Influence of depositional mobility (downwashing) on the accumulation of atmospherically supplied elements in peat cores: an experimental approach

Influence of depositional mobility (downwashing) on the

accumulation of atmospherically supplied elements in peat cores:

an experimental approach

Keyao Chen

Student

Degree Thesis in “Physical Geography” 30 ECTS Master’s Level

Report passed: 05 June 2014 Supervisor: Richard Bindler

Influence of depositional mobility (downwashing) on the accumulation of atmospherically supplied

elements in peat cores:   an experimental approach

Keyao Chen

Abstract

The potential influence of downwashing on atmospherically deposited elements is of rare focus compared with other geochemical processes related to peat. Downwashing may cause a rapid downward movement of atmospherically supplied elements before they bond to the peat organic substrate and thus reduce the reliability of age-depth models that rely on atmospherically supplied radioisotopes (e.g.

Pb,

Am,

Cs). However, the existence of downwashing has not been directly tested, and to which depth the deposited element can be washed down is not fully understood.   To address the question of downwashing, an experiment was set up to mimic wet deposition by applying a CuBr

solution during a three-week period in peat cores collected from Rödmossamyran. Through this, the experimental results clearly supported the existence of downward mobility. Added Cu

could be measured to a depth of 10 cm, similar to previous studies based on Be and Pb. As a similar metal to Cu, the age-depth model based on

Pb dating could underestimate the ages to some extent without consideration of downwashing.

Keywords: downwashing, atmospherically deposited elements,

Pb dating, peat, copper,

bromide

Table of contents

1 Introduction ... 1

1.1 Aims ... 1

1.2 Background ... 2

2 Materials and methods ... 3

2.1 Field site and sampling ... 3

2.2 Experimental set-up ... 4

2.2.1 Treatment of peat cores ... 4

2.2.2 Sample preparation and geochemical analyses ... 4

2.3 Statistical Analysis ... 5

3 ²¹⁰ Pb dating of peat cores: theoretical background ^{... 5}

4 Results ... 6

5 Discussion ... 9

5.1 Downward mobility of atmospherically supplied elements (downwashing) ... 9

5.2 Theoretical implications of downward mobility on

Pb dating .... 10

6 Acknowledgements ... 12

7 References ... 13

    1  

1 Introduction

Peatlands comprise a significant portion of the land surface in many regions of the world, with the largest peat deposits found in the northern hemisphere (Gorham 1991). In this water-logged environment, production of organic matter by peat forming plants is greater than the decomposition due to limited oxygen diffusion, which eventually results in a net vertical accumulation of peat (Turunen et al. 2004). Thus peatlands are usually considered as a carbon sink and peat accumulation closely relates to carbon cycling. Peat accumulation can be calculated based on the contemporary vegetation productivity and decomposition rate (Frolking et al. 2001). As the vegetation productivity (i.e. mass input) is more observable, numerous researches have focused widely on fields related to peat decomposition and carbon sequestration (Gorham 1991, Oldfield et al. 1995, Frolking et al. 2001, Moore et al. 2002, Belyea and Malmer 2004).

During the formation of peat, the peatlands consistently receive atmospheric signals at the surface and bury them gradually beneath as ongoing peat accumulation adds new materials above. This process makes peat a natural archive that preserves past environmental changes (Oldfield et al. 1995, Lamborg et al. 2002, Bindler et al. 2004, Turetsky et al. 2004). By reconstructions of past environmental changes, it is possible to investigate long-term carbon sequestration in peatlands (Oldfield et al. 1995, Turner et al. 2014). In addition, peat records are strongly coupled with atmospheric fluxes, thus it can be used to study the atmospheric deposition dynamics, e.g., soil dust and metal pollutants (Shotyk et al. 2003). Besides, dating recent peat allows the comparison and calibration of accumulation rates to monitoring data (van der Plicht et al. 2013).

However, several factors may influence the accuracy and precision of developing reconstructions from peat records. For example, decomposition may have a potential effect on the element concentrations in different depths of peat cores (Biester et al. 2003).

Moreover, several deposited metals in peat may experience post-depositional redistribution due to the transformation of dissolved organic complexes by oxidation around the water table (Urban et al. 1990, Oldfield et al. 1997, Biester et al. 2007). Downwashing has also been identified as a potential way to move and distribute the elements in peat cores (Damman 1978, Urban et al. 1990, Oldfield et al. 1995). Direct evidence to support the hypothesis of downwashing is very limited and less attention has been paid to how far can downwashing drive elements to (Vile et al. 1999, Wieder et al. 2010). As sound chronology is an essential prerequisite for meaningful reconstructions, understanding the mechanism of downwashing will definitely improve our knowledge of age estimation in peat.

1.1 Aims

Downwashing is a process related to wet deposition (mainly through rainfall), which may cause atmospherically deposited elements to be transported rapidly below the surface before they can bond to organic matter in peat through cation exchange. In order to understand the potential effect of depositional mobility by downwashing here, an experiment was designed to model the rainfall in peat cores by applying a CuBr

solution to mimic precipitation. The rationale behind the usage of CuBr

is mainly due to the fact that CuBr

is totally soluble in the water and that Cu

and Br

have different adsorption binding capacity in peat cores. This experimental approach used is similar to that in Vile et al. (1999) and Wieder et al. (2010), who studied Pb and Be retention in peat cores, respectively.

The potential occurrence of downwashing has been indicated from several previous studies

(Damman et al. 1978, Urban et al. 1990, Oldfield et al. 1995, Oldfield et al. 1997, Lamborg et

al. 2002). As early as 1978 Damman (1978) referred to downwashing through the

distribution and movement of several elements in ombrotrophic peat bogs. Other studies

suggested that downwashing could be one of the controlling factors related to a low retention

efficiency of

Pb in peat (Urban et al. 1990). In a paper summarizing the status of

radiometric dating in peat cores, Oldfield et al. (1995) proposed that downwashing could be

    2  

one of the mechanisms responsible for the displacement and net loss of lead (including

Pb) from peat profiles. Since the 1970s, atmospherically supplied radioisotope

Pb has been developed as a popular technique to construct age-depth models, specifically for recent peat accumulation (≤ 150 years) (Appleby and Oldfield 1983, Turetsky et al. 2004). The accuracy of chronologies established using

Pb as peat records would be immediately affected by downwashing. The discrepancy of

Pb activity profiles along with depth has been observed and posed difficulties in establishing a

Pb-chronology in some peat studies (Lamborg et al.

2002, Turunen et al. 2004).

Taken together, such studies demonstrate a need to specifically assess downwashing. Based on a previous experimental approach aimed at studying retention (Vile et al. 1999, Wieder et al. 2010), this experiment design concentrates on whether downwashing exists or not in peat cores. Besides, by comparing the two different elements (Cu

and Br

) with different binding capacity, this study tries to further answer the question of how far atmospherically deposited elements could be washed to. To address the above two scientific questions, the aims of the study are to   test the following two hypotheses: 1) Downwashing of Cu and Br both exist in peat cores, 2) As an anion, which has been shown to be mobile, Br can be washed to a deeper place than Cu.

1.2 Background

As mentioned above, the very first step to reconstruct past changes lies in accurate dating of the peat profiles. For decades, researchers tried varying measurements to develop reliable dating techniques. These techniques are either absolute (providing age independent of other methods) or relative (providing age information relative to an independent absolute dating technique). And they also are either continuous (providing a series of age information throughout some portion of the peat profile) or provide age information for a single event or period in time. Several examples are listed below.

Radiocarbon (isotope

C) dating is the most well known radiometric dating technique, which emerged as early as 1940s. The initial concentration of

C is taken as a constant in living organisms, while decay begins at the death of the living organism with a half-life for

C of 5730 years (Taylor 2000). By measuring the

C concentration in the residuals it is possible to determine the ages of organic materials (Taylor 2000).

C dating only provides single events, thus a detailed chronology in peat bogs needs to include multiple dated levels and wiggle-matching which involves resembling a series of

C dates available. Due to the variations in the atmosphere, there is a non-linear relationship between calendar age and

C age, thus wiggle-matching is necessary for calibration of a single radiocarbon date to true calendar year in case of handling peat profiles with multiple date levels (Ramsey et al. 2001).

Although

C dating can be applied to peat samples that are older than about 300 years to as long as 50,000 years in age, its ability of providing concise dates was under dispute (Plastino et al. 2001, Wieder et al. 2010).

Cosmogenic radionuclide dating is another effective dating technique based on the interaction of cosmic rays and nuclides (e.g.

Be,

Al,

Cl) at the surface of rocks (Granger and Muzikar 2001, Balco 2011). The cosmogenic nuclides are bombarded by cosmic rays, which originate from high-energy supernova explosions in space, resulting in spallation reactions. These spallation reactions decrease with depth, meaning the cosmogenic nuclide concentration of a rock surface is proportional to the length of time that it has been exposed on the earth’s surface. The time scale of cosmogenic radionuclide dating is from 1,000 to 10,000,000 years, depending on the isotope that is used for dating (Davies et al. 2012).

Luminescence dating is used extensively to provide absolute chronologies by measuring the sediment grains directly (Huntley et al. 1985, Murray and Wintle 2000, Duller et al. 2004).

Trapped electrons in sediment grains can be released by daylight, which erases the optically

stimulated luminescence signal. When the grains are buried and hidden from light, they

    3  

begin to accumulate trapped electrons due to ionizing radiation. By determining the radiation dose and elapsed time since the last exposure to sunlight, the timing for the deposition of sediment can be deduced. The intensity of the luminescence increases with age for sediments on a time scale of 1,000-100,000 years (Murray and Wintle 2000).

Besides, there is also incremental dating, allowing the construction of year-by-year annual chronologies. Tree-ring dating is a type of incremental dating. Tree-ring dating is based on the analysis of patterns of tree rings. Trees usually form a ring over a year, which begins at the growing season. The change of tree rings reflects the variation of climatic conditions, since the widths of the annual tree rings tend to be proportional to the overall warmth of the summer (Grudd et al. 2002). Trees from the same region will tend to develop the same patterns of ring widths for a given period. These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions. Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions and for sub-regions of the world.

210

Pb dating is another effective radiometric dating tool for determination of modern sediment accumulation rates (up to 150 years). The first introduction of

Pb dating was by Goldberg (1963) in Greenland ice cores. After then, Krishnaswamy et al. (1971) and Koide et al. (1972) applied this dating technique in lake sediments and marine sediments, respectively.

Recent decades have witnessed the remarkable development of

Pb dating (Appleby and Oldfield 1978, Appleby and Oldfield 1983, Oldfield et al. 1984, Bindford 1990, Nevissi et al.

1991, Moser et al. 1993, Oldfield et al. 1995, Lamborg et al. 2002, Piliposian and Appleby 2003, Brenner et al. 2004, Appleby 2008). Large numbers of studies conducted on measuring modern deposition take

Pb dating as a crucial technique with reliable chronology at a centennial scale.

Pb attached to atmospheric aerosols can accumulate on the surface of soils, peat or lakes through wet or dry deposition. By measuring

Pb inventory in each sediment slice it is possible to calculate the sediment accumulation rate or establish the age-depth relationship.

The establishment of

Pb dating was under two pre-assumed circumstances: First, the atmospheric

Pb flux was constant, and second the vertical accumulation of

Pb was immobilized once incorporated in the sediment, i.e., no post-depositional redistribution (Vile et al. 1999, Turetsky et al. 2004). However, in practice, several factors have been identified that could influence the accuracy of

Pb dating (Appleby 2008). The atmospheric flux is the most significant factor that could influence the

Pb supply rate. Besides, the rate of the transport from the catchment is likely to enhance

Pb supply rate. Moreover, sediment erosion often underestimates the net supply rate. Last but not the least, post-depositional mobility of

Pb is the most controversial topic, which is of particular importance for peat (Clymo et al. 1990, Urban et al. 1990, Oldfield et al. 1995, Vile et al. 1999).

2 Materials and methods

2.1 Field site and sampling

In September of 2012, four peat cores were collected from Rödmossamyran (63° 47’N, 20°

20’E; 40 m a.s.l; Figure 1a), which is characterized as an oligotrophic mire (Rydberg et al.

2010) near to the Umeå River and within the Grossjon Nature Reserve. Umeå has a subarctic climate, weather systems affected by Gulf of Bothnia. The mean annual temperature is around 2-3 °C, and the mean annual precipitation is about 600 mm with snowfall accounting for one third of the total precipitation (SMHI.se). Wind direction is mainly in the north-south direction, but westerly to south westerly winds appear frequently in the fall and winter (SNA 2003).

The cores were collected in an open Sphagnum-lawn area in the southern end of the mire.

The sampling area was dominated by different Sphagnum species. The size of each collected

core was 15 cm*14 cm in area and 25 cm in depth to fit the dimension of the sample box

    4  

(Figure 1b). The four sample cores collected included both the surface vegetation and the peat. The analysis of the peat cores was performed in the geochemical laboratory at Umeå University.

2.2 Experimental set-up 2.2.1 Treatment of peat cores

Each of the four sampled peat cores was put into a Plexiglas box with high density mesh placed at the bottom (Figure 1b) to allow excess water to drain through. These cores were marked as control core 1 (C1), control core 2 (C2), manipulation core 1 (M1) and manipulation core 2 (M2). All four cores were wet slightly every day with deionized water (in an Erlenmeyer flask; Figure 1b) to keep the vegetation moist. During the first week of pretreatment with deionized water, it was determined that 300 ml could be added without water flowing out of the open bottom of the sample boxes. This addition would equal 14 mm of rain. The cores of C1 and C2 only received 300 ml of deionized water, while M1 and M2 received 300 ml of deionized water with the addition of 0.15 g of CuBr

. CuBr

solution was added into M1 and M2 every Monday over a three-week period, totaling 0.45 g CuBr

(0.13 g Cu

; 0.32 g Br

). CuBr

was selected because it would dissolve fully in water without requiring additional acids.

2.2.2 Sample preparation and geochemical analyses

One week after the final addition, each core was removed from the treatment box and sectioned in 2 cm intervals using a saw sharpened to a knife blade. All of the samples were firstly homogenized in a food processer and then a subsample was oven dried at 70 ºC to a constant weight. Sample weights were recorded before and after drying to calculate the dry bulk density of each sample (Figure 1c). After drying, each subsample was ground in a Retsch Mixer Mill prior to further analysis.

Figure 1. a. The location of sampling site; b. Sample boxes with peat cores and Erlenmeyer flasks with

CuBr

solution; c. Preparation of sectioned samples for homogenization and drying.

    5  

All subsamples were analyzed for Cu and Br concentrations by wavelength-dispersive X-ray fluorescence (WD-XRF) spectrometer (Bruker S8 Tiger). Specifically, 2 g of dry sample was measured using methods modified from De Vleeschouwer et al. (2011), which were specially designed for an organic matrix (plant and peat standard reference materials). The lowest concentrations in the standard reference materials are 3.2 ppm for Cu and 1.2 ppm for Br, which exceed the instrument detection limit for both elements (0.3 ppm). The Cu analyses fall within the calibration range, while the Br concentrations in most samples from the treated cores exceed the calibration range, which is from 1.2 to 145 ppm. Thus, except for the control cores and the lower levels from the treated cores, the Br concentrations reported here can be considered as approximate. Two standard reference materials were included for quality control and certified and measured values for Cu and Br are listed in Table 1.

Table 1. Quality control analyses of two standard reference materials (SRM). Results are presented as mean ± stand deviation (ppm).

SRM n Certified Cu Measured Cu Certified Br Measured Br

DC73310 24 1230 1208±18 1.7 1.5±0.4

DC73322 4 40 42±1 4.0 4.3±0.4

2.3 Statistical Analysis

All statistical analyses in this study were performed by SPSS software (version 20, SPSS Inc., Chicago, IL, USA). The paired t-test was used to determine the significant difference between paired variables. A p value less than 0.05 was regarded as statistically significant.

3 ²¹⁰ Pb dating of peat cores: theoretical background

210

Pb is a short-lived radioisotope (half-life of 22.3 years) derived from the paternal isotope of

238

U. Through a number of decay series,

U decays to

Rn and subsequently decays to

210

Pb (Binford 1990). In order to give a visual description, the pathway of

Pb is shown in Figure 2. The total

Pb inventories in peat cores contain two components, namely supported

Pb and unsupported

Pb. The fraction of

Pb produced within (i.e. in situ) the peat sediment via radioactive decay of

Rn that never diffused to the atmosphere is called supported

Pb (Oldfield and Appleby 1984). On the other hand, the unsupported

Pb is directly removed from atmosphere through wet or dry deposition (Oldfield and Appleby 1984).

The calculation of age-depth relationships in peat cores is by quantifying the total

unsupported

Pb inventory from the surface to a depth where it is undetectable. Typically, a

core is collected and sectioned into contiguous depth increments, each of which is weighed

and processed for measurement of

Pb activity by either α, β, or γ spectrometry (Joshi 1989,

Nevissi 1991, Moser 1993). Ideally, the unsupported

Pb activity would decline

exponentially with peat accumulation if the peat accumulation rate was constant with time,

yielding a log-linear relationship between

Pb activity and depth. However, in practice,

because of ongoing decomposition and compaction, peat accumulation rates over the past

150–200 years have not been constant, leading in non-linear

Pb profiles (Appleby and

Oldfield 1983, Blaauw et al. 2003).

    6  

Figure 2. Pathways by which

Pb can reach peat (modified from Oldfield and Appleby 1984)

Three models have been developed in situations of varying peat accumulation rates. They are:

1) constant rate of

Pb supply (CRS) model, 2) constant initial concentration (CIC) model, and 3) periodic flux (PF) model. The CRS model assumes that the unsupported

Pb flux to the peat surface is a constant. The CIC model assumes the

Pb flux changes as the peat accumulation rate changes, however, the peat surface

Pb specific activity is constant. The PF model is the most complicated model, considering the

Pb flux deposited at the peat surface was constant at a period of time.

4 Results

The bulk density of the four peat cores exhibited similar sediment composition amongst

cores, varying from 0.02 g cm

at the uppermost to 0.07 g cm

at the depth of 22 cm (Figure

3a). Generally, bulk density was about 1.7-2.5 fold larger at the depth of 18-22 cm (0.06 g

cm

on average) compared with the upper 0-4 cm (0.03 g cm

on average). The values are

consistent with previous sampling from the mire (Rydberg et al. 2010).

    7  

Figure 3. a. Bulk density in four peat cores; b. Mn concentrations in manipulated (M) and control (C)  

cores. Black and grey dots represent core M1/ C1 and M2/C2 as in manipulated (M)/control (C) cores, respectively.

As a redox sensitive element, Mn may be mobile due to cation exchange, variation of water level or changes in the redox potential. The pattern for Mn concentrations was similar between the pairs of cores (Figure 3b). Data for Mn are shown to provide evidence that the CuBr

addition did not influence the geochemical composition of the peat besides for the added elements.

For the manipulated cores, 9654 ± 464 (equalled 3.98 ± 0.02 as logtransformed) ppm of the added Cu was retained at the top 2 cm slice. The Cu concentration per 2-cm slice displayed an approximately exponential decline with depth (Figure 4a). The Cu concentrations in control cores were 11 ± 1 (equaled 1.02 ± 0.02 as log transformed) ppm at the first slice, and also displayed a small declining trend with depth (Figure 4a). Until to the depth of 16 cm, the Cu concentration per 2-cm slice was significantly higher in the manipulated cores than in the control cores (Figure 4a). Even at the depth of 16 cm, the value of detected Cu concentration was 14 ± 8 ppm in the manipulated cores. Below the depth of 16 cm, the Cu concentrations in the manipulated cores were equal to the background values in control cores (≤ 3 ppm).

b. Mn concentrations (ppm)

0 150 300 450

Depth (cm)

0

5

10

15

20

0 150 300 450

M C

a. Bulk density (g cm

)

M

0.00 .05 .10

Depth (cm)

0

5

10

15

20

C

0.00 .05 .10

    8  

Figure 4. a. Cu concentrations in manipulated (M) and control (C) cores; b. Br concentrations in  

manipulated (M) and control (C) cores. In order to compare the added values and background values, both Cu and Br concentrations are log transformed. Black and grey dots represent core M1/ C1 and M2/C2 as in manipulated (M)/control (C) cores, respectively. White dots in the left-hand panels represent the mean background value of control cores.

Although the added Br decreased with depth, the sharpest decline occurred between the upper two slices, decreasing from 13277 ± 784 (equaled 4.12 ± 0.03 as log transformed) to 4250 ± 75 ppm (equaled 3.63 ± 0.01 as log transformed) (Figure 4b). After that, the decline of Br concentration per 2-cm slice was more moderate than the Cu decline pattern (Figure 4).

At the depth of 22 cm, a high value of Br could still be detected (133 ± 22 ppm). The background value of Br in the control cores was 51 ± 8 ppm at the first slice, and decreased to 9 ± 2 ppm in the deeper samples.

The total amounts of Cu and Br recovered in core M1 and M2 were 0.18 ± 0.01 and 0.35 ± 0.02 g, respectively (Table 2). These values were not significantly different from the added Cu and Br (approximately 0.13 g of Cu and 0.32 g of Br) at a statistical level (Table 2).

a. Log transformed Cu concentrations (ppm)

M

0 1 2 3 4 5

De pth (cm)

0

5

10

15

20

C

0.0 .4 .8 1.2 1.6 2.0

b. Log transformed of Br concentrations (ppm)

M

0 1 2 3 4 5

De pth (cm)

0

5

10

15 20

C

0.0 .4 .8 1.2 1.6 2.0

    9  

Table 2. T-test of added CuBr

solution and total Cu and Br recovered in the manipulated cores. Results are presented as mean ± standard deviation (g).

Elements Recovered in core M1 and M2 p t

Cu 0.18 ± 0.01 0.147 4.245

Br 0.35 ± 0.02 0.081 7.776

5 Discussion

5.1 Downward mobility of atmospherically supplied elements (downwashing)

Consistent with the two hypotheses in this study, both Cu and Br experience a certain degree of downward mobility, and the mobility of Br

is greater than Cu

in the peat cores (Figure 5).

Cu is the main focus here for downwashing. As a divalent metal, Cu has been shown to adsorb efficiently to organic matter similar to Pb

(Benedetti et al. 1996, Breault and Colman 1996, Brown et al. 2000). Because of the rich functional groups in peat, its potential ability to capture dissolved cations deposited by wet deposition, such as metals and polar organic molecules, is quite high. Ionizable organic acid functional groups (negatively charged) are exposed during the decomposition process of peat plants, i.e., alcohols, aldehydes, ketones, carboxylic acids, phenolic hydroxides and others (Pakarine and Tolone 1976, Shotyk 1988, Clymo et al. 1990, Urban et al. 1990, Proctor 1995, Vile et al. 1999). These functional groups can retain positively charged metals via chelation, exchange sorption, polar organic bonding and polar inorganic bonding.

Approximately 51% of the total Cu inventory is retained in the first 2-cm slice (Figure 5). The

binding mechanism of Cu is generally viewed as the reaction of Cu

ions with humic acids

(Vinkler and Meisel 1976, Boyd et al. 1981, da Silva et al. 2002). Specifically, ion-exchange

reactions take place by binding metal ions with the release of H

from adjacent aromatic

carboxylate COOH and phenolic OH groups or, less predominantly, two adjacent COOH

groups (Ho and McKay 2002). Br is secondary in this study because as a monovalent anion it

has been shown to be mobile (Shotyk 1997, Biester et al. 2006). It is no surprise to observe

less Br retained in the first slice (36% of total inventory) and greater downwashing occurred

compared with Cu (Figure 5). Although most experiments failed to show evidence of Br

sorption (Tennyson and Settergren 1980, Butters et al. 1989, Bowman and Gibbens 1992,

Davis et al. 1998), a few researchers have reported the sorption of dissolved Br

on dead

organic debris (Lundström and Olin1986, Seaman 1995).

    10  

Figure 5. Cu and Br inventories per 2-cm slice as % of total excess inventory (i.e. the concentrations from the control cores have been subtracted). Black and grey represent the two manipulated cores M1 and M2, respectively.

Compared with added Be in Wieder et al.’s study (2010), a similar pattern of Cu is observed in this study. According to their results, nearly half of the added Be was retained at the first 2-cm slice. Besides, both metals showed an exponential decline with depth. In addition the detectable depth of downwashing was 12-14 cm in their study, very close to the measured downwashing depth of 10 cm in this study. Within the top three slices (0-6 cm) more than 90%

of total Cu was retained (Figure 5). This result is comparable to what Vile et al. (1999) had observed in Pb retention in the top three sections. However, Vile et al. (1999) reported a lower Pb inventory in the first section of their low water table treatment (15 cm water table), and a deeper depth of detectable added Pb (18-20 cm). Although a larger fraction of metal can be retained effectively through cation exchange and Vile et al. (1999) and Wieder et al.

(2010) discussed their results in terms of immobility, results from this study add new evidence to support   downward mobility of atmospherically supplied elements.

5.2 Theoretical implications of downward mobility on

Pb dating

Although the peat has a very high cation exchange capacity and that soluble Pb

like Cu

could be retained effectively in peat through physiochemical binding to organic matter (Vile et al. 1999). Previous researches raised by Urban et al. (1990), Oldfield et al. (1995), Lamborg et al. (2002), Bindler et al. (2004) and a review by Biester et al. (2007) have hypothesized some downward movement of

Pb – and likely other atmospherically supplied elements- does occur during atmospherical deposition. Despite that the largest fraction of Cu is retained at the first slice, cation exchange is not able to prevent atmospherically deposited metals from downwashing. As a divalent metal, Pb has a similar nature like Cu, which in turn suggests a potential problem in

Pb dating related to downwashing.

Because of downwashing, a part of the atmospherically supplied

Pb may be transported to a deeper depth other than retained at the surface. A simplified description of the effect of downwashing is shown in Figure 6. If the unsupported

Pb flux is taken as a constant and the vertical accumulation of peat is undisturbed, then the yearly decreased

Pb activity is possible to be calculated through exponential decay based on its half life (Figure 6a). In this idealized

Pb model,

Pb activity of a certain year highly related to its depth in a peat core thus an age-depth model is possible to be built. However, when downwashing exists, part of the deposited

Pb is transported to a deeper depth (Figure 6b). The downward mobility of

210

Pb will smear the decay of in situ

Pb refers to the supported fraction only. Under this circumstance, a

Pb activity decay calculation based solely on half-life is not able to reflect

% of total added Cu inventory per 2-cm slice

0 20 40 60 80

0 1 2 3 4 5 6 7 8 9 10 11

0 20 40 60 80

% of total added Br inventory per 2-cm slice

Sa mpl e nu mbe r

    11  

the real age and is most likely to underestimate the age. According to research carried out in the same area of this study, background

Pb was reached at about 45 cm depth (Rydberg et al. 2010). Assuming that downwashing could transport Pb to the same depth as Cu showed, a rough estimate of 40 years of

Pb dating would be affected by downwashing, given a constant peat accumulation rate. In practice, the accumulation rate is higher in the upper layer than the deep ones, which suggests that 40 years would likely be an overestimate.

Figure 6. The comparison of

Pb model in ideal (a) and downwashing (b) situations.

This study aims to test the downward mobility of atmospherically deposited metals. As can be seen in the schematic diagram (Figure 6), downwashing changes the distribution of atmospheric fluxes received by peat. Different from other factors identified to influence the age-depth model in

Pb dating (Damman 1978, Urban et al. 1990, Oldfield et al. 1995, Biester et al. 2007), downwashing has the strongest influence on the recently accumulated peat of a few decades. Thus it brings special emphasis on downwashing of newly deposited peat, especially in those peatlands with frequent intense precipitation events. Further work is needed to have a better understanding of the mechanism of downwashing. Study on other factors that may interact with downwashing (e.g. rainfall intensity) will help to improve age-depth relationships in peat chronologies.

a. ideal situation

210

Pb activities of recent deposition

P ea t c ore lay ers 1 2 3 4 5

2013 2012 2011

b. effect of downwashing 2013

2012

2011

    12  

6 Acknowledgements

I feel much indebted to many people who have instructed and favored me in the course of

writing this paper. First of all, I would like to express my heartfelt gratitude to my supervisor

Richard, for his warm-heart encouragement and most valuable advice, especially for his

insightful comments and suggestions on the draft of this paper. Without his help and

guidance, I could not have completed this paper. High tribute shall be paid to Xinyue, her

spirits always guide me in the whole process of writing. I also would like to express my

thanks to Rolf, Tord, and Olof in Geography Department and my friends Xingru, Aihong,

Wanzhong, Ulf Hjalmars, Ulf Westin, Hao, Yang, Fu for their valuable help and spiritual

support during my study. Last but not least, I am indebted to my parents for their continuous

support and encouragement.

    13  

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