atmospheric heavy metal deposition, examining both the within-bog spatial variability and the processes that may affect the elemental records (20-22). However, although being subject to atmospheric pollutant deposition, many studies do not take into consideration the vegetation compartment (i.e., surface living vegetation and the layer of partially decomposed plant remains) in the analysis of peat profiles. In some cases, green plant material is not strictly considered to be peat and the moss/peat interface becomes the initial depth for sampling and analyzing (e.g., refs 23-26). Other studies remove living plants to facilitate the peat core extraction (e.g., refs 27-29). Hence, most studies only make a brief description of the vegetation constituents of peat, without describing whether the first layers of the core are completely decayed plants (peat) or sampling the aerial part of the vegetation. The objective of our study is to evaluate the role of this surface compartment in the total inventories and 210Pb-dating of peat profiles.

Materials and Methods Study Site. The Chao de Lamoso mire is part of the blanket bog complex located in Serra do Xistral (northwest Spain, Figure 1), situated 20-25 km south of the coast. The annual temperature in the area ranges from 8 to 12 °C and the annual precipitation from 1400 to 1800 mm. The sampling area lies at an altitude of 1039 m above sea level on a flat summit surface. Present-day vegetation is dominated by sedges (Carex durieui, Carex binervis, Carex panicea) and grasses (Agrostis cuurtisii, Agrostis heperica, Deschampsia flexuosa, Molinia caerulea, Festuca rubra, Juncus bulcosus, Potentilla erecta), while heathers (Calluna vulgaris, Erica mackaina, Erica cinerea) are more abundant in the drier areas of the bog (30). Sphagnum mosses are also present, but their abundance is much lower. Sampling and Sample Preparation. Two short (30 cm) peat cores separated by 50 m (CHL1, 29T, X ) 0617001, Y ) 4817281; CHL5, 29T, X ) 0616978, Y ) 4817302) were collected in June 1998 in Chao de Lamoso, using a Wardenaar peat

sampler (10 × 10 cm section) (31). The surface vegetation was also collected by cutting the aerial part. The fresh peat cores were immediately sliced using a stainless steel knife as follows: (1) the surface vegetation was separated into two sections: the aerial part and a section of 2 cm representing partially decayed plant remains, (2) five 1 cm thick sections, to provide high vertical resolution in the most recently accumulated peat, and (3) 2 cm thick sections below. The peat bulk density was determined by sampling three plugs from the middle of each section using a cylinder 2 cm in diameter (corresponding to volumes of 3.1 and 6.3 cm3 depending on the thickness of the peat sections). All samples were dried at 105 °C until constant weight, milled, and homogenized. To correctly evaluate the radionuclide and metal inventories of the aerial part of the vegetation, an accurate estimation of its cumulative mass is required. For this purpose, a second sampling was carried out in March 2008 in an area of 5 × 5 m around the CHL1 and CHL5 sampling sites. The vegetation (aerial part) was cut in a number 25 × 25 cm surfaces randomly selected. The plant material was dried at 105 °C until constant weight. Total metal concentrations were measured using an energy-dispersive miniprobe multielement analyzer (EMMAXRF) at the Institute of Environmental Geochemistry (University of Heidelberg, Germany) (32). 226Ra, 137Cs, and 241Am activities were determined by direct γ–spectrometry with a high-purity intrinsic Ge detector (CANBERRA, model GCW3523), which was calibrated by using peat standard geometries prepared in the laboratory. Dry samples were placed into calibrated geometries (Teflon vials). To ensure the equilibrium between 226Ra and its daughter radionuclides, the vials were hermetically sealed during three weeks before counting. 226Ra was determined via its daughter nuclide 214Pb emissions at 295 and 352 keV. 241Am and 137Cs were measured by their γ-emissions at 59.5 and 662 keV, respectively. 210Pb activities were determined by R-spectrometry measuring the activities of 210Po. The elapsed time between core sampling (June 1998) and 210Po analyses (August 2004) assured secular equilibrium with its parent nuclide. 210Po analyses were carried out following the method described by Sanchez-Cabeza et al. (33) (see the Supporting Information (SI)). 210Po was measured using PIPS VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2.

210Pb

(circles),

137Cs

(squares), and

241Am

(asterisks) activity profiles of CHL1 and CHL5 cores.

R-spectrometers (CANBERRA, model PD-450.18 A.M.) with a minimum detectable activity (MDA) of 0.40 Bq · kg-1.

Results Bulk Density. The accurate estimation of the bulk density of the aerial part of the vegetation is complex due to the difficulties in defining a reference volume and the variability of plant mass production with time. Nevertheless, we could estimate its cumulative mass by dividing the dry mass over the known sampling area. The values obtained ranged from 0.068 to 0.108 g · cm-2, with a mean cumulative mass of 0.079 ( 0.007 g · cm-2 for CHL1 and 0.09 ( 0.01 g · cm-2 for CHL5 (SI, Table S1). The mean peat density was similar in both cores (0.12 ( 0.01 and 0.10 ( 0.01 g · cm-3 for CHL1 and CHL5, respectively). Apart from the upper layers, CHL1 showed a linear increase in bulk density with depth. In core CHL5, the bulk density remained almost constant (SI, Figure S1). Radionuclides: 210Pb, 137Cs, and 241Am. Figure 2 shows the vertical distribution of total 210Pb, 137Cs, and 241Am in CHL1 and CHL5 cores (SI, Table S2). Both cores showed maximum 210Pb activity at 1 cm, with values of 583 ( 18 Bq · kg-1 for CHL1 and 598 ( 26 Bq · kg-1 for CHL5. Below, the 210Pb activity decreased exponentially to values of 5-10 Bq · kg-1 in the deepest sections. 226Ra was under the detection limit (mean value of 5.5 Bq · kg-1) in all samples due to the low proportion of mineral matter, consistent with the ombrotrophic status of the bog. Thus, we concluded that 210Pb was supplied exclusively from the atmosphere and was equal to excess 210Pb. The 137Cs activity profile of CHL1 showed a maximum of 136 ( 5 Bq · kg-1 at 2.5 cm. Two secondary maxima were also observed at 4.5 and 6.5 cm, with activities of 96 ( 7 and 90 ( 7 Bq · kg-1, respectively. For CHL5, a subsurface maximum of 196 ( 7 Bq · kg-1 was observed at 3.5 cm. Unlike CHL1, this core had only one secondary peak at 1 cm, with a value of 185 ( 13 Bq · kg-1. In both cores, the 137Cs activity decreased exponentially with depth, reaching around 15 Bq · kg-1 in the deepest sections. 241Am was detected between 2.5 and 6.5 cm. The 241Am record showed a resolved peak in both peat cores with 8860

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maximum activities of 4.6 ( 0.8 Bq · kg-1 (at 5.5 cm) and 5.3 ( 0.5 Bq · kg-1 (at 3.5 cm) for CHL1 and CHL5, respectively.

Discussion Vegetation Contribution to the Inventories. Most studies using bogs as archives of atmospheric deposition reconstruct metal emission trends exclusively from the analysis of peat layers (23-29) and rarely include the overlying vegetation compartment (i.e., the aerial part and slightly decayed plant remains). Thus, whereas some detailed studies about handling and preparing peat samples have been carried out (34), no study describes how to sample and consider this uppermost compartment. The metal content in plants rarely exceeds 10% of the total deposition (e.g., ref 35) and does not result in a significant underestimation if it is not included in the total metal inventory. Furthermore, the radionuclide dating may not be affected by removing vegetation when it corresponds to a few months of peat accumulation (e.g., ref 28). However, due to the atmospheric origin of 210Pb and the active uptake of 137Cs by roots, the presence of vegetation may play a significant role in the distribution of these and other radionuclides. In our peat cores, the 210Pb activities of the aerial vegetation were 396 ( 12 Bq · kg-1 for CHL1 and 507 ( 20 Bq · kg-1 for CHL5, revealing heterogeneity of the overlying vegetation. Similar activities were found at 1 cm (layer of plant remains), with values of 583 ( 18 Bq · kg-1 for CHL1 and 598 ( 26 Bq · kg-1 for CHL5. Since the transfer of 210Pb by roots is negligible (36), these activities come mainly from the atmosphere and must be taken into account when the atmospheric supply of 210Pb is estimated. Inferred from the cumulative mass, the 210Pb inventories in the aerial part of the vegetation were of 312 ( 10 and 439 ( 17 Bq · m-2 for CHL1 and CHL5, respectively. Higher inventories were accumulated in the plant remains section, with a mean value of 1635 ( 88 Bq · m-2. Not including the vegetation compartment in the record would lead to excess 210Pb inventories of 3026 ( 35 Bq · m-2 in CHL1 and 2148 ( 45 Bq · m-2 in CHL5 (Table 1), which correspond to annual 210Pb atmospheric supplies of 94 ( 1 and 67 ( 1 Bq · m-2 · year-1, respectively. The mean 210Pb inventory considering the vegetation compartment (4598 ( 397 Bq · m-2) leads to a 210Pb atmospheric

TABLE 1. Radionuclide (210Pb,

137Cs,

and

241Am)

and Pb Inventories Obtained Excluding and Including the Vegetation Compartment Inventories CHL1

210Pb

(Bq · m-2)

(Bq · m-2) 241Am (Bq · m-2) Pb (µg · cm-2) 137Cs

CHL5

peat layers

including vegetation

peat layers

including vegetation

3026 ( 35 1316 ( 29 19 ( 2 119

4994 ( 63 1746 ( 40 19 ( 2 132

2148 ( 45 1116 ( 23 13 ( 1 102

4201 ( 85 1734 ( 45 13 ( 1 118

supply of 143 ( 9 Bq · m-2 · year-1, 39-49% higher than the value obtained when it is not included. The activities of 137Cs in the vegetation were similar to those present in the upper peat sections (80% of Pb in grass is derived from the atmosphere (37). Consequently, Pb concentrations in the peat surface must originate from atmospheric deposition and, therefore, belong to the total metal inventory. However, due to the recent decrease of Pb atmospheric emissions associated with the introduction of unleaded gasolines during the 1970s (38), the metal concentrations in the surface layers were 2-6-fold lower than the maxima observed at depth. Consequently, the vegetation compartment contributes up to 12% to the total Pb inventory in both cores (Table 1). Since the decay and incorporation of plant material into the peat deposit depend on factors such as climatic and hydrological conditions (39, 40), mires located in different climatic areas may have a thicker vegetation compartment than that found for the CHL cores. Thus, the lack of surface plants, either by damage of the peat record or by removal during sampling, may involve significant underestimations of the total inventories that, for 210Pb, could lead to inaccurate chronologies when using the CRS model (29). Derivation of the Age Model. The application of the 210Pbdating models requires proper definition of the “zero” time. Usually, this time is ascribed to the sampling time and is given to the topmost layer of the core. In the particular case of peatlands, surface vegetation constitutes the first stage of peat formation and represents a time interval that depends on the balance between productivity and decomposition of the constituent plant species. Discarding this biologically active layer of growing vegetation from the peat and giving the zero age to the first peat layer would imply removing some years of peat accumulation and, consequently, losing

some part of the record. Thus, an accurate peat core chronology will require an estimation of the age of this layer. The fact that vegetation accumulates 210Pb can be used as a first approximation to estimate the age of the top section of the peat profile. Peat undergoes continuous organic matter decay, especially in the upper section of the deposit. Between 10% and 20% of the mass is lost by decomposition during the first year after accumulation, with a subsequent decay of 50-80% of the original mass present in the aerobic zone (39, 41). The CIC model infers ages from the specific activity of the surface section, and the overestimation of this activity due to organic matter decay implies underestimating subsequent ages (11), unless corrections were included in the model. Organic matter decomposition together with peat compaction alter the apparent peat accumulation rate, so the assumption of the CF:CS model of a constant accretion rate is excessively restrictive (7). The use of the CRS model enables explanation of the dilution of excess 210Pb based on varying accumulation rates (12). The age at certain depth by the CRS model is derived from the excess 210Pb inventory accumulated along the entire profile and below the considered depth, so the correct estimation of the excess 210Pb inventory is critical. As mentioned above, the significant fraction of 210Pb found in the vegetation compartment is due to atmospheric deposition and belongs to the total excess 210Pb inventory. For this reason, we applied the CRS model considering this layer as part of the peat profile. To compare these results with those of the common method, CHL peat cores were also dated considering exclusively peat material. As expected, the age-depth curves obtained including the vegetation compartment (i.e., the aerial part and the partially decayed plant remains) showed a shift in the chronology with regard to those derived without including it, which corresponds to its estimated age. The 210Pb inferred ages for the aerial part of the vegetation were 2 ( 1 years for CHL1 and 4 ( 1 years for CHL5. The section of partially decayed plant remains represented a higher time of accumulation, namely, 14 ( 1 years for CHL1 and 18 ( 1 years for CHL5. Thus, the vegetation compartment may represent 16-22 years of accumulation. It is worth noting that the vegetation layer could have higher thickness in ombrotrophic mires located in regions where climatic conditions are unfavorable for peat decay. Not taking into account these layers, therefore, would imply an even greater inaccuracy in the estimation of the peat chronology. Age Model Validation. Natural processes such as organic matter decay or element transport by interstitial pore waters may alter the excess 210Pb activity profile. For this reason, an independent validation of the chronology obtained from 210Pb-dating models is required. The fallout record of artificial radionuclides (e.g., 137Cs and 241Am) is often used for validating dates for the past 20-60 years (13-15). The high 137Cs activities detected in the first centimeters of the peat cores could be the result of the active uptake by living plants (10). Furthermore, the presence of 137Cs in the deepest layers of our cores where activities of excess 210Pb were detected provides clear evidence of downward migratVOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. 137Cs (squares) and 241Am (asterisks) activity profiles versus CRS chronologies obtained excluding (left) and including (right) the vegetation compartment. ion of this radionuclide in peat (10, 16, 17). Although these observations indicate a certain degree of 137Cs mobility in peat bogs, both 137Cs and 241Am activity profiles were represented versus CRS chronologies obtained for CHL1 and CHL5 when the vegetation compartment was (i) excluded and (ii) included in the record (Figure 3). It is apparent that the inclusion of vegetation in CHL1 produced a shift of 16 ( 1 years, meaning the assignment of the 241Am onset to 1955 ( 2 and the maximum to the 1960-1965 period, more consistent with the history of artificial radionuclide global deposition than the corresponding one obtained excluding the plant compartment. Similar results were obtained for the core CHL5: including vegetation in the record produced a shift of 22 ( 2 years, recording the first traces of 241Am in 1958 ( 4 and the maximum of its activity in 1965 ( 4. For 137Cs, the age-depth model obtained in the core CHL1 when the vegetation was included situated the maximum activities of the radionuclide in 1955-1965 and 1978 ( 4. In the CHL5 core, the time shift assigned the maximum activities of 137Cs to 1965 ( 4 and 1985 ( 9. Although the inclusion of the vegetation compartment did not allow detection of the shallower maximum of 137Cs linked to the Chernobyl accident in CHL1, the chronology was in good agreement with the history of artificial radionuclide deposition in both cores. 8862

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These results suggest that, despite the observed 137Cs mobility in bogs, the CHL1 and CHL5 cores may have preserved a significant immobilized fraction of the radionuclide after deposition since well-defined peaks in activity seemed to correspond to the history of atmospheric fallout of the radionuclide, and thus, the 137Cs records seem not to have been significantly altered. Furthermore, the mean peat accumulation rates obtained for the analyzed cores (CHL1, 225 ( 58 g · m-2 · year-1; CHL5, 180 ( 56 g · m-2 · year-1) were consistent with those observed in peats located in the North of Europe (41, 42). These results also seem to validate the chronology obtained when the surface vegetation compartment is included in the record. Our results point out that the vegetation compartment may contain a significant proportion of the total Pb and radionuclide inventories. For the CHL cores, this content implied a low to moderate underestimation (9-12%) for Pb, but a larger underestimation for radionuclides (25-36% for 137Cs and 39-49% for 210Pb) if the vegetation compartment was not included in the inventories. For 210Pb, the inclusion of the vegetation resulted in chronologies that agreed more closely with the history of artificial radionuclide emissions. An accurate peat chronology requires the integration into the record of the vegetation compartment and an evaluation of its estimated age resulting from the CRS model. Thus, in

line with other studies that have indicated that hydrological and biogeochemical characteristics of bogs may result in incorrect chronologies due to 210Pb mobility (ref 22 and references therein), incorrect sampling of the vegetation layer may also result in a significant underestimation of peat ages (16-22 years younger for the CHL cores). Although the degree of 210Pb mobility in the cores we have analyzed does not seem to affect the CRS chronology to a great extent, it does not rule out the possibility of higher mobility in other mires with a deeper acrotelm or higher porosity (as those discussed in ref 22). What this indicates is that bogs are far from being homogeneous systems and that chronologies and studies on metal accumulation using peat cores should consider the different processes that may affect the record and include all layers of the continuum between vegetation, plant remains, and peat. Incorrect dating not only results in incorrect chronologies in the reconstruction of atmospheric pollution, but also provides inconsistent peat and metal accumulation rates. As discussed in a recent review (22), this leads to an overestimation of recent pollution.

Acknowledgments We acknowledge the assistance in field and laboratory work from our colleagues at the Laboratori de Radioactivitat Ambiental (Universitat Auto`noma de Barcelona) and Edafolox´ıa e Quı´mica Agrıćola (Universidade de Santiago de Compostela). We also thank Andriy Cheburkin (Institute of Environmental Geochemistry, University of Heidelberg, Germany) for his help with the accurate EMMA-XRF analyses and Santi Hurtado (CITIUS, Universidad de Sevilla) and Mai Khanh Pham (International Atomic Energy Agency, IAEA) for their assistance with 241Am determination. The IAEA is grateful for the support provided to its Marine Environment Laboratories by the Government of the Principality of Monaco. This work was supported by the Spanish Project IBEROBOG (REN2003-09228-C02-01 Y 02).

Supporting Information Available Description of 210Po analyses, table of cumulative mass of vegetation (S1), table of radiouclide and Pb concentrations (S2), and additional figures (peat density profiles). This information is available free of charge via the Internet at http://pubs.acs.org.

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ES801552V

Role of Surface Vegetation in 210Pb-Dating of Peat Cores

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