Department of Ecology and Environmental Science

Umeå 2013

Incorporation and preservation of geochemical fingerprints in peat

archives

Sophia V. Hansson

This work is protected by the Swedish Copyright Legislation (Act 1960:729)

© Sophia Hansson 2013

ISBN: 978-91-7459-779-0

Omslagsbild:Illustration by Swedish artist André Högbom, www.redcrab.se

Elektronisk version tillgänglig på http://umu.diva-portal.org/

Tryck/Printed by: Print & Media

Umeå, Sweden 2013

Till minne av

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i

Table of Contents

Table of Contents i

Abstract ii

List of papers iii

Glossary iv

Introduction 1

Objective 4

Abstracts of the papers 5

Material & Methods 7

Study sites 7

Analytical methods 8

Statistical calculations 9

Results & Discussion 10

Decomposition - Three proxies - one story? 10

Downwashing of atmospherically supplied elements in the field:

Beryllium-7 as a tracer for downwash 13

Downwash in the laboratory:

Retention of metals in relation to rainfall intensities 17

The effect of downwash:

Implications on estimated accumulation rates 20

Conclusions 25

Future Perspectives 27

Author Contributions 28

Tusen tack… 29

References 31

ii

Abstract

The present status of the environment, including environmental problems such as

heavy metal accumulation in aquatic and terrestrial ecosystems, is in part the

consequence of long-term changes. Cores from peatlands and other natural archives

provide us with the potential to study aspects of the atmospheric cycling of elements,

such as metal pollutants, on timescales much longer than the decade or two available

to us with atmospheric deposition monitoring programs. The past decade especially

has seen a rapid increase in interest in the biogeochemical record preserved in peat,

particularly as it relates to environmental changes (e.g. climate and pollution).

Importantly, recent studies have shown that carbon dynamics, i.e., organic matter

decomposition, may influence the record of atmospherically derived elements such as

halogens and mercury. Other studies have shown that under certain conditions some

downward movement of atmospherically deposited elements may also occur, which

adds complexity to establishing reliable chronologies as well as inherent problems of

estimating accurate accumulation rates of peat and past metal deposition. Thus, we

still lack a complete understanding of the basic biogeochemical processes and their

effects on trace element distributions. While many studies have validated the general

temporal patterns of peat records, there has been a limited critical examination of

accumulation records in quantitative terms. To be certain that we extract not only a

qualitative record from peat, it is important that we establish a quantitative link

between the archive and the few to several decades of data that are available from

contemporary monitoring and research. The main objective of this doctoral thesis was

to focus on improving the link between the long-term paleorecord and the

contemporary monitoring data available from biomonitoring and direct deposition

observations. The main research questions have therefore been: Are peat archives an

absolute or relative record? And how are geochemical signals, including dating,

incorporated in the peat archive? What temporal resolution is realistic to interpret by

using peat cores?

iii

List of papers

This doctoral dissertation is based on the following four papers, which are referred to

in the text according to their Roman numerals.

I Hansson, S. V., Rydberg, J., Kylander, M., Gallagher, K. & Bindler,

R. 2013. Evaluating paleoproxies for peat decomposition and their

relationship to peat geochemistry. The Holocene 23(12): 1664-1669

II Hansson, S. V., Kaste, J. M., Keyo, C. & Bindler, R. Beryllium-7 as

a natural tracer for short-term downwash in peat. Accepted for

publication following minor revision in Biogeochemistry

III Hansson, S. V., Tolu, J. & Bindler, R. Testing the downwash of

atmospherically deposited metals in peat and the influence of rainfall

intensity. Submitted to Science of the total Environment.

IV Hansson, S. V., Kaste, J. M., Olid, C., & Bindler, R. Incorporation of

radiometric tracers in peat and implications for estimating

accumulation. Submitted to Atmospheric Environment

Paper I is reproduced with kind permission from The Holocene/SAGE Publishing.

iv

Glossary

Accumulation rate – The rate in which peat or elements accumulate in the peat column.

Acrotelm – The upper, actively growing and aerated layer of a bog. Consists of living parts of

sphagnum, litter and partly degraded material, and is only temporarily saturated as it lies above

the average water table; most of the decomposition and loss of organic matter occurs here.

Aerosols – A colloid of fine (smaller than 1 μm) solid particles or liquid droplets in air or

another gas. Examples of aerosols include clouds, haze, and air pollution such as smog and

smoke.

Americium-241 – (241Am), A radioactive isotope associated with, and released to the

atmosphere from, nuclear weapon test. 241Am is considered to attach onto organic material to a

higher extent than 210Pb and 137Cs and is therefore considered less mobile in peat. Due to its

specific origin from nuclear weapon test (occurring during the 1960s), 241Am should show a

distinctive peak in the upper peat profile.

Beryllium-7 – (7Be), A short-lived cosmogenic isotope formed in the atmosphere through

spallation of nitrogen and oxygen by high-energy cosmic-ray particles, where after it quickly

attaches to aerosol particles and deposits through both wet and dry deposition. Has a continuous

production in the atmosphere and the mean tropospheric residence time of 7Be is estimated to

be 22 ~ 48 days. Due to its natural origin and short half-life (53.3 days), 7Be is frequently used

as a tracer in environmental applications.

Bog – Ombrotrophic mire, which are acidic and poor in nutrients. Can either be raised bog –

dome shaped with low mineral content and completely fed by rainwater – or blanket bog –

found on lowlands with high rainfall intensities, shallower and forming a blanket-like cover

over the underlying soil

Bulk density - The mass of a material divided by the total volume that the mass occupy. Used

as an approximation for peat decomposition in geochemical studies. Bulk density increases with

depth through the peat column as decomposition results in a loss of strength in the organic

matrix, which leads to compaction as more mass is accumulated above.

Carbon/nitrogen ratio – The ratio between Carbon and Nitrogen is often used as an

approximation for peat decomposition in geochemical studies. Often abbreviated to C/N. The

C/N-ratio will decrease with depth through the peat column as carbon is lost from the acrotelm

to the lower catotelm during the decay process while nitrogen is considered generally

conservative.

Catotelm – The lower, completely saturated and mostly anoxic peat layer below the acrotelm.

Cesium-137 - (137Cs), A radioactive isotope that, like 241Am, is associated with, and released

to the atmosphere from, nuclear weapon tests. 137Cs is also associated with the failures of

nuclear power stations, notably the Chernobyl accident in 1986. 137Cs has been shown to be

mobile in peat where it is easily incorporated in the vegetation, therefore causing difficulties to

accurate date peat records independently, however it is widely used as supportive information

in 210Pb-dating techniques.

Copper – A chemical element with the chemical symbol Cu and atomic number 29. Although

an essential element to life organisms, Cu can also be found as contaminant metal in peat.

Decomposition – The process by which organic substances are broken down into simpler forms

of matter. Similar terms are “rotting” or degradation.

Downwash – the process which allows atmospherically deposited elements to penetrate the

peat surface following percolating rainwater thereby reaching further into the peat column

before attaching to the organic substrate and being deposited. Not a question of post-

depositional mobility but an extension of the actual deposition itself.

Fen – a minerotrophic mire with a permanent water table close to the surface. It receives input

not just through the atmosphere (aerosols, rain, snow, fog) but also from groundwater and

surface run-off. Tends to be more alkaline and nutrient rich than ombrotrophic mires (bogs).

Can be subdivided into oligotrophic mires or eutrophic mires based on their trophic status.

v

Hollows – The depleted area which is permanently or seasonally filled with stagnating water

and where aquatic plants and Sphagnum species are developing.

Humification – The process where the organic substrate is broken down (through

decomposition) into humus and humic or fulvic acids are released.

Hummocks - Hummock is a general geological term referring to a small knoll or mound above

ground. On peatlands hummocks are the elevated area (humps) raised by sphagnum

colonization.

Lawn – referred to here as the homogenous and flat area situated between hummocks and

hollows and is covered by mosses.

Lead – A chemical element with the chemical symbol Pb and atomic number 82. Regarded as

a heavy metal that is generally considered immobile after deposition in peat layers. Often found

in peat as a contaminant that is toxic to the environment. Although naturally occurring Pb is

usually connected to pollution.

Lead-210 - (210Pb), a radioactive lead isotope that is formed through the alpha decay of 226Ra

in soil into 222Rn, a noble gas, which escapes to the atmosphere and decays into 210Pb. It has a

continuous production, an atmospheric residence time of 4.8±0.3 days and a half-life of 22

years. 210Pb is widely used to date surficial peat deposits, sediments, and soils providing

chronology on a 100-150 year scale.

Light transmission – an approximation of humification. During humification (degradation of

organic matter into humus), humic or fulvic acids are formed and released which are both dark

brown in solution. A darker solution will transmit less light, indicating a higher degree of humic

acids and a higher degree of decomposition; that is, the darker the solution the more

decomposed material. Light transmission will decrease with depth through the peat column.

Mercury – A chemical element with the symbol Hg and atomic number 80. Also known as

quicksilver. Hg is heavy metal contaminant that is toxic to the environment. Although naturally

occurring Hg is usually connected to pollution.

Minerotrophic – See Fen above

Mire – An English variation of the Swedish word “myr” that includes all peatland habitats with

active peat formation. Similar to the term active peatland.

Monolith – In technical terms a geological feature consisting of a single massive stone or rock

formed by erosion, here it is referred to the single peat “square” cut out and removed from the

peat surface using a handsaw rather than a surface corer, thereby reducing the risk of

compaction through coring.

Oligotrophic – nutrient poor fens, comes from the Greek words oligos meaning few and

trophicos meaning feeding

Ombrotrophic – Comes from the Greek words ombro meaning rain and trophicos meaning

feeding.

Ombrotrophic mire - Usually dome shaped peat or raised bog which receives all its input

solely from atmospheric deposition (aerosols, rain, snow, fog). Acidic and poor nutrient

content. (See bog)

Peat – General term for the matrix in peatlands which is formed by accumulation of partially

decayed vegetation under wetland conditions where flooding obstructs flows of oxygen from

the atmosphere, slowing rates of decomposition. The major component is Sphagnum moss,

although many other plants can contribute. Soils that contain mostly peat are known as a

histosol.

Stratigraphy (peat-) - a branch of geology which studies rock or peat layers and layering

(stratification). Refers here to the various layers throughout the peat column which represents

various stages of peat formation and vegetation composition.

Titanium – A conservative chemical element, i.e. not involved in biogeochemical processes

that will lead to a redistribution of the element and therefore considered immobile, with the

symbol Ti and atomic number 22.

Zinc – A contaminant metal with the chemical symbol Zn and atomic number 30. Often

considered mobile in the peat column.

1

Introduction

Environmental changes occurring long-before modern monitoring series need to be

assessed using natural archives, such as ice-core, lake sediments and peat bogs, where

information about past conditions can be preserved. These natural archives, i.e., cores

from soils, lakes, and peatlands, provide us with a powerful means to study past

atmospheric deposition and carbon cycling on timescales longer than the years to few

decades available from field studies or direct monitoring programs. Since the late 19th

century the environmental archive represented by the net accumulation of organic

matter preserved in peatlands has been of interest to scientists aiming to reconstruct

long-term environmental changes (e.g., Sernander 1892; Weber 1899). Originally the

focus of this early peat research was on changes in the peat-forming plant macrofossils

and how these reflected the development of the bogs through time (von Post 1913;

Granlund 1932), as well as on changes in the pollen assemblage and how they could

be used in climate reconstructions (Granlund 1931). But with the groundbreaking

study from, e.g., Lee and Tallis (1973), who showed that peat records could be used

as a natural archive of past atmospheric contamination, the interest in exploiting the

potential of peat records expanded to include geochemistry and the information

geochemical records in peat could provide on past environmental changes (Aaby and

Jacobsen 1978).

Over longer timescales (decades to millennia) it is now well established that there is

a temporal coherence among peat records and between peat and other archives, which

supports the utility of peat records for studying long-term environmental changes. The

most prominent example is that of lead for which there are now many studies that

have shown a cohesive record of atmospheric pollution for the last few thousand years

preserved in peat and in other natural archives such as sediment records (Renberg et

al 2001). But along with establishing long-term patterns, studies have also tried to

quantitatively validate the peat record by comparing it to other media such as moss

samples from herbaria collections - at least in terms of matching the isotopic

composition of the lead between the peat archive and chronologically well-defined

herbaria samples (Weiss et al. 1999; Farmer et al. 2002).

Since the development of biomonitoring based on the use of forest mosses, several

studies have shown that the concentrations of many pollutant metals in forest moss

are strongly correlated to annual atmospheric wet deposition rates (Ross 1990; Berg

et al. 1997; Rühling and Tyler 2001). Because bryopohytes (i.e., forest mosses such

Hylocomium splendens and Pleurozium schreberi in Scandiinavian biomonitoring)

and lichen have a relatively large surface area to intercept deposition, and a strong

binding capacity for metals as well as the fact that they lack roots and therefore take

up metals supplied essentially via only atmospheric deposition (wet and dry

2

Figure 1. Conceptual figure of the “missing link”, i.e. the link from long-term paleorecords to the contemporary monitoring data by which the accuracy of the paleoarchive could be validated.

deposition), they are highly suitable as tools to monitor the deposition of metals

(Rühling and Tyler 1968, 1970; Ross 1990; Steinnes et al. 1992). Although the main

focus has been on the record of Pb contamination, which includes the use of stable Pb

isotopes, the fact that peat records, forest moss and herbaria mosses all show a

cohesive pattern of Pb deposition over the last 150 years is an important factor that

supports the utility of peat records as an archive of past deposition (Weiss et al. 1999;

Farmer et al. 2002; Bindler 2011).

Because ombrotrophic bogs are by definition strongly coupled with the atmospheric

supply of many elements, peat records from such bogs are considered particularly

useful as a biogeochemical archive for studying the past atmospheric deposition of

soil dust (Shotyk et al. 2002; Björck and Clemmensen 2004; Kylander et al. 2007;

2013), pollutant trace metals (Shotyk 1998; Martinez-Cortizas et al. 1999), as well as

climate change (Chambers et al. 2007; Martinez-Cortizas et al. 2012). Given the

importance of peatlands as a carbon sink, peat records are also valuable for

establishing changes in net carbon accumulation over time in response to past and

especially recent climate changes (Gorham 1991; Fenner et al. 2007; Malmer et al.

2011).

However, a number of studies over the past few decades have also identified several

factors that can affect the integrity or temporal accuracy of the recent peat record.

These factors potentially include decomposition (Biester et al. 2003, 2012), nutrient

cycling (Malmer 1962; Damman 1978; Malmer 1988), and spatial heterogeneity in

accumulation that can occur due to variations in vegetation type and microtopography

(Bindler et al., 2004; Le Roux et al., 2005; Coggins et al., 2006; Rydberg et al., 2010;

Martínez-Cortizas et al. 2012). During the decomposition process organic compounds

3

are reorganized and the peat will experience a loss of mass, which might have a strong

influence on metal concentrations and their accumulation rates (Biester et al., 2003;

Kuhry and Vitt, 1996). An important component of interpreting the geochemical

record is therefore to understand the influence of organic matter decomposition and

how it affects the geochemical record.

Furthermore, a number of researchers have hypothesized that downwashing or post-

depositional remobilization can occur, which could smear the profile of

atmospherically supplied elements and even physical particles such as pollen within

the uppermost peat layers (e.g., Damman 1978, Clymo and Mackay 1987; Urban et

al. 1990; Oldfield et al. 1997; Biester et al. 2007). Of the factors that could affect the

integrity and temporal accuracy of the peat record, downwashing is particularly

pertinent. Downwashing would have an immediate influence on how and where

atmospherically deposited metals are accumulated in the peat record, which would

potentially include the atmospherically supplied radiometric isotopes most commonly

used for radiometric dating of the recent peat record; for example, lead-210 (210Pb),

cesium-137 (137Cs), and americium-241 (241Am). Should these be distributed

downwards into the peat during deposition it would thus have a direct effect on age-

depth modeling and the establishment of reliable and precise chronologies in short-

term peat records (Urban et al. 1990; Mitchell et al. 1992; Oldfield et al. 1995;

Lamborg et al. 2002; Bindler et al. 2004; Bindler and Klaminder, 2006; Biester et al.

2007). This would in turn lead to inaccurate estimations of peat accumulation and thus

also past deposition rates.

Although a great deal of effort has been placed on establishing the integrity of the

long-term accumulation record in peat, we still lack a complete understanding of some

of the basic biogeochemical processes and how these may affect trace element

distributions. It is still unclear whether peat records fully represent a quantitative

record or a qualitative record for the recent past (years to decades). If it is a

quantitative record then it should be possible to quantitatively calibrate the peat record

with direct monitoring data. The main research questions of this doctoral thesis have

therefore been: Are peat archives an absolute or relative record? And, how are

geochemical signals, including those relating specifically to dating, incorporated in

the peat archive?

4

Objective

The main objective with this doctoral thesis could best be described by the conceptual

sketch in Figure 1, which illustrates the potential to specifically link the long-term

paleorecord to the few decades of contemporary monitoring data available from

biomonitoring and direct deposition observations, as well as experimental studies.

With current knowledge, we know that as the peat record is buried in the catotelm

much of the information it contains will be preserved, enabling it as a functional

environmental archive of long-term environmental changes. Although discussed

initially during earlier studies on peat geochemistry exploring the processes

controlling element distributions (e.g., Damman 1978), little focus in subsequent

years, however, has been put on the processes in the acrotelm that might affect how

the geochemical signals we wish to retrieve are actually incorporated into the archive.

If we want to truly validate the accuracy of the peat archive then we must take

advantage of the few decades of monitoring data now available to us and link the peat

record to those from contemporary monitoring. But before we can compare long-term

paleo records to biomonitoring and direct deposition data, we must first understand

the processes that incorporate and ultimately preserve geochemical signals in peat and

how these can affect the integrity and accuracy of peat records as atmospheric

deposition archives. More specifically; are peat archives an absolute or relative

record? And how are geochemical signals, including dating, incorporated in the peat

archive? To answer this I have therefore chosen to work solely with surface peat cores

(30-50 cm length, spanning ≤500 years) thereby focusing on the transition between

the aerated acrotelm and the anoxic catotelm. By doing so I could specifically target

the interface between long-term and short-term records, thus enabling me to create a

better link between paleo records and monitoring data.

The objective of each of the four papers included in this thesis was therefore to address

and respond to the following research questions;

I. Does decomposition in peat bulk density, light transmission and

carbon/nitrogen ratios (C/N) reflect the same patterns? Furthermore, how do

each of these decomposition proxies relate to other geochemical data?

II. Is there evidence to support the hypothesis of an existing downwash in

natural peat cores?

III. How do differences in precipitation intensities affect downwash of

commonly studied pollutant metals in peat cores?

IV. How might downwashing affect reconstructions of peat mass accumulation,

as well as past metal deposition, and how well will these reconstructions

correlate with contemporary monitoring data?

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Abstracts of the papers

Paper I. Evaluating paleoproxies for peat decomposition and their relationship to peat geochemistry Sophia V. Hansson, Johan Rydberg, Malin Kylander, Kerry Gallagher and Richard Bindler. 2013. The Holocene 23(12) 1666-1671 Abstract

The past decade has seen a rapid increase in interest in the biogeochemical record preserved in peat,

particularly as it relates to carbon dynamics and environmental change. Importantly, recent studies show that carbon dynamics, i.e., organic matter decomposition, can influence the record of atmospherically

derived elements such as halogens and mercury. Most commonly bulk density, light transmission or

carbon/nitrogen ratios (C/N) are used as a proxy to qualitatively infer the degree of decomposition in peat, but do these three proxies reflect the same patterns? Furthermore, how do each of these proxies relate to

other geochemical data? To address these questions we analyzed bulk density, light transmission and C/N,

as well as multi-element geochemistry (WD-XRF), in 3 hummock cores (70 cm in length, ca 500 years) from an ombrotrophic Swedish bog. To compare the proxies we applied principal component analysis

(PCA) to identify how the proxies relate to and interact with the geochemical matrix. This was coupled

with changepoint modeling to identify and compare statistically significant changes for each proxy. Our results show differences between the proxies within and between cores, indicating each responds to a

different part of the decomposition process. This is supported by the PCA, where the three proxies fall on

different principal components. Changepoint analysis also showed that the inferred number of changepoints and their depths vary for each proxy and core. This suggests that decomposition is not fully captured by

any one of these commonly used proxies, and thus more than one proxy should be included.

Paper II. Beryllium-7 as a natural tracer for short-term downwash in peat Sophia V. Hansson, James M. Kaste, Keyao Chen and Richard Bindler

Accepted for publication following minor revision in Biogeochemistry Abstract

Several factors can affect the integrity of natural archives such as peat records, e.g., decomposition and nutrient cycling, and it has also been hypothesized that some rapid downward transport of atmospherically

derived elements may occur. We test this hypothesis by analyzing the short-lived, natural tracer beryllium-

7 (t ½=53.4 d) in five cores from two peatlands. In triplicate hummock cores from a raised bog in southern Sweden, 7Be could be measured to 20, 18 and 8 cm depth, and in a nutrient-poor mire in northern Sweden

to a depth of 16 cm in a Sphagnum lawn core, but only 4 cm in the dominant, more-decomposed fen peat,

indicating some spatial variability both within and between sites. Total 7Be inventories were 320–450 Bq m-2 in the bog, and 150 Bq m-2 (lawn) and 240 Bq m-2 (fen peat) in the mire. 25–79% of the total inventory

of 7Be was located in the upper 2-cm layer. To further test downwashing, in the laboratory we applied a

CuBr-solution to two cores and a Cu-solution to one core taken from the mire Sphagnum lawn, all with low water table conditions. About 50% of the added Cu and ~35% of the added Br were retained in the surface

(2 cm) layer; 1-3% of the Cu was found at 8-12 cm depth and ~1% of the Br was measured in the lowest

level (20-22 cm). Based on our novel approach using 7Be and experimental work we show that short-term downwashing can occur in peatlands and the depth of this depends on the properties of the peat, e.g., bulk

density and decomposition as well as peatland hydrology.

6

Paper III. Testing the downwash of atmospherically deposited metals in peat and the influence of rainfall intensity Sophia V. Hansson, Julie Tolu and Richard Bindler. Submitted to Science of the Total Environment Abstract

Accumulation records of pollutant metals in peat cores have been frequently used to reconstruct past

atmospheric deposition rates. While there is good support for peat as a record of relative changes in metal deposition over time, questions remain whether accumulation records represent a quantitative or a

qualitative record. Several processes can potentially influence the quantitative record of which

downwashing is particularly pertinent as it would have a direct influence on how and where atmospherically

deposited metals are accumulated in peat. The aim of our study was two-fold: first, to compare and contrast

the retention of Pb, Cu, Zn and Ni in peat cores; and second, to test the influence of different precipitation

intensities on the potential downwashing of metals. Four different ‘rainfall’ treatments were applied to 13 peat cores over a 3-week application period, including both daily (2 or 5.3 mm d-1) and event-based

additions (37 mm d-1, added over 10 hr or 1 hr). From our results two main trends were apparent: 1) there

was a clear difference in retention of the added metals in the surface layer (0-2 cm), i.e. 21-85% for Pb, 18-63% for Cu, 10-25% for Zn and 10-20% for Ni. 2) For all metals and peat types the addition treatments

resulted in different downwashing depths, i.e., as the precipitation-additions increased so did the depth at

which added metals could be detected. Although the largest fraction of Pb and Cu were retained in the surface layer and the remainder effectively immobilized in the upper peat (≤10 cm), there was a smearing

effect on the overall retention, where precipitation intensity exerts a strong influence on the vertical

distribution of added trace metals. These results suggest that the position of a deposition signal in peat records would be preserved, but it would be quantitatively attenuated.

Paper IV. Incorporation of radiometric tracers in peat and implications for estimating accumulation Sophia V. Hansson, James M. Kaste, Carolina Olid and Richard Bindler

Submitted to Atmospheric Environment

Abstract

Accurate dating of peat accumulation is essential for quantitatively reconstructing past changes in

atmospheric metal deposition and carbon burial. By analyzing fallout radionuclides 210Pb, 137Cs, 241Am and

7Be, and total Pb and Hg in five cores from two Swedish peatlands we addressed the consequence for estimating accumulation rates due to downwashing of atmospherically supplied elements within peat. The

detection of 7Be down to 18-20 cm for some cores, and the broad vertical distribution of 241Am without a

well-defined peak, suggest some downward transport by percolating rainwater and smearing of

atmospherically deposited elements in the uppermost peat layers. Application of the CRS age-depth model

leads to unrealistic peat mass accumulation rates (400-600 g m-2 yr-1), and inaccurate estimates of past Pb and Hg deposition trends based on comparisons to monitoring data (forest moss biomonitoring and wet

deposition). After applying the new IP-CRS model, that assumes a potential downward transport of 210Pb

through the uppermost peat layers, recent peat accumulation rates (200-300 g m-2 yr-1) comparable to published values were obtained. Furthermore, the rates and temporal trends in Pb and Hg accumulation

correspond more closely to data from monitoring. We suggest that downwashing can be successfully traced

using 7Be, and if this information is incorporated into age-depth models, it allows for a better calibration of peat records with monitoring data and better quantitative estimates of peat accumulation and past

deposition.

7

Materials and methods

Study sites

By definition ombrotrophic bogs (i.e., ombro = rain and trophic=fed) are supplied

with their nutrients, and thus also contaminants, only through atmospheric deposition

and are essentially isolated from lateral groundwater inputs from surrounding mineral

soils. In contrast, minerotrophic peatlands receive inputs not only from the

atmosphere but may also be supplied to some extent by lateral groundwater influenced

by the surrounding or underlying mineral soils. Most research on pollution records

from peat cores has therefore focused on ombrotrophic bogs because of the fact they

are supplied exclusively by atmospheric inputs (e.g., rain, aerosols, fog, snow), and

thus relating these records directly with the atmospheric signal is simplified in part as

compared to other continental archives. However, with careful evaluation many

minerotrophic peatlands can also serve as good records of atmospheric metal

deposition (cf. Shotyk et al. 1997). In this doctoral thesis, both mire types are included.

Rationale for the choice of study sites

Due to the geographical limitation of ombrotrophic bogs, which have their northern

limit in central Sweden (Sjörs 1983, Almquist-Jacobson and Foster 1995), as well as

the scientific question at hand, it is not always possible to choose between sites, but

rather it is necessary to adjust to the peatlands that are available and assess their

applicability. It is therefore important to include different types of peatland systems -

and different types of peat - when studying the effects of processes influencing the

peat record. For that reason this thesis is based on peat cores collected from two

contrasting peatlands; Store mosse, a classic ombrotrophic bog in south-central

Sweden, and Rödmossamyran, a minerotrophic fen in northern Sweden that includes

a small open sphagnum lawn with some sphagnum hummock features, and denser

oligotrophic or poor fen that transitions towards a pine mire at the edges (Korpela

2004). Both sites have also been subject to previous studies on both peat stratigraphy

and mire development as well as geochemical studies, which provided detailed

background information for both sites. For brevity only a short site description is

included here. More detailed description and further details on sampling and sample

preparation can be found in papers I-IV and references therein.

Store mosse

Store mosse (Figure 2) is a ~8000 ha bog complex on the South Småland Archaean

plain in the boreal-nemoral zone of south-central Sweden (57° 15´ N, 13° 55´ E; 160-

170 m a.s.l.). The glacial deposits in the area consist of moraine and fluvial sediments

with some occurrences of postglacial sand dunes (Svensson, 1988a), and peat

formation was estimate to have started by 8000 BP (Svensson, 1988a); Kylander et

8

al. 2013). Extensive studies on the development of Store mosse have been previously

published (Malmer et al., 1997; Malmer and Wallen, 1999; Svensson, 1986, 1988a,

b,) as well as some geochemical studies e.g. Bindler et al (2004); Bindler and

Klaminder (2006), Kylander et al (2013).

Figure 2. Location of Store mosse and Rödmossamyran in Sweden.

Rödmossamyran

Rödmossamyran (Figure 2) is a relatively small, ~7 ha, nutrient-poor (or oligotrophic)

fen located near the present-day coastline of the Gulf of Bothnia in northern Sweden

(63° 47´ N, 20° 20´ E; 40 m a.s.l.). The boreal forest surrounding the mire is underlain

by thin, poorly-to-moderately sorted till re-worked during the isostatic rebound of the

region. Based on the basal ages of two other mires in the region – 2400 BP for Stor

Åmyran and 3800 BP for Sjulsmyran (Oldfield et al. 1997; Nilsson et al. 2001), which

were formed by the same isostatic rebound and emergence from the sea – Rydberg et

al (2010) estimated that Rödmossamyran transitioned from a marsh to a mire about

2500–2800 years ago. Rödmossamyran, although being one single site has two

contrasting environments separated by a sharp vegetation boundary; a small open

sphagnum lawn (~0.3 ha) dominated by S. centrale and S. subsecundum, and a more

oligotrophic fen representing the dominant characteristics of the mire with a mixture

of Sphagnum (S. centrale, S. subsecundum and S. palústre), shrubs (e.g., Calluna

vulgaris and Ledum palustre L.) and small, thinly spaced pines (Pinus sylvestris) that

increase in density towards the mire edge. Based on analyses of mercury in a transect

from the mire edge in towards the sphagnum lawn, there was no indication of a

groundwater influence on the metal inventories (Rydberg et al. 2010).

Analytical methods

The analytical techniques used to estimate the various geochemical parameters

throughout the work of this doctoral thesis are all well-established, some (e.g. bulk

density) even on a very fundamental level, and will therefore only be described here

9

in brief as detailed information, including preceding sample preparation work, can be

found in paper I-IV respectively.

Analysis of bulk density, light transmission, tot-C/N, Tot-Hg and WD-XRF were all

conducted at the geochemical laboratory at Umeå University, Sweden. The

instrumentations used was Hitachi U-1100 spectrophotometer with a 1-cm quartz

cuvette for light transmission, Perkin-Elmer Series II CHNS/O analyzer 2400

operating in a CHN-mode only for tot-C/N, Perkin-Elmer SMS 100 thermal

decomposition atomic absorption spectrometer for tot-Hg and a Bruker S8 Tiger WD-

XRF with a Rh anticathode x-ray tube for the XRF analysis. The gamma spectrometry

analysis were conducted at the radiometric laboratory at College of William & Mary,

Williamsburg VA, USA. 7Be, 137Cs 241Am and 210Pb were all measured using a

Canberra Broad Energy 5030 with high-purity intrinsic Ge detectors and ultra-low

hardware with copper-lined 1000 kg lead shields.

Statistical calculations

Z-scores

To avoid the effects of scaling and variance between variables, the raw data in paper

I were transformed into Z-scores (Z = (Xi - Xavg)/ Xstd), where Xi is the measured

concentration of an element in a sample, and Xavg and Xstd are the average

concentration and standard deviation. The purpose of using Z-scores is that it reduces

all variables to a similar range of variation, while still keeping the relative variation

of the original data.

Principal component analysis (PCA)

Principal component analysis (PCA) is a useful tool for interpreting large geochemical

datasets (Biester et al., 2012; Kylander et al. 2013). By reducing the dimensionality

of the data, the PCA transforms the interrelated variables into a new set of

uncorrelated variables and a smaller number of factors, which provides insight into

the structure of the variance of the parameters (Jolliffe 2002). In paper I we therefore

applied a PCA to the data using SPSS Statistics v.18 to assess how bulk density, light

transmission and C/N-ratio relate to the geochemical composition of the peat.

Change point analysis

Change point analysis is a statistical tool to overcome factors that introduce variation

or noise within the geochemical records, allowing us to identity significant changes

in the data set that might otherwise be clouded by noise (Gallagher et al., 2011). In

paper I we therefore applied changepoint modeling to identify the locations of

significant changes in each decomposition proxy within each core. Further details on

the method can be found in Gallagher et al. (2011) and references therein.

10

Results & Discussion While many studies have validated the general temporal patterns of peat records at

large, with a focus on pollutant metal accumulations (Shotyk et al., 1996; Weiss et al.,

1999; Renberg et al. 2001; Farmer et al., 2002; Novak et al., 2003), there has been a

limited critical examination of accumulation records in quantitative terms where the

metal accumulation record is expressly compared to data from monitoring (Le Roux

et al., 2005). To be certain that we extract not only a qualitative record from peat, it is

important that we establish a quantitative link between the archive and the few to

several decades of data that are available from contemporary monitoring and research.

More importantly: Are peat archives an absolute or relative record? And how are

geochemical signals, including dating, incorporated in the peat archive?

Decomposition: Three proxies – one story?

Studies of peat-based environmental change reconstructions have utilized a range of

proxies such as fossil lipids, plant macrofossils, testate amoeba, and carbon and

nitrogen isotopic ratios to infer the degree of decomposition (humification) in peat

(Blackford, 2000; Borgmark and Schoning, 2006; Menot and Burns, 2001; Nott et al.,

2000) and, although proven reliable, they are often time consuming, operationally

defined or relatively expensive procedures. The most common techniques used to

assess the degree of peat decomposition have been bulk density (Chambers et al.,

2011; Clymo, 1965; Fenton, 1982; Franzén, 2006; Johnson et al., 1990; Malmer and

Wallen, 1993), light transmission or adsorption (reported as %T or %Abs,

respectively) (Blackford and Chambers, 1993; Borgmark, 2005; Borgmark and

Schoning, 2006; Caseldine et al., 2000; Yeloff and Mauquoy, 2006) or

carbon/nitrogen ratio (C/N-ratio) (Kuhry and Vitt, 1996; Malmer and Holm, 1984;

Malmer et al., 1997; Malmer and Wallen, 2004), all being relatively inexpensive and

time efficient. All three of these proxies are well studied, including the driving

mechanisms behind each proxy, yet there are few or any direct comparisons or critical

evaluations of all three proxies in relation to each other within a geochemical

framework. If they are proxies for one and the same thing, then by default they should

also reflect the same information. In paper I we therefore aimed to answer the key

question; do these three proxies show the same pattern and do they record the same

information regarding the degree of peat decomposition?

Our first step was just to examine the raw data and determine whether we could

extrapolate any general trends. All three proxies in all three cores did roughly follow

the general behavioral patterns as expected from literature; i.e., bulk density decreased

with increasing depth whilst light transmission and C/N-ratio decreased with

increasing depth (Figure 3). However, clear variations not just in the absolute values

11

but also the depths at which they occurred was evident between all three proxies and

among cores. To avoid any effects of scaling or variance between variables, the data

were also transformed in Z-scores, yet the differences remained.

Decomposition proxies vs. PCA

By including data on major and trace elements from WD-XRF analysis, and applying

a principal component analysis (PCA) to the data set, we sought to identify the main

factors influencing the geochemical composition, and how the three proxies related to

these (Joliffe, 2002; Biester et al. 2012). If the three proxies record and describe the

same decomposition pattern then they should group together within the same PC

loading plot, yet regardless of whether the proxies were included as active or passive

variables, the main component of each of the three proxies consistently placed on

different PC loadings. Rather than helping us explain the similarities between the

proxies, the differences between them became even clearer once the PCA was applied.

Bulk density grouped primarily with the biophilic elements (elements associated with

organic matter), light transmission with the relatively mobile or redox-sensitive

elements and C/N-ratio with the conservative lithogenic elements. Based on the PCA

outcome it therefore became clear that each decomposition proxy did not fully reflect

the same signal, however, some common underlying process(es) must exist because

the pattern of where the proxies placed within the PCA-scatterplots in relation to the

geochemical data remained consistent. We therefore interpreted this as an indication

that the proxies do not reflect one decomposition signal but merely different aspects

of it.

Figure 3. Results from the analysis of the three Store Mosse peat cores for a) bulk density (g cm-3), b)

light transmission (%Trans) and c) C/N-ratio (SM1, filled circles; SM2, gray squares; SM3, open triangles). (Reproduced with permission from SAGE Publishing.)

12

Decomposition proxies vs. changepoint analysis

To further test the cohesiveness of the three proxies within each core and to test

whether these three decomposition signals were spatially cohesive or spatially

variable (i.e., consistent between cores), we applied a Changepoint model (Gallagher

et al. 2011). This model statistically identifies the locations of significant changes in

each decomposition proxy within each core. The output confirmed the results as seen

in both the raw data and through the PCA; that is, there was a lack of correlation and

common pattern in changepoints both within each core and among the cores. This

indicated some within-bog spatial variability in the decomposition signal, which has

previously been shown for both Pb and Hg as well as plant macrofossils in Store

mosse (Bindler et al. 2004). Our conclusion was that although bulk density, light

transmission and C/N-ratio all provide valuable information on peat decomposition,

our results indicate that the decomposition signal is not fully captured by any one

proxy. To get a more representative assessment of long-term biogeochemical changes

and responses to environmental factors in peatlands, ideally one should always use a

multi-proxy and, when possible, multi-core approach.

Paper I in a nutshell:

Decomposition is not fully captured by any one proxy, as bulk

density, light transmission and C/N-ratio reflect different patterns.

Thus when studying decomposition patterns of peat bogs, a multi-

proxy and multi-core approach should always be used to ensure that

the full decomposition process is included in the interpretation.

13

Downwashing of atmospherically supplied elements in the field: Beryllium-7 as a natural tracer for downwash

Over the last quarter of a century several researchers have hypothesized that the

fundamental assumption that 210Pb is immobile in the peat archive may not be entirely

correct. Several studies have suggested with good evidence that some downward

movement of atmospherically deposited elements may occur under certain conditions,

which may then create difficulties in establishing reliable chronologies as well as

inherent problems of estimating accurate accumulation rates of peat and past metal

deposition (Damman, 1978; Urban et al. 1990; Mitchell et al. 1992; Oldfield et al.

1995; Benoit el al. 1998; Lamborg et al. 2002; Biester et al. 2007). However, to date

no one has sought to actively test the question of a potential downwash of

atmospherically deposited elements in peat. In paper II our aim was therefore to

specifically address the question: Is there evidence to support the hypothesis of an

existing downwash in natural peat cores?

Beryllium-7 is the key

In order to address our aim we first had to identify a tool to trace the potential

downwash through the peat column, preferably in a natural way without any additions

that could influence the results through an operationally defined procedure rather than

measuring unimpeded downwash occurring under natural conditions. With a half-life

of only 53.4 days and a continuous production in the atmosphere, beryllium-7 (7Be)

is a unique tracer of recently supplied (<6 months) atmospheric elements and

environmental processes (Baskaran et al. 1993; Blake et al. 1999; Fitzgerald et al.

2000; Yoshimori 2005; Short et al. 2007; Papastefanou 2009; Walling et al. 2009; Zhu

& Olsen 2009; Kaste et al. 2011a; Kaste et al. 2011b). In the atmosphere 7Be quickly

attaches to aerosols and is delivered to the earth’s surface primarily via wet deposition

(Wallbrink & Murray 1996; Whiting et al. 2005). It also has a similar behavior as 210Pb (T1/2 = 22 yr, Lal & Suess 1968; Short et al. 2007) and Pb, which has been the

foremost focus of previous studies raising the question downward

transport/downwash in peat. Based on these characteristics, and the fact that 7Be can

be measured at same time as 210Pb using gamma spectrometry, 7Be was an ideal tool

to study short-term downwashing of atmospherically deposited elements in peatlands.

Any 7Be activities measured below the peat surface would have to be have deposited

on the mire within the past ca. 6 months or less.

The hypothesis of downwash

Our hypothesis was that if rapid downwashing occurred then dissolved 7Be would

follow the percolating rainwater, thereby reaching deeper into the peat column before

being immobilized through binding to the organic matter substrate. Originally the

study only included the triplicate hummock cores from Store mosse, but in order to

14

meet some critique that was raised at an early stage regarding spatial variability and

the representativity of the initial results for peat cores in general, we also later included

two cores from Rödmossamyran. By doing so we could gain a more comprehensive

view of downwash in peat, because our results would then be based on a total of five

peat cores from two contrasting peatlands which, more importantly, represented three

different types of peat; sphagnum hummocks in ombrotrophic bogs, and sphagnum

lawn and fen-peat in an oligotrophic mire. We hypothesized that the open environment

and well-aerated peat structure found on the sphagnum hummocks on Store mosse

would facilitate a deeper gradient of downwashing, whereas on Rödmossamyran with

a more compacted and dense fen peat would show an accumulation gradient of 7Be

downcore more similar to those found in soils. For the sphagnum lawn, which has a

high water table (at or near the surface), it was uncertain as to whether downwashing

would be of a similar scope as for the hummocks, because of the open structure of the

peat with a low degree of decomposition or whether the high water table would inhibit

downwashing as shown in experimental additions in the laboratory (Wieder et al.

2010). It should be noted though, that at this point in time the aim of our study was to

empirically study downwashing and was not implicitly mechanistic because we first

had to address the hypothesis of downwash itself before delving into which processes

that could cause it. The results from the gamma-spectrometry, however, confirmed

our hypothesis on all accounts, yet both the depth at which 7Be could be detected, as

well as the within site variability of the 7Be distribution were greater than anticipated.

Figure 4. Distribution with depth shown as % of the total 7Be inventory in the three hummock

cores from Store mosse and the sphagnum-lawn and fen core from Rödmossamyran.

Empirical evidence

Due to its high affinity to bind to organic matter, which leads to strong bonds with

oxygen atoms on surfaces of organic compounds and inorganic minerals, 7Be is

rapidly bound to soil and organic particles once deposited (Hawley et al. 1986; You

15

et al. 1989; Taylor et al. 2012). Experimental studies have also indicated that 7Be is

not readily remobilized after deposition (Taylor et al. 2012). Thus, any 7Be measured

at depth in peat must have been rapidly transported to these underlying levels,

especially considering the short half-life of only 53.4 days. The deepest level at which

we could detect 7Be activities ranged from only 2–4 cm in the fen core down to as

deep as 18–20 cm in one of the sphagnum hummock cores, indicating variable degrees

of downward movement in different peat environments. Even more interesting was

the distribution pattern (Figure 4) that clearly showed that although the largest fraction

(25-80%) was measured in the top 0-2 cm slice, the remaining 7Be was distributed

down to the approximate height of the water table in two of three hummock cores and

well below the water table in the sphagnum lawn, where the water table was at the

lawn surface at the time of sampling. In addition, two of the hummock cores also

exhibited an almost bi-modal distribution with secondary peaks found in the 12-20

cm depth interval. The implications of this are two-fold; first, some downward

movement within the peat must exist in order for 7Be to be located at those depths,

and second, the differences in distribution between cores and between sites indicates

that there is a spatially variable component.

If not downwash, then what?

During the initial review process of this study, a critical concern was the differences

in 7Be distributions between cores, particularly the two secondary peaks found in 2 of

5 cores. The critique put forward was that the differences in distribution undermined

our conclusions of an existing downwash, i.e. since the depths of downwash was not

conclusive between cores our result could not be interpreted as evidence for

downwash. Yet, the question of spatial variability is not novel as several studies have

shown that variations in geochemistry (as well as plant macrofossil composition) can

occur even within very small distances. Considering the current knowledge on the

spatial variability of geochemistry in peatlands (Benoit et al. 1998, Bindler et al. 2004,

Coggins et al. 2006, Rydberg et al 2010), one cannot and should not expect that each

core should show record the exact same distribution with depth. Since the design of

this study was empirical rather than mechanistic in nature, we could only speculate as

to why the second peak in activity occurred in 2 of 5 cores; however, the fact that any 7Be could be detected below the surface is strong evidence to indicate that a significant

downward redistribution of recently deposited 7Be occurs. One could of course invoke

a range of additional hydrologic processes and argue, for example, that lateral flow

may be a possible explanation for the secondary peaks, but given the short half-life of 7Be, that alone cannot give a full explanation of the exponential decline of 7Be found

throughout the cores. These deeper peaks are also a lesser component of the total 7Be

inventory – the bulk of the inventory is retained in the upper aerated zone. Any

relocation of 7Be by lateral flow still requires that recently deposited 7Be must first be

rapidly transported to the groundwater table and transferred rapidly laterally (without

16

binding) through the peat. Whether this transport is downward or lateral would

support our hypothesis in either case.

Based on the behavioral characteristics of 7Be, including its short half-life, we

interpret our results as direct evidence that downwashing of atmospherically supplied

elements does occur in certain environments, regardless of whether the 7Be was

transported vertically through percolating rainfall or secondarily through horizontal

redistribution by lateral flow. The differences in depths between the various peat

types, indicates that the extent of downwash must be dependent upon peat

characteristics, such as bulk density, fragmentation (and thus connectivity of pore

spaces) and vegetation composition. The latter two would have implications for

binding based on increasing surface area and the properties of the organic matter

compounds present.

Paper II in a nutshell:

Yes, downwash of atmospherically deposited elements in peat does exist

based on empirical evidence from five peat cores showing that 7Be can be

measured as far down as 20 cm in well-aerated Sphagnum peat. The extent

of this downwash will depend upon the properties of the peat (e.g. density,

fragmentation) yet mechanistic processes e.g. hydrology cannot be excluded.

17

Downwash in the laboratory: Retention of metals in relation to rainfall intensities Based on the natural 7Be results of the five cores from Store mosse and

Rödmossamyran, we had empirical evidence verifying our hypothesis that

downwashing can occur in peatlands. However, we wanted to test to what extent this

applies to other elements as well, in particular pollutant metals that are frequently

studied using peat cores, and that we did not “just got lucky” with our field data, as

was implied by one anonymous reviewer. We therefore conducted an additional

experiment in the laboratory based on that used by Vile et al. (1999) and Wieder et al.

(2010), who conducted their studies to demonstrate the immobility of atmospherically

supplied elements. In those experiments they added dissolved Pb (Vile et al.) and

dissolved Be (Wieder et al.) to ombrotrophic peat cores with different water table

levels (3 cm, 15 cm and varying between 3 and 15 cm depth) to study the retention or

mobility of these elements throughout the peat column.

CuBr-addition

With the assistance of a master’s student, four peat monoliths were collected from

Rödmossamyran and placed in Plexiglas-boxes fitted with a mesh-bottom and brought

back to the laboratory. Over a three week treatment period, a solution of 0.15g CuBr2

dissolved in 300 ml deionized water was gently added to two cores over ~1 hour - a

rate equivalent to 14 mm of rain – and the additions occurred on a weekly basis over

a three-week period. An equal amount of water was added to two control cores. We

specifically chose CuBr2 as it completely dissolves in water and also that it would

give us two separate tracers; Cu+2, a metal cation that binds rapidly to the organic

matrix (Bunzel et al. 1976; Qin et al. 2006) and Br- an anion that is bound to organic

matter through interaction with humic substances and exists in peat mainly in organic

form (Yamada 1968; Maw and Kempton 1982; Bietser et al. 2004; Zaccone et al.

2008).

This treatment yielded a total addition of 0.13 g Cu and 0.32 g Br, which were

excessive concentrations of Cu and Br, 1000-fold greater than the ambient

concentration; however, the resulting concentrations are similar to those found in the

vicinity of metal smelters (Ukonmaanaho et al. 2004) after decades of loading. The

reason for this high concentration addition was that we wanted to better capture the

tail end of the downwashing, which we predicted would occur from the Vile et al.

(1998) and Wieder et al. (2010) studies. This came at the cost of unrealistically high

concentrations of Cu and Br. Recognizing that these high Cu treatment could

potentially exceed the binding capacity of the peat, we re-performed the laboratory

addition on a third monolith using a lower Cu concentration (2.523 mg over three

weeks, further details see paper II). Despite the high addition in the first Cu treatment,

18

the new results showed distributions comparable to both the first Cu treatment, but

also the distribution of Be shown by Wieder et al. (2010), i.e. 49 ±1 % of the added

Cu was retained in the upper 0-2 cm section whereas the rest showed an exponential

decline down to the height of the water table. The results of our laboratory treatments,

in combination with the results presented previously by Vile et al. (1999) and Wieder

et al. (2010), therefore became indicative that the hypothesis of downwashing would

include not just 7Be, but Cu, Pb and total Be as well. Whereas the studies of Vile et

al. and Wieder et al. had concluded the treatments provided evidence for limited

mobility, and thus confirmation of a central premise in age-depth modelling that the

dating radioisotopes are immobile, our field study and first experimental study

together indicated that initial downwashing has a substantial effect on how some

atmospherically supplied elements are distributed.

Downwash and rainfall intensities

During the review process of paper II critique was raised against not only the high

concentrations applied in the experiment but also to the rate of addition, which was

determined to be “a weakness that could not be overlooked”. Considering that our

experimental set-up was based on that of Vile et al. (1999) and Wieder et al. (2010),

and that the rate of addition (14 mm h-1), in fact was in close agreement with naturally

occurring rainfall events (see paper III), this comment was surprising. However, as

we ourselves had begun to think of the effect various rainfall intensities might have

on the extent of downwash, the idea of a new and more extensive experimental set-

up, which would address this critique that was raised, came to life.

A total of 13 peat cores were collected from adjacent sides of the vegetation boundary

on Rödmossamyran and placed in Plexiglas core boxes (designed and assembled

solely for the study at hand). The cores were then randomly selected and subjected to

4 different rainfall treatments: Steady-low, Steady, Event and Flush, which were

applied over a 3-week application period and consisted of both daily (2 or 5.3 mm d-

1) and event-based additions (37 mm d-1, added over 10 hr or 1 hr). The artificial rain

that was added to the cores followed that of Wieder et al. (2010) but was adjusted to

the precipitation chemistry of the Umeå region with a pH of 4.7. In addition, the bulk

artificial rain also included EPA Method 200.7 spiking standards 3 and 5, which

contained four metals commonly studied in peat records and which we had chosen to

study in terms of retention in peat (Pb, Cu, Zn and Ni).

Besides considering differences in mobility between the metals, the design of the

experiment was set up to test how various rainfall intensities would influence the

retention of metals in peat. Based on the knowledge that most rainfall is episodic and

does not occur at an evenly distributed rate (SMHI.se; Hellström and Malmgren,

2004), but also that previous studies had ignored this factor (Vile et al. 1999; Wieder

19

et al. 2010) we wanted to capture a range of intensities varying from the annual daily

average of the region (2 mm d-1, i.e., Steady-low) to high, yet not extreme, episodic

addition (37 mm 10 h-1, i.e., Event). The hourly distribution of this Event treatment

followed a rainfall event on 27 September 2012, as recorded at the weather station

maintained by the Department of Applied Physics and Electronics, Umeå University

(http://www8.tfe.umu.se/weather/1024/1440/10/sv-SE/history.htm). Similar events

have been recorded at SMHI’s weather station at Holmön, 4 km east of Umeå, where

hourly rainfall data are also available. In addition, we also wanted to trace any

difference in downwash between different types of peat, hence the inclusion of both

sphagnum lawn and fen cores.

The results from our study were two-fold: First, there was a clear difference in the

depth to which elevated concentrations of each of the added metals could be detected

with the order from the shallowest to the deepest being Pb<Cu<Zn≤Ni. This pattern

is consistent with earlier studies on adsorption rates of metals in peat (e.g. Bunzl et al.

1976; Kerndorff and Schnitzer, 1980). This difference is also reflected in the retention

of the added metals in the surface layer (0-2 cm), i.e., 21-85% for Pb, 18-63% for Cu,

10-25% for Zn and 10-20% for Ni, with the range in values dependent upon the

varying treatment. Second, our results also showed that for both peat types and all

metals the different treatments resulted in different downwashing depths, i.e., as the

precipitation increased so did the depths of downwashing (Steady-

low<Steady≤Event<Flush). From these results we can conclude that downwashing,

in connection to rainfall events with higher intensities, exerts a strong influence on

the vertical distribution of recently deposited metals in the upper, aerated, section of

the peat. Although the largest fraction of Pb and Cu were retained in the surface, there

was still a smearing effect on the overall retention. This suggests that the general

position of a deposition signal, i.e., past metal pollution, would be preserved in peat

records but that this signal would be quantitatively attenuated.

Paper III in a nutshell:

Based on results from 13 peat cores and 4 rainfall treatments,

the retention of metals decreases in the order Pb<Cu<Zn<Ni.

Higher precipitation, in terms of both quantity and intensity,

leads to deeper downwashing of metals. It is unambiguously

shown that downwashing during intense rainfall events exerts a

strong influence on the vertical distribution of metals in peat.

20

The effect of downwash: Implications on estimated accumulation rates

Based on the empirical evidence in paper II and the experimental evidence in paper

III, we concluded that we had sound evidence to support our hypothesis of

downwashing of atmospherically supplied elements in peat. The continuance of this

was, of course, to see what implications this would have on interpretations of the time

trends for geochemical data in peat. For instance, as previous work had shown,

difficulties in establishing reliable chronologies in peat records based on 210Pb age-

depth models were believed to be caused by a downward transport or movement,

which could also include remobilization (Malmer and Holm, 1984; Urban et al. 1990;

Mitchell et al. 1992; Lamborg et al. 2002). We therefore wanted to address the

consequence for estimating accumulation rates in the light of downwashing of

atmospherically supplied elements, as previously shown by the analyses of 7Be; but

also to look further at the representativity of single cores and the incorporation of

atmospheric signals in peat. Our specific question was: How might downwashing of

the radioisotopes used for age-depth modeling influence our quantitative estimates of

peat mass accumulation rates and accumulation rates of metal pollutants?

To facilitate this we analyzed 210Pb, 137Cs, 241Am and 7Be as well as total

concentrations of Pb and Hg in five peat cores from Store mosse and Rödmossamyran

(7Be are also included in paper II). The results from these analyses are presented in

Figure 5. A more detailed discussion regarding the distribution of these elements can

be found in paper IV, but a brief synopsis follows here. Based on the results of our

analyses, i.e., the lack of well-defined peaks in 241Am activities and the non-

monotonic decline of 210Pb, we concluded that a rapid downwashing or smearing of

atmospherically deposited radionuclides occurred not only for 7Be as shown

previously in our cores (paper II), but for 210Pb as well. 241Am may likewise have been

downwashed at the time of deposition or re-mobilized at a later time, but this cannot

be separated here. The downwashing shown by 7Be and as we suggest from the

initially increasing 210Pb activities in the upper layers contradicts a main assumption

of conventional 210Pb dating models, e.g., the constant rate of supply (CRS) model

(Appelby and Oldfield, 1978) which is the most applied dating model, that the

radioisotopes upon which the model relies are immobile. To test this specifically, we

applied two age-depth models to our data set: the conventional constant rate of supply

(CRS) model (Appleby and Oldfield, 1978), as well as a new age-depth model, the

so-called initial penetration CRS (IP-CRS) (Olid et al. In revision), which considers

an initial downward movement in the upper section of the peat. To test the quantitative

accuracy of our accumulation records we compared our data to reported peat mass

accumulation rates for similar sites (and including previous work on Store mosse) and

to deposition data available from monitoring programs (forest moss biomonitoring

and wet deposition; IVL.se).

21

Figure 5. Bulk density, activities of 210Pb, 137Cs, 241Am and 7Be, as well as total concentrations of Pb and Hg as measured in the three cores from Store mosse (upper panel) and in the two cores from

Rödmossamyran (lower panel).

Monitoring data – the “facit”

Previous studies in Sweden and Norway have shown that concentrations of trace

metals in forest mosses can be directly related to absolute deposition rates (Ross,

1990; Berg et al., 1995; Rühling and Tyler, 2001). Using the equations relating the

metal concentrations of the moss to annual wet deposition of the metals (Rühling and

Tyler 2001), it was possible to reconstruct the past Pb deposition trend for the period

from 1970 to 2010, based on biomonitoring moss data from southern Sweden (data

host, IVL.se), which could then be extended back to 1870s based on analyses of Pb in

herbaria samples (Naturvårdsverket Monitoring 1987). These results could then be

compared to our accumulation rates from the five cores from Store mosse and

Rödmossamyran. The monitoring data indicates a threefold increase (3.2 – 10.3 mg

m-2 y-1) in lead deposition from the 1870’s to the maximum peak recorded in 1970

(the timing of which is supported by varved sediments from northern Sweden;

Brännvall et al. 1999), after which Pb deposition decreased by >90% to 0.4 mg m-2 y-

1. Wet deposition measurements of Pb (IVL.se) show a decline from about 20–50 mg

m-2 y-1 in the late 1980’s to about 0.2–0.3 mg m-2 y-1 in present time. Mercury was not

regularly included in monitoring programs until the mid-1980’s and therefore fewer

data points are available for comparison (Munthe et al., 2001; Munthe et al., 2007).

22

However by comparing a limited forest moss survey in ca. 1970 to regular monitoring

from 1985, there is indication that mercury deposition had declined about 50% over

this period (from 186 to 78 ng g-1, Rühling and Tyler 2001). Wet deposition of Hg in

the late-1980s was 10–27 µg m-2 y-1 (Iverfeldt et al., 1995) and about 5–10 µg m-2 y-1

in the 1990’s–early 2000’s.

Conventional CRS model

After applying the conventional CRS modeling (Appleby and Oldfield, 1987), the

maximum peat mass accumulation rates in the uppermost layers of our cores were

calculated to be in the range of 415-629 g m-2 y-1 in Store mosse and 415-427 g m-2 y-

1 in Rödmossamyran. These values, however, appeared unrealistically high because

they are more than twice those typically reported for ombrotrophic bogs, based mostly

on direct measurements of moss growth (30 to 300 g m-2 yr-1, Clymo, 1970; Malmer

and Wallen, 1999; Kempter and Frenzel, 2007). Based upon our 7Be-data, our

interpretation of the mass accumulation rates estimated by the CRS model in our

cores, was that our two-fold higher mass accumulation rates were caused by

downwashing of 210Pb below the peat surface, which resulted in an underestimation

of the ages of the upper layers. This problem of underestimating ages was previously

observed by Oldfield et al. (1997) in a study of a peat core from Stor Åmyran in

northern Sweden (~10 km from Rödmossamyran), where a tephra layer, identified as

originating from the AD 1875 eruption of Askja, had a CRS date of 1927.

The underestimation of peat ages in the CRS model output resulted in overestimates

of peat accumulation rates, but also, more importantly, inaccurate estimations of past

Pb and Hg accumulation rates. By comparison to the biomonitoring data, the

conventional CRS-modeled accumulation records show two main differences. First,

the CRS-model output suggests an increasing accumulation trend for both Pb and Hg

in recent peat, which is opposite of the declining trend known from biomonitoring and

wet deposition data (IVL.se; Rühling and Tyler 2001). The decline in Pb and Hg

accumulation in the peat cores is not consistent with the established temporal pattern

for deposition indicated by moss monitoring since ca. 1970 (and by sediment studies,

cf. Renberg et al. 2001). Second, the conventional CRS model shows a ~two-fold

higher (18 mg m-2 y-1) Pb accumulation than the Pb deposition estimated from

biomonitoring (10 mg m-2 y-1). The accumulation rates based on the conventional CRS

model therefore appear unrealistically high in comparison.

Adjusted (IP)-CRS model

By applying the IP-CRS model, which takes an instantaneous downwashing of

atmospherically deposited elements into account, our estimated peat mass

accumulation rates in the uppermost layers was reduced to 105-132 g m-2 y-1 in Store

23

mosse and to 103 and 198 g m-2 y-1 in Rödmossamyran, which are in closer agreement

with published values for these mire types. Although the Pb accumulation still

remained higher (4 -18 mg m-2 y-1) than biomonitoring (10 mg m-2 y-1) the IP-CRS

model yielded accumulation rates of Pb and Hg that were in better agreement with the

temporal trends and deposition rates from the biomonitoring data than those estimated

from the conventional CRS-model. More importantly the increasing trend in

deposition during recent years as seen in the CRS-model output, which is not

supported by monitoring or direct deposition data, is no longer evident in the IP-CRS

model output. However, for both Pb and Hg, the IP-CRS chronology of metal

accumulation still indicates a lag relative to monitoring data. Further to this the peaks

of Hg accumulation are not perfectly synchronous, particularly in Rödmossamyran.

Two possible explanations for the remaining difference between monitoring and our

reconstructions are: 1) that our one sampling occasion and the depth of downwashing

indicated at this time by the 7Be does not perfectly capture the redistribution caused

by downwashing; and 2) that the IP-CRS model does not include a redistribution of 210Pb that matches the complexity of the 7Be distribution, particularly as seen in the

two Store mosse cores also having higher activities near the water table, which could

underestimate the complete extent of downwashing. In spite of this, the IP-CRS model

is still a first important step towards addressing the occurrence of downwashing in

peat and it provides a valuable framework for moving forward to make better

quantitative comparisons of peat records with the increasing number of years of

monitoring data.

Based on the output from the two models, and from the comparison to monitoring

data, we concluded that applying a conventional CRS model without considering the

possibility of downwashing can lead to an overestimation of peat mass accumulation

rates – and thus also inaccurate trace metal accumulation rates and incorrect temporal

patterns. Our results also suggest that more reliable chronologies and accurate

estimates of past metal accumulation rates could be obtained if downwashing was

considered within the age-depth modeling process. However, a continued

development of age-depth models is still required to fully account for downwash in

peat.

24

Paper IV in a nutshell:

210Pb, 137Cs, 241Am and 7Be, along with tot-Pb and Hg were measured in

five peat cores. Two age-depth models were applied yielding different peat

mass accumulation - and metal deposition - rates which was compared to

monitoring data. The conventional CRS-model lead to unrealistic peat mass

accumulation rates and inaccurate estimations of past Pb and Hg deposition

trends. The new IP-CRS-model, which considers downwash, rendered in

mass accumulation comparable to published values and temporal trends of

metal accumulation in better agreement with monitoring data. More

reliable chronologies and accurate peat mass accumulation rates – thus also

trace metal accumulation - can therefore be obtained if the possibility of

downwash is considered in the age-depth model used.

25

Conclusions

The main research questions of this doctoral were: how are geochemical signals,

including those relating specifically to dating, incorporated in the peat archive? And,

are peat archives an absolute or relative record? The answers to these are neither

precise nor simple. In answer to the first, there is a general consistency regarding the

incorporation of decomposition signals and also deposition signals of atmospheric

metal pollutants, exemplified through lead and mercury. Regarding decomposition

proxies, the data indicate that they are not fully interchangeable and that

decomposition as a process is reflected in different ways. Given the differences

between the three proxies for decomposition evaluated here, the ideal would be to

include more than one both for studies of past decomposition, and how this relates to

climate changes, and for studies aimed at geochemical questions. For the second

question, peat archives can be considered as both a qualitative and a quantitative

record depending on the temporal resolution required. The qualitative aspects for past

trace metal deposition and for soil dust are well established, but as it pertains to

quantitatively matching the recent peat record to measurements performed in

monitoring programs the answer is less clear. The aim of this thesis and each paper

has not been to question the fundamental utility of peat as an archive. Numerous

studies, based on sound evidence, have validated peat as an archive (Shotyk 1998;

Weiss et al. 1999; Renberg et al. 2001; Farmer et al. 2002), and our general results

here do not contradict previous work. However, some critical questions have been

raised in terms specifically regarding whether peat is a relative or absolute record. The

precise accuracy and temporal resolution in recent time remains unclear. As an archive

on millennial, centennial or even decadal scale peat records preserve a consistent

record of patterns in past deposition, and the data presented here are consistent with

the established patterns, but with regard to a higher temporal resolution, i.e. annual

scale, several processes can affect the integrity of the signal being incorporated

thereby altering the accuracy of the record. As the combined results of this thesis have

shown, care must be taken as to which processes are involved in the incorporation and

preservation of geochemical signals (exemplified in this doctoral thesis by

decomposition and downwashing), because these may alter the signal and thereby the

integrity of the record.

From the results and conclusions of the four papers this doctoral thesis is based upon,

the following two conclusions can be made. First, decomposition is not fully captured

by any one proxy (paper I). Although bulk density, light transmission and C/N-ratio

are all informative they represent different aspects of the decomposition process.

More work is required before we can determine exactly which aspects these might be

and how this influences, or is influenced by, other geochemical data. Second, the

combined results of paper II – IV proves that downwashing, that is, the rapid

26

advection of atmospherically deposited elements downwards through the surface

layers of peat, does exist and that the extent of it will depend upon precipitation and

peat characteristics. Although there are many questions that remain unanswered, such

as the influence of pH, within-site hydrological flow and the precise peat structure

(i.e. vegetation composition, density and compaction) that will specifically influence

the extent to which downwashing may occur, the results here from field and laboratory

studies provide convincing evidence that downwashing does occur in bogs and in

fens, in hummocks and in lawns, and that this has implications for studies interested

in accumulation rates of any element – pollutant metals as well as carbon.

27

Future perspectives

The logical next steps from this studies combined can be summarized into the

following bullet points;

Within-site hydrology: Water movement is obviously a key mechanism of

downwashing and as pointed out in our discussion, the influence of within-

site hydrology may be more important than just downward movement as

studied in this thesis. It would be useful to consider the rapid movement of

rainwater and potential short-term fluctuations in the water table due to major

rainfall events, which would be a potential factor for downward

transport/redistribution of atmospherically deposited elements in peat. More

work is therefore needed which would entail a mechanistic study design

enabling studies of peat wetness, water table height, lateral flow as well as

monitor rainfall, DOC and water chemistry.

Influence of pH/DOC on 7Be: Although experimental studies on the

adsorption rates and retention of metals in peat extends back to the early

1970’s (e.g., Rashid 1974; Bunzel et al. 1976; Kerndorff and Schnitzer,

1980) little work has been done, with the exception of Taylor et al (2012),

on the behavior of 7Be in in peat. Taylor et al. showed that the sorption time

of 7Be is almost instantaneous at pH levels of 4.5, after which it is not readily

remobilized. However, because many peatland environments have pH-

ranges below this (e.g., pH 3.7-6.5, Sjörs 1950; pH 3.4-5.8, Malmer, 1986)

more work is required to know in which cases 7Be is a good analogue to trace

the movement of 210Pb and in which cases it is not.

Isotopic labeling through artificial addition under natural field conditions:

Without undermining the results of our (and others’) experimental studies, it

still has to be said that no laboratory experiment can ever fully capture the

complexity that reigns under natural conditions. If we really want to test

downwashing using additions, we should do so directly onto controlled field

plots using isotopically labeled metals (e.g., Kaste et al. 2003). The addition

levels could then be held to a level comparable to ambient levels of metal

atmospheric deposition rates as measured by monitoring data. With isotopic

labeling, such as with stable Pb isotopes, this would facilitate the separation

of small additions of metals from those already present in the peat, which

would provide a more natural assessment of downwashing in natural field

conditions.

28

Author contributions

Paper I

JR, MK and RB carried out the fieldwork; SH prepared all samples and performed the

laboratory analysis (except CN together with JR); KG carried out the change point

modeling analysis; SH performed all other data analysis and wrote the manuscript.

All co-authors contributed with comments during the writing.

Paper II

SH, KC and RB performed the fieldwork on Rödmossamyran (for the Store Mosse-

cores see Paper I); SH prepared the Rödmossamyran samples; JK carried out the

radiometric measurements (SH took part in some of the analyses); KC performed the

initial high Cu-addition experiment including XRF analysis with RB; SH performed

the low Cu-addition experiment and those XRF analysis; SH performed the data

analysis with input from RB and JK; SH wrote the manuscript with input and

comments from JK and RB.

Paper III

SH planned the experimental set-up; SH and RB carried out the fieldwork; SH

performed the laboratory experiment, prepared all samples and ran the XRF-

measurements; SH analyzed the data with input from JT and RB; SH wrote the

manuscript with comments from JT and RB.

Paper IV

SH and RB performed the fieldwork on Rödmossamyran (for the Store Mosse-cores

see Paper I); SH prepared all samples and carried out the XRF and Hg analyses; JK

performed the radiometric analysis (SH took part in some of the analyses) and

calculated the conventional CRS-model, CO calculated the IP-CRS model, SH

analyzed the data with input from RB; SH wrote the manuscript with input and

comments from all co-authors.

Authors:

SH: Sophia V. Hansson, JR: Johan Rydberg; MK; Malin Kylander, KG: Kerry

Gallagher, RB: Richard Bindler, JK: James M. Kaste, KC: Keyao Chen, JT: Julie

Tolu and CO: Carolina Olid

29

Tusen tack…

First and foremost; Rich – where do I even begin? You are truly an inspiration in so

many ways. As a supervisor, a boss, a teacher, a co-worker and a mentor – I couldn’t

have wished for a better supervisor even if I had tried. So thank you. Thank you for

giving me the opportunity and for introducing me to the world of science. Thank you

for never, ever turning me away no matter how busy you were or how stupid my

question was. For guiding me through the scientific process - fieldwork to submission

to “stupid” reviews - you were always ready to lend a helping hand or a sound advice.

Thank you for all the conversations ranging from science, teaching, and

communication to music, food and coffee. Thank you for believing in me even in the

times when I didn’t. Thank you! Thank you! Thank you!

Tusen tack till Jonatan - Del två av Supervisor-teamet. Tack för alla gånger du fick

rycka ut ”akut” med råd, tips och hjälp om allt från cover letters till oengagerade

studenter. Du är en enorm inspiration som lärare och en dag ska även jag få hjärtan på

kursutvärderingarna… Tack för att du alltid sett bortom forskningen och för att du

ständigt påmint mig om ta vara på personen bakom doktorandrollen.

Tusen tack till Francois - The third part of the holy supervision-trinity. Thank you for

showing me the value of networking, for taking me in and giving me the opportunity

to work in Toulouse, and above all for always sharing your larger-than-life personality

and passion when it comes to food, wine and fashion. You are a great scientist, a great

supervisor and a great friend. Merci beaucoup!

Tusen tack till Jon & Carsten – my academic kid-brothers! Tack för att ni stått ut

med mitt svängiga humör i stressade stunder, för alla gånger ni fått vika servetter och

måla blompinnar, för hjälp med idéer och fältarbete och it-support. Tack för alla

luncher, alla middagar och alla pubrundor. Tack för att ni alltid, alltid funnits där!

”My life is a little bit better because of you”… I’m doooooin sciiiiienceeeee…

Tack till Lovisa för tiden i BM, alla drinkar och för alla välbehövliga lång-luncher!

A huge thank you to everyone in the Paleogroup: Dominique, Julie, Ingemar,

Christian, Åsa, Carolina, Marina, Wenting, Manuela, Erik and Erik. Och tack till

Johan för att du tog mig under dina vingar då jag började (och förlåt för alla miljoner

frågor innan jag fått kläm på EMG-admin-systemet…).

Tusen tack till alla medförfattare; Johan, Malin, Kerry, Jim, Keyao, Julie, Carolina

and Rich!

Tusen tack till Antonio – “Martniez Cortizas et al.” Thank you for showing me that

being a big-shot-name can still equal modesty and humbleness. Thank you for

unwaveringly teaching me PCA no matter how stupid my questions were. Thank you

for opening your home to me, showing me the enchanting streets of Santiago and for

introducing me to the “real” tapas. Muchas gracias!

Tusen tack till Malin – You are my academic role model! Thank you for showing me

that you can be a woman with a family AND be a successful researcher who rock as

scientist! “When I grow up I wanna be like you…” So thank you for just being you!

30

A great big thanks also goes to Gael LeRoux for acting as an objective mentor

throughout these years. Your comments and insights, no matter how critical, have

truly been helpful and I appreciate it tremendously!

Ett stort tack till Ingrid och Ann-Sofie och alla andra av EMGs administratörer.

Utan er skulle EMG stanna på en dag!

Thank you “Klassiker-gänget” for sharing hours of training and competition, sweat

and frozen fingers, laughter and tears. It was a great experience and I’m so happy that

I got to shear it with you all.

Till Kristina och Helena; Vänner är som stjärnor, man ser dem inte alltid men man

vet att de finns där! Tusen tack till Helena för att du påminner mig om att det finns en

hel värld utanför Universitet (till och med i Umeå!) och för att du alltid ger din ärliga

åsikt. Tusen tack till Kristina för att du alltid finns där redo för nya äventyr i vått och

torrt! Ni två är helt underbara och jag är så tacksam över att få ha er som vänner!

Henke – Tack för 14 år av vänskap och för alla fika-stunder där teman om allt mellan

himmel och jord har avhandlats! Kraaaam! En fika-date så snart jag kommer hem?

Tusen tack till Moster Gun & ”Morbror” Leif för att ni visat er uppmuntran och

stolthet från dag 1, det har fungerat som en sporre i alla stunder det känts motigt.

Tusen tack till Linda, min stora idol, för att du finns och att du är du. Jag är så oerhört

stolt över att få vara din syster! (Och väntar med spänning på din nästa bok!) Skrufsan

och Doffe förtjänar också tusen och åter tusen tack för all glädje de sprider omkring

sig. Moster Iia lovar att komma och hälsa på igen snart! Tusen tack till Janne för att

du alltid håller mig alert med dina snabba kommentarer och dråpliga skämt. Men

framförallt tack för tar hand om mina gudbarn och för att du gör min syster så lycklig.

Tusen tack till Mamma & Pappa – Vad vore jag utan er? Ni är min klippa, mitt

bollplank, min uppbackning och min frizon. Tack för allt ert stöd, er uppmuntran och

er stolthet. Tack för att ni tar ner mig på jorden då jag spårar iväg och för att ni jagar

på mig då jag kört fast. Jag älskar er så mycket, och orden räcker inte till att beskriva

hur oerhört tacksam jag är för att ni finns. Så tusen och åter tusen tack!

Tusen tack till Charlie för att du accepterar mina tokigheter, mina knasiga regler och

allt som hör Cirkus-Hansson till. Tack för att du är du och att du är bäst. Och på

begäran; “I want to say a special thank you to the person who fought, sweated, cried,

and laughed at my side during this amazing period in my life, Thank you Karl-

Christian Högbom!” Din tok-stolle…

People have asked how I can have a cat and a dog while living alone and doing a Phd,

yet the answer is quite simple; it’s for the sake of my sanity. So to Lucifer & Chino:

Tusen tack! Tack för er odelade uppmärksamhet, er obevekliga lojalitet och er

villkorslösa kärlek. Ni är värd varenda hårtuss, dregel och missad sovmorgon!

Finally I’d like to thank someone who may not even remember me but whose impact

made a long lasting impression for which I am forever grateful: my old social science

teacher Tomas Frank. Tack för att du vågade ställa krav, för att du fick mig att sätta

höga mål och för att du gav mig viljan att arbeta för att nå dem. Jag är nära nu…

Tusen tack!

31

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