Environmental Changes and Development of the Nutrient Budget of Histosols in North Iceland During the Holocene

Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene

Susanne Claudia Möckel

Faculty of Life and Environmental Sciences University of Iceland 2016

Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene

Susanne Claudia Möckel

60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Geography

Advisors Guðrún Gísladóttir Egill Erlendsson

Master’s Examiner Ian Thomas Lawson

Faculty of Life and Environmental Sciences School of Engineering and Natural Sciences University of Iceland Reykjavik, September 2016

Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene 60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Geography

Faculty of Life and Environmental Sciences School of Engineering and Natural Sciences University of Iceland Sturlugata 7 101, Reykjavik Iceland

Telephone: 525 4000

Bibliographic information: Susanne Claudia Möckel, 2016, Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene, Master’s thesis, Faculty of Life and Environmental Sciences, University of Iceland, pp. 74.

ISBN XX

Printing: Háskólaprent Reykjavik, Iceland, September 2016

Abstract

Little work has been done in Iceland regarding vegetation changes in peatlands in the context of soil chemical properties. This study examines interactions between climate, Histosols, vegetation and land use during the Holocene. Emphasis is on the development of cation exchange capacity (CEC), base saturation (BS), and decomposition rates using C:N and von Post humification. Soil physical properties were also determined. Vegetation development was reconstructed based on pollen analysis. In order to examine the impact of different geographic settings (coastal, inland and highland fringe), results from three sloping fens in Northwest Iceland were compared. Minerogenic soil content is highest in the proximity of the active volcanic belt, reflected in higher C:N values and greater ability to bind nutrients. The site closest to the sea reveals exceptionally high BS values. Overestimation of CEC due to oceanic precipitation may explain this pattern. Contrary to an expected decline of C:N with depth, values were stable or increased with depth. Evidently, C:N alone is not a reliable indicator of decomposition rates, but depends on the chemical composition of the organic parent material. The pollen record suggests optimal plant growth conditions at intermediate fertility levels. Differences in plant species richness between fertility levels are minor, but species diversity, species evenness and pollen concentrations are greatest at intermediate nutrient content. Environmental conditions driven by climate changes caused some changes in vegetation and soil properties before the settlement, but overall the Histosols showed resilience towards adverse impacts and severe degradation. After the settlement, they struggled to buffer the severe impact caused by destruction of vegetation and enhanced erosion. By connecting soil chemical and physical characteristics with palaeobotanical data, this study increases our understanding of environmental and anthropogenic determinants of soil- and vegetation development.

Útdráttur

Gróðurfarsbreytingar í mýrum með tilliti til efnaeiginleika jarðvegs hafa lítið verið rannsakaðar á Íslandi. Fyrirliggjandi rannsókn snýr að víxlverkunum loftslags, mýrarjarðvegs, gróðurfars og landnotkunar á nútíma (s. l. 10000 ár). Sérstök áhersla er á katjónrýmd (e: cation exchange capacity; CEC), mettun jónrýmdar af katjónum (e: base saturation; BS) og ákvörðun niðurbrotsstigs á grundvelli hlutfalls lífræns kolefnis og köfnunarefnis (C:N) sem og matskvarða von Post fyrir niðurbrot lífræns efnis. Eðliseiginleikar jarðvegs voru einnig ákvarðaðir. Frjókornagreiningar voru notaðar til að draga upp mynd af gróðurfarsbreytingum. Til að rannsaka áhrif landfræðilegrar legu voru niðurstöður frá þremur hallamýrum á Norðvesturlandi bornar saman, ein staðsett nálægt sjó, önnur inn í landi á láglendi og sú þriðja á hálendisbrún. Innihald steinefna reyndist mest nálægt virka gosbeltinu, sem endurspeglast í hærra hlutfalli C:N og meiri bindingu næringarefna. Nærri sjó endurspeglast áhrif sjávar í óvenjulega háu BS. Möguleg skýring er ofmat á CEC vegna hafrænna áhrifa á efnainnihald úrkomu. Hlutfall C:N er stöðugt eða eykst með dýpi, sem er andstætt því sem búist var við. Eitt og sér er C:N hlutfallið ekki nógu

góður mælikvarði á niðurbrotsstig en það tengist efnafræðilegri samsetningu lífræns móðurefnis. Frjókornagreiningin bendir til að bestu vaxtarskilyrði séu þegar næringarefnainnihald jarðvegs er í meðallagi. Ekki er mikill munur á tegundafjölda (e: species richness) við breytilegt næringarefnainnihald jarðvegs, en tegundafjölbreytni (e: species diversity), tegundajafnvægi (e: species evenness) og þéttleiki frjókorna (e: pollen concentration) eru mest þegar næringarefnainnihald jarðvegs er í meðallagi. Þær breytingar á gróðri og jarðvegseiginleikum sem urðu fyrir landnám orsökuðust af loftlagsbreytingum, en í heildina litið er seigla mýrarjarðvegs gagnvart hnignandi umhverfisskilyrðum mikil. Eftir landnám, í kjölfar gróðureyðinar og aukins jarðvegsrofs, minnkar geta mýrarjarðvegs til að veita utan að komandi breytingum viðnám. Með því að tengja efna- og eðliseiginleika jarðvegs við upplýsingar um fornt gróðurfar eykur rannsóknin þekkingu okkar á áhrifum umhverfisþátta og mannvistar á þróun jarðvegs og gróðurs.

Dedication

For my sister

Table of Contents

List of Figures ...... viii

List of Tables ...... ix

Abbreviations ...... x

Acknowledgements ...... xi

1 General Information ...... 1 1.1 Iceland – a brief overview ...... 1 1.2 Climate change and vegetation development ...... 2 1.3 Soils in Iceland ...... 4 1.3.1 Andosols ...... 5 1.3.2 Vitrisols – soils of the deserts ...... 5 1.3.3 Histosols – soils of the wetlands ...... 6 1.4 Use of palynology in pedologic research ...... 7 1.4.1 Pollen-analytical studies ...... 7 1.4.2 Pedologic research on peatlands ...... 8

References ...... 10

2 Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene ...... 15 2.1 Introduction ...... 15

3 Methods ...... 18 3.1 Research area ...... 18 3.2 Sampling ...... 19 3.3 Soil morphology and physical properties ...... 20 3.4 Chemical soil properties ...... 21 3.4.1 C:N ratio based on organic C and N ...... 21 3.4.2 pH in water and NaF ...... 21 3.4.3 Cation Exchange Capacity ...... 22 3.5 Chronology, soil accumulation rate and carbon sequestration...... 22 3.6 Analysis of pollen and spores ...... 23

4 Results ...... 24 4.1 Soil accumulation rate ...... 24 4.2 Soil morphology ...... 25 4.3 Physical and chemical soil properties ...... 28 4.3.1 Dry bulk density, soil organic matter and soil water content ...... 28 4.3.2 pHwater and pHNaF ...... 31 4.3.3 Base cations, CECbases and CECpot ...... 31 4.3.4 C:N ratio and carbon sequestration ...... 32 4.3.5 Stratigraphic pattern of pollen ...... 34 vi

5 Discussion ...... 39 5.1 The impact of location ...... 39 5.1.1 Decomposition and nutrient availability ...... 39 5.2 Decomposition rates, species composition and nutrient availability ...... 40 5.3 Driving factors behind changes in species composition ...... 41 5.4 Impact of climate and vegetation changes on peat properties – pre- settlement development ...... 41 5.5 Post-settlement development of vegetation and peat properties ...... 44

6 Conclusions ...... 45

References...... 47

Appendices ...... 54

vii

List of Figures

Figure 1.1: Geographical location of the active volcanic systems in Iceland ...... 1

Figure 3.1: Location of the sample sites...... 20

Figure 4.1: Profile development of physical soil properties DBD (g cm´3), water by dry ´1 soil mass wd (g g ), SOM (%) and MS...... 29

Figure 4.2: Profile development of %N, %C, C:N ratio and CSQ (g C m-2 yr-1) ...... 33

Figure 4.3: Summarized pollen percentage diagram for Torfdalsmýri...... 36

Figure 4.4: Summarized pollen percentage diagram for Tindar...... 37

Figure 4.5: Summarized pollen percentage diagram for Hrafnabjörg...... 38

viii

List of Tables

Table 3.1: Main weather characteristics for the three sample sites...... 19

Table 4.1: SAR [mm yr-1] and age model based on tephra layers of known eruption date and one 14C age (2σ)...... 25

Table 4.2: Selected soil morphologic properties...... 26

Table 4.3: Profile development of soil physical and chemical properties (DBD: g cm- 3 -1 -1 -2 -1 ; wd: g g ; BS: %; P: mg kg ; CSQ: g C m yr )...... 30

Table 4.4: Main characteristics of each pollen zone...... 35

Abbreviations

BS = Base Saturation

CEC = Cation Exchange Capacity

CECpot = potential Cation Exchange Capacity

CSQ = Carbon Sequestration

DBD = Dry Bulk Density

MS = Magnetic Susceptibility

SAR = Soil Accumulation Rate

SOM = Soil Organic Matter wd = Soil water content based on dry weight

Acknowledgements

I would like to give special thanks to my supervisors Guðrún Gísladóttir and Egill Erlendsson - for their professional support and advice, but also for their warmth and positive attitude. I also want to thank Utra Mankasingh, who substantially contributed to this project through all her help in the lab and many subject-specific discussions. Julia Brenner proof- read the thesis for English grammar mistakes, which I appreciate greatly. Of great importance were my friends and colleagues, both within- and outside of the University. They played a very vital part in making life abroad enjoyable. My friends and family in Germany have also always been of great support. Special thanks go to my parents, who always encouraged and supported me in all my plans and never complain about me living so far away. Last but not least I want to thank my sister, Sabine, who has always been a very important companion. She is even up for exploring the beauty of the Icelandic nature with me during her holidays, despite being constantly cold and longing for subtropical temperatures. Additionally, I want to thank The University of Iceland Research Fund, Rannís (grant no. 141842-053) and Orkurannsóknarsjóður Landsvirkjunar Research Fund for funding this research.

1 General Information

This MS thesis consists of an introduction on environmental conditions in Iceland and a scientific journal article, to be found in the next chapter. In the introduction, an overview will be given on present geologic and climatic conditions and past vegetation and climate development. The main Icelandic soil types and existing pedologic research work are discussed. 1.1 Iceland – a brief overview

Iceland is an island of approximately 103,000 km2, situated in the North Atlantic close to the Arctic Circle, between 63°23´-66°32´N and 13°30´–24° 32´W (Einarsson 1984). The country is mainly shaped by its northern position and its geological development. The origin of the island is volcanic, owing to the geographical superposition of the Mid-Atlantic ridge, a spreading plate boundary, and a mantle plume (Allen et al. 1999; Ribe et al. 1995). Volcanism is still active today, owing to the coincidence of these two structures. The most active volcanic systems are arranged in the vicinity of the Mid-Atlantic Ridge, presently crossing the country in a southwest-to-northeast direction (Thordarson and Hoskuldsson 2002).

Figure 1.1: Geographical location of the active volcanic systems in Iceland (Thordarson and Larsen 2007) 1

Although controversy exists around when, exactly, the development of the country began, it is clear that about 14 to 16 million years ago parts of the island had emerged, as the oldest rocks discovered date back to that time (Thordarson and Hoskuldsson 2002). Undoubtedly, Iceland is a comparatively young country, shaped by volcanic eruptions which have been occurring approximately 27 times per century during the last 400 years (Thordarson and Larsen 2007). The climate of Iceland is oceanic, characterized by cool summers and mild winters. In the context of global weather patterns, the country is situated at the border of warm and cold ocean currents, and at the frontier of warm and cold air masses (Einarsson 1984). These conditions have a major influence on weather characteristics of the country. A low-pressure centre in the southwest of the island, the so-called Icelandic Low, signifies the frequent occurrence of cyclones. The topography of the country also plays a significant role in shaping local and regional climates. Orographic conditions vary between regions and modify microclimatic conditions. Amongst the most dominating weather characteristics are strong winds, frequent precipitation, mild winters and cool summers, accompanied by significant regional variations and abrupt shifts in weather conditions (Einarsson 1984; Ólafsson et al. 2007). Mean temperatures in coastal regions typically fluctuate around 0°C in the winter months and 10°C in the summer months, with a tendency towards slightly higher temperatures in the south than the north. Mean annual precipitation is also higher in South Iceland, where it typically fluctuates around 1000 mm yr-1, but can also reach over 3000 mm yr-1 in some lowland areas, particularly in the southeast. In North Iceland, precipitation typically fluctuates around 600 mm yr-1, but can also be less than 400 mm yr-1 in some areas (Einarsson 1984; IMO n.d.a; n.d.b; Ólafsson et al. 2007) Like its geological age, the history of human occupation in Iceland is comparatively young. The first permanent settlers arrived on the island around AD 870. Currently, the population of the country is around 332,000 inhabitants (StatisticsIceland n.d.), which results in a mean population density of approximately 3 inhabitants per km2. This low population density, coupled with the fact that about two thirds of the inhabitants live in the greater area of the capital Reykjavik, makes clear that the majority of the country is very sparsely populated compared to other North European countries (Eurostat 2015). The short settlement history and low population density may easily allow for the assumption that vast areas of the country are unaffected by human activities – a field for discussion that will be given space in the following paragraphs. 1.2 Climate change and vegetation development

The history of climate and vegetation change since the last Ice Age in Iceland has been investigated by means of various paleoecological methods. Methods used have included: analysis of microfossils such as pollen and spores (Caseldine et al. 2006; Einarsson 1961; Erlendsson 2007; Erlendsson and Edwards 2009; Erlendsson et al. 2009; Hallsdóttir 1990; Lawson et al. 2007); macrofossils such as leaves, seeds etc. (Eddudóttir et al. 2015; Rundgren 1998; Wastl et al. 2001); and chironomid analysis and use of sediment proxies both of lacustrine and marine sediments (Axford et al. 2007; Caseldine et al. 2006; Gathorne- Hardy et al. 2009). Thanks to such studies, a general picture of the Icelandic history of climate change and vegetation development during the Holocene can be reconstructed. Studies on marine cores and ancient shorelines suggest a main phase of deglaciation between 15 ka BP and 11 ka BP, with variations in timing between regions (Andrews and

Helgadóttir 2003; Ingólfsson and Norddahl 2001). Over the following millennia, increased sedimentation rates in marine and lake cores have indicated a highly dynamic terrestrial environment with sparse vegetation cover and enhanced erosion, leading to increased sediment fluxes from land to sea/lakes (Andresen et al. 2005; Larsen et al. 2012). Indeed, reconstructions of vegetation cover by pollen analysis supports such a scenario. Caseldine et al. (2006) found that the time period between 10.8 ka BP and the deposition of Saksunarvatn tephra around 10.2 ka BP was at first characterized by open ground taxa, indicative of disturbed environments such as Caryophyllaceae, Apiaceae, Brassicaceae, Saxifragaceae. Towards 10.2, periglacial ground disturbance seemed to decrease with denser vegetation cover and the spread of dwarf shrubs and sedges. The deposition of the Saksunarvatn tephra led to disturbance and instability over several centuries, followed by a period of recovery and the expansion of dwarf shrub heath, dominated by Juniperus communis (juniper) and Betula nana (dwarf birch). Slowly, downy birch (Betula pubescens Ehrh.) became established as climate conditions warmed. The birch woodland reached its maximum extent between c. 8.0 and 6.0 ka BP (Eddudóttir et al. 2015; Eddudóttir et al. 2016; Hallsdóttir 1990; 1995). During this period the birch tree line reached maximum elevations, growing up to an altitude of 450 - 500 m a.s.l. on the Tröllaskagi peninsula in North Iceland and over 400 m a.s.l. in Húnavatnssýsla Northwest Iceland (Eddudóttir et al. 2016; Wastl et al. 2001). Climate and temperature reconstructions for different parts of the country after 6.0 ka BP indicate variable scenarios. For instance, a Holocene temperature reconstruction by Axford et al. (2007) conducted with subfossil chironomids from two lakes in north- and northeast Iceland suggested peak warmth around 5.0 ka BP with comparatively high temperatures lasting until ca. 3.0 ka BP in those areas. A study by Larsen et al. (2012) conducted on sediments from lake Hvítárvatn in the central highlands indicated a shift towards colder temperatures from approximately 5.5 ka BP, eventually leading to a period of Neoglaciation after 4.2 ka BP. Palynological studies indicate that growth conditions for birch deteriorated after 6.0 ka BP, expressed by the onset of birch woodland retreat in favour of heathlands and mires (Eddudóttir et al. 2015; Eddudóttir et al. 2016; Hallsdóttir 1995; Hallsdóttir and Caseldine 2005). The apparent climate deterioration was not linear, but characterised by several periods of increased warmth. Such periods are depicted in pollen records as periods of increased birch pollen (Einarsson 1961). Erlendsson and Edwards (2009) dated the last period of possible birch expansion to between AD 600 and AD 800, only shortly before the settlement of Iceland. After the settlement, vegetation was affected not only by climatic conditions, but also, and to a great extent, by human activities (Erlendsson 2007; Erlendsson and Edwards 2009). As described e.g. in Hallsdóttir (1987) and Erlendsson (2007), post-settlement environmental impacts were reflected in drastic vegetation changes, especially in the disappearance of birch woodlands in the vicinity of farms within a few decades of settlement. The woodlands were eradicated for farming space, fodder, fuel and for charcoal production and could never regenerate due to intensive grazing of sheep and other livestock (Arnalds 1987). Other plant communities, such as grass heath, dwarf-shrub heath and mires colonised the space created by the woodland removal (Hallsdóttir and Caseldine 2005). Significant climate deterioration took place between 1250 and 1900, commonly referred to as “The Little Ice Age” (LIA; (Ogilvie and Jónsson 2001). The changing climate conditions provided an additional stress on the environment, which was already under pressure from land use (Axford et al. 2007; Stötter et al. 1999). Consequently, widespread ecosystem degradation was facilitated through damage of vegetation cover, which in turn accelerated the soil erosion processes set in motion after the settlement (Dugmore et al. 2009; Gísladóttir et al. 2011; Gísladóttir et al. 2010; Ólafsdóttir and Guðmundsson 2002). 3

Estimates of vegetation cover at the time of the settlement vary, but according to Arnalds (2005), vegetation might have covered up to two thirds of the country, with woodlands accounting for at least 25 % of the total land area. At present, natural birch woodlands cover only about 1.5 % of the island (Traustason 2015), and approximately 50 % of the original vegetation cover have been destroyed or severely damaged (Arnalds 2005). Today, the pressure from grazing livestock on the remaining vegetation and soil cover is still high. Currently, there are about 375,000 winterfed ewes in the country, accompanied by nearly 80,000 cattle and, as of 2013, an estimated minimum of about 70,000 horses (StatisticsIceland n.d.). 1.3 Soils in Iceland

A rather unique combination of environmental conditions has led to the development of soils in Iceland that are uncommon in the European context (Arnalds 2004). This starts with the parent material, which is mainly basaltic tephra of volcanic origin (Arnalds 2004). Consequently, most Icelandic soils belong to the volcanic soil order, called Andosols in the World Reference Base for Soil Resources (WRB; (FAO 2014) and Andisols in the Soil Taxonomy of the United States Department of Agriculture (USDA; (USDA 1999). As Wada (1985) points out, Andosols commonly differ considerably from soils of non-volcanic parent material, even if they are formed under similar climatic conditions and vegetation cover. Even among volcanic soils, Icelandic soils are quite unique (Arnalds 2004; Arnalds et al. 1995). First of all, soil development is still very young, starting about 10.0 ka BP, following deglaciation (Arnalds and Kimble 2001). Since then several factors – volcanism, climatic conditions, cryoturbation and soil erosion – have been modifying the soils steadily (Arnalds 2008; Arnalds and Kimble 2001). With eruptive frequency of about 27 eruptions per century (Thordarson and Larsen 2007), and rapid deposition rates of eolian material in some parts of the country (Arnalds 2008), existing soils in Iceland are gradually being fed by new parent material. They can also get buried under thick tephra deposits, a process which is prevalent in soils near active volcanoes (Shoji et al. 1994). High altitude areas in the close vicinity of active volcanic zones are commonly barren. These conditions, and the destruction of vegetation cover by human activities since the settlement, have led to unusually high desertification rates and the prevalence of desert soils in vast areas of the country. Contrary to this, wetland soils can form where the influence of eolian input is less prominent and the bedrock is less permeable (Arnalds 2004). Based on the processes and characteristics described above, Icelandic soils have been divided into two main groups: vegetated and un-vegetated soils. Within un-vegetated soils, so-called Vitrisols are the most common, making up for about 30 % of the Icelandic soils. Other groups, such as Cryosols, Regosols and Leptosols, as well as complexes of the different un-vegetated soil types, make up about 15 %. When it comes to vegetated soils, the extent and type of input of new parent material and drainage conditions lead to a division of soils with andic and/or histic properties. Andosols cover about 54 % of Icelandic soils and Histosols only 1 % (Arnalds 2004; 2008). In the following chapters, the properties of the different soil categories will be discussed more deeply. First, Andosols and Vitrisols will be discussed. Then, Histosols in the broader context of wetlands will be outlined.

1.3.1 Andosols

As stated above, soils in Iceland are divided into two main groups depending on vegetation cover. Such separation is not applied for volcanic soils in the major soil classification systems: WRB and Soil Taxonomy (FAO 2014; USDA 1999). In the Icelandic system, only vegetated soils are referred to as Andosols (Arnalds 2015). General characteristics of Icelandic Andosols are low dry bulk density (DBD), high water retention and infiltration rates, high accumulation of organic material, high cation exchange capacity (CEC) and considerable quantity of amorphous clay minerals such as allophane, ferrihydrite and immogolite. As a result of bases released by weathered basaltic tephras, the pHwater is commonly near neutral. The soil colour is dark, potentially with a reddish hue as a result of high organic content and ferrihydrite. While the presence of amorphous clay has a positive impact on water content, CEC and soil fertility, the structure of Andosols is usually poorly developed. Amorphous clay exhibits only poor cohesion abilities, making the soils very susceptible to cryoturbation, wind erosion and collapse beyond water saturation (Arnalds 2004; 2008). Icelandic Andosols are subdivided into several groups, depending on drainage conditions and organic carbon content: Brown Andosols, Gleyic Andosols and Histic Andosols. Brown Andosols are freely drained and can contain up to 12 % C, even though values commonly observed are 2-7 % (Arnalds 2008). They occur in freely drained areas and are usually less acidic than wet or more organic types of soil, with pHwater between 5.5 and 7.5. Tephra layers within the soil profiles are often quite distinct, with dark basaltic layers being more frequent than lighter coloured rhyolitic ones. The allophane content of Andosols is, with 15-30 %, comparatively high (Arnalds 2004). Gleyic Andosols are more poorly drained than Brown Andosols, resulting in higher water content. They are wetland soils located in the close vicinity of the active volcanic belt and receive increased input of eolian material. Their organic carbon content is 1-12 %, a lower range than is commonly observed in wetland soils. The range of observed acidity, pHwater 4.5-7.2, demonstrates rather acidic to near-neutral soils. Where boundaries of tephra layers are characterized by textural changes, redox features such as mottles are often clear, and weak iron cementation may occur (Arnalds 2004). Histic Andosols are poorly drained and contain between 12 and 20 % C. These soils exhibit properties characteristic of both Histosols and Andosols. Their organic content is high and often incompletely decomposed, and their dry bulk density ranges between 0.2 and -3 0.4 g cm , values very similar to that of Histosols. Soil acidity has pHwater between 4.5 and 5.5, also within the pHwater range for Histosols. The organic carbon content is too low to meet the criteria of Histosols and the andic properties are pronounced enough to count as Andosols. Consequently, these soils are defined as Andosols with histic properties (Arnalds 2004; Arnalds and Óskarsson 2009).

1.3.2 Vitrisols – soils of the deserts

In a global context, Vitrisols are rather unique desert soils, both because of the dark basaltic parent material as well as the humid climate, which is commonly not associated with deserts (Arnalds 2004; Arnalds and Kimble 2001). According to the WRB, soils with vitric properties are less weathered than soils with andic properties. This is expressed in more pronounced presence of volcanic glass, lesser concentration of amorphous clay and higher DBD (FAO 2014). Main differences between the Icelandic soil types Vitrisols and Andosols (as outlined e.g. by Arnalds and Kimble (2001) and Arnalds (2004)) correspond in many 5

ways to that description. The texture of Vitrisols is coarser because of greater volcanic glass content, less organic matter (< 1 % C) and less allophane content. In Vitrisols DBD is higher than in Andosols, typically between 0.8 g cm-3 and 1.2 g cm-3, and they are also more alkaline, usually with pHwater 7-7.9. The profile development of Vitrisols is shallower, and their potential for land use very limited under prevailing vegetation cover. The high grain surface area of tephra, in comparison to other sandy soils with less tephra content, leads to a comparatively high water holding capacity and high CEC. Arnalds (2015) argues that Vitrosols exhibit properties that could potentially support reasonable plant growth, at least under favourable environmental conditions, such as temperature and protection from grazing and eolian input.

1.3.3 Histosols – soils of the wetlands

Histosols, often called peat, are the dominating soils in mires formed in cold-temperate and boreal climates (Bridgham et al. 1995; FAO 2014; USDA 1999). Histosols are organic soils; their parent material is mainly plant remains that decompose slowly due to waterlogged, anaerobic conditions (Bridgham et al. 1995; Clymo 1987; FAO 2014; Gorham 1991; USDA 1999). Within the USDA soil taxonomy and WRB, Histosols do not belong to volcanic soils (FAO 2014; USDA 1999). Histosols in Iceland differ from those found in other Nordic regions. Although not located in the immediate vicinity of the active volcanic ridge, Histosols can contain considerable quantities of inorganic material in the form of volcanic ash and eolian deposits. As a result, Icelandic Histosols possess both histic and andic soil characteristics (Arnalds 2008; Arnalds and Óskarsson 2009). The limited spatial coverage of about 1 % is also unusual compared with regions at similar latitudes (Arnalds 2004). This feature is intriguing given that the total wetland cover in Iceland is estimated to be around 10 % (Óskarsson 1998). Icelandic Histosols exhibit some of the properties usually observed in Histosols. The content of organic carbon is high (> 20 %) and DBD low (0.17-0.4 g cm-3), both of which lead to a high water holding capacity (Arnalds 2004). The chemical properties of Icelandic Histosols show high variability, with acidity and nutrient status mainly depending on quantity and form of mineral material present (Guðmundsson 1978). The pHwater of Histosols in Iceland is commonly 4-5.5 (Arnalds 2004). Availability of cations, CEC and base saturation (BS) has been shown to vary between regions, depending on precipitation and eolian deposition (Jóhannesson and Kristjánsdóttir 1954), but is overall comparatively high -1 with values ranging between 50 and 100 cmolc kg (Jóhannesson 1960). The soil properties, not least the chemical properties, vary between wetland types. Icelandic wetlands have broadly been divided into two groups, so called bogs (flói) and sloping fens (hallamýri) (Einarsson 1961; Steindórsson 1936). Bogs are located in basins underlain by relatively impermeable bedrock. They are ombrotrophic, meaning they are mainly fed by precipitation. Sloping fens are, as the name implies, located on slopes of varying degrees, on valley floors and hill- and mountain slopes. Their hydrology is dependent on water from, precipitation, runoff, and groundwater and they are called rheotrophic or minerotrophic, as they are partly fed by water that has flowed through or over mineral soil. Because of the higher input of minerals in fens than bogs, fens are usually less acidic and more nutrient rich (Clymo 1987; Einarsson 1961; Guðmundsson 1978; Moore 1986). Wetlands with characteristics of both types are very common in Iceland (Arnalds 2015; Steindórsson 1964).

1.4 Use of palynology in pedologic research

Palynology has been used as a tool to support studies on soil quality in Iceland (Gísladóttir et al. 2011; Gísladóttir et al. 2010); conversely, many palynological studies are supported by sedimentary information (Caseldine et al. 2006; Hallsdóttir 1987; Lawson et al. 2007; Vickers et al. 2011). The focus of these studies has primarily been to link vegetation changes to alterations of physical properties of soil. For instance, a study by Gísladóttir et al. (2011) on Histosols in West Iceland investigated the profound impact of human-induced changes in vegetation composition and coverage on soil quality. Changes in soil quality after the settlement were assessed by means of organic carbon concentration, bulk density, and soil moisture content, and were supported by reconstructions of vegetation changes through pollen analysis. The response of soil to human activities associated with woodland decline was decreased carbon concentration and water content, and increased bulk density. Increased mineral input by eolian deposition enhanced decomposition rates and changed the texture of the soils. Results of the pollen analysis supported the scenario of physical soil deterioration as woodlands declined sharply, associated by a spread of open ground taxa. Little work has been done on vegetation changes in the context of soil chemical properties. Therefore, separate overviews will be given about palynological and pedological research.

1.4.1 Pollen-analytical studies

The first modern pollen percentage diagram was published in 1916 by the Swede Lennart von Post (Manten 1967), and in 1934 Sigurður Þórarinsson introduced the method to the audience of the Icelandic journal Náttúrufræðingurinn (e. The Natural Scientist). There he described the great scientific potential offered by the analysis of pollen preserved in peatlands, and called on the Icelandic scientists to start using this method (Þórarinsson 1934). Indeed, only a few years later for his doctoral thesis on historical agriculture, he became the first researcher to employ this method on Icelandic material (Þórarinsson 1944). In the following years only a few researchers have conducted pollen analysis on Icelandic material. In 1961 Einarsson published his work on the late- and post-glacial climate history of Iceland based on pollen analysis (Einarsson 1961). The main emphasis of his study was on fluctuations in the relative occurrence of Betula pollen, as derived from the only woodland- forming species of the country. From this data, he inferred the climate development of the Holocene, with decreasing Betula pollen percentages reflecting deteriorating climatic conditions and increasing percentages being associated with warmer and drier climates. Whereas the first researchers conducted their work mainly on pollen samples derived only from peat profiles, lake sediments were increasingly used later on when suitable coring equipment became available (Hallsdóttir 1995). The choice of source material should depend on the aim of the research, but as Einarsson (1961) points out: pollen deposits from lake sediments are often better preserved than those from peat. Peat deposits can provide a better overview of local vegetation, whereas those from lake sediments reflect regional vegetation (Vasari et al. 1972). Vasari et al. (1972) was one of the first to reconstruct post-glacial vegetation development in Iceland using only lake sediments. The results of his work reveal a very similar pattern to that of Einarsson (1961), apart from an overall chronological delay in Vasari’s pattern compared to Einarsson’s. In the late eighties Hallsdóttir (1987) defended her doctoral thesis on the impact of human settlement on vegetation, concluding that the most striking change detected after the settlement was the disappearance of birch woodlands and the following expansion of grass-heath and mires. Even though not commonly applied

in Iceland, it should be noted that a pollen record from marine sediments has also been used. This was done by Andrews et al. (2001) who conducted a multi proxy study on a marine core from a Fjord in North-West Iceland. The aim of the research was to reconstruct changes in the nearshore marine environment since approximately 4.0 ka BP, and the effect of the settlement, using palynology amongst other methods. Overall, the tool of palynology has been used increasingly in Iceland in various fields, such as climatology, ecology and archaeology. These studies have enhanced our understanding of climate and vegetation change during the Holocene (Caseldine et al. 2003; Caseldine et al. 2006; Eddudóttir et al. 2015; Eddudóttir et al. 2016; Erlendsson and Edwards 2009; Hallsdóttir 1990; Rundgren 1995; 1998). Others have focused on the ecological influence of land use after the settlement and/or the reconstruction of the lifestyle of our ancestors (Erlendsson 2007; Erlendsson et al. 2009; Gathorne-Hardy et al. 2009; Gísladóttir et al. 2010; Hallsdóttir 1987; Lawson et al. 2009; Lawson et al. 2007; Vickers et al. 2011; Zori et al. 2013).

1.4.2 Pedologic research on peatlands

Since the beginning of the settlement of Iceland, wetlands have played a considerable role in people’s lives. This is reflected in the recurrence of wetlands in both early and modern literature. They were on one hand a valuable resource of peat, ferric oxide and hay production, but on the other a dangerous geographical obstacle (Huijbens and Pálsson 2009). The inconveniences associated with wetlands are reflected in the fact that the arrival of the first rubber boots (Wellingtons by popular name) to Iceland, in the beginning of the 20th century, is frequently considered a historical landmark (Johannesson 2013). In fact, it was at about the same time that wetlands were increasingly seen as troublesome impediments to progress (Huijbens and Pálsson 2009). From the middle of the 20th century until the nineties, extensive mechanised digging of drainage ditches took place with governmental financial support (Eylands 1967; Geirsson 1998; Óskarsson 1998), leading to the excavation of nearly 30,000 km of ditches (Arnalds et al. 2016; Gísladóttir et al. 2009). Since the nineties, there has been an awakening regarding the ecological and environmental value of mires and wetlands. Wetland restoration projects were initiated as a response, motivated by nature conservation and, more recently, mitigation of greenhouse gas emission (Aradottir et al. 2013). In spite of the generally accepted importance of wetlands, research on Icelandic wetland soils is limited, especially with respect to the nutrient state and other chemical properties. More research emphasis has been directed towards dryland soils over recent years, notably Andosols (Arnalds et al. 1995; Arnalds and Kimble 2001). Much emphasis has also been on human impact on soil physical properties and carbon sequestration in relationship with increased erosion after the settlement (Gísladóttir et al. 2011; Gísladóttir et al. 2010) and the effect of different fertilization methods (Guðmundsson et al. 2005). As Óskarsson and Halldórsson (2006) point out, results of studies on wetland soils in neighbouring countries cannot be directly applied to Iceland, due to their differing nature. Nevertheless, foreign research can stimulate ideas on soil processes and their causes under Icelandic conditions. As organic soils formed under waterlogged conditions, peatlands are an important store of carbon in form of accumulated organic matter (Bridgham et al. 1995; Clymo 1987; FAO 2014; Gorham 1991; USDA 1999). Peatland development is highly influenced by climatic conditions, especially due to varying hydrologic conditions under changing climate (Bridgham et al. 1995). However, it is not only climate that exerts control over peatlands, peatlands also influence the microclimate of their surroundings, not least by having an impact on hydrologic conditions such as groundwater levels and runoff (Minayeva 8

and Sirin 2012). The same is true for the impact of peatlands on vegetation cover. The type of vegetation and biomass is not only influenced by climate, but also by properties of the peatland soils, Histosols. It is therefore important to understand processes and characteristics of soils in peatlands in order to understand how and to what extent they react to climate changes. Vegetation cover and soil properties of peatlands are interdependent. Soil properties and climatic conditions influence vegetation characteristics, whereas the type and quantity of vegetation has a considerable impact on soil development and processes, such as organic matter and nutrient input and nutrient demand (Bridgham et al. 1995). Clymo (1983) describes in more detail the factors that the chemical properties of Histosols depend on, agreeing that plant material functioning as parent material plays a critical role and the nature and quantity of soluble inorganic material present. He also emphasizes the role of climate, especially temperature and hydrologic conditions, due to their influence on the activity of soil organisms. Some of the processes and characteristics reported in the overseas literature have been investigated in Iceland. Jóhannesson and Kristjánsdóttir (1954) compared Histosols from North- and South Iceland and found noticeably higher content of exchangeable base cations, base saturation, pH, CEC and soil organic matter content in the samples from the north. The authors linked this to the considerable difference in precipitation, which is far greater in South Iceland than in the north. Guðbergsson and Einarsson (1995) investigated the relationship between the mineral content and acidity of Histosols from West- and North Iceland. They found that the mineral content and pH values in all the samples were higher than usually found in comparable soils in other countries. They also found differences in mineral content between samples sites, which were not reflected in difference in acidity. Their conclusion was that the overall higher mineral content of Icelandic Histosols makes them less acidic than Histosols in neighbouring countries. Factors other than minor differences in mineral content exert greater control on differences in acidity between Icelandic sites. This includes climate and vegetation. The C:N ratio of Histosols in Iceland can span a wide range within deep profiles: C:N values of both < 20 and > 30 within a single profile were observed by Guðmundsson (1978). He found the C:N ratio of Icelandic Histosols to be low compared with similar Histosols in neighbouring countries - a feature that he attributes to the unusually high content of mineral material in Icelandic Histosols. This effect has indeed also been observed in other volcanic regions. Research on Histosols in Japan has shown that frequent input of tephra tends to increase decomposition rates of organic material, and as a result decrease accumulation rates (Damman 1988).

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2 Environmental changes and development of the nutrient budget of Histosols in North Iceland during the Holocene

Möckel SC1, Gísladóttir G1,2., Erlendsson E1.

1. Department of Geography and Tourism, Faculty of Life- and Environmental Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland

2. Earth Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland

Keywords. Decomposition, Histosol, Minerogenic content, Nutrient state, Soil resilience, Vegetation changes 2.1 Introduction

Since the initial settlement of Iceland around AD 870, people’s perception of wetlands was ambivalent. Clearly, wetlands had substantial impact on daily life, reflected in their recurrence in literature. While being a valuable resource of peat, ferric oxide and hay, they also constituted a dangerous geographical obstacle (Huijbens and Pálsson 2009). With the onset of the industrialisation of agriculture in Iceland in the beginning of the 20th century, wetlands were seen as troublesome impediments to progress. From the middle of the 20th century until the nineties, extensive mechanised digging of drainage ditches took place with governmental financial support (Eylands 1967; Geirsson 1998; Óskarsson 1998). Since the nineties there has been an awakening regarding the ecological and environmental value of mires and wetlands. Wetlands in Nordic regions are dominated by peatlands (Bridgham et al. 1995), which are important stores of carbon (C) in the form of accumulated organic matter (Bridgham et al. 1995; Clymo 1987; FAO 2014; Gorham 1991; USDA 1999). Drainage of theses wetlands will increase decomposition of C and thus reduce their C pools (Armentano and Menges 1986; Bridgham et al. 1995). Wetland restoration projects were initiated in Iceland from the 1990s onwards (Garðarsson et al. 2006; Óskarsson 2011), motivated by nature conservation and mitigation of greenhouse gas emission (Aradottir et al. 2013). In spite of the generally accepted importance of wetlands, pedologic research emphasis in Iceland has been directed towards dryland soils (Arnalds et al. 1995; Arnalds and Kimble 2001). The focus has been on human impact on soil physical properties and carbon sequestration in relation to increased erosion after the settlement (Gísladóttir et al. 2011; Gísladóttir et al. 2010) and effects of different fertilization methods (Guðmundsson et al. 2005). Rather than functioning the research topic as a focus, peatlands have widely been used as information archive in palaeoecological research. They preserve valuable information on environmental conditions and land use in the past, not least in form of preserved pollen (Einarsson 1961; Erlendsson 2007; Erlendsson and Edwards 2009; Gísladóttir et al. 2010; Hallsdóttir 1987; Vickers et al. 2011; Zori et al. 2013). Palynological information has also been used as a tool to support studies on soil quality (Gísladóttir et al.

15 2011; Gísladóttir et al. 2010), and many palynological studies are supported by sedimentary information (Caseldine et al. 2006; Eddudóttir et al. 2015; Eddudóttir et al. 2016; Hallsdóttir 1987; Lawson et al. 2007; Vickers et al. 2011). An important focus of these studies is to link vegetation changes to alterations of physical properties of soil. Little work has been done regarding vegetation changes in the context of soil chemical properties in Iceland. As Óskarsson and Halldórsson (2006) point out, results of studies on wetland soils in neighbouring countries should be applied to Icelandic peatlands under reserve only. An important factor, which is likely to influence wetland soil development in Iceland considerably, is the frequent deposition of tephra deposits from volcanic eruptions, and eroded surfaces (Arnalds et al. 2016; Gísladóttir et al. 2011), but Icelandic wetland soils possess both histic and andic soil characteristics (Arnalds 2008; Arnalds and Óskarsson 2009). Research on wetlands has shown that input of tephra tends to increase decomposition rates of organic material and decrease peat accumulation (Damman 1988; Farrish and Grigal 1988; Thormann et al. 1999). A study in Iceland by Guðmundsson (1978) supports these findings. He found that the carbon:nitrogen (C:N) ratio of peatland soils in Iceland can span a wide range, between < 20 and > 30 within a single profile. Overall, it is low compared with similar peatland soils in northern peatlands (Loisel et al. 2014), which attributes to the unusually high content of mineral material (Arnalds et al. 2016; Guðmundsson 1978). Also, recent tephra deposits situated rather shallow within a profile have proven to have an impact on present vegetation (Damman 1988). The source of water also exerts great influence on the mineral content of wetlands. Based on water source, Icelandic wetlands have broadly been divided into bogs (flói) and sloping fens (hallamýri) (Einarsson 1961; Steindórsson 1936), but wetlands with characteristics of both types are very common (Arnalds 2015; Steindórsson 1964). Bogs are located in basins underlain by relatively impermeable bedrock and are mainly fed by precipitation. Sloping fens are located on slopes of varying degree, on valley floors, and hill and mountain slopes. Their hydrology is dependent on water from precipitation, runoff and groundwater. Partly fed by water that has flowed through or over mineral soil, they receive higher input of minerals than bogs. Soil properties vary between wetland types. The varying quantity and form of mineral material causes high variability in chemical properties (Guðmundsson 1978). Fens are usually less acidic and more nutrient rich (Clymo 1987; Einarsson 1961; Guðmundsson 1978; Moore 1986). Availability of cations, cation exchange capacity (CEC) and base saturation (BS) has been shown to vary between regions, depending on precipitation and eolian deposition (Jóhannesson and Kristjánsdóttir 1954), but overall -1 these values are high, ranging between 50 and 100 cmolc kg (Jóhannesson 1960). The comparatively high mineral content in Icelandic wetland soils leads to decreased acidity compared with neighbouring countries (Guðbergsson and Einarsson 1995). Despite the special conditions for wetland formation in Iceland, foreign research can stimulate ideas on processes and their causes under Icelandic conditions. Peatland development is highly influenced by climate, which contributes to hydrologic conditions (Bridgham et al. 1995). Likewise, peatlands influence the microclimate of their surroundings by shaping hydrologic conditions such as groundwater levels and runoff (Minayeva and Sirin 2012). It is important to understand processes and characteristics of soils in peatlands in order to understand how and to what extent they react to climate changes. Vegetation cover and soil properties of peatlands are interdependent. Soil properties and climatic conditions influence vegetation characteristics, whereas the type and quantity of vegetation has a considerable impact on soil development and processes in soils, such as organic matter and nutrient input and demand (Bridgham et al. 1995; Kolka et al. 2012). Chemical properties depend on factors such as plant material functioning as parent material, and the nature and

16 quantity of minerogenic content, but also on soil organisms, which have a great impact on soil temperature and hydrologic conditions (Clymo 1983). By influencing the activity of soil organisms, climate plays an important role here as well. This study examines the interactions of climate, land use, peatlands and vegetation. The focus is on the impact of climate and land use on peatland soil quality, nutrient state and the vegetational response over the course of the Holocene. A matter of consideration in this context is the geographical location of the peatlands, such as proximity to oceanic and eolian input of mineral material, and its potential impact on nutrient state and vegetation cover. Soil samples were taken from three sloping fens in the Austur-Húnavatnssýsla county in North Iceland (Figure 3.1). The sites are at different distances to the sea, different heights above sea level and at different latitudes. One of the key factors of interest was the nutrient state of the Histosols, particularly CEC and BS. The development of organic carbon (OC) and nitrogen and decomposition rates, indicated by C:N ratio and degree of humification, was also investigated, and subfossil pollen were analysed. Prehistoric measurements were compared with measurements on historic samples in order to increase the understanding of human impact on Histosols. The main purpose was to examine the validity of the following hypotheses: 1. Minerogenic content in wetlands has a positive impact on nutrient content of Histosols and decomposition of organic material. The geographic location of the wetlands plays an important role in this context, with increasing eolian input in the proximity of erosion areas and the active volcanic belt. Rationale: Research in other countries indicates pronounced minerogeneity in peatlands close to erosion areas and active volcanic regions, leading to enhanced decay (Damman 1988; Farrish and Grigal 1988; Thormann et al. 1999). Decomposition of organic material is commonly positively related to the nutrient content of Histosols (Kolka et al. 2012; Petersen 1980). 2. The nutrient state of the wetlands exerts influence on plant species composition in such a way that species diversity and species evenness are greatest at intermediate levels of nutrient content. Rationale: Other studies on vegetation response to soil fertility revealed optimal growth conditions and a peak in plant species diversity not at highest, but rather at intermediate fertility levels (Huston 1979; Tilman 1983). This pattern is attributed to the predominance of only few species in nutrient rich soils. 3. Climatic conditions exert great influence on vegetation cover. Vegetation cover affects decomposition of organic material. Increased decomposition has a positive effect on the ability of material to bind nutrients. Rationale: Different types of vegetation cover in peatlands lead to changes in the original chemical composition of the organic parent material (Hoorens et al. 2003; Hornibrook et al. 2000; Lawson et al. 2014). The original chemical composition of plants affects their decomposability (Hoorens et al. 2003; Wardle et al. 1997), and variabilities in plant species have also been shown to alter decomposition rates (Anderson and Hetherington 1999; McTiernan et al. 1997; Wardle et al. 1997). Decomposition rates and the ability of organic material to bind nutrients is positively related (Kolka et al. 2012; Petersen 1980). 4. Climatic changes affect properties of Histosols, but Histosols are resilient to soil degradation caused by climatic factors. Degradation of peatlands is mainly caused by land use. Rationale: Previous research on the impact of human settlement on soil quality in Iceland, on both dryland and wetland soils, indicate a profound negative impact of land

17 use on vegetation cover and soil properties. The detected changes started widely only short after the settlement in c. AD 870 and are reflected, for instance, in decreased soil organic carbon and moisture content, increased bulk densities, enhanced eolian deposition and erosion (Gísladóttir et al. 2011; Gísladóttir et al. 2010). Once erosion triggered by land use has started, adverse climate conditions can potentially enhance degradation (Dugmore et al. 2009).

3 Methods 3.1 Research area

Torfdalsmýri is the northernmost and most low-lying of the three sample sites (Figure 3.1). It is located on the northwestern tip of Skagi peninsula at a distance of approximately 1 km to the open sea to the west. East and south of the sample site are lowlands characterized by wetland areas. Generally, the area is rather open and unprotected, with the nearest mountain range about 15 km to the south. Hrafnabjörg is the southernmost sample site, located in Svínadalur valley. The valley follows a north-south direction. About 10 km to the south it merges into the elevated plains of Auðkúluheiði, a sparsely vegetated area, widely affected by severe soil erosion (Arnalds et al. 1997; Eddudóttir et al. 2016). To the east, the sample site is surrounded by hills up to approximately 500 m a.s.l., to the west by mountain ranges up to about 1000 m a.s.l.. Hrafnabjörg is located farthest from the sea with a distance of approximately 15 km to the north-northwest. Tindar is also located in Svínadalur, approximately 15 km farther north than Hrafnabjörg. Here the valley is broader and opens up to the northwest. The distance to the sea is about 12 km to the northwest. To the east, the site is surrounded by hills up to approximately 300 m a.s.l. As a proxy for climate data of the three sites, measurements of the weather station Hraun á Skaga were used for the site Torfdalsmýri, for Tindar measurements of the weather station Blönduós and for Hrafnabjörg, which is located at the highland fringe, measurements of the lowland station Blönduós and the highland station Kolka are used (Table 3.1; (IMO n.d.a). Note that means of the climate variables in Table 3.1 are an approximation only as measurements at a given station are sometimes incomplete due to short interruptions in observations. Generally, the climate of Iceland is oceanic and largely influenced by its situation at the border of warm and cold ocean currents and the frontier of warm and cold air masses (Einarsson 1984). Weather characteristics are dominated by strong winds, frequent precipitation and frequent weather changes, mild winters and cool summers. Depending on topographic conditions, especially orography, weather patterns can vary considerably between regions. There is a noticeable difference in precipitation between northern and southern parts of the country, with lower precipitation in the north than the south (Einarsson 1984; Ólafsson et al. 2007). In the proximity of Torfdalsmýri, mean annual temperatures have been fluctuating between 1.1 and 4.7 °C since the middle of the 20th century, with mean tritherm temperatures (mean temperatures for June, July, August, the usually three warmest months of the year) being 7.9 °C and mean annual precipitation between 344-697 mm. The range of mean annual wind speed is 4.8-6.5 m s-1. In the area around Tindar, mean annual temperatures have been within the range of 1.9-4.2 °C between the middle of the 20th century and the beginning of the 21st century, with

18 mean tritherm temperatures being 9.1 °C. Annual precipitation was between 303-786 mm. The range of mean annual wind speed is 2.7–4.7 m s-1. Weather conditions at Hrafnabjörg are characterized by both the lowlands and highlands. By comparing weather data from Blönduós and Kolka (data available since 1994), it is clear that temperatures and precipitation must be lower in Hrafnabjörg than Tindar, whereas wind speeds can be considerably higher (Table 3.1; (IMO n.d.a).

Table 3.1: Main weather characteristics for the three sample sites; name of according weather station(s) in brackets; data are available for the following time spans: Hraun á Skaga: 1956-2015; Blönduós: 1949-2001; Kolka: 1994-2015 (IMO n.d.a).

Torfdalsmýri Tindastaðir Hrafnabjörg (Hraun á Skaga) (Blönduós) (Blönduós / Kolka)

Mean annual temperature range (°C) 1.1 - 4.7 1.9 - 4.2 1.9 - 4.2 / -0.3 - 1.7 Mean annual temperature (°C) 2.9 3.1 3.1 / 0.7 Mean tritherm temperature (°C) 7.9 9.1 9.1 / 7.8 -1 Mean precipitation range (mm yr ) 344 - 697 303 - 786 303 - 786 / 253 - 504 -1 Mean precipitation (mm yr ) 512 481 481 / 391 Mean wind speed range (m s-1) 4.8 - 6.5 2.7 - 4.7 2.7 - 4.7 / 3.7 - 8.1 Mean wind speed (m s-1) 5.7 3.8 3.8 / 7.4 3.2 Sampling

Three potential sample areas were chosen by purposive sampling, based on previous knowledge of the research area obtained from aerial photographs and previous visits. In order to illuminate the impact of different geographical parameters on soil development and vegetation cover, areas at different distances to the sea, different heights above sea level and different latitudes were chosen. Within each of those areas, random spot checks were performed in order to find sample sites located within rather undisturbed wetlands, with soils consisting of peaty material throughout the profile. Spot checks were performed with a JMC Backsaver corer. The most suitable site within each area was chosen, a soil pit dug and a vertical profile cleared for in situ morphologic description. For analysis of soil physical and chemical properties in the laboratory, bulk samples of each layer and a profile core were taken. All sample sites are located within sloping fens and the locations of the chosen sites are (Figure 3.1): Torfdalsmýri (N66°3.376', W20°22.821; 39 m a.s.l.), Tindar and Hrafnabjörg (N65°26,252, W19°59,889; 329 m a.s.l.). At Tindar it was decided to excavate two cores at slightly different locations in order to cover as much of the Holocene as possible in terms of soil development. The first core (N65°34,665, W20°07,196; 108 m a.s.l.) was taken in the upper part of the sloping fen and covers the time span between the sampling date and Hekla 4 tephra, stemming from a volcanic eruption of Hekla dated back to approximately 4.25 ka BP (Dugmore et al. 1995). The second core (N65°34,680, W20°07,402; 101 m a.s.l.) was taken several meters below in a drainage ditch at the edge of the same fen. It covers the time span between Hekla 4 and the so called S-layer, a tephra with the geochemical properties of Katla, radiocarbon dated to approximately 6.6 ka BP (Eddudóttir et al. 2015).

Figure 3.1: Location of the sample sites (IS50 3.1V database of the National Land Survey of Iceland). 3.3 Soil morphology and physical properties

Morphologic characteristics such as layer thickness, structure, color, and abundance of roots were described both in the field and in the laboratory, depending on weather conditions (Schoeneberger et al. 2012). Sediment description was based on the Troel-Smith system

20 (Troels-Smith 1955) described in Aaby and Berglund (1986). Humification of organic substances was estimated by using the von Post evaluation (as described e.g. by (FAO 1988; Verry et al. 2011). A visual scale is applied, which divides the degree of peat humification into 10 grades (H1 to H10), based on colour and turbidity of free pore water and the nature and volume of the remaining fibrous and amorphous material. The lowest number of the scale indicates no decomposition or humification, the highest number indicates complete decomposition. Determination of physical soil properties such as dry bulk density (DBD), soil water content based on dry soil mass (wd) and soil organic matter content (SOM) was conducted in the facilities of the University of Iceland. Mean DBD, wd and SOM were determined for each layer. Three cubes of 1.8 cm x 2.1 cm x 2.1 cm volume per layer were carved out of the profile core at equal intervals. Their total mass was recorded, both at field water content and after drying at 105°C for 24 hours. The dried samples were sieved through a 2 mm mesh sieve and volume and mass of the > 2 mm fraction recorded. Calculations of wd and DBD are based on the weight difference before and after drying and the weight of the dried fine earth fraction < 2 mm divided by the volume of the sample cubes. For determination of SOM by loss on ignition, the fine earth fraction was homogenized and duplicate subsamples of approximately 0.5 g weighed. After moisture correction at a constant temperature of 105°C for 24 hours, samples were ignited at 550°C for four hours in a muffle furnace. The difference in mass before and after ignition is assumed to conform to the SOM of the soil (Heiri et al. 2001). Patterns in magnetic susceptibility (MS) of the profile cores were determined, used as an indicator of mineral particles and a tool to locate minor tephra layers. Measurements were undertaken on contiguous 1 cm intervals using a Bartington MS2 meter and Bartington MS2F probe (Dearing 1994). 3.4 Chemical soil properties

3.4.1 C:N ratio based on organic C and N

Total C and N content was determined by dry combustion at 950 °C in an elemental analyzer (USDA 2014). Subsamples of 10-15 g were milled, sieved through a 150 µm mesh sieve and dried for 4 hours at 70 °C. For each layer a subsample of the dried material was weighed into tin caps. According to the SOM content of the layers, three weight ranges were chosen: for samples with 30-40 % SOM 20-25 mg, for those with SOM content within the range of 40-60 % 15-20 mg and for samples with ≥ 80 % SOM 10-15 mg of crushed soil were used. Generally, the C:N ratio reflects the degree of decomposition and quality of organic matter (Batjes 1996) and can be used as a proxy to reconstruct climatic and hydrological conditions (Biester et al. 2012). Higher C:N values indicate lower levels of decomposition, occurring during times of high water table, and vice versa. Changes in plant composition and decomposability of organic material can also cause variabilities in C:N ratios (Biester et al. 2012). The C:N ratio for each layer was calculated based on total organic carbon and nitrogen content, and the molar weight of C and N.

3.4.2 pH in water and NaF pH was determined potentiometrically in both water and 1 N NaF solution, conducted in triplicates for each layer. For measurement of pHwater, air-dried soil was suspended in deionized water, using a soil:water ratio of 1:10. The comparatively wide soil:water ratio

21 was chosen in order to achieve a workable slurry of the highly organic soils (Blakemore et al. 1987; Rayment and Lyons 2011). The suspensions were shaken for one hour before measurement of pH with a calibrated pH electrode. Averages of the triplicates were then calculated. Determination of pHNaF is a standard method to obtain information on the presence of amorphous material resulting in andic soil properties, common in soils of volcanic regions. High abundance of amorphous material results in a pHNaF ≥ 9.4. Air dried soil was suspended in 1 N NaF solution with a pH between 7.5 and 7.8, using a soil:NaF ratio of 1:50. The pH of the suspension was read exactly two minutes after addition of 1 N NaF and under constant stirring of the suspension (Blakemore et al. 1987; USDA 2014).

3.4.3 Cation Exchange Capacity

For determination of exchangeable base cations Ca2+, Mg2+, Na2+ and K+ as well as 3+ extractable Al and P, extraction with 1N pH 7 ammonium acetate NH4OAc was conducted in the facilities of the University of Iceland. Subsequently, the extracts were sent to the Innovation Centre of Iceland for analysis of Ca2+, Mg2+, Na2+ and K+ content by atomic absorption spectrophotometer (USDA 2004). The cation exchange capacity (CEC) was calculated by summing up Ca2+, Mg2+, Na2+ and K+. For determination of potential cation exchange capacity (CECpot), displacement of an + index cation (NH4 ) after washing was conducted in the facilities of the University of + + Iceland. Soil samples were saturated with NH4 , and excess NH4 not absorbed by the soil + colloids was removed by washing with ethanol. Finally, the adsorbed NH4 was extracted by leaching the soil with 2M KCl. The extracts were sent to the Innovation Centre of Iceland + for determination of adsorbed NH4 , using steam distillation and titration (Blakemore et al. 1987; USDA 2004). Based on CEC and CECpot the percent base saturation (BS) was calculated. 3.5 Chronology, soil accumulation rate and carbon sequestration

Chronologies for the profile cores were established based on tephra layers and one 14C age determination. Occurrence of tephra layers in the soil cores were identified both visually and via anomalies in magnetic susceptibility. Subsamples of each tephra layer were prepared for analysis of major chemical elements by electron probe microanalysis (EPMA). Subsamples were suspended in 10 % NaOH and placed in a hot water bath of approximately 95 °C for 20 minutes. The suspension was then sieved through 250 µm and 150 µm mesh sieves and the portion > 150 µm was dried for preparation of polished thin sections. Analysis of the thin section by EPMA was conducted on a JEOL JXA-8230 electron probe microanalyser using a beam diameter of 5 µm and beam current of 5 nA for silicic and small-grained tephra. For basaltic tephra grains of larger size, a beam diameter of 10 µm and beam current of 10 nA was used. Accelerating voltage was 15 kV. Geochemical results were compared with results of previous analyses (Boygle 1999; Dugmore et al. 1995; Eddudóttir et al. 2015; Grönvold et al. 1995; Larsen et al. 1999; Larsen and Eiríksson 2008; Mangerud et al. 1986) and thus linked to the source volcano. Anomalies and analyses with sums of < 96% and > 101% were removed from the dataset. Based on that and the position of the tephra layers within the profiles, tephra layers could be connected to previously dated volcanic eruptions. Because of the rather large time span between some of the tephra layers in the profiles of Torfdalsmýri and Hrafnabjörg, several samples were prepared for AMS 14C age

22 determination and analyzed in the Laboratory of Ion Beam Physics at the ETH in Zurich. Due to the lack of suitable plant macrofossils, all but one sample, which was woody material from a single twig, consisted of a mixture of above-ground terrestrial plant material, dominated by leaves of graminoids. Based on the core chronologies and total carbon content, annual soil accumulation rate (SAR) and carbon sequestration (CSQ) were calculated. 3.6 Analysis of pollen and spores

For analysis of pollen deposits, three subsamples from each soil layer were taken from the core at regular intervals. A volume of 1.5–2.1 cm³ per layer (0.5-0.7 cm³ from each subsample) was used for pollen extraction by means of standard techniques, involving washing in 10 % HCl, 10 % NaOH and acetolysis solution to remove calcium carbonates, humic acids and organic matter (Berglund and Ralska-Jasiewiczowa 1986; Faegri and Iversen 1989; Moore et al. 1991). Dense media separation (Nakagawa et al. 1998) using LST fastfloat was used instead of 40 % HF treatment to remove minerogenic material. Additionally, samples were sieved (150 µm) to remove coarse particles following the 10 % NaOH treatment. For calculation of pollen concentration, two marker tablets (batch no. 177745) containing Lycopodium clavatum spores were added to each sample (Stockmarr 1971). A minimum of 300 terrestrial pollen grains was identified using a Leica light microscope with a minimum proportion of 50 accounting for other species than those belonging to the Cyperaceae family. Betula pollen diameters were measured and recorded, unless the pollen were too degraded to obtain reliable measurements. With reference to other publications on size distribution of Betula pollen of different species (Caseldine 2001; Mäkelä 1996), Betula nana pollen were defined as ≤ 20 μm and Betula pubescens pollen as > 20 μm. Pollen identification was based on Moore et al. (1991), as well as the pollen reference collection of the University of Iceland. Coprophilous fungi were identified from descriptions and images provided by van Geel et al. (2003). The results were plotted in final pollen diagrams produced with the program TILIA (version 1.7.16). Pollen categories and calculation of pollen sums are based on Caseldine et al. (2006). Additionally, fungal spore percentages were calculated based on the sum of terrestrial pollen and fungal spores. Pollen and spore taxonomy is mainly based on Bennett (2007), whereas certain divergences may occur due to the specific nature of the Icelandic flora (Erlendsson 2007). As an indicator of plant species composition and species diversity, species richness was calculated by rarefaction analysis using the CRAN package ‘vegan’ (Oksanen et al. 2016) in the programme R, version 3.2.5. Species evenness was estimated by (Hurlbert 1971) probability of interspecific encounter (PIE), calculated in Excel. The index ranges between 0 and 1; the higher the values, the higher the probability that two randomly selected pollen grains represent two different taxa and the more even is the species composition (Hurlbert 1971; Olszewski 2004).

23 4 Results 4.1 Soil accumulation rate

Six key tephra layers were detected in Tindar, and five key tephra layers were detected in Torfdalsmýri and Hrafnabjörg (Appendix A, Table 4.1). Tephra layers that were present at all sites are: Hekla 1766 (0.23 ka BP; (Þórarinsson 1968), Hekla 1104 (0.90 ka BP; (Larsen and Thorarinsson 1977), Hekla 3 (3.06 ka BP; (Dugmore et al. 1995) and Hekla 4 (4.25 ka BP; (Dugmore et al. 1995). In Torfdalsmýri, so-called Saksunarvatn tephra forms the bottom of the profile, a tephra layer most likely stemming from the Grímsvötn volcanic system, dated back to approximately 10.18 ka BP (Andrews et al. 2002; Grönvold et al. 1995). In Tindar, Ssn2, a tephra layer from the Snæfellsjökull volcanic system (Jóhannesson et al. 1981) dated back to approximately 4.6 ka BP and the so called S-layer, a tephra with the geochemical properties of Katla, radiocarbon dated back to approximately 6.6 ka BP (Eddudóttir et al. 2015). In Hrafnabjörg, Hekla 5 tephra from approximately 7.05 ka BP (Larsen and Thorarinsson 1977) could be identified. As the base of the profile in Hrafnabjörg was not defined by a tephra layer of known age, basal age of the sequence (c. 8.9 ka BP) was estimated by linear extrapolation from the two nearest control points. In Torfdalsmýri one 14C age was used for the age interpolation, dated back to c. 8.36 ka BP. Ages for every cm and annual soil accumulation rates (SAR) were interpolated based on the tephra layers of known age (Table 4.1). In Torfdalsmýri, SAR was consistently low with values between 0.05 and 0.09 mm yr-1. In Tindar, SAR was moderately high between Katla S and Hekla 3 (0.18–0.24 mm yr-1), but decreased above Hekla 3 to a minimum of 0.10 mm yr-1. Above Hekla 1766 there was a sharp increase to 0.50 mm yr-1. In Hrafnabjörg, SAR was low (0.05–0.06 mm yr-1) between Hekla 5 and Hekla 3. Above Hekla 3 this number doubles, but remained relatively low (0.12–0.14 mm yr-1). Overall, SAR was considerably higher in Tindar than at the other two sites. Tindar was the deepest profile, with nearly 120 cm, but ended at around 6.60 ka BP and covered the shortest time period.

24 Table 4.1: SAR [mm yr-1] and age model based on tephra layers of known eruption date and one 14C age (2σ). An alternative model based on excluded radiocarbon dates is indicated by the dashed line.

SAR Depth [cm] Tephra layer Age [BP] Age model as interpolated from tephra layers of known eruption date and 14C ages [mm yr-1] Torfdalsmýri H1766 H1 0 Top of profile 0.00 0 0.06 Excluded radiocarbon dates 20 H3 2-3 Hekla 1766 0.23 646 ± 54 H4 0.06 40 Included tephra layers and 7-8 Hekla 1104 0.90 radiocarbon dates 0.09 60 3574 ± 55 26.5-29 Hekla 3 3.06 8356 ± 86 0.07 [cm] Depth 80 Saksunarvatn 37.5-42.5 Hekla 4 4.25 0.09 100 14 8.36 81-82 C 120 0.05 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 90.5-94.5 Saksunarvatn 10.18 Age [BP] Tindar 0 Top of profile 0.00 0 H1766 0.50 H1 Included tephra layers 13-13.5 Hekla 1766 0.23 20 0.11 H3 21-22 Hekla 1104 0.90 40 0.10 60 H4 43-43.5 Hekla 3 3.06 Ssn2 0.18 80 65-71 Hekla 4 4.25 [cm] Depth 0.24 100 79-79.5 Ssn 2 4.60 S-layer 0.19 120 117-118.5 S-layer 6.60 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Age [BP] Hrafnabjörg H1766 0 H1 0 Top of profile 0.00 Excluded radiocarbon dates 0.14 20 4970 ± 56 4-4.5 Hekla 1766 0.23 H3 40 H4 0.12 Included tephra layers and H5 radiocarbon dates 12.5-13 Hekla 1104 0.90 60 0.13 2398 ± 21 80 41.5-42.5 Hekla 3 3.06 3379 ± 65

0.06 [cm] DEpth 100 49-50 Hekla 4 4.25 0.05 120 64.5-65.5 Hekla 5 7.05 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Age [BP] 4.2 Soil morphology

Some of the selected soil morphological properties exhibited distinct changes with profile depth (Table 4.2). Quantity and/or size of roots declined, and degree of humification increased. Soil structure became weaker with depth and is associated by a decrease in compound size. Colour changes and sediment development based on Troel-Smith system did not show a clear pattern in all profiles. Overall, all layers except silicic tephra layers revealed dark colours, ranging from black over very dark brown/very dark grey to dark brown. Main differences in morphological properties between sample sites were the higher degree of decomposition and content of ligneous material in Tindar (layers 6–10) and coarser material in layers 1 and 2 in Hrafnabjörg.

25 sapric

[moist] rk brown) rk brown) rk brown) wn) wn) ≙ = gradual, = s) (see also 10 - ial; H9 Munsell colour code 7,5YR 2,5/3 da (very 7,5YR 2,5/2 da (very 7,5YR 2,5/2 da (very 10YR 2/1 (black) 7,5YR 3/3 (dark bro 7,5YR 3/2 (dark bro 10YR 2/2 dark(very brown) 7,5YR 3/1 dark(very gray) 7,5YR 2,5/2 dark (very brown) 10YR 2/2 dark(very brown) 7,5YR 2,5/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) structural unit hemic mater

≙ Von Von Post Humification index H4 H3 H3 H5 H8 H5 H6 H6 H3 H4 H4 H5 H5 H5 H6 H6 H5 H6 8 - H6 = massive (no

ndary description: C = clear, G clear, = description:C ndary ith , Ga+ , Ga+ , Ga+ , Ag1 cription built

material, ) fibric

= medium; bou medium; = , pl = platy, MA ≙ Sediment des upon Troel-Sm Ag2, Th1, Sh1 Th2, Sh1, Ag1 Th2, Sh1, Ag1 Th1, Sh1, Ag1, As1, Ga+ Ag2, As1, Sh1, Th+, Ga+, Dg1, Th1, Sh1 Ag2, Th1, Sh1, Ag3, Sh+, Ga+ Dl1, Th+, Th3, Ag1, Ga+ Sh+, Th2, Sh1, Ag1, Ga+ Ag2, Th1, Sh1, Ga+ Ag3, Sh1, Th+ Ag2, Th1, Sh1, Ga+ Th1, Sh1, Dl1, Ag1, Dl2, Sh1, Ag1, Th+ Ag2, Th1, Sh1, Dg+ Dl+, Ag2, Th1, Sh1, Dg+ Dl+, Ag2, Th1, Sh1, Dg+ Dl+, 5 - Smith 1955 - Structure [grade, size, type] pl2, m, 3, tn, pl 3, tn, pl 1,pl vn, MA 1,pl vn, 2,pl vn, MA 3, tn, pl 3, tn, pl 2, tn, pl 2,pl vn, 2, tn, pl 2,pl vn, 1,pl vn, 1,pl vn, 2,pl vn, 1,pl vn, fine, f = fine, m fine, = fine, f , vn = very thin

n Boundary [distinctness, topography] SC, SC, SC, S C, S C, S C, S C, S C, G, S A, S A, S A, S A, S A, S A, S A, S G, S S C, ed humification (H1 rlund 1986; Troels s any, vf = very = vf any, common, 3 = m = 3 common, strong, m = medium, tn = thi Smith system (Aaby and Be - igher numbers indicate numbers increa igher els Roots [quantity, size] moderatelym 3 few few f, very vf, f few ver 2 vf, m few f, very few ver 2 vf, few vf, very few vf very few vf very few vf very 33 f, 1vf, m 33 f, 1vf, m m very few f, few very 3 vf, 3 vf m very few f, few very 3 vf, m very few f, few very 3 vf, m few very 3 vf, f few very 3 vf, 3 vf 13 m vf, moderate, 3 = ed on Tro Depth Depth [cm] 0-7 7-8 8-26.5 26.5-29 29-37.5 37.5-42.5 42.5-52.5 52.5-54.5 54.5-82 82-86 86-90.5 90.5+ 0-9 9-21 21-22 22-24.5 24.5-27 27-43 43-43.5 43.5-57 57-65 65-71 71-79 79-79.5 79.5-93.5 93.5-117 117-118.5 bas

ak, 2 = properties. Roots: 1 = few, 2 = 2 few, = 1 Roots: properties. Horizon Oi 2C - Hekla 1 3Oi 4C - Hekla 3 5Oi 6C - Hekla 4 7Oe 8Oe 9Oe 10Oe 11Oe 12C - Saksunarvatn Oi 2Oi 3C - Hekla 1 4Oi 5Oe 6Oe 7C - Hekla 3 8Oe 9Oe 10C - Hekla 4 11Oe 12C - Ssn2 13Oe 14Oe 15C - S-layer iment descriptioniment von Post Humificationh Post index: von tructure: 1 = we s dar ); sed oth; Soil layer Torfdalsmýri 1 2 3 4 5 6 7 8 Tin 1 2 3 4 5 6 7 8 9 10 oil morphologic oil 5.3 5.5 8.4 9.2 0.2 1.2 1.4 3.8 5.3 (Ssn) 4.6(Ssn) (Clymo 1983 : Selected s Selected : 2 . (S-layer) 6.6(S-layer) 4 brupt, S = smo (Hekla3) 3.06 (Hekla4) 4.25 (Hekla3) 3.06 (Hekla4) 4.25 brackets) le (Hekla1104) 0.9 (Hekla1104) 0.9 tephra layer in tephra layer Cal yr. BP (ocurring Cal yr. (Saksunarvatn) 10.2 Tab A = a Schoenebergeral., 2012); et material; 26 rk brown) rk brown) rk brown) wn) ll colour code [moist] Munse 5YR 2,5/1 (black) 7,5YR 2,5/2 da(very 7,5YR 2,5/3 da(very 7,5YR 2,5/2 da(very 7,5YR 3/2 (dark bro 7,5YR 2,5/1 (black) 7,5YR 2,5/1 (black) 10YR 2/1 (black) 10YR 2/1 (black) Von PostVon Humification index H3 H3 H4 H4 H6 H7 H6 H7 H7 , Ga+ , Ga+, Gs+, Ga+, , Ga+ , Ga+ , Ga+ Ga+,, Sh+, Dl+ , Ga+, Gs+, Ga+, pon Troel-Smith ediment descriptionediment built S u Th2, Sh1, Ag+ Ag2, Th1, Sh1 Th2, Sh1, Ag1 Ag2, Th1, Sh1 Ag2, Th1, Sh1 Ag3, Sh1, Th+ Ag2, As1, Th1 Ag2, Th1, Sh1, Ag2, As2, Sh+ Th+, Structure [grade, size, type] pl2, m, pl3, m, 3, tn, pl 3, tn, pl 1, pl vn, MA 2, pl vn, 1, pl vn, MA undary pography] istinctness, Bo [d to S C, S C, S C, S C, A, S A, S S C, S C, S C, Roots [quantity, size] 3 f, 3 m 33 vf, f, 2 m 33 vf, f, 2 m 2 3 m few vf, f, very 1 moderatelym few f, very few vf, verym few few vf, very m few f, very very few 2 vf, m few veryf, very few moderately few vf, 22 vf, f cm] Depth [ 0-4 4-4.5 4.5-10 10-12.5 12.5-13 13-29 29-41.5 41.5-42.5 42.5-49 49-50 50-64.5 64.5-65.5 65.5-69 69-75 66 orizon H Oi 2C - Hekla 17 3Oi 4Oi 5C - Hekla 1 6Oi 7Oe 8C - Hekla 3 9Oe 10C - Hekla 4 11Oe 12C - Hekla 5 13Oe 14Oe Soil layer Hrafnabjörg 1 2 3 4 5 6 7 8 9 0.7 2.1 7.7 8.9 ocurring ets) ayer in ayer (Hekla3) 3.06 (Hekla4) 4.25 (Hekla5) 7.05 brack (Hekla1104) 0.9 tephra l (Hekla 1766) 0.25 Cal yr. BP ( Calyr. 27 4.3 Physical and chemical soil properties

4.3.1 Dry bulk density, soil organic matter and soil water content

In Torfdalsmýri, SOM content spanned the widest range, with values between 33 and 82 %. SOM was lowest in the top and bottom layer, 45 % and 33 %, respectively. In the remaining layers it was constantly very high (67–82 %), except in layer 5, where SOM dropped to 45 %. wd in Torfdalsmýri also spanned the widest range of all sites, ranging -1 between 1.85 and 4.72 g g . This pattern was very similar to SOM, with higher wd in layers with high SOM content and vice versa. DBD was very low in most layers in Torfdalsmýri, ranging between 0.18 g cm-3 and 0.24 g cm-3 in all layers except 5 and 8, where DBD rose to 0.34 g cm-3 and 0.41 g cm-3 respectively (Figure 4.1; Table 4.3). In Tindar, the SOM pattern was less variable than in Torfdalsmýri, where values fluctuated within the range of 43–74 %. The highest values, ≥ 60 %, were found in layers 5, 6, 8 and 9. The same layers also revealed very high wd and low DBD, between 3.09 and 4.73 -1 -3 g g and 0.17 and 0.24 g cm , respectively. Similarly high wd and low DBD values were also measured in layers with lower SOM content. Lowest wd in Tindar is 2.01–2.02 g g-1 in layers 3 and 4. DBD in these layers increased slightly (0.34 g cm-3; Figure 4.1; Table 4.3). Hrafnabjörg was the site with the lowest SOM content and the narrowest range (32–58 %). Highest SOM content was reported in layers 3 and 8 (58 and 57 %, respectively). Four out of nine layers (2, 5, 6, and 9) did not exceed 33 %. Those four layers also revealed -1 comparatively low wd and high DBD (1.67–2.23 g g and 0.34–0.41 g cm-3; Figure 4.1; Table 4.3) Overall, DBD and SOM were negatively correlated, with about 64 % (r2 = 0.64) of the variation in DBD depending on variations in SOM. wd and SOM content were positively 2 correlated, with 78 % (r = 0.78) of the variation in wd brought about by variations in SOM. The strength of correlation between MS and DBD and MS and SOM could not be determined because of the higher resolution of data for MS than the other variables. Nevertheless, trends can be observed, which indicate a positive relationship between MS and DBD, and negative between MS and SOM (Figure 4.1; Table 4.3). Overall, the most pronounced peaks in MS were related to the occurrence of tephra layers. Between tephra layers, MS fluctuated around zero in Torfdalsmýri, which corresponds to the high SOM content and low DBD in most layers. From a depth of 14 cm upwards, the pattern changed as MS stayed above zero between tephra layers, coinciding with a decrease in SOM content. A slight increase in MS was also measured within layer 5, which also stands out in terms of SOM and DBD. In Tindar, MS values were low between tephra layers from Hekla 3 downwards. Above Hekla 3, they increased, reflecting diminished SOM content and increased DBD in the upper part of the core. In Hrafnabjörg, MS was consistently above zero between tephra layers, reflecting the comparatively low SOM and high DBD.

28 , Pronounced peaks in MS

and MS. (%) , SOM ) ´1 (g g

d w

mass

dry soil , water by ) ´3 (g cm

perties DBD ment of physical soil pro rrence of tephra layers in the profile, are marked by a +. : Profile develop 1 . 4 Figure that coincide with the occu

29 5.94 9.28 8.23 9.92 4.08 2.62 7.37 8.08 9.60 8.95 4.16 5.81 3.15 2.55 CSQ 10.60 10.69 27.22 11.81 10.00 16.29 18.68 17.61 16.74 12.22 13.94 10.91 10.73 20 24 28 26 27 32 29 29 21 19 21 24 22 21 21 24 23 21 18 18 19 21 20 19 18 18 21 C:N .88 .31 .98 .46 .14 .70 .84 .30 %C 33 43 50 41 31 46 44 12 30.21 25.26 19.79 26.85 32.31 44.37 42.45 37.28 48.15 44.44 29.98 20.63 29.39 23.72 14.23 16.41 33.18 19.39 12.37 %N %N 1.955 2.078 2.149 1.844 1.351 1.693 1.827 0.492 1.677 1.519 1.119 1.305 1.718 2.506 2.405 1.827 2.469 2.514 1.903 1.327 1.842 1.333 0.844 0.995 2.120 1.264 0.681 -3 0.36 0.35 0.39 0.57 0.46 0.10 0.16 0.34 4.68 2.54 0.87 0.57 0.35 0.37 0.37 1.38 0.90 0.39 1.01 1.02 0.55 0.87 0.81 0.62 0.82 0.58 0.54 mgdm

P |

-1 1.51 1.61 1.92 2.36 1.36 0.46 0.89 0.82 9.85 2.55 1.66 1.48 2.09 1.66 7.81 5.43 1.73 4.44 2.96 2.39 2.86 1.98 1.60 2.83 2.36 1.54

30.92 ). 1 mg kgmg - yr

BS 29 33 36 77 52 54 49 54 74 70 72 79 71 68 84 54 54 58 50 44 40 2 331 167 330 178 263 103 - : g C m pot 6.89 0.79 1.65 1.01 2.98 1.09 10.59 14.71 9.01 10.70 16.18 28.72 14.67 15.63 21.01 17.52 17.51 25.41 16.01 14.61 10.21 16.67 14.77 14.09 24.30 14.13 18.60 CEC .06 .65 .09 .16 .77 .20 .95 3 8 4 8 5 29 59 35.58 59.53 41.45 47.45 84.18 61.79 88.59 95.52 99.32 70.23 42.39 44.61 54.58 36.14 36.26 83.48 57.84 52.47 ; CSQ 105.72 111.35 1 - 1.98 2.62 2.76 3.34 5.29 2.87 3.48 5.24 6.89 5.60 8.67 14.01 7.98 11.52 14.72 12.68 13.81 18.10 16.43 9.90 8.62 8.99 7.98 8.23 12.24 6.20 7.53 bases CEC 8.34 12.08 13.54 13.75 15.61 13.66 19.67 12.67 45.55 21.70 25.44 41.05 33.62 65.29 66.92 71.87 83.38 79.32 72.09 28.74 37.69 29.43 19.52 21.18 42.05 25.38 21.24 -3 dm .01 .10 .30 .66 .32 .09 .11 .05 .02 .06 .00 .04 .01 .01 .04 .03 .08 .06 .10 c 0.05 0.14 0.12 0.26 0.74 0.14 0.16 0.48 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3+ ; BS: %; P: mg kg Al 1 - 0.23 0.62 0.60 1.05 2.19 0.68 0.90 1.17 0.06 0.40 0.89 1.93 1.33 0.52 0.52 0.26 0.11 0.24 0.02 0.12 0.05 0.03 0.10 0.09 0.29 0.24 0.29 cmol

| : g -1 d kg + 0.05 0.02 0.01 0.01 0.01 0.00 0.02 0.05 0.22 0.14 0.06 0.03 0.02 0.02 0.02 0.02 0.00 0.00 0.32 0.07 0.05 0.00 0.00 0.00 0.00 0.00 0.00 c K ; w 3 - 0.19 0.08 0.04 0.04 0.04 0.00 0.14 0.11 1.48 0.54 0.16 0.10 0.07 0.11 0.10 0.13 0.03 0.00 1.42 0.20 0.20 0.00 0.00 0.00 0.00 0.00 0.00 cmol .24 .32 .28 .29 .42 .24 .29 .31 + 0 0 0 0 0 0 0 0 0.18 0.21 0.22 0.24 0.13 0.18 0.22 0.24 0.22 0.31 0.24 0.18 0.13 0.16 0.16 0.14 0.15 0.11 0.14 Na 1.01 1.46 1.37 1.18 1.25 1.13 1.64 0.76 1.18 0.80 0.65 0.70 0.56 1.00 0.98 1.36 1.33 1.35 1.04 0.51 0.58 0.53 0.40 0.36 0.50 0.43 0.39 2+ 0.67 0.82 0.90 1.00 1.42 0.89 1.14 1.23 1.57 1.34 1.91 3.13 1.73 2.69 3.41 3.17 3.70 4.46 3.90 2.26 2.22 2.75 2.04 1.97 2.78 1.23 1.53 Mg 2.83 3.76 4.42 4.10 4.17 4.23 6.44 2.98 5.18 5.61 9.18 7.30 6.57 9.70 8.99 5.00 5.07 9.54 5.05 4.31 10.39 15.26 15.50 17.96 22.37 19.55 17.10 al properties (DBD: g cm 2+ 2+ 1.02 1.47 1.57 2.05 3.44 1.74 2.02 3.65 4.92 3.92 6.48 10.60 6.10 8.63 11.07 9.25 9.88 13.33 11.97 7.39 6.22 6.08 5.77 6.12 9.32 4.86 5.86 Ca 4.31 6.78 7.71 8.43 8.30 8.82 10.15 11.45 32.49 15.19 19.01 31.07 25.69 48.93 50.34 52.42 59.65 58.42 52.53 21.45 27.20 19.91 14.12 15.75 32.01 19.89 16.53 F Na 9.0 8.8 8.5 9.5 8.7 9.5 7.9 9.7 9.4 8.1 8.0 9.4 7.6 8.1 8.3 9.3 8.4 9.0 9.5 9.7 9.8 10.5 11.5 10.7 10.8 10.2 10.3 pH .8 .5 .5 .6 .7 .4 .3 water 4 4 4 4 4 4 4 4.9 5.4 5.1 5.2 5.2 4.2 4.5 4.2 5.1 5.1 4.9 5.6 5.3 5.1 4.9 5.0 5.1 4.8 5.1 5.1 pH SOM 45.47 67.40 82.40 71.77 44.83 79.37 81.12 33.11 51.81 46.00 42.71 46.62 59.65 72.72 50.03 60.98 74.37 49.66 46.52 31.60 58.18 41.95 32.58 32.83 48.10 57.18 31.72 d ent of soil physical and chemic w 2.8 3.3 3.8 3.2 2.2 3.9 4.7 1.8 3.5 3.0 2.0 2.0 3.1 4.2 3.2 4.0 4.7 3.5 2.8 1.8 3.0 2.2 1.7 1.8 2.7 3.1 2.2 lopm 0.24 0.22 0.20 0.24 0.34 0.21 0.18 0.41 0.15 0.26 0.34 0.34 0.24 0.18 0.22 0.18 0.17 0.23 0.23 0.34 0.23 0.31 0.41 0.39 0.29 0.24 0.35 DBD Layer 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 5.3 5.5 8.4 9.2 0.2 1.2 1.4 3.8 5.3 0.7 2.1 7.7 8.9 : Profile deve 3 Tindar . (Ssn) 4.6(Ssn) 4 (S-layer) 6.6 (Hekla 3) 3.06 3) (Hekla 4.25 4) (Hekla 3.06 3) (Hekla 4.25 4) (Hekla 3.06 3) (Hekla 4.25 4) (Hekla 7.05 5) (Hekla Hrafnabjörg Torfdalsmýri Cal Cal yr. BP (Hekla 1104) (Hekla 0.9 1104) (Hekla 0.9 1104) (Hekla 0.9 ksunarvatn)10.2 Table (Hekla 1766) 0.25(Hekla (ocurring tephra (ocurring tephra layer in brackets)

(Sa 30 4.3.2 pHwater and pHNaF pHwater was most stable in Torfdalsmýri. Values fluctuated within the narrow range of 4.3–4.9, and were consistently low. Even though the highest measured pH occured in the layers with the lowest SOM, a clear correlation between pHwater and SOM was not found. In Tindar, there was more variation in pHwater, with a span of 4.2–5.4. A clear relationship between pHwater and SOM could not be detected here either. This was expressed by the fact that comparatively high pHwater of ≥ 5.1 was both measured in layers with low (layers 1–4) and high (layers 8 and 9) SOM content. The span of pHwater in Hrafnabjörg was 4.8–5.6 and only layers 4 and 7 revealed a pHwater < 5. Again, there was no clear relationship between SOM and pHwater, which was illuminated best by comparing the pHwater of layers 3 and 9. Both layers had a pHwater of 5.1 despite the fact that layer 3 had the highest SOM content (58 %) while layer 9 sustained 32 % SOM, amongst the lowest in the profile. Overall, only 4 % (r2=0,04) of variation in pHwater was reflected in variation of SOM content. Andic soil properties are clearly at work in some layers of all profiles, as indicated by pHNaF ≥ 9.4. Amongst those layers are 4, 5, 7 and 8 in Torfdalsmýri, 2–5 in Tindar and 5–9 in Hrafnabjörg (Table 4.3).

4.3.3 Base cations, CECbases and CECpot

Due to low DBD of Histosols, it has been argued that major cations and CEC for those soils should rather be based on unit soil volume than mass (Kolka et al. 2012). The delineation of the results and discussion in this paper will thus not be based on the SI units, which is cmolc -1 -3 -1 kg , but on cmolc dm . For means of comparison, values based on both mass (cmolc kg ) -3 and volume (cmolc dm ) are reported in Table 4.3. There was a moderate inverse relationship between CEC and SOM (r²=0.19) and 2+ 2+ 2+ + CEC and C:N ratio (r²=0.24). CEC, as sum of the base cations Ca , Mg , Na and K -3 (Table 4.3), was lowest in Torfdalsmýri (1.98–5.29 cmolc dm ). Highest CEC was reported -3 in Tindar (5.60–18.10 cmolc dm ), and intermediate levels in Hrafnabjörg -3 (6.20–16.43 cmolc dm ). In Torfdalsmýri, where the lowest CEC was found, the availability + -3 of the major nutrients was lowest, with the exception of Na (0.24–0.42 cmolc dm ). In Tindar, similarly high levels were reported in some layers, but not all -3 + -3 (0.13–0.31 cmolc dm ). In Hrafnabjörg Na content ranged between 0.11 cmolc dm and -3 0.24 cmolc dm . 2+ -3 Ca content in the different sample sites was 1.02–3.65 cmolc dm in Torfdalsmýri, -3 -3 3.92–13.33 cmolc dm in Tindar and 4.86–11.97 cmolc dm in Hrafnabjörg. Interestingly the highest Ca2+ content in Torfdalsmýri (layer 8) was lower than the lowest Ca2+ content in Tindar (layer 2) and Hrafnabjörg (layer 8). Hrafnabjörg revealed increased Ca2+ content in the uppermost layer. Tindar showed an overall decline in Ca2+ with time with highest values occurring downwards in the profile, and lowest values occurring in layers 2 and 3. The temporal development from high to low Ca2+ content was not as pronounced in Torfdalsmýri and Hrafnabjörg, even though there was a declining trend with time in Torfdalsmýri. In Hrafnabjörg, an opposite trend was observed where Ca2+ content increased over time. Mg2+ availability reflected the pattern of Ca2+ levels in all sample sites. Highest -3 availability was observed in Tindar (1.34–4.46 cmolc dm ), and highest availability was found in the lowermost layers 7-10, lowest in layers 1 and 2. In Hrafnabjörg, levels fluctuated -3 -3 between 1.23 cmolc dm and 3.90 cmolc dm , increasing overall with time. In Torfdalsmýri there was an overall decline through time, with a maximum of -3 -3 1.42 cmolc dm in layer 5 and a minimum of 0.67 cmolc dm in layer 1.

31 + In Torfdalsmýri, K availability was highest in the top and bottom layer (0.05 cmolc dm-3). From layer 2 downwards, K+ decreased gradually until no measurable quantity was + detected in layer 6. In Tindar K , availability decreased downwards, with the highest -3 availability (0.22 cmolc dm ) in the top layer and lowest availability (under detection limit) in the bottom layer. In Hrafnabjörg, only layers 1, 2 and 3 revealed measurable K+ quantities -3 (0.05–0.32 cmolc dm ). Extractable elemental P (Table 4.3) reached the highest levels by far in Tindar. A steady decline from 4.68 mg dm-3 in the top layer to 0.39 mg dm-3 in layer 5 was detected. From layer 6 downwards, values oscillated with relatively high values in layers 8 and 9. In Hrafnabjörg, P was highest in layer 1 and 2 (1.01 and 1.02 mg dm-3) and lowest in the bottom layer (0.54 mg dm-3). The decline downwards in the profile followed an oscillating pattern. In Torfdalsmýri, P was lowest of all sample sites overall, ranging between 0.10 and 0.57 mg dm-3. Exchangeable Al3+ (Table 4.3) content was comparatively high in most layers in -3 Torfdalsmýri. Most of the values fluctuated within the range of 0.12–0.48 cmolc dm . -3 Higher content was only reported in layer 5 (0.74 cmolc dm ), and lower in layer 1 (0.05 -3 3+ cmolc dm ). In Tindar, there was more variability in exchangeable Al between layers. The -3 range was 0.01–0.66 cmolc dm . Lowest quantity was detected in the top layer (0.01 cmolc -3 -3 dm ) and highest in layer 4 (0.66 cmolc dm ). There was no apparent temporal pattern. In 3+ Hrafnabjörg, exchangeable Al was constantly very low (0.00–0.10 cmolc dm-3) and there was an overall increase downwards in the profile. Despite the low CEC in Torfdalsmýri, the site revealed the highest base saturation (BS) in some layers (Table 4.3). Quite a peculiar picture is drawn in Torfdalsmýri. Even though CEC was comparatively low in all layers, it exceeded CECpot in layers 2–6. This lead to a very high BS, greatly exceeding 100 %. Contrary to that, CECpot was approximately three times as high as CEC in layers 1, 7 and 8, resulting in rather low BS levels of 29–36 %. In Hrafnabjörg and Tindar this unexpected pattern was not shown. It was only in layer 1 in Hrafnabjörg where BS slightly exceeded 100 %. In other layers, BS was between 40 and 84 %, steadily decreasing with increased depth. In Tindar BS was relatively stable, between 49 and 54 % in layers 2–5 and between 70 and 79 % in the remaining layers.

4.3.4 C:N ratio and carbon sequestration

C:N ratios (Figure 4.2, Table 4.3) reached highest values in Torfdalsmýri, with values between 26 and 32 being constantly above 25 in layers 3-8. Above layer 3 there was a drop in C:N ratio, declining to 20 in the top layer. In Tindar and Hrafnabjörg, C:N ratios were constantly below 25. The range in Hrafnabjörg was narrowest, with ratios fluctuating between 18 and 21. In Tindar the span was between 19 and 24, with highest values observed in layers 4, 8 and 9. CSQ in Tindar was between 7.4 and 27.2 g C m-2 yr-1 and rather high compared to Torfdalsmýri and Hrafnabjörg, which comprised similar ranges (2.6–10.7 g C m-2 yr-1 and 2.5–13.9 g C m-2 yr-1, respectively). Contrary to Torfdalsmýri, CSQ increased towards the top of the profile in Tindar and Hrafnabjörg.

) 1 - yr 2 - SQ (g C m ent of %N, %C, C:N ratio and C : Profile developm 2 . 4 Figure

33 4.3.5 Stratigraphic pattern of pollen

Based on major changes in species composition and/or pollen concentrations (compare Figure 4.3, Figure 4.4 and Figure 4.5), major pollen zones were defined for each sample site, either comprising one or several layers. The most important characteristics of each pollen zone, including mean Betula pollen diameter, are summarized in Table 4.4. There are some prominent differences between sample sites (compare Figure 4.3, Figure 4.4 and Figure 4.5). Concentrations of terrestrial pollen were very different. Highest concentrations were found in Torfdalsmýri, between 71,000 and 328,000 grains cm-3. In -3 Tindar, concentrations ranged between approximately 26,000 and 91,000 grains cm , and in Hrafnabjörg the highest concentration of approximately 61,000 grains cm-3 did not even reach levels of the lowest concentrations in Torfdalsmýri. Lowest concentration in Hrafnabjörg was around 14,000 grains cm-3. Species richness was very similar in all sample sites, but species evenness, as indicated by Hulbert’s index, was highest in Torfdalsmýri (compare Figure 4.3, Figure 4.4 and Figure 4.5). The most prominent taxa in Torfdalsmýri were, just as in Tindar and Hrafnabjörg: Betula undiff., Salix, Cyperaceae and Poaceae. Contrary to the other two sites, Ericales were also rather dominant in Torfdalsmýri, and Juniperus communis and Thalictrum alpinum reached noteworthy percentages. Ranunculus acris-type, Rosaceae and Apiaceae were frequently recorded and show development through time in Torfdalsmýri and Hrafnabjörg.

34 Table 4.4: Main characteristics of each pollen zone. Pollen zones are based on major changes in species composition and/or pollen concentrations (compare Figure 4.3, Figure 4.4, Figure 4.5, Appendix 2, Appendix 3 and Appendix 4) and comprise either one or several soil layers.

Torfdalsmýri Tindar Hrafnabjörg 0 Decrease of Betula; increase of Poaceae and Dominance of Cyperaceae continues, accompanied by Continuous decrease of Betula Thalictrum alpinum; Cyperaceae by far most increased presence of Salix and Thalictrum alpinum. accompanied by a decrease of Cyperaceae. abundant; Horizon 1 represents the only pollen sample Betula pollen remain sparse and Poaceae decreases to Poaceae, Thalictrum alpinum and Ericales where spores of Sordaria-type and Sporormiella-type its lowest values. gain importance. reach significant values. Mean Betula size: 18.24 µm Mean Betula size: 20.81 µm Mean Betula size: 21.96 µm 1000

The trend of the preceding millenias is reversed: Betula decreases and Cyperaceae increases. Mean Betula size: 21.38 µm Prominent expansion of Ericales and noticeable increase of Salix; Decline of Cyperaceae; Betula 2000 remains fairly stable. Pollen concentrations reach their lowest levels. Mean Betula size:19.81 µm Continuous decrease of Betula Betula reaches its local maximum at 29 %; Cyperaceae accompanied by a strong increase of declines slightly Cyperaceae. Mean Betula size: 21.07 µm Salix nearly vanishes. Mean Betula size: 23.64 µm 3000

Slight increase of Betula and prominent increase of Cyperaceae; few prominent species result in decreased heterogeneity; relatively high pollen concentrations. Overall decrease of Betula pollen and increase of Mean Betula size: 22.24 µm 4000 Salix . Thalictrum alpinumand spores of Selaginella selaginoides reach their local peak of 13 % each. Towards Hekla 4 highest pollen concentrations are reached. Development towards maximum Betula Mean Betula size: 21.23 µm occurrence. Establishment of Cyperaceae as the most dominant taxon, overall decrease of Salix. 5000 Mean Betula size: 23.87 µm Cal. yr. BP yr. Cal. Taxa such as Ranunculus-type, Apiaceae, Lactuaceae, Homogeneous vegetation composition and Rosaceae gain importance; Cyperaceae and with Cyperaceae as the most dominant Betula decrease; very low pollen concentrations and taxon, local maximum in Salix pollen and the most heterogeneous vegetation composition of the strong presence of Betula and Polaceae; 6000 whole research period. minor evidence of Juniperus communis. Mean Betula size: 22.80 µm Other herbs than Cyperaceae clearly dominated by Rumex Acetosa. Establishment of Betula; Juniperus communis and Mean Betula size: 23.22 µm Poaceae remain; Ericales lose their dominance;Salix remains rare; despite some 7000 decrease Cyperaceae keep their position as the most dominant taxon; establishment of Thalictrum Species composition loses heterogeneity. Betula and alpinum; spores of Selaginella selaginoides appear. Cyperaceae become more abundant; Thalictrum Mean Betula size: 19.20 alpinumgains greater importance; Poaceae, Salix and Pteropsida monolete indet. decline.

8000 Dominance of Cyperaceae, Betula, Salix and Poaceae; noteworthy abundance of Apiaceae, Rosaceae and Diversification of species composition; Poaceae Ranunculus-type as well as spores of Selaginella decrease; Salix nearly vanishes; Cyperaceae increase; selaginoides and Pteropsida monolete indet. Rosaceae gain importance; new taxa appear, Mean Betula size: 22.50 µm particularly Juniperus communis and Ranunculus 9000 acris-type.; Galium and Betula pollen recorded at minor values. Mean Betula size: 21.67 µm Salix, Poaceae, Ericales and Cyperaceae dominant; minority of 2 % consists of herbs such as Rosaceae, Apiaceae, Polygonum aviculare, Caltha palustris, Thalictrum alpinum and Caryophyllaceae; Betula and Juniperus communis still absent. 10000

35 re re ed nd del and the natu nted by Hurlbert´s index. A higher index indicates increas

s. layer ollen percentage diagram for Torfdalsmýri. Dots signify percentages <1 %. Mean species richness per sample site a d by rarefaction analysis (n=300). Species evenness is represe rized p fferent ee e.g. Hurlbert, 1971; Olszewski, 2004). The lithology comlumn shows key tephra leyers used in the age mo Summa : 3 . 4 was calculate

Figure layer species evenness (s of the peat in the di

36 by ski, was calculated layer

s. layer by Hurlbert´s index. A higher index indicates increased species evenness (see e.g.evenness Hurlbert, 1971; Olszew (see indexincreased indicates species higher Hurlbert´s index. A by e diagram for Tindar. Dots signify percentages <1 %. Mean species richness per sample site and ness is represented ness is ephra leyers used in the age model and the ofnature the peat in the different shows key t ollen percentag ithology comlumn Summarized p : 4 . 4

Figure rarefactionSpecies even analysis (n=300). 2004). The l

d by Hurlbert´s index. A higher index indicates increased shows key tephra leyers used in the age model and the nature the and age model the key tephra leyers used in shows

The lithology comlumn

ert, 1971; Olszewski, 2004). ert, 1971; Olszewski, layers ollen percentage diagram for Hrafnabjörg. Dots signify percentages <1 %. Mean species richness per sample site and

rent e.g. Hurlb zed p by rarefaction analysis (n=300). Species evenness is represente Summari : 5 . 4 was calculated

Figure layer (see species evenness of the peat in the diffe

38 5 Discussion 5.1 The impact of location

5.1.1 Decomposition and nutrient availability

The lower absolute nutrient availability in Torfdalsmýri may be partially caused by the comparatively low pH. Increased acidity is known to adversely affect nutrient availability, and the CEC of organic soils is even more pH dependent than that of mineral soils (Brady and Weil 2014). The CEC of organic matter has proven to increase faster with increasing pH than that of clay (Helling et al. 1964). With respect to SOM, which was highest in Torfdalsmýri, CEC was surprisingly low. But the amount of SOM plays a lesser role than the type and degree of humification. The CEC of organic matter commonly rises as humification progresses (Kolka et al. 2012; Petersen 1980). Degree of humification, estimated using the von Post scale, did not show considerable differences between sample sites. The C:N ratio as a proxy for the degree of decomposition suggests that humification is less advanced in most layers of Torfdalsmýri than the other two sites. This is probably one of the causes underlying the comparatively low CEC. Here, the geographical setting of the sample sites is likely to exert a considerable influence. Increased eolian input in Tindar and Hrafnabjörg, because of the relative proximity to erosion areas around the active volcanic belt, lead to more pronounced minerogeneity and enhanced conditions for decay (Damman 1988; Farrish and Grigal 1988; Thormann et al. 1999). The impact of mineral content on decay rates is supported by changes of the C:N ratio after the human settlement, which were most pronounced in Torfdalsmýri. After the settlement, values decreased rapidly to levels comparable to Tindar and Hrafnabjörg. At the same time, MS increased and SOM decreased sharply. The higher nutrient availability and higher pH in Tindar and Hrafnabjörg are probably a result of the geographic setting and topographic conditions at the sample sites. Increased Ca2+ and Mg2+ content in the top layer of Tindar and Hrafnabjörg, in contrast to the very low values in Torfdalsmýri, reflect the influence of eolian input. Increased Ca2+ and Mg2+ in peatlands close to erosional areas is not surprising. Icelandic rocks are mainly basaltic, which contain relatively high levels of CaO and MgO (Jakobsson et al. 2008). This effect is especially pronounced in Hrafnabjörg, caused by the relative proximity to the active volcanic belt and erosional areas such as Auðkúluheiði (Arnalds et al. 1997; Eddudóttir et al. 2016). The highest Na+ content was reported in Torfdalsmýri, especially in comparison with Hrafnabjörg. This is most likely a result of increased oceanic composition of precipitation due to the proximity of the sample site to the coast (Damman 1988; Gíslason 1993). The main source of Na+ in soils is not the parent material, but marine input in precipitation (Nanzyo et al. 1994). Hrafnabjörg is located farthest from the sea, explaining the considerably lower Na+ content. The pattern of BS within the profile of Torfdalsmýri is striking. The low BS in layers 1, 7 and 8 could be expected as nutrient availability usually decreases with increasing acidity (Brady and Weil 2014) and the pH is generally rather low in Torfdalsmýri. All the more surprising is the extraordinarily high BS in layers 2-6, which greatly exceeded 100 %. At the same time other soil properties, such as pHwater and SOM, were comparable to layers 1, 7 and 8. Torfdalsmýri is the only sample site with CEC exceeding CECpot to that extent in some layers. Only assumptions can be made to explain this unsual pattern. Generally, a higher BS indicates that actual fertility is closer to potential fertility (Hazelton and Murphy

39 2007). Interpretation here needs to take into consideration that the cause of the high BS in the present case is not even partially due to an increase in the absolute availability of nutrients, but solely due to the unusually low CECpot in layers 2-6. Given the extremely high organic matter content in most of layers 2-6, one would rather expect the CECpot to be high as well (Brady and Weil 2014). Here, part of the CEC might be a result of nutrients not bound to exchange sites, but freely available in the soil water. Free salt in Histosols derived from oceanic precipitation can lead to the overestimation of available base cations (van Reeuwijk 2002). Marine influence on peatlands leads to relatively high concentrations of various elements, amongst others Na+, K+, Ca2+ and Mg2+ (Franzén 2006). The adsorbability of Na+ and K+ is comparatively weak and they get easily washed out, whereas other elements, such as Mg2+ and Ca2+, can be accumulated in anaerobic subsurface layers. 5.2 Decomposition rates, species composition and nutrient availability

As stated before, the C:N ratios indicate differences in decomposition rates between sample sites. Decomposition rates are lowest in Torfdalsmýri, as the site revealed the highest C:N ratios with exception of layers 1 and 2. The low decomposition rates are probably partly explaining the comparatively low CECpot in layers 2–6 (Kolka et al. 2012; Petersen 1980). Factors other than decomposition rates may also exert influence on the Histosol’s capacity to bind nutrients. In layers 7 and 8 in Torfdalsmýri the high C:N (29) indicates rather poorly decomposed material. Despite that, CECpot was rather high in those layers, comparable to levels detected in some layers of Tindar and Hrafnabjörg, which revealed lower C:N ratios. Generally, it should be questioned whether C:N ratio alone is suitable as a proxy for decomposition rates. A commonly observed pattern in northern peatlands is a rather steady decline of C:N ratio with depth, reflecting the higher degree of decomposition in older layers than younger (Kuhry and Vitt 1996; Malmer and Holm 1984). In this study, the contrary is true. C:N ratios were either quite stable throughout the whole profile (Hrafnabjörg), oscillating with a tendency towards higher values in the lower parts (Tindar), or revealing a prominent increase with depth (Torfdalsmýri). This pattern implies that the C:N ratio is not only reflecting the degree of decomposition, but also depends on changes in vegetation cover (compare Table 4.4) at the time of peat formation, leading to changes in the original chemical composition of the organic parent material (Hoorens et al. 2003; Hornibrook et al. 2000; Lawson et al. 2014). The original C:N ratio differs between plant species, and so does the decomposability of the plant litter. A study by Wardle et al. (1997) found that the original nitrogen concentration is positively related to decay rates. Many herbaceous plants reveal higher decomposability than woody plants (Hoorens et al. 2003). Phenolic compounds are generally considered to restrict decay rates (Brady and Weil 2014), but some studies have revealed that their effect on decomposition can be both positive and negative (Hättenschwiler and Vitousek 2000; Hoorens et al. 2003). Other studies suggest that, rather than the original chemistry of each species, litter composition exerts a significant influence on decay rates (Anderson and Hetherington 1999; McTiernan et al. 1997; Wardle et al. 1997). It is clear that C:N ratio alone is not a reliable tool to estimate the degree of decomposition. The humification estimated using the von Post scale revealed little differences between the sample sites. Further analysis of the degree of humification of the material, and the type and quality of organic material present, is necessary to allow for conclusions on decomposition rates (Silamikele et al. 2010).

40 5.3 Driving factors behind changes in species composition

Pollen deposits did not indicate considerable differences in species richness despite obvious differences in nutrient availability and CECpot between Torfdalsmýri, on one hand, and Tindar and Hrafnabjörg, on the other. Tindar and Hrafnabjörg are overall more minerogenous, reflected in lower SOM and/or higher MS and DBD. They also revealed higher content of extractable P in most layers. This supports the findings of other studies on northern peatlands that species richness does not vary remarkably between nutrient poorer and nutrient richer sites (Vitt et al. 1995). Evenness of detected pollen taxa was greatest in Torfdalsmýri, resulting in higher species diversity, and therefore a more heterogeneous species composition. Pollen concentrations in Torfdalsmýri also exceeded those detected in Hrafnabörg and Tindar by far, likely indicating denser vegetation cover. This supports the findings of other studies that revealed optimal growth conditions and a peak in plant species diversity not at highest, but rather at intermediate fertility levels (Huston 1979; Tilman 1983). This pattern is attributed to the predominance of only few species in nutrient rich soils, which is strongly reflected in Tindar and Hrafnabjörg. Factors other than nutrient availability also exert a great influence on vegetation patterns. Habitat diversity has widely been detected to be a major driver of species diversity (Vitt et al. 1995; Vivian-Smith 1997). Microhabitat diversity, including for instance hummocks, tree bases and adjacent open lawns, pools etc. (Vitt et al. 1995), was possibly more pronounced during the past in Torfdalsmýri than the other two sites. 5.4 Impact of climate and vegetation changes on peat properties – pre-settlement development

The first centuries after deposition of Saksunarvatn tephra are, in this study, covered only by Torfdalsmýri. Taxa characteristic for open ground vegetation (Caseldine et al. 2006) are common, and the picture of a species-poor environment is drawn. Pollen concentration is comparatively low and soil properties are indicative of soil development in its early stages. Base cation availability and base saturation is low. Most characteristic of the soil morphology at this stage is the absence of roots in the layer, and the structureless nature of the material, consisting of an unconsolidated mass (compare layer 8 in Torfdalsmýri). The physical soil properties are characterized by high DBD, high MS, and low wd and SOM. This is probably caused by intermixing of tephra from the layer below and is typical for an unstable, recently deglaciated environment with sparse vegetation cover and considerable eolian deposition (Larsen et al. 2012). Indicative of increased tephra content in the soil is also the comparatively high pHNaF (Blakemore et al. 1987; USDA 2014) and low N and C content (Axford et al. 2009). The high Salix percentages are most likely from Salix herbaceae, a typical species of nutrient poor and/or unstable minerogenic soils in periglacial environments (Beerling 1998). Even though the cores of Tindar and Hrafnabjörg do not cover the centuries after Saksunarvatn tephra, previous studies in the region conducted by Eddudóttir et al. (2015) and Eddudóttir et al. (2016) indicate that regions inland and at the highland fringe were already at least partly vegetated at the time of the occurrence of Saksunarvatn tephra. Vegetation around palaeolake Kagaðarhóll, located approximately 2.5 km northwest of Tindar, is indicative of rather warm summer temperatures, reflected in the establishment of dwarf shrubs such as Betula nana and Juniperus communis (Eddudóttir et

41 al. 2015). Vegetation records from lake Barðalækjartjörn at the highland margin, approximately 7 km southeast of Hrafnabjörg, show relatively low occurrence of pioneer species, whereas dwarf shrubs such as Salix spp., Betula nana and Juniperus communis are already present (Eddudóttir et al. 2016). Despite the lower occurrence of open ground taxa, there is also evidence of partly barren surfaces and unstable environmental conditions, reflected in rather high MS and DBD of the lake sediments, both at the inland site and at the highland fringe (Eddudóttir et al. 2015; 2016). During the following centuries (layer 7 in Torfdalsmýri; 9.2-8.4 ka BP) environmental stabilization is indicated in Torfdalsmýri, both by vegetation development and soil properties. Open ground taxa decrease, and Betula and Juniperus communis appear. The relative peak of Juniperus communis pollen and the occurrence of Betula towards 9.2 ka BP conform to the findings of previous studies in North Iceland (Björck et al. 1992; Caseldine et al. 2006). The local Juniperus communis maximum of 3.5 % may appear comparatively low, but it needs to be taken into consideration that Juniperus communis tends to be heavily underrepresented in pollen deposits, in contrary to overrepresentation of taxa such as Poaceae, Cyperaceae and Betula (Schofield et al. 2007). Overall, species composition is rather heterogeneous, which is indicative of little competition (Hallsdóttir 1990). Herbs such as Rosaceae and Ranunculus spp. gain more dominance. The vegetation changes are also reflected in ameliorated soil chemical and physical properties (Beerling 1998). The soil properties indicate environmental stabilization: minimal eolian influx, increased vegetation cover, advanced morphological development, preserved roots, and a platy soil structure. SOM and wd are high, contrary to low DBD and MS. Decreased input of eolian material and reworking of tephra is reflected in pHNaF, which has decreased sharply. The same accounts for pHwater (4.3), which is likely a result of both decreased mineral content and increased SOM. BS is, with 33 %, only slightly lower than in the preceding layer, but CEC has increased and so has CECpot. Vegetation records and soil properties are indicative of ameliorated climatic conditions and ongoing environmental stabilization over the following millennia, until at least 7.0 ka BP in Hrafnabjörg, 6.0 ka BP in Torfdalsmýri and 4.25 ka BP in Tindar. Low pHNaF indicates little eolian deposition. Betula becomes established and Cyperaceae, Salix and Poaceae percentages decline (Hallsdóttir 1990). Main vegetation differences between the sample sites are most likely due to the different geographic settings. Torfdalsmýri, close to the sea and not surrounded by sheltering mountains, is more prone to the impact of adverse weather conditions while the lowland site, Tindar, is the most sheltered. Present dispersal of Betula pubescens suggests that the species grows predominantly in more sheltered areas (Elkington 1968), whereas Betula nana is adapted to more exposed settings. Size distribution of measurable Betula pollen indicate the dominance of Betula nana in Torfdalsmýri, also supported by the higher predominance of the light demanding Juniperus communis (Grubb et al. 1996; Hallsdóttir 1990). These results conform to the study by Eddudóttir et al. (2015) at palaeolake Kagaðarhóll, that reports a decrease in Juniperus communis pollen frequency as Betula pubescens-sized pollen occurrence increases. Pollen of the size of Betula pubescens are most common in Tindar, but also frequent in Hrafnabjörg. The content of woody material is very high in some layers of Tindar, expressed in layers of woody peat. In Hrafnabjörg a scenario of open birch-woodland and nutrient poor soils is likely. Growth conditions for Betula were apparently good, but not optimal as indicated by high percentages of Selaginella selaginoides spores, which tend to decline with increasing temperature and nutrient availability (Klanderud 2008). Environmental degradation is first indicated in Hrafnabjörg between 7.00 and 4.25 ka BP, where pollen concentration decreases sharply. The reduced vegetation cover facilitates the development of the highest locally detected species diversity, as indicated by

42 maximum species richness and a high Hurlbert’s index (0.86). Herbs that could not reach noteworthy amounts under conditions of more dense vegetation cover become more abundant (Hallsdóttir 1990). Despite sparser vegetation cover and reduced biomass, soil properties show resilience towards adverse changes; SOM even increases, and MS decreases. It is not before the deposition of Hekla 4 tephra that environmental conditions have some adverse impact on soil properties. Above Hekla 4, Cyperaceae percentages rise sharply, likely a result of a wetter and cooler climate, leading to reduced evaporation and the expansion of mires (Hallsdóttir 1990). Increasing DBD and MS and decreasing SOM are indicative of increased eolian input. This development conforms to the results of Eddudóttir et al. (2016) at the nearby lake Barðalækjartjörn. A stable environment is reflected in the resilience of sediment properties, despite the opening up of the woodlands between c. 6.7 and 4.0 ka BP. After the deposition of Hekla 4, environmental destabilization is indicated by a rise of MS and DBD and a drop in SOM. In Torfdalsmýri, climatic conditions for birch growth deteriorate between 6.0 and 3.06 ka BP, leading to a relative decline in Betula pollen and the spread of Thalictrum alpinum and Salix. This, together with the relative increase of Selaginella selaginoides between c. 5.5 and 4.2 ka BP, allows the assumption that woodlands opened up, leading to increasingly nutrient-poor soils (Beerling 1998; Eddudóttir et al. 2016; Klanderud 2008). Even though present chemical composition of the soil does not exactly reflect conditions in the past, it can provide insight to the past. The nutrient pattern drawn in Torfdalsmýri is very peculiar and only assumptions can be made about possible causes. Even though present BS is very high, CECpot is very low. It is likely that very little exchangable cations were present in the past, especially based on the assumption that a great part of the measured available nutrients is due to freely available elements in the present soil water. Soil physical properties are also indicative of a disturbed environment. A sudden and sharp increase in DBD and decrease in SOM in layer 5 (5.5–5.3 ka BP) is accompanied by high pHNaF and the structureless nature of the material. Increased input of detrital particles from sources outside the peatland are demonstrated. Enhanced erosion, induced by reduced vegetation cover or the occurrence of a single or few sudden extreme weather events associated with flooding, and the unique deposition of fine-grained detrital material, might be the cause. Despite these very sudden and adverse changes, the soil recovers quickly. The disturbed layer is very thin (2 cm), and in layer 4 and 3 (5.3–3.06 ka BP), restabilization of soil morphological and physical properties is widely achieved, despite the Hekla 4 tephra fall event separating the two layers. Vegetation cover also restabilizes in layer 4, indicated by local maximum pollen concentration. Between Hekla 3 and Hekla 1104 tephras, pollen records and soil properties draw a picture of deteriorating environmental conditions. In Torfdalsmýri, a prominent drop in pollen concentrations and the comeback of open ground taxa such as Ericales and Salix indicate less dense vegetation cover than the preceding millennia. Mean MS increases and SOM decreases. In Hrafnabjörg and Tindar, Salix and Ericales remained a minority and Cyperaceae are more dominant, indicating wetter and cooler climatic conditions (Hallsdóttir 1990). In Tindar, the high occurrence of Sphagnum spores is a further indicator of wetter and colder climate. Many species of Sphagnum either prefer hummocks or hollows and pools as their habitat, and the development of these peatland features is enhanced by wetter climate (Clymo 1987). Despite the strong presence of Cyperaceae, there is also evidence of Betula pollen in Hrafnabjörg, where the local maximum is reached. Soil properties at Hrafnabjörg draw a contradictive picture. Stable SOM and comparatively low mean MS can be interpreted as a result of dense vegetation cover, but increased SAR provides a picture of environmental degradation associated with increased eolian input (Gísladóttir et al. 2010).

43 In Tindar, increased DBD and MS and a simultaeneous decrease in SOM clearly point towards increased eolian input caused by environmental destabilization. Overall, the pollen record and development of soil properties before the settlement indicate, that environmental conditions driven by climate changes caused both vegetation changes and changes in soil properties. Despite that, the Histosols clearly show resilience towards adverse impact and severe soil degradation and have the ability to recover from periods of decreased vegetation cover and increased eolian input. 5.5 Post-settlement development of vegetation and peat properties

The centuries after the settlement in AD 870 are characterized by the clear predominance of Cyperaceae and Poaceae and an overall decrease of Betula, reflecting the eradication of woodlands (Eddudóttir et al. 2016) and consequent spread of peatlands caused by human activities (Hallsdóttir 1987; Hallsdóttir and Caseldine 2005). It is possible that this development was not equally rapid at all sites. In Hrafnabjörg, records of Betula pollen declines rapidly between AD 1104 and AD 1310, but regains strength between c. AD 1310 and AD 1766 (layer 2). In Torfdalsmýri the decline from 10.7 % to 8.3 % is not as prominent as expected. There are different explanations for this pattern. Hrafnabjörg was possibly not used extensively as pasture earlier than the late 18th/early 19th century. Evidence of coprophilous fungi, generally associated with the dung of grazing animals (van Geel et al. 2003), is sparse c. AD 1310–1766, but increases sharply after AD 1766 until present. The high occurrence of Betula pollen in bogs after the settlement might be a result of pollen reworking by erosional processes (Erlendsson 2007; Erlendsson et al. 2009; Gísladóttir et al. 2010; Lawson et al. 2007). The decline in soil quality in Hrafnabjörg is particularly prominent c. AD 1310-1766, with increased DBD, decreased wd and SOM. In Torfdalsmýri, a decline in soil quality is predominantly reflected in decreased SOM, but the strongest indicator of pollen reworking at the site is the high occurrence of Pteropsida monolete indet. spores, a common effect of reworking (Gathorne-Hardy et al. 2009; Lawson et al. 2007). Last, production and dispersal of pollen might actually have been enhanced as a result of thinning out of the woodlands, as dense woodlands can have an impeding impact on dispersal (Lawson et al. 2007). Decline in soil quality as a result of environmental degradation is also reflected in soil properties. Mean MS increased at all sample sites, due to increased eolian input following vegetation cover disturbances and soil erosion. In Torfdalsmýri, SOM decreased considerably, perhaps also causing the noticeable decrease in CEC and low BS. Changes in SAR and CSQ vary between sample sites. In Torfdalsmýri SAR was constantly low and there are no additional indicators of major environmental changes due to human disturbance. At the same time, CSQ decreased. The ocean proximity and the greater distance of Torfdalsmýri from large erosion areas and eolian sources around the active volcanic belt might explain why the site did not face an increase in SAR and CSQ after the settlement. In Tindar, SAR remained rather stable until Hekla 1766, but underwent more than a fourfold increase between Hekla 1766 and present. This was accompanied by a sharp increase in CSQ, indicating a rather recent aggravation of soil degradation. According to Gísladóttir et al. (2010) land use and deteriorating climatic conditions after the settlement led to the loss of soil and soil organic carbon from sparsely vegetated areas. At the same time, vegetated areas were facing influx of sediments and rising CSQ. Wetland areas are known to function as a trap for aeolian material, and the increasing SAR in Tindar is a strong indicator of increasing aeolian input from enhanced soil erosion in the surrounding areas (Dugmore et

44 al. 2009; Gísladóttir et al. 2010). In Hrafnabjörg, SAR and CSQ remained around the increased levels detected above Hekla 3, possibly due to the closer proximity of Hrafnabjörg to eolian sources in the highlands (Eddudóttir et al. 2016). This indicates that soil deterioration already started before the human settlement at the site, and is consistent with the results of Eddudóttir et al. (2016) that the environment close to the highland margin could not resist adverse changes caused by unstable conditions after the Hekla 4 eruption. Overall there is a strong positive relationship between SAR and CSQ (r²=0.81) and it is likely that the increased CSQ in Tindar and Hrafnabjörg is a result of increased SAR caused by influx of minerogenic material.

6 Conclusions

1. The results of this study support the hypothesis that minerogenic content in peatlands has a positive impact on nutrient content and decomposition of organic material in Histosols. The samples with more pronounced minerogeneity revealed higher nutrient availability and a higher decomposition state as indicated by lower C:N ratios. Those sites are located in closer proximity to potential source areas of minerogenic eolian material, proving that the geographic location of peatlands has a great impact on their soil properties. Enhanced eolian input of material of volcanic origin leads to increased Ca2+ and Mg2+ content. Peatlands at greater distances from eolian source areas, but in closer proximity to the sea, reveal elevated Na+ caused by the oceanic composition of precipitation. Oceanity may also increase the content of nutrients freely available in the soil water and can lead to overestimation of available base cations. It needs to be emphasized, though, that there is evidence that C:N ratios as a proxy for decomposition rates should be interpreted critically, especially in soils with a heterogeneous vegetation history. Besides factors such as temperature, hydrology and microbial activity vegetation changes exert a great influence on decomposition of organic material. 2. The pollen record provides evidence of better growth conditions at intermediate fertility levels. This supports the hypothesis that the nutrient state of the wetlands exerts influence on plant species composition. Overall, species richness and species evenness were greater in Torfdalsmýri, the site with the lowest nutrient content, than Tindar and Hrafnabjörg. Though the differences in species richness and species evenness were not great between the sites, there is a tendency towards higher species diversity and a more heterogeneous species composition in Torfdalsmýri. In the more nutrient rich soils of Tindar and Hrafnabjörg only a few competitive species dominate. Greater species diversity and species evenness in Torfdalsmýri might not only be connected to lower nutrient content, but also to a more diverse microtopography than at the other two sites. 3. The sites with lower C:N ratios revealed an enhanced ability to bind nutrients. If those differences in C:N ratios between sample sites are referred to as differences in decomposition rates the hypothesis that a greater state of decomposition has a positive effect on the ability of the material to bind nutrients is supported. The sample sites that were closer to source areas of minerogenic influx revealed lower C:N ratios and higher nutrient content. The pattern of change in C:N ratios within sample sites indicates, however, that decomposition rates derived from C:N ratios need to be interpreted with care. Within samples sites, C:N ratios either increased with depth or were rather stable throughout the profiles, contrary to declining values frequently observed in Histosols.

45 Supported by the pollen data and the changing lithologic character of the profiles, it may be inferred that the type of vegetation contributing to the parent material at the time of Histosol formation affects decomposition. Evidently, C:N ratio alone is not a reliable indicator of decomposition rates, but highly depends on the chemical composition of the organic material. 4. The pollen record provides evidence that environmental conditions driven by climate changes caused some changes in vegetation and soil properties before the settlement. Peatlands close to the highlands, especially, suffered from unstable environmental conditions during the last two millenia before the settlement. But overall, the Histosols showed resilience towards adverse impact and severe degradation brought about by climatic factors only. After the settlement they struggled to buffer the severe impact caused by human-induced destruction of vegetation and enhanced erosion.

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53 Appendices

Appendix A: Key tephra layers in the profiles and their geochemical composition.

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

Torfdalsmýri 2-3 cm, Hekla 1766 58.84 1.47 14.94 10.16 0.28 2.13 5.58 3.17 1.52 0.66 98.74 58.81 1.47 15.28 10.06 0.27 2.10 5.67 3.44 1.46 0.66 99.22 58.42 1.45 15.24 10.31 0.25 2.19 5.83 3.28 1.43 0.68 99.09 59.01 1.42 15.22 9.98 0.25 2.05 5.55 3.66 1.48 0.64 99.26 59.01 1.40 15.28 9.94 0.25 2.06 5.67 3.48 1.48 0.59 99.17 61.03 1.17 15.51 9.54 0.25 1.61 5.10 3.36 1.58 0.46 99.61 58.56 1.46 15.16 10.31 0.25 2.11 5.82 3.39 1.46 0.68 99.20 60.64 1.24 15.39 9.78 0.23 1.65 5.19 3.42 1.62 0.49 99.66 60.75 1.21 15.37 8.82 0.25 1.59 5.06 3.54 1.65 0.41 98.65 61.73 1.15 15.53 8.64 0.24 1.48 4.84 3.55 1.71 0.46 99.33

Torfdalsmýri 7-8 cm, H1 71.23 0.22 14.11 2.94 0.14 0.08 1.92 3.99 2.42 0.06 97.11 71.19 0.15 13.82 3.31 0.11 0.12 1.75 4.65 2.67 0.03 97.79 71.78 0.22 13.86 3.58 0.13 0.13 1.65 4.50 2.65 0.07 98.56 71.28 0.20 13.48 3.37 0.11 0.11 1.64 4.64 2.75 0.11 97.70 71.13 0.25 14.29 3.33 0.14 0.10 1.80 3.57 2.69 0.03 97.33 71.37 0.22 13.95 3.39 0.09 0.11 1.63 3.65 2.72 0.00 97.14

Torfdalsmýri 27-28 cm, H3 64.55 0.54 15.23 7.14 0.22 0.64 3.85 4.23 1.83 0.16 98.38 63.92 0.60 15.12 7.59 0.21 0.68 3.71 3.34 1.81 0.18 97.16 71.86 0.26 14.34 3.35 0.10 0.13 1.85 4.25 2.51 0.02 98.67 61.50 0.76 15.51 8.86 0.29 0.99 4.69 4.34 1.63 0.24 98.82 63.39 0.45 14.36 6.17 0.23 0.53 3.47 2.98 1.88 0.14 93.60 71.33 0.22 14.29 3.08 0.09 0.12 1.87 4.61 2.44 0.06 98.10 66.97 0.34 14.92 5.66 0.20 0.33 3.30 4.52 2.01 0.14 98.38 66.93 0.37 14.95 5.65 0.17 0.33 3.06 3.14 2.15 0.08 96.84 62.22 0.67 15.01 8.04 0.22 0.87 4.31 3.39 1.76 0.24 96.73 65.55 0.60 15.03 7.04 0.15 0.54 3.61 4.20 1.95 0.17 98.84

Torfdalsmýri 41-41 cm, H4 74.10 0.05 13.04 1.98 0.09 0.03 1.24 3.12 2.90 0.06 96.61 74.47 0.10 13.23 2.02 0.09 0.00 1.21 4.31 2.81 0.01 98.25 73.70 0.12 12.96 2.03 0.07 0.02 1.27 3.37 2.77 0.02 96.34 74.36 0.12 13.48 2.06 0.14 0.03 1.28 4.16 2.79 0.03 98.45 70.55 0.07 16.88 1.34 0.06 0.01 2.17 5.86 1.82 0.04 98.80 73.14 0.09 13.01 1.99 0.08 0.01 1.24 4.13 2.74 0.00 96.43

Torfdalsmýri 93-94 cm, Saksunarvatn 50.35 3.04 13.22 14.93 0.26 5.72 10.05 2.55 0.55 0.32 100.99 49.55 3.03 13.14 14.31 0.21 5.70 9.85 2.80 0.40 0.33 99.33 50.27 3.05 13.29 14.84 0.26 5.60 9.96 2.34 0.43 0.34 100.37 49.59 2.97 13.14 14.28 0.22 5.59 9.93 2.82 0.41 0.35 99.30 49.76 3.03 13.31 14.21 0.22 5.58 10.01 2.90 0.43 0.34 99.79 49.70 2.85 13.39 13.72 0.24 5.73 10.05 2.56 0.40 0.32 98.96 49.38 2.85 12.87 14.76 0.26 5.61 9.99 2.71 0.43 0.32 99.18 49.78 3.02 13.02 14.12 0.23 5.50 9.99 2.69 0.44 0.35 99.14 49.32 3.02 12.84 14.39 0.24 5.48 9.99 2.73 0.43 0.29 98.73 49.48 3.00 13.11 14.46 0.21 5.44 9.90 2.79 0.42 0.36 99.17

54 SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

Tindar, 13-13.5 cm, Hekla 1766 60.50 1.29 15.57 9.29 0.24 1.72 5.13 3.46 1.60 0.53 99.33 60.37 1.27 15.39 9.79 0.27 1.82 5.28 3.17 1.60 0.46 99.42 60.59 1.18 15.45 9.15 0.25 1.61 4.83 3.35 1.60 0.39 98.41 60.93 1.17 15.38 9.56 0.26 1.59 5.14 3.64 1.68 0.41 99.76 58.68 1.46 15.43 9.91 0.26 2.05 5.77 3.43 1.49 0.59 99.07 61.07 1.21 15.57 9.36 0.22 1.56 5.04 3.66 1.65 0.46 99.80 58.97 1.52 15.37 9.54 0.23 2.12 5.64 3.66 1.50 0.68 99.23 58.51 1.47 15.45 10.19 0.26 2.19 5.82 3.34 1.46 0.69 99.38 61.13 1.16 15.56 9.30 0.29 1.54 5.06 3.55 1.65 0.43 99.67 60.96 1.16 15.39 9.49 0.30 1.51 5.04 3.60 1.65 0.43 99.52 58.19 1.51 15.28 10.27 0.25 2.13 5.84 3.58 1.48 0.63 99.16 60.75 1.20 15.26 9.44 0.26 1.57 5.21 3.14 1.63 0.39 98.85

Tindar, 21-21.5 cm, Hekla 1104 72.18 0.19 14.15 3.19 0.09 0.10 1.96 4.89 2.32 0.05 99.12 71.39 0.21 14.13 3.29 0.09 0.10 1.83 3.74 2.68 0.03 97.50 71.51 0.18 14.25 3.42 0.13 0.11 1.88 4.73 2.68 0.08 98.97 71.57 0.24 14.16 3.31 0.14 0.11 1.82 4.57 2.69 0.05 98.65 72.00 0.21 14.13 3.33 0.09 0.11 1.78 3.66 2.75 0.00 98.07 71.60 0.23 13.98 3.36 0.11 0.12 1.82 4.67 2.65 0.02 98.56 72.16 0.25 14.22 3.51 0.09 0.12 1.81 4.29 2.78 0.00 99.23 71.12 0.19 14.02 3.26 0.08 0.11 1.88 4.64 2.70 0.03 98.03

Tindar, 43-43.5 cm, H3 71.74 0.19 14.23 3.10 0.14 0.09 1.97 4.54 2.54 0.01 98.55 71.50 0.16 14.33 3.21 0.10 0.12 1.90 4.67 2.64 0.04 98.67 71.11 0.21 13.92 3.32 0.13 0.14 1.88 4.71 2.39 0.03 97.83 67.60 0.44 14.91 5.75 0.15 0.37 2.98 3.07 2.19 0.12 97.58 71.72 0.28 14.00 3.21 0.15 0.12 1.92 4.61 2.53 0.00 98.53 67.26 0.40 15.04 5.53 0.17 0.36 2.93 4.48 2.02 0.15 98.34 64.59 0.50 15.16 7.34 0.16 0.61 3.78 4.29 1.83 0.16 98.43 70.74 0.20 13.97 3.26 0.10 0.12 1.96 4.77 2.46 0.00 97.58

Tindar, 68.5-69.5 cm, H4 73.35 0.10 13.14 2.05 0.07 0.02 1.34 4.46 2.91 0.05 97.49 73.79 0.16 13.28 2.15 0.09 0.01 1.33 4.74 2.89 0.04 98.47 72.91 0.20 13.66 2.64 0.11 0.02 1.59 4.31 2.67 0.06 98.17 73.92 0.11 13.30 2.07 0.05 0.00 1.15 3.55 2.81 0.01 96.98 73.99 0.09 13.20 2.11 0.11 0.02 1.23 4.50 2.89 0.01 98.15 73.74 0.13 13.26 2.04 0.02 0.01 1.29 3.72 2.73 0.05 96.99 74.12 0.09 13.12 2.02 0.06 0.01 1.25 3.81 2.87 0.04 97.39

Tindar, 79-79.5 cm, Ssn 63.22 0.75 16.03 6.43 0.21 0.83 3.00 3.65 3.28 0.21 97.61 66.67 0.36 15.66 4.28 0.15 0.27 1.64 4.63 4.25 0.05 97.95 62.14 1.00 16.01 6.66 0.19 1.03 3.48 3.95 3.09 0.23 97.78 61.16 0.90 16.13 6.84 0.22 1.09 3.37 3.73 3.09 0.25 96.78 66.34 0.45 15.99 4.04 0.14 0.27 1.70 3.93 4.24 0.05 97.15 67.45 0.45 15.92 4.18 0.15 0.30 1.61 4.51 4.24 0.03 98.84 66.46 0.38 15.80 4.19 0.15 0.30 1.58 4.12 4.24 0.05 97.27 66.76 0.32 16.07 4.19 0.15 0.29 1.65 4.57 4.06 0.09 98.15 66.58 0.35 15.59 4.13 0.16 0.26 1.66 4.21 3.97 0.08 96.98 63.09 0.87 16.33 6.35 0.21 0.93 3.38 4.99 3.13 0.20 99.47 65.29 0.60 16.20 5.56 0.19 0.54 2.42 4.84 3.63 0.10 99.37 66.18 0.36 15.88 4.25 0.14 0.25 1.66 3.90 4.20 0.07 96.90 61.98 0.83 15.98 6.53 0.20 1.03 3.31 4.58 3.14 0.25 97.82 66.55 0.31 15.85 3.80 0.12 0.26 1.57 3.93 4.24 0.02 96.65 65.14 0.59 16.30 4.94 0.23 0.45 2.04 5.02 3.75 0.11 98.56 67.16 0.38 15.96 3.96 0.19 0.24 1.70 3.91 4.12 0.08 97.71 65.22 0.41 15.66 4.05 0.14 0.27 1.60 4.58 4.19 0.05 96.18 66.20 0.34 15.31 4.05 0.17 0.27 1.43 4.44 4.41 0.03 96.65 63.72 0.72 16.19 5.88 0.24 0.79 2.96 4.82 3.24 0.18 98.73

55 SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

Tindar, 117-118 cm, Katla S-layer 48.13 4.17 13.15 15.01 0.23 4.97 9.86 3.27 0.81 0.50 100.10 48.12 4.27 13.33 14.64 0.20 5.07 9.79 3.25 0.81 0.46 99.94 47.79 4.01 13.07 14.56 0.22 5.15 10.01 3.13 0.76 0.40 99.10 48.01 4.20 13.29 14.86 0.22 5.08 9.87 3.14 0.83 0.51 100.01 48.29 4.07 13.21 14.78 0.22 4.78 9.74 3.07 0.84 0.46 99.47 47.49 4.21 13.06 14.74 0.21 5.04 9.75 3.07 0.77 0.46 98.81 47.64 4.14 13.04 15.01 0.22 5.02 9.60 3.09 0.78 0.46 98.99 48.23 4.32 13.16 15.04 0.23 5.01 9.93 3.07 0.77 0.51 100.27 48.11 4.12 13.08 14.70 0.24 4.94 9.88 3.00 0.81 0.48 99.35 48.13 4.04 13.20 14.84 0.20 5.10 9.83 3.02 0.83 0.50 99.69 47.90 4.23 13.01 14.72 0.20 5.02 9.78 3.20 0.79 0.45 99.30 48.05 4.01 13.06 14.57 0.21 5.15 9.60 3.27 0.78 0.49 99.18

56 SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

Hrafnabjörg, 4-6.5 cm, Hekla 1766 58.44 1.41 15.42 10.29 0.27 2.10 5.94 3.47 1.41 0.63 99.38 59.83 1.32 15.30 9.69 0.26 1.83 5.46 3.52 1.57 0.52 99.30 58.53 1.46 15.25 10.10 0.27 2.11 5.88 3.51 1.46 0.66 99.23 58.83 1.52 15.60 9.92 0.22 2.08 5.76 3.42 1.50 0.67 99.52 59.66 1.36 15.48 10.09 0.26 1.86 5.25 3.42 1.52 0.54 99.44 61.00 1.20 15.70 9.41 0.27 1.55 5.15 3.59 1.62 0.50 99.99 61.76 1.17 15.56 8.79 0.21 1.40 4.68 3.41 1.75 0.42 99.14 60.97 1.16 15.31 9.42 0.22 1.59 5.12 3.34 1.54 0.49 99.16 58.92 1.46 15.33 10.16 0.25 2.03 5.52 3.57 1.52 0.64 99.40 58.46 1.47 15.04 10.32 0.27 2.18 5.67 3.39 1.48 0.68 98.97

Hrafnabjörg, 12.5-13 cm, Hekla 1104 72.11 0.21 14.35 3.33 0.15 0.12 1.87 4.56 2.68 0.00 99.38 71.14 0.23 14.80 3.42 0.11 0.11 1.89 4.11 2.71 0.03 98.55 71.87 0.22 14.18 3.32 0.08 0.10 1.83 2.98 2.60 0.02 97.20 71.54 0.19 14.30 3.24 0.15 0.10 1.76 4.25 2.62 0.00 98.15 72.06 0.23 14.05 3.29 0.12 0.10 1.98 4.85 2.41 0.01 99.11 71.43 0.20 14.02 3.40 0.11 0.11 1.86 3.56 2.74 0.02 97.45 72.03 0.21 14.29 3.36 0.10 0.10 1.72 4.51 2.74 0.05 99.11 63.15 0.07 22.78 0.75 0.05 0.05 5.65 7.62 0.57 0.06 100.75

Hrafnabjörg, 41.5-42.5 cm, H3 74.36 0.13 12.74 1.95 0.04 0.04 1.23 3.84 2.74 0.04 97.10 73.77 0.08 13.23 2.11 0.13 0.03 1.28 4.63 2.74 0.00 98.01 72.58 0.13 12.86 2.11 0.06 0.02 1.25 3.73 2.83 0.07 95.65 74.61 0.11 13.39 2.15 0.07 0.02 1.22 4.21 2.83 0.00 98.62 71.50 0.20 14.34 3.22 0.11 0.14 1.84 4.06 2.48 0.00 97.89 74.30 0.11 13.25 2.21 0.08 0.03 1.23 4.24 2.95 0.00 98.40 72.92 0.17 13.78 2.62 0.13 0.02 1.45 3.29 2.75 0.00 97.12 69.66 0.28 14.43 4.07 0.13 0.15 2.37 4.66 2.33 0.06 98.14 73.92 0.15 13.18 2.18 0.06 0.01 1.18 4.62 2.85 0.00 98.15

Hrafnabjörg, 49-50 cm, H4 73.78 0.12 13.20 2.06 0.09 0.01 1.21 4.29 2.81 0.01 97.58 73.07 0.11 13.17 2.00 0.08 0.02 1.15 3.94 2.70 0.01 96.25 73.04 0.10 13.14 2.09 0.03 0.03 1.17 4.62 2.78 0.02 97.03 74.13 0.11 13.26 2.01 0.10 0.02 1.17 3.33 2.83 0.00 96.96 72.62 0.11 12.95 2.04 0.07 0.01 1.20 4.07 2.77 0.02 95.87 74.31 0.16 13.35 2.11 0.10 0.01 1.29 4.30 2.87 0.02 98.53 74.21 0.07 12.66 1.94 0.07 0.05 1.24 3.86 2.68 0.00 96.78 74.62 0.11 13.30 2.04 0.04 0.01 1.26 4.22 2.89 0.00 98.49 73.33 0.15 13.39 2.04 0.07 0.03 1.31 4.30 2.83 0.00 97.46

Hrafnabjörg, 64.5-65.5 cm, H5 73.04 0.11 13.05 2.19 0.09 0.03 1.28 3.76 2.73 0.03 96.31 64.82 0.51 15.68 4.97 0.22 0.40 1.99 4.40 3.84 0.11 96.93 73.69 0.05 12.86 2.03 0.10 0.03 1.22 4.24 2.52 0.00 96.74 73.46 0.12 13.48 2.07 0.09 0.03 1.27 3.21 2.83 0.00 96.56 74.48 0.07 13.59 2.17 0.05 0.02 1.30 4.49 2.83 0.00 99.00 74.13 0.13 13.31 1.97 0.09 0.02 1.23 4.51 2.93 0.02 98.34 74.32 0.08 13.32 2.05 0.11 0.01 1.20 4.46 2.89 0.04 98.49

57 e ncreased del and th r sample site and layer ndex indicates i used in the age mo x. A higher i cies richness pe urlbert´s inde umn shows key tephra leyers percentages <1 %. Mean spe evenness is represented by H ri. Dots signify ski, 2004).ski, The lithology coml =300). Species

tage for diagram Torfdalsmý the different layers. e e.g. Hurlbert, 1971; Olszew ed by rarefaction analysis (n Pollen percen : 2 Appendix was calculat species evenness (se nature of the peat in

58 refaction y ra y 1971; Olszewski, alculated b alculated

s. nness (see e.g. Hurlbert, eve ferent layer er sample site and layer was c was layer and site sample er ecies richness p richness ecies e nature of the peat in the dif ex indicates increased species y percentages <1 %. Mean sp Mean %. <1 percentages y used in the age and th model urlbert´s index. A higher ind agram for Tindar. Dots signif Dots Tindar. for agram evenness is represented by H lumn shows key tephra leyers ercentage di ercentage p : Pollen : 3

Appendix analysis (n=300). Species 2004). The lithology com

59 sed rea inc

del and the del and x indicates sample site and layer site sample sed in the age sed in mo s richnesss per rlbert´s index. A higher inde mn shows tephra leyers u mn key ercentages <1 %. Meanercentagesspecie <1 %. venness is represented by Hu ki, 2004). The lithology 2004). The ki, comlu 300). Species e r Hrafnabjörg.Dots signify p

. ge diagram fo ge diagram e.g. Hurlbert, 1971; Olszews the different layers d by rarefaction analysis (n= : Pollen percenta 4 Appendix was calculate evenness(see species nature of the peat in