Mid-Holocene Mineral Dust Deposition in Raised Bogs in Southern Sweden

Meddelanden från Stockholms universitets institution för geologiska vetenskaper 382 Jenny Sjöström Mid-Holocene mineral dust deposition in raised bogs in Mid-Holocene mineral dust deposition in raised Swedenbogs in southern southern Sweden

Processes and links

Jenny Sjöström

Jenny Sjöström I have a BSc in Geography, a MSc in Geography, and a MSc in Quaternary Stratigraphy and Climate Reconstruction from the Department of Physical Geography, Stockholm University. [email protected]

ISBN 978-91-7911-488-6

Department of Geological Sciences

Doctoral Thesis in Marine Geology at Stockholm University, Sweden 2021

Mid-Holocene mineral dust deposition in raised bogs in southern Sweden Processes and links Jenny Sjöström Academic dissertation for the Degree of Doctor of Philosophy in Marine Geology at Stockholm University to be publicly defended on Friday 4 June 2021 at 10.00 in William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14 and online via Zoom, public link is available at the department website.

Abstract Atmospheric mineral dust is a key component of the climate system, which affects insolation, brings nutrients to marine and terrestrial ecosystems, and acts as a cloud condensation nuclei. To reconstruct past patterns in terrestrial dust deposition natural archives may be utilized, such as loess, dunes, lakes, and peat bogs. Bogs became an established dust archive in the early 2000s, and the number of studies has since increased. However, most studies use single records to represent dust deposition, meaning that we have limited understanding of regional paleodust dynamics or about the representativeness of single bog records. This thesis aims to address these uncertainties by comparing paleodust deposition between bogs located on a 65 km transect. The thesis includes a methodological development for organic matter removal from peat samples for XRD mineral analysis (Paper I) and two peat paleodust reconstruction studies (Paper II, III). The first paleodust reconstruction from Draftinge Mosse (mosse translates to bog in English), Småland, showed that four dust events (DE) were recorded during the ombrotrophic stage (Paper II). These results were compared to a previously conducted study on Store Mosse, 20 km northeast of Draftinge Mosse, which showed similar patterns in DE and peat accumulation rate (PAR), indicating that the events were at least regional in character. However, the magnitude of the DE differed, which was related to differences in the sizes of the two bogs. The second paleodust reconstruction, from (Davidsmosse) located c. 25 km from the west coast, recorded many more DE (14) compared to the more inland sites (Paper III). Two longer periods saw numerous DE, dominated by coarse particles: between 2800 and 2130 cal BP, and from 1000 towards 490 cal BP. These two periods occurred during regionally cold periods. Human activities also intensified during the latter period, possibly amplifying the DE. Most of these episodic events were not recorded at the inland sites, and the Davidsmosse record seemed to be more in line with previously constructed coastal paleostorm records. That the bog located closer to the coast recorded many more events compared to the inland sites suggests that the location of a bog will influence the aeolian events recorded. However, the DE observed at the inland sites were also recorded at Davidsmosse, indicating that the inland events might represent winds that were sustained over longer distance, or alternatively, that regionally dry conditions prevailed during these periods. The paleostorm records from south-western Sweden, including the new results from Davidsmosse presented here, suggest that storm intensities have varied during the last 3000 years, with increased storminess frequency coupled to colder episodes related to extended sea ice and a southward shift of storm tracks. When comparing DE and PAR at both sites studied here, a recurring pattern of increased accumulation rates were observed during a majority of DE, supporting the suggestion of previous studies that dust deposition may affect peat growth, and thus also peat carbon sequestration. Combining elemental data with XRD mineral analysis enabled anchoring of elemental inferences with mineral observations, allowed identification of authigenic minerals, and aided in source tracing. Despite the fact that local factors affect mineral deposition and PAR, this work has outlined some of the possible mechanisms behind these observations (e.g. distance to the coast, or bog size difference) which may be important for future peat paleodust studies to consider. For example, future studies should include grain size analysis (down-core, as well as across a bog surface); pollen analysis to further elaborate on human activities and vegetation cover; and further investigate differences in mass accumulation rates between bogs.

Keywords: Paleodust, peat, geochemistry, mineralogy, REE, Holocene, peat accumulation rates, paleostorms.

Stockholm 2021 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-192114

ISBN 978-91-7911-488-6 ISBN 978-91-7911-489-3

Department of Geological Sciences

Stockholm University, 106 91 Stockholm

MID-HOLOCENE MINERAL DUST DEPOSITION IN RAISED BOGS IN SOUTHERN SWEDEN

Jenny Sjöström

Mid-Holocene mineral dust deposition in raised bogs in southern Sweden

Processes and links

Jenny Sjöström ©Jenny Sjöström, Stockholm University 2021

ISBN print 978-91-7911-488-6 ISBN PDF 978-91-7911-489-3

Printed in Sweden by Universitetsservice US-AB, Stockholm 2021 Till Emil och Ingrid

Abstract Atmospheric mineral dust is a key component of the climate system, which affects insolation, brings nutrients to marine and terrestrial ecosystems, and acts as a cloud condensation nuclei. To reconstruct past patterns in terrestrial dust deposition natural archives may be utilized, such as loess, dunes, lakes, and peat bogs. Bogs became an established dust archive in the early 2000s, and the number of studies has since increased. However, most studies use single records to represent dust deposition, meaning that we have limited understanding of regional paleodust dynamics or about the representativeness of single bog records. This thesis aims to address these uncertainties by comparing paleodust deposition between bogs located on a 65 km transect. The thesis includes a methodological development for organic matter removal from peat samples for XRD mineral analysis (Paper I) and two peat paleodust reconstruction studies (Paper II, III). The first paleodust reconstruction from Draftinge Mosse (mosse translates to bog in English), Småland, showed that four dust events (DE) were recorded during the ombrotrophic stage (Paper II). These results were compared to a previously conducted study on Store Mosse, 20 km northeast of Draftinge Mosse, which showed similar patterns in DE and peat accumulation rate (PAR), indicating that the events were at least regional in character. However, the magnitude of the DE differed, which was related to differences in the sizes of the two bogs. The second paleodust reconstruction, from (Davidsmosse) located c. 25 km from the west coast, recorded many more DE (14) compared to the more inland sites (Paper III). Two longer periods saw numerous DE, dominated by coarse particles: between 2800 and 2130 cal BP, and from 1000 towards 490 cal BP. These two periods occurred during regionally cold periods. Human activities also intensified during the latter period, possibly amplifying the DE. Most of these episodic events were not recorded at the inland sites, and the Davidsmosse record seemed to be more in line with previously constructed coastal paleostorm records. That the bog located closer to the coast recorded many more events compared to the inland sites suggests that the location of a bog will influence the aeolian events recorded. However, the DE observed at the inland sites were also recorded at Davidsmosse, indicating that the inland events might represent winds that were sustained over longer distance, or alternatively, that regionally dry conditions prevailed during these periods. The paleostorm records from south-western Sweden, including the new results from Davidsmosse presented here, suggest that storm intensities have varied during the last 3000 years, with increased storminess frequency coupled to colder episodes related to extended sea ice and a southward shift of storm tracks. When comparing DE and PAR at both sites studied here, a recurring pattern of increased accumulation rates were observed during a majority of DE, supporting the suggestion of previous studies that dust deposition may affect peat growth, and thus also peat carbon sequestration. Combining elemental data with XRD mineral analysis enabled anchoring of elemental inferences with mineral observations, allowed identification of authigenic minerals, and aided in source tracing. Despite the fact that local factors affect mineral deposition and PAR, this work has outlined some of the possible mechanisms behind these observations (e.g. distance to the coast, or bog size difference) which may be important for future peat paleodust studies to consider. For example, future studies should include grain size analysis (down-core, as well as across a bog surface); pollen analysis to further elaborate on human activities and vegetation cover; and further investigate differences in mass accumulation rates between bogs. Sammanfattning Atmosfäriskt mineraldamm, mineralpartiklar som transporteras via eoliska processer (vind), interagerar med klimatet genom att påverka inkommande och utgående strålning, transporterar näringsämnen till ma- rina och terrestra ekosystem samt fungerar som kondensationskärnor vid molnbildning. Terrestra arkiv för mineraldamm inkluderar loess, sanddyner, sjöar samt högmossar. Högmossar etablerades som naturliga arkiv för mineraldamm i början av 2000-talet, och antalet studier har sedan dess ökat. Ökningen till trots så har få studier av mineraldamm genomförts i samma geografiska område, vilket gör det svårt att bedöma representativiteten av studier baserat på enskilda högmossar. Denna avhandling inkluderar en bakgrund till forskningsfältet, ett metodavsnitt, samt tre artiklar (Artikel I–III). Den första artikeln redogör för hur torv- prover ska förbehandlas för att möjliggöra identifikation av mineral med röntgendiffraktion (XRD) (Artikel I). De följande två artiklarna (II, III) beskriver hur mineraldamms- depositionen har varierat över tid, re- konstruerat från geokemiska analyser av torvsekvenser från Draftinge Mosse, Småland (Artikel II) samt Davidsmosse, Halland (Artikel III). Resultaten från Draftinge Mosse visade att torvackumulation startade för ca 8300 år sedan och att för- höjd mineraldamms- deposition skett under fyra perioder. Resultaten från denna studie jämfördes med en tidigare utförd studie och jämförelsen visade på många likheter, men också vissa olikheter. Likheterna in- dikerar att mineraldamms- studier från enskilda mossar representerar (åtminstone) regionala händelser me- dan skillnaderna tyder på att lokala faktorer också påverkar signalen. Till exempel så skiljde mängden mineraldamm mellan de olika mossarna, liksom hur de reagerat på hydrologiska förändringar. Dessa skillnader härleddes till storleksskillnaden mellan mossarna (400 hektar respektive 7700 hektar). Den andra re- konstruktionsstudien (Artikel III), från Davidsmosse, belägen ca 30 km från västkusten, visade att torvackumulation skett de senaste 5400 åren och fjorton episodiska ökningar av mineraldamm skett under denna period, många fler episoder än vad som noterades i de småländska mossarna. Resultaten visade också att under två perioder var frekvensen av dessa episodiska ökningar särskilt hög, med deposition av större partiklar. Den första av dessa perioder varade från 2800 t.om. 2130 år före nutid och den andra från 1000 t.o.m. 490 år före nutid. Dessa perioder sammanfaller med regionalt kallare klimat, ökande utbredning av havsis i Atlanten samt ett skifte söderut av stompassager. De flesta av dessa episodiska händelser noterades inte i de två inlandsmossarna, utan stämde bättre överens med stormrekonstruktionsstudier från både när- området (Halland), men även från studier av sanddyner i Danmark och en högmosse i Skottland. Sammanfattningsvis så visar stormrekonstruktions- studierna, inklusive resultaten från Davidsmosse (Artikel III), att stormfrekvensen i regionen varierat under de senaste 3000 åren och att en ökad stormfre- kvens skett under kallare perioder. Resultaten från de två rekonstruktionsstudierna visar också att perioder med ökad mineraldeposition sammanföll med ökad tillväxt i mossarna, vilket styrker tidigare forskning som föreslagit att mineraldamm bidrar med näringsämnen, vilka kan påverka tillväxten. Resultaten från dessa studier visar att lokala faktorer, som högmossens storlek och geografiskt läge, är viktiga att beakta vid studier av mineraldamm. Resultaten tyder också på att mineral som avsätts på mossar troligen har ett lokalt ursprung. Framtida studier föreslås inkludera kornstorleksanalys (både i torvsekvensen och lateral ytprovtagning), pollenanalys, samt vidare analys av skillnader i massackumulation mellan olika högmossar.

List of papers

This doctoral thesis consists of a summary and three papers. The presented papers are referred to as Paper I –III.

Paper I Sjöström, J.K., Bindler, R., Granberg, T., Kylander, M.E., 2019. Procedure for organic matter removal from peat samples for XRD mineral analysis. Wetlands 39, 473– 481. https://doi.org/10.1007/s13157-018-1093-7 .

Paper II Sjöström, J.K., Martínez Cortizas, A., Hansson, S.V., Silva Sánchez, N., Bindler, R., Rydberg, J., Mörth, C-M., Ryberg, E.E.S., Kylander, M.E., 2020. Paleodust Deposi- tion and peat accumulation rates – Bog Size Matters. Chemical Geology 554, 119795. https://doi.org/10.1016/j.chemgeo.2020.119795 .

Paper III Sjöström, J.K., Bindler, R., Martínez Cortizas, A., Hansson, S.V., Kylander, M.E. Peat paleodust and accumulation rates in southern Sweden during the last 5400 years: processes and links. In preparation.

This thesis builds partly on the author’s previously published licentiate thesis (defended March 2018). The following chapters have been re-written and updated, using around 10% from the licentiate: Introduction, Peat bogs as paleodust archives, and Mineral stability in peat bogs. Paper I was included as a manuscript in the licentiate, and some early data from Paper II was also presented (peat composition, chronology, WD-XRF data, and XRD mineral data)

Author’s contribution

Paper I: JS outlined the study design, conducted the bulk of the analysis, interpretation, and discussion. RB contributed with adaptions to the study design, interpretation, paper structure, and discussion. TG participated in the analysis and reviewed the interpretation and discussion. MK reviewed the manuscript and contributed with both interpretation and to the discussion.

Paper II: MK designed initial study and fieldwork. AMC, RB, MK, SVH participated in the 2014 field sampling, interpretation and discussion. SVH conducted peat subsampling, freeze drying, bulk density, LOI analysis, some elemental analysis (C, N, 13C) and submitted samples for radiocarbon dating, reviewed results, interpretation, and discussion. NSS submitted samples for radiocarbon dating, and contributed with interpretation and to the discussion. JS led the writing of the paper, compiled all the data, conducted the statistical analysis, conducted the bulk of the interpretation which was sent out continuously for re-view to all co-authors. JS also made additional LOI analyses, submitted additional samples for radiocarbon dating, constructed the age- depth model, prepared samples for the WD-XRF analysis, sample digestion and ICP- OES analysis, XRD mineral analysis, and collected two additional sequences for pH (2017) and macro-fossil analysis (2019). JR conducted the WD-XRF analysis and reviewed the manuscript and contributed with feedback. ER supervised the macro-fossil analysis and reviewed the manuscript.

Paper III: MK outlined the initial study design, which was adapted by JS. AMC, SVH, RB, and MK participated in the peat sampling. Subsampling, freeze drying, bulk density, and LOI was done by SVH. JS led the writing of the paper, designed the soil sample collection and analysis, picked out samples for radiocarbon dating, constructed the age-depth model, performed statistical analysis, interpretation of results, and discussion, and sent out the paper for continuous review by all the co-authors. AV conducted elemental analysis (WD-XRF, C, N, 13C), which was presented in her a BSc thesis. AMC, MK, and RB reviewed results, and contributed with interpretation and to the discussion.

AV: Anna Virolainen; AMC: Antonio Martínez Cortizas; ER: Eleonor Ryberg; JR: Johan Rydberg; JS: Jenny Sjöström; MK: Malin Kylander; NS: Noemí Silva Sánchez; RB: Rich Bindler; TG: Therese Granberg

Related publications not included in this thesis

Kylander, M.E., Martínez Cortizas, A., Bindler, R., Kaal, J., Sjöström, J.K., Hansson, V.S., Silva Sánchez, N., Greenwod, L.S., Gallagher, K., Rydberg, J., Mörth, C.-M., Rauch, S., 2018. Mineral dust as a driver of carbon accumulation in northern latitudes. Sci. Rep. 8 (6878), 1–10. https://doi.org/10.1038/s41598-018-25162-9 .

Martínez Cortizas, A. Sjöström, J.K., Ryberg, E., Kylander, M., Kaal, J., López-Cos- tas, O., Álvarez Fernández, N., Bindler, R. 9000 years of changes in peat organic matter composition in Store Mosse (Sweden) traced using FTIR-ATR. Boreas. Accepted.

Schillereff, D., Chiverrell, R., Sjöström, J.K., Kylander, M.E., Boyle, J., Davies, J., Toberman, H., Tipping, E. Phosphorus supply controls long-term functioning of mid- latitude ombrotrophic peatlands. Submitted. Pre-print: https://eartharxiv.org/repository/view/2165/

Ryberg, E.E.S., Väliranta, M., Kylander, M.E., Sjöström, J.K., Martínez Cortizas, A., Ehrlén, J. Factors driving vegetation changes in a (Scandinavian) boreal bog: Store Mosse, south-central Sweden. In preparation.

Contents

Introduction ...... 1 Research objectives ...... 5 Background ...... 6 Peat bogs as paleodust archives ...... 6 Mineral stability and weathering in peat ...... 8 Provenance tracing in peat paleodust studies ...... 9 Reconstruction of bog water-table fluctuations ...... 10 Holocene climate in Scandinavia ...... 12 Methods ...... 14 Fieldwork, subsampling, and bulk density ...... 14 Radiocarbon dating and chronology building ...... 14 XRD mineral analysis ...... 17 Carbon, Nitrogen, and stable isotopes of carbon ...... 18 WD-XRF ...... 19 ICP-OES ...... 19 Statistical analysis ...... 21 Result: Paper I – III ...... 23 Paper I: Procedure for organic matter removal from peat samples for XRD mineral analysis ...... 23 Paper II: Paleodust deposition and peat accumulation rates – Bog size matters .. 25 Paper III: Peat paleodust deposition and accumulation rates during 5400 years in south-central Sweden: processes and links ...... 28 Discussion...... 30 XRD mineral analysis in peat paleodust studies ...... 30 Peat paleodust and accumulation rates in southern Sweden ...... 31 REE as a source tracing tool...... 32 Peat accumulation rate variability...... 33 Conclusions ...... 35 Future studies ...... 36 Regional comparison of lithogenic mass accumulation rates ...... 36 REE content and ratios in different phases ...... 37 Grain-size analysis ...... 38 GPR, depth measurements and carbon content ...... 38 References ...... 41 Acknowledgements ...... 55 Appendix A: Paper I; Appendix B: Paper II, Appendix C: Paper III.

Introduction

Atmospheric mineral dusts consist of soil particles that are uplifted by aeolian processes, transported in the atmosphere, and deposited through either dry (gravity) or wet (precipitation) deposition. In the atmosphere, mineral dust particles are the most commonly occurring (by mass) particle, being more common than soot from fires, sulfates from industrial combustion, and ash from volcanic eruptions (Knippertz & Stuut, 2014, Fig. 1). Mineral dust particles can be of any size, although generally of finer size with increasing transport distance (Knippertz and Stuut, 2014). When suspended in the atmosphere, mineral particles interact with the Earth’s climate system by affecting incoming solar radiation, atmospheric chemistry, and by bringing nutrients to marine and terrestrial ecosystems (Albani et al., 2015; Gu et al., 2019; Knippertz & Stuut, 2014; Kylander et al., 2018; Yu et al., 2015). Dust particles also impact human health, causing respiratory disease, infections, and carry pathogens (Griffin et al., 2001). About ~75 % of the global dust load is deposited over the continents (Harrison et al., 2001; Shao et al., 2011). In a global context, lower latitudes are dustier than higher latitudes, and the northern hemisphere is dustier than the southern. Three continental regions dominate as sources of global atmospheric dust: arid regions in North Africa (40–60% of the global dust load), the Arabian Peninsula (10–20%) and eastern Asia (15-20%) (Maher et al., 2010). During dry periods, vegetation cover is reduced and the spatial extent of dust source areas increases (Harrison et al., 2001), which, in combination with aeolian activity, causes increased atmospheric dust loads. During the Holocene, global dust loads have varied with high, but decreasing, dust loads in early Holocene, followed by lower dust loads in mid-Holocene (8000–6000 cal BP), and increasing dust fluxes observed in late Holocene (Mayewski et al., 1997; Albani et al., 2015), although regional differences have been observed (Albani et al., 2015). Globally, peatlands store ~500 Gt of C, with around 415 Gt estimated to be stored in northern peatlands (Loisel et al., 2014; Yu et al., 2010). Despite covering only 3% of the Earth’s surface, this stored C represents a third of the total soil C pool. Recent studies suggest that this C pool may be volatilized with future warming (Loisel et al., 2021), although the response of individual peatlands depends on location, type of mire, and external pressures (Hodgkins et al., 2018; Loisel et al., 2021). Furthermore, recent studies have shown that nutrient input through atmospheric dust deposition might affect carbon sequestration in peatlands (Kylander et al., 2018; Ratcliffe et al., 2020).

Paleodust reconstructions have to date mainly been focused on low latitude marine sediment cores (Kohfeld and Harrison, 2001), but terrestrial paleodust records have progressively increased during the last decades. While marine records capture dust loads from distal source areas, terrestrial records also capture local and regional dust variability. Terrestrial regional dust loads can be affected not only by climate factors, but also by human activities such as forestry, agriculture, and industrialization (Mar- tínez Cortizas et al., 2005; Silva-Sánchez et al., 2014). Terrestrial archives of paleodust deposition include lakes, loess, and peat bogs, with each archive being associated with certain strengths and weaknesses in terms of paleoclimate reconstructions. Loess deposits accumulate exclusively aeolian particles but are often constrained to relatively dry regions in close proximity to extensive dust source such as deserts. They are also low resolution, can be affected by soil leaching processes and restricted to luminescence dating which gives ages with relatively low precision (5–10%) (Kohfeld and Harrison, 2001). Lakes are geographically widespread but can only be used for paleodust studies if the aeolian dust is mineralogically distinct from fluvial inputs (Albani et al., 2015). Ombrotrophic bogs are abundant in northern latitudes, receive all their nutrients and moisture from the atmosphere (e.g. Rydin et al., 2006) and thereby act as a natural archive of atmospheric mineral dusts (Shotyk, 1988). They also host ample material for radiocarbon dating. Some of the challenges within peat paleodust studies include: successfully identifying peatland succession and governing transport processes (fluvial vs. aeolian) (Sjöström et al., 2020); separating local and regional events (Albani et al., 2015); deciphering climate signals from human land- use changes (Martínez Cortizas et al., 2005; De Vleeschouwer et al., 2012); and establishing a reliable proxy for calculation of dust mass accumulation rates (DMAR) (Shotyk et al., 2002; Kylander et al., 2016; Martinez Cortizas et al., 2019). Further- more, despite an increasing number of peat paleodust studies conducted in the recent decade (Table 1) the representativeness of individual peat records are rarely considered, which limits the possibility to verify the spatial representation of dust loads inferred from individual bog records.

Figure 1. Visualization depicting atmospheric aerosols: mineral dust (red), sea salt (blue), carbon bursts from fires (green), and sulfates (white) from volcanic eruptions and combustion. Source: Nasa/Goddard

In the context of Scandinavia, the first peat paleodust study was published in 2013 (Kylander et al., 2013), where 8900 years of dust deposition was reconstructed in a peat sequence from Store Mosse (mosse means bog in English). This was followed by further analysis of the same sequence, where links between dust deposition, nutrient content, and peat accumulation rates (PAR) were explored (Kylander et al., 2016, 2018). The authors identified four dust events (DE), which were mostly coupled to drier conditions. The authors also found that the mineral composition changed throughout the studied period indicating a shift in source area and/or wind direction. A period of exceptional growth was also recorded, a high peat accumulation event (HPAE), between c. 5600–3900 cal BP, which was coupled to raised water tables and deposition of nutrient rich minerals fertilizing the bog. This doctoral thesis builds on this earlier work, and aimed to couple down-core elemental data with XRD mineral observations, extend the number of peat paleodust studies in Sweden, explore the spatial representativeness of single-site reconstructions, analyze local soils for mineral and elemental content, and further probe links between dust deposition and PAR. In order to conduct XRD analysis on peat samples, methodological development was necessary because no established protocol for efficient organic matter removal from highly organic peat samples with the purpose to conduct XRD analysis existed (Paper I). To extend the number of peat paleodust studies in Sweden, paleodust deposition and PAR was reconstructed from two bogs 1) Draftinge Mosse, a bog located nearby (~18 km) to the previously studied Store Mosse bog (Kylander et al. 2013; 2016; 2018), and, 2) Davidsmosse a bog located closer to the coast compared to the two previous paleodust records (Fig. 2). Finally, the results were put into a regional dust and climate context in order to explore links between dust deposition, PAR, and Scandinavian climate variability.

Figure 2. Location map of sites mentioned in the text. (A.) Orientation map. (B.) Geological overview of southern Swedish (adapted from Kylander et al. 2018), including indication of the two sites studied here, from west to east: 1) Davidsmosse and, 2) Draftinge Mosse, and the previously studied bog 3) Store Mosse (Kylander et al., 2013; 2016; 2018). (C.) Satellite image of Davidsmosse, and (D.) Draftinge Mosse. Pic- tures of coring locations, Davidsmosse (E.) and Draftinge Mosse (F.). Satellite images from Google Earth, 2021.

Research objectives The aim of this project was to study how dust deposition have varied through time and space in a northern latitude terrestrial setting. Within this overarching goal, the specific study aims were to improve the terrestrial coverage of paleodust deposition in Sweden using raised peat bogs in South-central Sweden as archive; compare dust deposition and peat accumulation rates between individual sites to verify the repre- sentativness of individual peat dust records; and explore both observed similarities and differences to increase knowledge about local factors that may affect peat dust records; and investigate relationship between mineral dust deposition, mineralogy, and PAR. In order to fulfill these study aims, the following objectives were developed:

1. Develop a standard protocol for organic matter removal from highly organic peat samples for XRD mineral analysis (Paper I). This will be done by testing organic matter removal efficiency on a homogenised bulk peat sample, dived into subsamples subjected to distinct pre-treatments: no pre- treatment; combustion at different temperatures (300–550°C); oxidation with H2O2, and oxidation with Na2S2O8.

2. Reconstruct mineral dust deposition and PAR at Draftinge Mosse, based on elemental analysis coupled with XRD mineral analysis. The peat sequence will also be analysed for peat composition, bulk density, accumulation rates, and pH. Finally, dust events and PAR will be compared to the nearby Store Mosse to establish local and regional patterns (Paper II).

3. Extend the regional assessment of past dust deposition and peat accumulation rates through analysis of Davidsmosse, a bog located closer to the coast compared to the two previous paleodust studies (Paper III). Local dust sources will also be analysed, and compared to the peat mineral content. Finally, the results will be compared with the previously studied bogs, as well as with coastal paleostorm records, and related to climate development during the studied period.

Background

Peat bogs as paleodust archives Peatlands have a long history of being used as a natural archive for past environmental change. Already in 1894, Sernander (1894) proposed a first Holocene climatic division, based on peat macrofossil data – the Blytt-Sernander scheme. In the Blytt- Sernander scheme Scandinavian climate since the last glaciation was divided into periods with predominantly oceanic (Atlantic) or continental influence (Boreal). This scheme was later adapted and updated with the inclusion of pollen zones (Von Post, 1916; Mangerud, 1982). In Sweden, peatland mapping campaigns were conducted in the early 1900s, mainly driven by an interest in peatlands as a fuel resource, but these studies also resulted in detailed knowledge about the distribution and properties of peatlands in Sweden. The last 100 years have seen extensive proxy and dating devel- opments, allowing reconstruction of past vegetation, hydrology, and temperature with increasing chronological accuracy (Mangerud, 1982; Berglund and Ralska- Jasiewiczowa, 1986; Barber et al., 2003; Chambers et al., 2012; Väliranta et al., 2012; Lowe and Walker, 2015). The field of paleodust reconstructions based on peat records evolved from peat pollution studies (e.g. Aaby & Jacobsen, 1978; Madsen, 1981; Weiss et al., 1999). Shotyk et al., (2001) conducted a Holocene Pb study coupled with the analysis of lithogenic trace element content (scandium (Sc), titanium (Ti), yttrium (Y), rare earth elements (REE)), to improve source tracing of Pb, in a ~14 000-year peat sequence from Switzerland. This study established some of the basic concepts of peat paleodust studies and remains influential in the research field today. The authors conducted a review of the geochemistry of the studied elements, the associated mineral hosts, as well as fractionation during weathering and/or transport. Based on this, a number lithogenic elements were described as displaying a conservative and immobile behavior in the peat, meaning that they could be used as proxies for soil mineral dust: Sc. Ti, Y, REE. The authors found that periods of elevated mineral dust deposition were related to dry periods, with reduced vegetation cover, and higher wind strengths. Whereas lithogenic trace elements were enriched during periods of low mineral deposition, related to deposition of fine grained, long distance transported dust (Shotyk et al., 2002). The authors related mineral DE to large scale atmospheric circulation changes, such as input of coarser particles during the cold and dry Younger Dryas, and elevated input of fine grained, long-distance, particles after the end of the African Humid Period and related desertification of Sahara. This study also introduced the

6 concept of enrichment factors (EF) to peat paleostudies. This approach had been commonly applied within peat pollution studies (e.g. Martínez-Cortizas et al., 1997), to identify periods of elevated dust deposition, changes in grain size, and/or source change. Shotyk et al. (2001, 2002) also suggested that a single element (e.g. Sc or Ti) could be used to calculate total aeolian soil dust (ASD) based on the immobile behavior of these elements. This approach is based on two assumptions, 1) that the chosen element deposited on the bog occurs in equivalent concentration to that in the earth’s crust (i.e. upper continental crust (UCC)), and 2) that the element is not enriched or depleted during weathering or transport (Shotyk, 1988; Shotyk et al., 2002). This allows the total ASD (µg cm-2 yr-1) flux to be calculated as follows, using Ti as an example:

(1) 100 �� = × Tisample × density × accumulation rate 0.4

-1 Where 0.4 is the Ti concentration in the UCC, the TiSample (µg g ) is the sample concentration, which is multiplied by the density of the sample (g cm-3) and the accumulation rate (cm yr-1). This approach to calculate ASD is still commonly applied (e.g. Bao et al., 2012; De Vleeschouwer et al., 2012; Li et al., 2020; Pratte et al., 2019; Sapkota et al., 2007). As the research field developed, later studies have suggested that local factors also needs to be taken into account because local vegetation changes may also affect mineral dust deposition (Martínez Cortizas et al., 2005; De Vleeschouwer et al., 2012; Silva-Sánchez et al., 2014), emphasizing the need for multiproxy approaches within peat paleodust studies. Subsequent studies have also chal- lenged the use of a single element to calculate dust fluxes (i.e., ASD or dust mass accumulation rate (DMAR)) because the calculated total dust values will vary based on the element used in the calculation (Kylander et al., 2016), and in some cases give values higher the total ash content (Martínez Cortizas et al., 2019). However, a viable alternative to this approach to calculate dust fluxes in bog archives has yet not been developed. During the last two decades the number and spatial representation coverage of peat paleodust studies have steadily increased (Table 1), analytical methods have been re- fined (Krachler et al., 2002; Ferrat et al., 2012), provenance tracing tools developed (Kylander et al., 2007; Marx et al., 2005), and application of quantitative analysis have increased, e.g. correlation matrices (Martínez Cortizas et al., 2005), principal component analysis (Muller et al., 2008), and change point modelling to determine timing of changes (Kylander et al., 2007). The term “dust” is within this thesis operationally defined as all minerals derived via aeolian transport to the studied bogs. The term “local” refers to changes reflected within a 5 km radius, whereas “regional” refers to between 5 to 100 km radius.

Table 1. Peat paleodust studies, ordered by region. Region, country Reference Asia China (Bao et al., 2012; Pratte et al., 2019; Peng et al., 2021) Tibet (Ferrat et al., 2011) Indonesia (Weiss et al., 2002) Oceania Australia (Kylander et al., 2007; Marx et al., 2009) Easter Island (Chile) (Margalef et al., 2014) Indian ocean Amsterdam Island (Li et al., 2020) Europe Belgium (De Vleeschouwer et al., 2012; Allan et al., 2013) Romania (Longman et al., 2017; Panait et al., 2019) Spain (Martínez Cortizas et al., 2005, 2019; López-Buendía et al., 2007; Silva-Sánchez et al., 2014) Sweden (Kylander et al., 2013; Sjöström et al., 2020) Switzerland (Steinmann and Shotyk, 1997; Shotyk et al., 2001, 2002; Le Roux et al., 2012) North America Canada (Pratte et al., 2017) USA (Kamenov et al., 2009) South America Argentina (Hooper et al., 2020) Chile (Sapkota et al., 2007)

Mineral stability and weathering in peat The geochemical environment in bogs is characterized by low concentrations of cations and anions, acidic conditions (pH 3–5), with an excess of organic acids, reducing conditions, aerobic decomposition in the acrotelm and slow anaerobic organic decay in the catotelm, and limited microbial activity (Vitt, 2006; Heller et al., 2015). Some minerals will weather rapidly in the acidic peat (carbonates, apatite) (Le Roux et al., 2006), whereas contrasting evidence has been put forward regarding silicates. Within the silicate mineral group, weathering stability differs, with quartz, K-feldspar, and muscovite being the most stable, and Ca-plagioclase and olivine being least stable (Table 2). The rate of mineral weathering (dissolution) is dependent on several factors such as pH, type of mineral, temperature, geologic substrate and microbial community inhabiting the different parts of the peat (Bennett et al., 1991; Bennet and Rogers, 2001). Bennet et al. (1991) showed that quartz and feldspar dissolved rapidly in the deeper part of the peat column in near neutral conditions of a fen, whereas Le Roux et al. (2006) found that silicate minerals (quartz and feldspars) were preserved on millennial time scales in an acidic bog. Quartz is generally stable in acidic conditions, whereas weathering of feldspars increase at lower pH (Boyle and Voigt, 1973; Nesbitt et al., 1997). And within the feldspars, Ca-plagioclase is least stable in acidic conditions, and K-feldspars most resistant (Table 2) (Nesbitt et al., 1997). Steinmann and Shotyk (1997) suggested that feldspars were not weathered to any great extent, based on a constant ratio between quartz and feldspars during the last 2000 years. Le Roux et al. (2006) suggested that limited feldspar weathering might be related to the presence of organic coatings on mineral grains, potentially inhibiting weathering. 8

Table 2. Schematic stability scheme of common silicates and heavy minerals. Adapted from Nesse 2017. Least stable Olivine. Ca-plagioclase. Pyroxene. Amphibole. Biotite. Ilmenite. Na-plagioclase. Magnetite. K-feldspar. Muscovite.

Most stable Zircon. Rutile. Quartz.

Provenance tracing in peat paleodust studies Within peat paleodust studies downcore changes in the mineral composition are commonly inferred through either elemental ratios or through stable isotope analyses (Pb, Nd, or Sr) . Elemental ratios can be used to depict changes in grain size or mineralogy because some elements are enriched, or depleted, in certain mineral groups or grain sizes. For example, ratios of Ti/Sc and Zr/Sc have been used to infer deposition of heavy minerals (Shotyk et al., 2002), Ga/Al to detect plagioclase deposition (Kylander et al. 2016), and Si/Ti to infer grain size variability (Sjöström et al., 2020). Ratios between light (LREE), middle (MREE), or heavy rare earth elements (HREE) have been used both to distinguish between different dust sources (Ferrat et al., 2011); as well as to infer the mineral composition, such as periods of elevated clay, phosphate, or feldspar input (e.g. Kylander et al., 2016; Kylander et al., 2007; Le Roux et al., 2012; Pratte et al., 2017, 2019). In addition, total REE (ΣREE) content in peat deposits have also been used to infer input of fine grained particles (Pratte et al., 2019). The lanthanide REE are a group of elements, from La to Lu, that, based on their chemical properties show ordered responses during chemical and physical processes such as rock formation, weathering, and sedimentation, making the REE useful as tracers of geological processes (McLennan, 1989). Different classifications of the REE group exists in different disciplines, with Sc and Y sometimes also included based on their chemical resemblance (Nomenclature of Inorganic Chemistry, 2005). Within peat paleodust studies REE generally refer to La to Lu, often including Y, which is the classification used in this thesis. In the context of peat, Yliruokanen and Lehto (1995) studied the occurrence of REE in Finnish mires and concluded that downcore REE content largely reflected that of the surrounding bedrock. A conservative and immobile behavior of REE in peat was suggested by Shotyk et al. (2002) and later confirmed by Aubert et al., (2006). REE occur, often together, in accessory minerals such as phosphates, carbonates, fluorides, and silicates in pegmatites, granites and other metamorphic and igneous rocks (Tyler 2004), with enrichment or depletion of LREE, MREE, HREE associated with distinct mineral phases. This feature, allows REE ratios to be used as a mineral composition and source proxy tool in peat paleodust studies (Kylander et al., 2007).

The Draftinge Mosse sequence was analysed for REE content (Paper II), with the aim to explore potential source changes within the sequence. In addition, the elemental data were also coupled with downcore XRD mineral analysis to anchor the elemental ratio interpretations with mineral observations (Paper II).

Reconstruction of bog water-table fluctuations Decomposition of peat is influenced by several factors such as dominant vegetation, nutrient content, successional stage, fires, temperature, and moisture access (e.g. Charman et al., 2015; Loisel et al., 2014; Yu et al., 2010), with depth of the water- table table likely having the greatest impact because it governs the depth of the acrotelm (Clymo, 1984; Chambers et al., 2012). During drier periods, the water-table drops and the peat litter stays in the oxic acrotelm longer compared to periods with a higher water-table, increasing the decomposition rates. When the peat litter has been transported down to the water-logged catotelm, decomposition rates are greatly reduced. Temperature also influences decomposition, although mostly indirectly by affecting the precipitation-evaporation balance (Charman et al., 2013). Reconstruction of decomposition rates have been inferred using various methods, such as the von Post Humification Index (H1–H10), macro-fossil analysis, bulk density, humification (colorimetry), C/N ratios, and testate amoebae analysis (Kuhry and Vitt, 1996; Chambers et al., 2012; Väliranta et al., 2012; Biester et al., 2014). In addition to these established approaches, recent advances in non-destructive mid-infra- red spectroscopy have shown to enable distinction between compositional changes related to long-term (i.e. age-dependent) and short-term decomposition (related to lowered water table) (Martínez Cortizas et al., accepted). Within this project, decomposition rates have been interpreted from multiple proxies: visual inspection of the peat cores directly after core retrieval for composition and color; by measuring bulk density (1 cm resolution); and from down-core C/N ratio analysis (Paper II, III). Pho- tographs of each core were also taken during core retrieval for later reference (Fig. 3). The C/N ratios can be used as a decomposition proxy based on the greater mass loss of C relative to N during decomposition (Kuhry and Vitt 1996). However, elemental concentrations, and thus C/N ratios, are also affected by variations in the dominating vegetation with generally higher N concentrations in sedges compared to Sphagnum mosses (e.g. Kuhry and Vitt 1996), and therefore the ratio can reflect both botanical changes and decomposition which needs to be taken into consideration when using this proxy (Biester et al. 2014). The bulk density can inform about past decomposition rates based on that the peat becoming denser during decomposition compared to less decomposed peat (Kylander et al., 2013). However, bulk density can also vary with other factors, such as dominant vegetation and compaction, which needs to be taken into consideration when interpreting the data.

Figure 3. Pictures of the Draftinge Mosse cores, taken at core retrieval. The figure also includes alignment information (Paper I, supple- mentary material).

Holocene climate in Scandinavia The current climate in southern Sweden is to a large extent influenced by North At- lantic atmospheric circulation and sea surface temperatures (SST), with dominant wind directions being from the west or south-west (Seppä et al., 2005; Alexandersson, 2006; Jong et al., 2009). The strength of this influence is regulated by the North At- lantic Oscillation (NAO), which refers to a meridional oscillation in atmospheric mass between the Icelandic Low and Azores High, a pressure gradient that is most pro- nounced during winter (e.g. Hurrell and Van Loon, 1997). This pressure difference is reported against a long term mean (Hurrell and Van Loon, 1997; Trigo et al., 2002). During a positive oscillation (NAO+) the westerlies are strengthened, causing mild and moist winters in northern Europe while a negative (NOA-) causes cool and dry winters (Hurrell and Van Loon, 1997; Trigo et al., 2002). In southern Sweden, the strongest winds typically occurring between October and March (Alexandersson, 2006; Jong et al., 2009). In the following paragraphs the Holocene subdivision follows Walker et al. (2019). During the early Holocene summer temperatures increased rapidly (Seppä et al., 2005; Väliranta et al., 2015; Kaufman and et al., 2020), and reached above Holocene mean temperatures after 10 000 cal BP, an effect of the high insolation. This was pararelled by a trend of gradually increased moisture availability (Wanner et al., 2011). At this time, the westerlies were likely enhanced during the winters, causing mild and moist winters in south-central Sweden (Seppä et al., 2005). This is also when a first peat initiation period has been recorded in Scandinavia (Rundgren 2008), and agrees with the start of peat accumulation at Store Mosse and Draftinge Mosse. Around 8200 cal BP, a major cooling event was recorded, coupled to re-arrangement of the North Atlantic ocean circulation from large fresh water input to the North At- lantic (Seppä et al., 2005; Walker et al., 2019). In Scandinavia this event is reflected as a shift to cooler temperatures and mostly dry conditions (Almquist-Jacobson, 1995; Seppä et al., 2005; Wanner et al., 2011; Digerfeldt et al., 2013), although some studies report that wet summers prevailed, indicating that the seasonality may have been enhanced (Hammarlund et al., 2005). The mid-Holocene (8.2–4.2 cal BP) was generally characterized by a relatively warm and stable climate, with enhanced oceanic influence in southern Sweden. Early in this period relatively wet conditions prevailed, with a trend towards drier conditions (lower lake levels) until c. 6000 cal BP, followed thereafter by wetter conditions and lake level increases (Almquist-Jacobson, 1995; Hammarlund et al., 2003; Digerfeldt et al., 2013). During this period of enchanced moisture access a second peatland ex- pansion occurred in Sweden (Rundgren, 2008), and within this phase peat initiation occurred at Davidsmosse. Within the general trend of warmer and wetter conditions, colder episodes were recorded in and around the North Atlantic around 6300 and 4700 cal BP, while drier episodes occurred around 6500, 5700, 5100, and 4700 cal BP (Wanner et al., 2011). Dry summer conditions were recorded in bogs in Halland (south-western Sweden) between 4800 and 4500 cal BP (Jong et al., 2006). The late

Holocene (4.2 cal BP to the present) is characterized by a long-term trend of decreasing temperatures, replaced by an abrupt temperature increase during the last 150 years caused by human activities (Wanner et al., 2011; Marcott et al., 2013; PAGES 2k Consortium, 2019). Reconstructions of NAO variblity during late Holocene suggest that NAO+ dominated, but that larger varilabity of the index, including NAO- conditions, were inferred between 4500 and 2000 cal BP, particulary between 3000 and 2000 cal BP (Olsen et al., 2012). Shifts in moisture access and temperature have been recorded within the general trends with cooler conditions observed around (cal BP): 2700; 1500, and during the Little Ice Age (LIA, c. 700 to 150 cal BP) (Wanner et al., 2011) noted. Drier periods occurred (cal BP) between 3200 and 2800 (Borgmark, 2005; Wanner et al., 2011); 1500; and 800. The LIA represents the coldest temperatures during the entire Holocene (Wanner et al., 2011; Marcott et al., 2013), and was likely a variable period, both in terms of temperature and moisture access (Wanner et al., 2000; PAGES 2k Consortium, 2019). Reconstructions related to moisture availability in southern Sweden show that mostly drier conditions prevailed during the early LIA, replaced by gradually wetter conditions (Gunnarson et al., 2003; Seppä et al., 2009), while a bog record from south-western Sweden reported increasingly dry conditions over the last 600 years (Jong et al., 2006). A shift to greater variability and intermittent NAO- index was also inferred during early LIA (Olsen et al., 2012). In- creased wind strengths have been recorded towards the end of the late Holocene, based on both dune and peat paleostorm records (Clemmensen et al., 2001; Björck and Clemmensen, 2004; Jong et al., 2006; Bernhardson et al., 2019).

Methods

Fieldwork, subsampling, and bulk density The majority of peat sequences analysed within this thesis were collected during a field campaign in the spring of 2014, initiated and planned by M. Kylander. Selected raised bogs, located on a west to east transect were cored using a Russian peat corer (100 x 7.5 cm barrel). At each site, complete peat sequences were retrieved from two parallel boreholes, with a 25 cm overlap for each core. Directly after retrieval, the cores were wrapped in plastic film and put in PVC tubes. After transport back to Stockholm University the cores were stored in a freezer (-18°C). The frozen cores where sectioned into 1 cm slices with a stainless-steel saw, followed by freeze drying. Weight and volume of the freeze dried subsamples were recorded to calculate bulk density (Le Roux & De Vleeschouwer, 2010). The bulk density was then used to align the overlapping cores and build a composite sequence. Ash content was determined by recording the sample weight at 105°C and after loss on ignition (LOI) at 500– 550°C (Dean, 1974). The sites studied here, Draftinge Mosse and Davidsmosse, were revisited at several times during this project, to measure the depth of the basin (Draft- inge Mosse), collect additional sequences for pH and macro fossil analysis (Draftinge Mosse), surface bog samples (Davidsmosse), and local source sampling (around Draftinge Mosse and Davidsmosse). Furthermore, Store Mosse was also revisited to collect additional peat sequences, measure elemental content in peat waters, and to collect source samples from areas surrounding the bog. Two winter campaigns to measure basin topography with ground penetrating radar (GPR) at Draftinge Mosse and Store Mosse were also conducted. Early results from the GPR work are presented in the Future studies chapter.

Radiocarbon dating and chronology building Radiocarbon is widely used within paleoenvironmental reconstruction studies to re- late observed variability in a sequence to calendar ages. The basis of this dating method is that organisms (plants) will take up atmospheric CO2 through photosynthe- sis, alternatively through ingestion of plants (animals), and incorporate the carbon into the organism’s cells. This carbon will largely reflect the atmospheric ratio at the time of incorporation between unstable radionuclide 14C and stable C isotopes (12C and 13C). When the organisms dies 14C will decay, and because the half-life of the 14C radionuclide is known (5730 years), it is possible to establish the age by measuring 14 the ratio between unstable and stable isotopes by Accelerator Mass Spectrometry (AMS) (e.g. Blaauw & Christen, 2005). Since the 14C content of the atmosphere has varied through time, the radiocarbon ages need to be calibrated to calender years, and the ages of in-between dated levels need to be estimated.

Figure 4. Example of macro-fossils used for radiocarbon dating and their calibrated ages (from the Davidsmosse sequence). (A.). Sphagnum branch, including leaves and, (B.) Betula seeds, and (C.) Sphag- num leaves.

The chronologies constructed within this thesis are based on AMS radiocarbon dating of macrofossil samples (Fig. 4) (Paper II, III). In a few cases, bulk peat samples were also used (Paper II) to get an initial age estimate. The age-depth model for Paper II was based on 16 AMS radiocarbon-dated samples that were submitted to two different laboratories: (BETA (US) and DirectAMS (Spain). For Paper III five samples were submitted to BETA (US) and one to the Radiocarbon Laboratory, Uppsala Uni- versity (Sweden). All samples were subjected to acid-alkali-acid washing to remove 14 carbonates, bacterial CO2 or humic/fluvic acids prior to C measurement. The age-depth models were constructed using Bacon, a script developed to take into account the uncertainty of radiocarbon ages, the increasing uncertainty in-between dated levels, and to improve the estimation of accumulation rates by applying Bayesian statistics (Blaauw and Christen, 2011). Bacon applies an iterative Markov Chain Monte Carlo process to create the age depth mode, where several thousands of independent likely age-depth models and accumulation rates are constructed between each dated level (Blaauw and Christen, 2011). In this process the sequence is divided into sections with a priori memory of accumulation rate inherited from the previous sections. This process avoids sharp accumulation rate changes from occurring at the radiocarbon-dated levels (Blaauw and Christen, 2011). From these data, calendar age distributions are constructed, with uncertainties depicted in grey scales. The uncertainties in-between dated levels become larger the further away modeled ages are from a dated point (Fig 5, Paper II, Paper III). The calibrated ages, accumulation rates, and in-between estimated ages are all subject to uncertainty and the obtained resolution can therefore vary from decadal to centennial (or more) resolution. The obtained resolution is also dependent on resolution of dated samples, accumulation rates of the record (which can also vary within a sequence), and type of material used for radiocarbon dating (e.g. bulk material or plant 15 macro-fossils) (Nilsson et al., 2001; Blaauw et al., 2004; Blaauw and Christen, 2005; Mauquoy and van Geel, 2007). These uncertainties make inter-site and proxy comparison complex (Blaauw and Christen, 2005; Blaauw et al., 2010). To illustrate this in the context of the work done here, the age-depth model for Davidsmosse (Fig. 5, A)(Paper III) and the difference between the maximum and minimum ages (within 95% confidence) was plotted (i.e., the minimum and maximum calibrated age range) (B). Figure 5 (B.) depicts both the increasing uncertainty between dated levels as well as shows that the uncertainty increases after 1000 cal BP towards the present. In addition to this, the last 300 years are difficult to obtain reliable dates from because the combustion of fossil fuels has affected the atmospheric 14C budget, complicating com- parisons between different proxy archives during this period. One way to depict chronological uncertainty in proxy records is to use the Ghost plot function in Bacon, whereby the uncertainty from the age-depth model is included in the proxy plot (Blaauw and Christen, 2011). This Bacon function, was applied in Paper II were mineral dust deposition and accumulation rates were compared between Draftinge Mosse and Store Mosse.

Figure 5. (A.) Age-depth model for Davidsmosse (Paper III), and (B.) Depiction of difference between minimum and maximum calibrated ages (95% confidence) for Davidsmosse.

XRD mineral analysis X-ray diffraction (XRD) analysis was applied within this thesis to identify minerals both in the peat (Paper I, II, III) and in soil samples (Paper III). With XRD analysis, minerals are identified based on their structure and chemical composition. During the analysis a sample is subjected to X-rays, and the diffraction of the rays are used to calculate the distance (d-spacing) between the planes of the atoms (crystal lattice) by applying Bragg’s equation: (2) �� = 2� sin � where n = integer, λ = wavelength, d = distance between lattice plane, θ =incident beam angle. This gives information about the crystal structure, which in turn is dependent on the chemical composition. The result is visually presented in diffracto- grams, where the angle (2θ) is presented on the x-axis and d-spacing counts per second (cps) is presented on the y-axis (Fig. 6). Mineral phases are then identified based on the location, as well as the shape of peaks, by comparing with a reference mineral database.

Figure 6. Example of the visual representation of XRD results. The diffractogram shows the result of two surface samples from Davidsmosse (Paper III). 2020A (red) represents the detrital minerals, rinsed out from a living peat sample with distilled water (see Paper III for more details). Identified minerals included mica, hornblende (Hbl) quartz (Qz), plagioclase feldspar (Pl), and k-feldspar (Kfs). The second example, 2020B, is the result of XRD analysis of the vegetation residue remaining after the washing out of detrital minerals. Identified minerals included a layered double hydroxide (LDH), calcite (Cal), apatite (Ap), and quartz.

The pre-treatment of the peat samples analysed within this thesis followed the protocol developed and outlined in Paper I, which includes organic matter removal by combustion at 500°C, washing the sample in distilled water, followed by mounting on an XRD plate (Fig. 7). Determination of background, peak localization, and identification was conducted in Highscore, a Panalytical software (Degen et al., 2014). The XRD analyses for Papers I and II were conducted in a PANalytical X–ray diffraction system (X'Pert Powder), equipped with a point detector, at the Department of Geo- logical Sciences, Stockholm University. In addition, two samples were also analysed at the Museum of Natural History (NRM), Stockholm, to verify the results. The NRM host a similar PANalytical XRD to that at the Department of Geological Sciences but is equipped with a multidetector, that allows both reduced analysis time, reduction of

17 peak to noise ratio, as well as an increased potential to detect minor mineral phases. For Paper III all XRD analyses were conducted at the NRM.

Figure 7. Pictures depicted the pre-treatment steps prior to XRD analysis. (A.) Freeze dried peat slice, (B.) ash residue left after combustion, which were washed three times prior to analysis, and (C.) sample mounted on of axis cut silica quartz plate.

For Paper III, quantification of the relative occurrence of phases was conducted using the Rietveld refinement method. This method was developed in the 1960s by Hugo Rietveld and fits a modelled profile to the data (Rietveld, 1969). The success of the refinement is dependent on several factors such as pretreatment of the sample, quality of the analysis, and successful identification of mineral phases (Toby, 2006). Here, the default settings of the Rietveld refinement in Highscore plus were used (De- gen et al., 2014).

Carbon, Nitrogen, and stable isotopes of carbon Carbon and N concentrations, as well as stable isotopes of C were measured in both bulk peat samples (Paper II, III) and ash residues (Paper I and II). Measurements were conducted at the Department of Geological Sciences, Stockholm University, in a Carlo Erba NC2500 elemental analyzer, connected to a Thermo Fisher Delta V advantage mass spectrometer. Carbon and N concentrations are reported as dry weight percentage (%), whereas the stable isotopes are reported per mille (‰) ratios relative to V-PDB (δ13C). The measurement error for C and N concentration and isotope measurement were <1% and <0.15‰, respectively. C content and δ 13C values were also measured in the ash residues, with a somewhat higher error reported for the C content (<3%), related to the small sample size. For Paper I ash residues were measured to determine residual organic content following organic removal procedures (i.e., combustion or wet oxidation) and for Paper II the analysis was conducted on the ash residues, before and after a dilute acid treatment, to evaluate the presence of carbonates.

WD-XRF X-ray fluorescence spectrometry (XRF), as used here, is a non-destructive analytical approach applied to determine the elemental composition of a sample. As with XRD analysis, the sample is irradiated with X-rays, but instead of measuring diffraction, the wavelength and intensity of emitted light by exited atoms (fluorescence) is measured. This fluorescence is distinct for each element, allowing identification and quantification of the elemental composition (e.g. Müller, 1972). Some of the limitation with this analytical approach includes inter-elemental matrix effects (the concentration of individual elements affects the concentration of all other elements) and that quantification can be affected by grain size, homogeneity, and surface roughness (Rydberg, 2014). To reduce quantification uncertainties, milling of the sample into a fine powder is required (Rydberg, 2014). The WD-XRF analysis was conducted at the Department of Ecology and Environmental Science, Umeå University, using a Bruker X8-Tiger analyzer equipped with a Rh-anticathod X-ray tub. Milled and dried samples (peat samples: 400–500 mg, soil samples: 50 mg) were put in plastic vessels, covered with Mylar® film and analysed for elemental content. Calibration was done based on the powdered sediment method (Rydberg 2014) adapted to peat samples by (Hansson et al., 2013). Accuracy and precision were estimated based on both certified and in- ternal peat standards (Paper II, Paper III).

ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is a destructive analytical approach that requires samples to be fully digested and dispersed in a liquid prior to analysis (Givelet et al., 2004). During analysis the sample is vaporized and transported to an ionization chamber where the atoms are excited. When the elec- trons fall back to their ground state emission of electromagnetic radiation occurs (Boss and Fredeen, 1997). Within ICP-OES analysis elemental composition is identified based on the distinct radiation emission of elements and the intensity of the radiation allows quantification of its concentration (Boss and Fredeen, 1997). For Paper II, ashed peat samples were digested following the method outlined by Krachler et al. (2002) but adapted so that the HBF4 concentration was adjusted to the sample weight. Ashing has the advantage of improving the detection of elements occurring in trace concentrations, because element concentrations in the ash are about 50-times higher. All sample pre-treatment was done in a clean lab at the Department of Geological Sciences, Stockholm University. Prior to analysis the sample was weighed, transferred to microwave vessels, and digested in a MARS6 microwave at 180°C during 20 mins, followed by evaporation to near dryness on a hotplate (Fig. 8). Finally, the samples were diluted with 1% HNO3 either 10 times (major elements) or 30 times (trace elements). The results are reported in ppb and requires division by the liquid dilution factors and sample weight to obtain the concentration in the ash residues. 19

Figure 8. (A.) Samples loaded microwave chamber, (B.) hotplate evaporation in Teflon beakers in a clean lab fume hood, and (C.) samples diluted with 1% HNO3 in preparation for the ICP-OES analysis.

The Draftinge Mosse sequence was analysed for elemental content both by WD-XRF and ICP-OES, enabling inter-analytical comparison for a few elements (Al, Ca, K, P, and Ti). The comparison showed that both methods captured relative changes well, but that the results of the ICP-OES underestimated the content of some elements (Al, Ti) (Fig. 9, Paper II). This information was used to correct the ICP-OES data, which was analysed at a higher resolution compared to the WD-XRF analysis. Both analytical approaches have weaknesses and strengths with WD-XRF requiring minimal pre- treatment and being non-destructive, which enables further analysis of the same sample, but has higher detection limits compared to ICP-OES, which can capture a wider range of elements, including those occurring in trace amounts. The drawback with ICP-OES are that it requires lengthy sample preparation, must be conducted in a clean lab facility, and requires the use of strong acids. The under-representation of some elements shown here (Fig. 9) likely indicates that either precipitates were formed in the digested solution prior to the ICP-OES analysis or alternatively, that the acid digestion was not complete.

Figure 9. Comparison of concentration values by WD-XRF (circles) and ICP-OES (crosses), respectively. The comparison showed that some elements where under-represented from the ICP-OES analysis (Al, Ti), especially in the lowermost (fen) part of the sequence. This information was used to correct the Al concentrations in the ICP-OES data-set (Sjöström et al., 2020).

Statistical analysis Geochemical data-sets are often information-rich, yielding downcore concentration profiles for numerous major and trace elements. To quantitatively asses associations within the data-set statistical analyses are often applied, such as correlation matrices (e.g. Martínez Cortizas et al., 2005) or principal component analysis (PCA) (Muller et al., 2008). In PCA, the number of dimensions of the data-set is reduced, and the co-variability of variables (e.g. elemental concentrations) to a dimension is reported with numerical values ranging from -1 to 1, with values closer to -1 or 1 indicating stronger association. Prior to statistical analysis the data need to be converted to average centred values (z-scores) to reduce scaling affects (i.e. that the average is affected by large or small outliers) and differences in measured units (e.g. g, mg or µm g-1). The elemental data-sets retrieved for both Paper II and III were analysed by PCA. To verify the results, and evaluate if the conversion to z-scores affected the results, PCAs were conducted both on concentration values and z-scores values and the results were compared. Furthermore, analysis including both the entire sequence (i.e. fen and bog) as well as only part of the sequence (i.e. only bog, or only fen) was carried out (Paper II, III), which allowed identification of distinct associations depending on which part of the sequence was included. For example, when including the entire Draftinge Mosse sequence the lithogenic elements were split into two components, whereas 21 when only the bog was included the elements were associated to on component, related to different grain sizes (and transport processes) in the fen vs the bog (Paper II).

Result: Paper I – III

Paper I: Procedure for organic matter removal from peat samples for XRD mineral analysis Sjöström, J.K., Bindler, R., Granberg, T., Kylander, M.E.

Peat paleodust deposition is commonly reconstructed from down-core elemental data- sets, which are used to infer mineral dust deposition, mineral composition, and/or grain sizes (e.g. Shotyk et al., 2002). A drawback with this approach is that many minerals host a similar elemental composition, making it difficult to distinguish between minerals as well as between biogenic and minerogenic phases. Although protocols for organic matter (OM) removal from soil samples existed, the most suitable approach to remove OM from highly organic peat samples (up to 99% OM), with the purpose of conducting XRD mineral analysis had yet to be developed. Established approaches to remove OM include combustion at temperatures ranging from 450–550°C and wet chemical oxidation (e.g. H2O2). However, minerals can go through alterations, or even complete dissolution, depending on applied pre- treatment (Brindley and Brown, 1984; Mikutta et al., 2005). Due to this, the general recommendation is to apply minimum pre-treatment prior to mineral analysis. This is problematic when considering another general recommendation, which is to remove excess organic matter (>3–5%) prior to XRD analysis (Moore and Reynolds, 1997). Based on these conflicting recommendations, and the particularity of Sphagnum peat samples (highly organic, decomposition resistant), a test scheme to evaluate which method was most efficient for the removal of OM but at the same time most gentle to the mineral component. The test scheme (Fig. 10), included an initial exploratory study (Fig. 10, A.) where combustion at 450°C was compared to oxidation by H2O2. The result from the exploratory study was used to guide the test scheme in the replicate study (Fig. 10, B.) Based on the results of the exploratory study H2O2 was not included in the replicate study, instead Na2S2O8 was included. In the replicate study a large composite peat sample was homogenized and was subsequently divided into subsamples that were subjected to the different pre-treatments (n=3, per pre-treatment), followed by XRD mineral analysis. Clay mineral standards were also added to the sub-samples, to verify the effect the OM removal method had on these minerals.

Figure 10. Test procedure for evaluation of efficacy and effect of the different pre-treatments. (A.) An initial exploratory study was conducted, followed up with (B.) a replicate study on homogenised peat sub-samples subjected to combustion in different temperatures (300–550°C), or chemical oxidation with Na2S2O8. The residues were then analysed by XRD and for residual carbon content. Figure from Paper I (Fig. 1).

The results showed that combustion at 500°C was most efficient in removing OM, while oxidation with Na2S2O8 preserved the clay minerals but relatively large amounts of OM still remained (~16%). Based on the results of the replicate study, the recommended protocol is combustion at 500°C, which will preserve the majority of common dust minerals. Phase changes can occur during the pre-treatment, but the changes can mostly be anticipated when the temperature and pressure of the combustion setting are known. One advantage in using the combustion approach is that this is a standard procedure within peat studies to determine organic matter content, thereby reducing additional pre-treatment steps and enabling multiple analysis of the ash residue. If particular clay mineral phases need to be examined, wet chemical oxidation by Na2S2O8 can be considered, although this approach requires that larger organic particles are sieved away prior to oxidation, which also risks removing mineral particles.

Paper II: Paleodust deposition and peat accumulation rates – Bog size matters Sjöström, J.K., Martínez Cortizas, A., Hansson, S.V., Silva Sánchez, N., Bindler, R., Rydberg, J., Mörth, C-M., Ryberg, E.E.S., Kylander, M.E.

This paper presents a peat paleodust and accumulation rate reconstruction study covering the last 8300 years from Draftinge Mosse bog, located in south-central Sweden. Three peat sequences were retrieved in 2014, 2017, and 2019, respectively. The bulk of the analyses (e.g. 14C dating, peat composition, bulk density, ash content, major and trace elemental content, δ 13C, and PAR were conducted on the 2014 sequence, while the 2017 sequence was analysed for downcore pH variability and bulk density, and the 2019 sequence for bulk density and macro-fossil content. The results showed that Draftinge Mosse initiated as a minerotrophic fen that tran- sitioned into ombrotrophic conditions around 5600 years ago (Fig. 11). From the elemental data-set it was inferred that the transition was coupled to a decrease in the grain size of the minerals deposited, suggesting dominantly fluvial transport of minerals during the fen stage. This finding highlights the importance of establishing the successional stages of bogs if the goal is to reconstruct atmospheric mineral dust deposition. Four DE were recorded during the ombrotrophic phase at (c. cal BP): 2200; 1385–1150; 830–590, and from 420 to the present. Draftinge Mosse recorded large variability in PAR (between 8 – 220 g m-2 yr-1) with a period of exceptional growth between 6090 and 4530 cal BP (HPAE) followed by a potential hiatus in accumulation between 4000 and 2500 cal BP. Throughout the studied period, increases in PAR co- occurred with most of the recorded DE, indicating a coupling between dust deposition and accumulation rates. The non-humified peat observed during the HPAE, in combination with the high accumulation rates, indicates that water tables were high during his period. The elemental profiles also indicate that a distinct mineral composition may have been deposited during this time, enriched in minerals hosting important nutrients (P, K, and Ca). XRD mineral data from this period identified quartz, calcite, and a doubled-layered hydrotalcite like mineral. This was coupled to decreases in the L/HREE ratio, indicating that the ratio may reflect the carbonate content because HREE may be enriched in these minerals (Tyler, 2004). The identification of calcite and hydrotalcite was surprising as these minerals are not stable in acidic conditions. The presence of carbonates suggest that they may either have been formed during the pre-treatment (ignition at 500°C), alternatively precipitated by the vegetation within the cell wall, which may have protected the minerals from dissolution. In support of the latter, previous studies have found calcite precipitates within the cell wall in Sphagnum peat (Rudmin et al., 2018). The presence of these minerals, together with the elemental profiles, nevertheless support that a distinct mineral composition was deposited during this period, as previously suggested by Kylander et al. (2018) in nearby Store

Mosse, but the lack of observation of these minerals by XRD suggests that the minerals may have been weathered away.

Figure 11. Draftinge Mosse summary figure depicting peat composition, pH, and (A.) Ash accumulation rates (AAR), peat accumulation rates (PAR), Sc mass accumulation rate (MAR), (B.) mineral content, (C.) elemental ratios, and (D.) Result of the statistical analysis (PCA) showing the weight of the components throughout the studied period (based on the full sequence PCA). Figure from Paper II (Fig. 5).

When comparing mineral DE and PAR between Draftinge Mosse (400 ha) and Store Mosse (7700 ha), many similarities were observed (Fig. 12) but also a few differences. When comparing DE (as inferred from Sc MAR profiles) during the ombrotrophic stage they occur at roughly the same time at Draftinge Mosse and Store Mosse, but the magnitude of the DE differed. Overall higher accumulation rates were recorded at the more moderately sized Draftinge Mosse, suggesting that local sources likely dominate the mineral assemblage deposited on bog, but that a common forcing caused the DE (e.g. moisture changes and/or increased wind strengths). The PAR at the two sites displayed similar variability, but also here some differences were noted. For example, larger variability was noted at Draftinge Mosse compared to Store Mosse, indicating that the size of the bog might affect its’ buffering capacity.

Figure 12. Comparison of PAR and Sc MAR at A). Draftinge Mosse, B.) Store Mosse. Figure from Paper II (Fig. 6)

Paper III: Peat paleodust deposition and accumulation rates during 5400 years in south-central Sweden: processes and links Sjöström, J. K., Bindler, R., Martínez Cortizas, A., Hansson, S.V., Kylander, M.E

In this paper paleodust deposition and accumulation rates were reconstructed from a peat sequence from Davidmosse, located c. 30 km from the west coast, southern Swe- den. The sequence covers the period from 5400 to 360 cal BP, and was studied for peat composition, bulk density, elemental, mineral and ash content. In addition, local soils from the area surrounding the bog, as well as from aeolian dunes located closer to the coast were analysed for grain size and mineral content. The Davidsmosse sequence recorded 14 episodes of increased mineral deposition (cal BP): 3580–3490 (1); 3280 (2); 3140 (3); 3010–2840 (4); 2740 (5); 2610 (6); 2480 (7); 2340 (8); 2240– 2130 (9); 2050 (10); 1890 (11); 1690 (12); 1240 (13); and 960–490 (14) (Fig. 13). Increases in PAR were observed during the majority of the mineral events. The comparison betwee elemental and mineralogical content of local sources and bog minerals, suggest that mainly local sources dominate the mineral assemblage deposited on the bog, in line with the suggestion of Paper II. Two phases recorded numerous episodic and coarse-grained DE (cal BP): 2835–2130 (Fig. 13: Unit IV) and 1000–490 cal BP (Fig. 13: Unit VI). These periods of increased activity are not clearly expressed at the two inland sites of Draftinge Mosse and Store Mosse (Fig. 13). Rather these periods seem to agree with periods of increased storm frequencies seen in two proxi- mal coastal peat paleostorm records (located c. 30 and 60 km from Davidsmosse) (Björck and Clemmensen, 2004; Jong et al., 2006). These periods of increased stormiess have also been noted in dune reconstructions in Denmark as well as in a Scottish coastal bog (Clemmensen et al., 2001; Kylander et al., 2020). Periods of increased (winter) storm frequency in the North Atlantic region have been related to extended sea ice south of Iceland coupled to a southward shift of polar storm tracks. If the DE observed during Units IV and VI are a results of winter storms this suggest that the DE represents a winter signal, whereas the observed increases in PAR must have occurred during the active growing season (i.e. spring, summer, autumn). Colder periods in Scandinavia are often coupled to wetter summer conditions (Seppä et al., 2009), so the increases in PAR observed contemporary with the mineral events likely also reflect moister conditions, in combination with increased nutrient input from the mineral deposition. The similarity of the Davidsmosse sequence to these storm records indicates that coastal bogs records episodic events (i.e. storms) that are coupled to North Atlantic atmospheric circulation changes, whereas the inland bogs record saw fewer but more prolonged events. Interestingly, the events observed at the inland bogs are all reflected at Davidsmosse. Wheather this means that events observed at the inland bogs represent stronger winds that were sustained further inland compared to events that were only observed at Davidsmosse, or alternatively that they represent prolonged drier periods with reduced vegetation cover

28 over a greater region remains to be established. The increased PAR, however, rather suggest that high water tables persisted. Further studies are required to untangle which processes are responsible for this pattern, for example by examining the grain size distribution of an events recorded at all three bogs coupled to pollen analysis to elaborate on the variability in vegetation cover (see Future studies for further details). The observed differences between the three sites nevertheless suggest that the location of the bog matters for which type of aeolian events that will be recorded. The type of source also differs between the coastal bogs and the inland sites, with coastal sites being located closer to beaches which host abundant, non vegetated, source material available for uplift, whereas the inland sites hosts dunes that may have been re-acti- vated during drier and/or windier periods.

Figure 13. Summary figure from Paper III depicting (A.) Peat accumulation rate (PAR), C/N ratio and mineral dust events (as inferred from the statistical associations, CP2) and the Si/Al ratio, used to infer the grain size. (B.) Depiction of dust deposition at Draftinge Mosse and Store Mosse respectively, using the lithogenic element Ti, and the result of a nearby bog storm record (Hyltemossen, Björck and Clemmensen 2004) where storm events was inferred by point counting of sand sized quartz grains.

Discussion

The aims within this doctoral thesis were to develop a protocol for pre-treatment of highly organic peat samples, with the purpose to conduct XRD mineral analysis (Pa- per I), and to extend the number of peat paleodust studies in southern Sweden through paleodust reconstruction at Draftinge Mosse (Paper II) and Davidsmosse (Paper III), and compare dust deposition and PAR between the sites. During the thesis work, another objective developed – to explore links between paleodust deposition and PAR (Paper II, Paper III).

XRD mineral analysis in peat paleodust studies The analytical development and subsequent XRD analysis of peat samples allowed for identification of downcore mineral content (Paper II, III), an approach that to date had only rarely been applied within peat paleodust studies. This approach allowed for linking local mineral sources with minerals deposited on the surface of the bog (Paper III). A potential drawback with XRD mineral analysis is that it identifies only the major phases (95–99%), so that minor minerals may go undetected. Some authors therefore argue that this makes XRD less useful as a source tracing tool, because source changes are often manifested by the minor minerals (Knippertz and Stuut, 2014). Despite this, the work conducted within this thesis has shown that XRD mineral analysis is a valuable complement to the elemental data-sets since it allows for anchoring inferences from elemental data (Paper II, III). The XRD mineral data allowed the identification of mineral phases that otherwise might have gone undetected (e.g. calcite, hydrotalcite), which opened new questions about the environment that allowed for these minerals to be preserved or precipitated (Paper II). From the XRD mineral results, it was also shown that feldspars were absent after 3000 cal BP at both Draftinge Mosse and Davidsmosse (Paper II, III). This cor- responds to a period of generally low detrital mineral input. The XRD mineral analysis, coupled with Rietveld quantification, also aided in source tracing (Paper III).

Peat paleodust and accumulation rates in southern Sweden The multi-proxy analysis of the Draftinge Mosse sequence showed that peat initiation started around 8300 cal BP and that five periods of elevated mineral input occurred throughout the studied period. Based on both the elemental data and macro-fossil content of the sequence the system became ombrotrophic around 5600 cal BP, with inferred coarser particles deposited in the fen stage compared to the ombrotrophic successional stage, which likely indicates that distinct transport processes dominated (fluvial vs. aeolian) in the different stages. Four aeolian DE were recorded in the ombrotrophic section. The HPAE, identified both in Draftinge Mosse and Store Mosse, suggests that moist conditions and high water tables persisted regionally during this period, coupled to deposition of a nutrient rich mineral assemblage. Interestingly, the timing of the HPAE (initiation, peak) was somewhat different between the two bogs, suggesting that local factors, such as size of the bog, may also regulate the response of bogs. This pattern repeated in the period following the HPAE, a period of regionally colder and drier conditions. At Draftinge Mosse peat accumulation halted during this period, while accumulation continued at Store Mosse, albeit at a lower rate. The four DE observed at Draftinge Mosse were also coupled to increased PAR, the same pattern was also mostly observed at Store Mosse, supporting the previous suggestion that mineral deposition affects these nutrient-poor bog ecosystems (Kylander et al., 2018), and furthermore showed that this is a recurring process (Paper II, III). Despite the similarities in DE, the magnitude of the recorded events differed with greater MAR observed at the more moderately sized bog. If the bulk of the minerals deposited on the bogs were fine grained and suspended in the atmosphere prior to deposition the mass accumulation rates should have been more in line. This difference, suggest that the bulk of the minerals deposited on the bog have a local origin, possibly deposited via creep or saltation. However, since periods of elevated mineral input were mostly synchronous at the two bogs, the events were likely caused by a similar forcing mechanism, e.g. drier climate with more exposed soils, and/or stronger winds. The results from the second site, Davidsmosse bog, located c. 30 km from the coast, showed that peat initiation occurred around 5400 years ago and that ombrotrophic conditions were reached around 4760 cal BP. Many more periods of elevated mineral input were recorded at this site (14), compared to the inland sites (4). Two phases recorded numerous episodic events of inferred coarser grain sizes. The character of these events, and resemblance to regional storm records, suggest that they might have been deposited by winter storms. Three bogs in Halland, located 2.5–17 km from the coast, have previously been studied for aeolian sand input by point counting quartz sand grains (Björck and Clemmensen, 2004; Jong et al., 2006). The authors concluded that the size of the recorded particles (100–350 µm) implied niveo- aeolian (saltation, creep) transport. The similarity of the Davidsmosse record to these bog storm records, suggest that also coarse particles were deposited at Davidsmosse, also supported by an increasing Si/Al ratio during these periods (Fig. 13, B.). The peaks of recorded events in Davidsmosse and the Halland storm records were not always synchronous, particularly not during the last 600 years when the uncertainty of

31 the age-models of the sequences are higher, but the general trends agree. Human activities increased during the last 3000 years, with greater influence inferred during the last 1500 years, which may have increased the number and size of source areas and amplified the dust input (Jong et al., 2006, 2009), but the coarse grained character of the events still implies strong winds. Taken together, the results from the paleostorm records, and from Davidsmosse, indicate that storm frequencies have varied during the last 3000 years and that they seem to have been related to atmospheric circulation changes in the North Atlantic during colder periods. The storm events recorded at Davidsmosse may thus reflect winter storminess, whereas the PAR represents the conditions during the growing season. To further increase our understanding of these processes, including the forcing mechanisms that are involved, are important for future storm predictions, which is also important as storms may have profound societal impacts (Feser et al., 2015). When comparing events between Store Mosse, Draftinge Mosse, and Davidsmosse knowledge about the analytical resolution is important because similarities or differences may be biased by the analytical resolution. The average resolution of the elemental analyses covering the last 4760 years was 48 years at Store Mosse (n=100), 82 years at Draftinge Mosse (n=58), and 44 years at Davidsmosse (n=108), indicating that the sample resolution does not account for the observed differences in recorded mineral events as the resolution at Store Mosse and Davidsmosse were more or less comparable. Despite the differences in resolution between Store Mosse and Draftinge Mosse, DE and peat accumulation variability were more similar compared to the numerous events recorded at Davidsmosse. That the more inland bogs did not record a majority of the Davidmosse DE (Fig. 13, Fig. 15), whereas the DE of the inland bogs coincide with events at Davidsmosse suggests that the location, and available sources, of the study site will governs which aeolian events will be recorded. This is important to take into consideration for peat paleodust studies ahead.

REE as a source tracing tool The results of the trace elemental analyses of Draftinge Mosse showed that a signifi- cant REE and Sc enrichment occurred in the fen stage, which was difficult to explain only in terms of changing mineral composition considering both the magnitude of the increase (1–3 orders of magnitude) and the statistical decoupling of REE from the other lithogenic elements. When turning to studies on REE behavior in wetlands out- side of peat paleodust studies, a somewhat more complex behavior of REE in wetlands are depicted. Where REE patterns in wetlands are related not only to the source material but also to redox processes and related organic matter and Fe dynamics (Davranche et al., 2016; Broder and Biester, 2017). Fractionation between HREE and LREE between different phases has furthermore been observed, where HREE are bound to organic and carbonate ligands whereas LREE are more prone to binding to Fe and Mn oxides, at least in stream water (Stolpe et al., 2013). The behaviour of REE is also described as being pH dependant, with increased adsorption with increasing pH (Davranche et al., 2015). Biological activity such as REE enrichment by bacteria 32 and fractionation by plant has also been demonstrated (Davranche et al., 2015; Sjöberg et al., 2020). The bulk of these studies were undertaken on fens or wetland stream waters, and a somewhat different behaviour of REE might be expected due to the specific geochemical context in bogs. But because REE analyses peat paleodust studies commonly are done on bulk peat samples (i.e. including REE both in solution and in solid phases), including samples from both the fen and bog stage, these studies nevertheless indicate that REE content and ratios might not only be affected by detrital (solid) mineral phases. This suggests that interpretation of REE profiles needs careful consideration, especially if variation of REE patterns follows trophic status changes (i.e. fen to bog transition) as in Draftinge Mosse, which entails shifts in the geochemical context, e.g. decoupling from soligenous ground water, pH changes, increased/de- creased microbial activity etc (Paper II). In addition, during the HPAE, when greater presence of authigenic minerals (carbonates) was observed, a decrease in the L/HREE ratio was noted.

Peat accumulation rate variability In both Draftinge Mosse and Davidsmosse mineral DE were mostly coupled to increased PAR (Paper II, III) suggesting that nutrient-poor bog ecosystems are affected by dust deposition. The elevated PAR also suggests that effective precipitation was high during the periods of elevated mineral deposition, at least during the growing season. When comparing PAR between the three sites, the two inland sites show larger variability in accumulation rates compared to Davidsmosse (Fig. 14). This is likely an effect of the differences in annual average precipitation where the more the inland bogs are located in an area with lower average precipitation (800 mm yr-1 vs. 1100 mm yr-1) and are therefore likely more vulnerable to precipitation changes. This is supported by previous studies that suggested that the location will govern the limiting factors of bog growth, such that sites located in areas with high moisture access are less sensitive to changes in precipitation, compared to bogs located in areas closer to a temperature or moisture threshold (Charman et al., 2015).

Figure 14. PAR at Davidsmosse, Draftinge Mosse and Store Mosse during the last 5400 years

Conclusions • XRD mineral analysis is a valuable tool in peat paleodust studies that can be used to identify peat minerals, verify or anchor inferences from elemental data-sets, and identify biogenic minerals. By applying the protocol developed here (LOI 500°C) the ash residue can be used to both determine OM content (a standard procedure) as well as for mineral identification.

• REE is a useful trace source tracing tool, but may need to be carefully interpreted because not only detrital minerals may explain their variability, but also allogenic factors such as changing pH, presence of geogenic water, authigenic minerals, and interaction with organic matter and Fe colloids being potentially relevant.

• The comparison between DE at Draftinge Mosse and Store Mosse showed that these nearby records displayed many similarities both in terms of mineral deposition, as well as in PAR. This indicates that a similar forcing mechanism drove both the DE and variability in PAR. However, a few differences were also noted, such as the magnitude of lithogenic mass accumulation, and precise timing and intensity of PAR changes. These differences were related to the size difference between the two bogs (400 ha and 7700 ha, respectively), indicating that local factors (e.g. successional stage, bog size) need to be taken into consideration when reconstructing mineral dust deposition and PAR.

• The reconstruction of mineral deposition and PAR at Davidsmosse, a bog located closer to the coast compared to Draftinge Mosse and Store Mosse, showed many more DE (14) compared to the more inland sites (4) and overall seemed to be more in line with both nearby and more distal storm records, suggesting that the location of the bog influences which type, or intensity, of aeolian event that will be recorded.

• The events recorded at the inland bogs were all reflected at Davidsmosse, indicating that DE recorded inland either represents particularly strong winds, or alternatively, that regionally dry conditions persisted during these periods.

• The majority of minerals deposited on these south Swedish bogs seems to- have had a local origin, but a similar forcing mechanism, e.g. strong winds, caused mobilization and deposition of the particles.

• Increases in PAR were here recorded in parallel with most of the DE, suggesting that that mineral deposition may affect bog ecosystems and carbon uptake as suggested by previous research (Kylander et al., 2018). The work conducted here showed that this seems to be a recurring process in bogs.

Future studies

Regional comparison of lithogenic mass accumulation rates The result in Paper II suggested that the size of the bog might affect mineral deposition, based on the fact that the moderately sized Draftinge Mosse accumulated lithogenic elements at higher rates compared to the extensive Store Mosse. To further explore if this finding is relevant for other bog systems could increase our understanding about how the size of the bog might affect nutrients loads as well as how local factors influence the peat paleodust record. Previous studies, using another aeolian transported particle, namely pollen, have shown that the size of the lake or bog influences the spatial scale that the reconstruction represents (Sugita, 2007), with larger basins and bogs representing a wider regional scales compared to smaller. The existence of a similar relationship can be tested by comparing mass accumulation rates over several bogs, and what each might tell us about past climate patterns. Although the work conducted within this thesis compared DE between three bogs, differences in the magnitude of the deposition was only compared between Draftinge Mosse and Store Mosse. This was due to that the analytical quality of the elemental data at the three sites varied. Ideally, we would like to calculate and compare the same elemental MAR across the sites. Unfortunately, the only available lithogenic element studied at all three sites was Ti (Fig. 15), with reported recoveries of around 60% from Store Mosse (Kylander et al., 2013), and 75% from Draftinge Mosse (Sjöström et al., 2020) (Fig. 15). Additional elemental analysis would enable a more detailed comparison of differences in mass accumulation rates. A study with this approach could also potentially elaborate and test, using the three bogs studied here, alternative methods to calculate ASD based on single elements (Shotyk et al., 2002), which has been shown to give biased deposition rates (Kylander et al., 2016; Martínez Cortizas et al., 2019). For example by calculating elemental concentration into oxide content, and combining the major lithogenic oxides (e.g. SiO2 + AlO3).

Fig. 15 Comparison of Ti MAR between Davidsmosse, Draftinge Mosse, and Store Mosse using the Ghost proxy tool (Blaauw and Christen, 2011). The analytical recovery of Ti at Draftinge Mosse (~75%) and Store Mosse (~60%) prevents a detailed comparison but the DE and dust depsotion trends can still be observed. Note the difference in magnitude of Ti MAR between the different bogs.

REE content and ratios in different phases Within peat paleodust studies the REE content of bulk peat samples are assumed to mainly reflect detrital mineral content, whereas recent studies describe a more complex behavior of REE in wetlands (see Discussion chapter). Some of these differences likely stem from how REE in solution behave compared to REE hosted in minerals. However, many REE containing minerals are not stable in bogs (e.g. phosphates, carbonates, clay minerals). To also consider the solution geochemistry of REE might therefore be applicable. In a first step, investigating REE phase association in the Draftinge Mosse sequence might inform on if ratio changes (ie between L/HREE) is related to shifts in phase associations, authigenic minerals, detrital minerals, or a combination. By using an already well studied sequence, such as Draftinge Mosse, would enable comparison with an already existing data-set.

Arbuzov (2018), studied a peat sequence from Western Siberia for REE content and reported that most of the REE (40–80%) was associated with hydrolyzable sub- stances, with fractionation between the REE also observed in different phases. To verify if this pattern is repeated in other peatlands analysis of peat samples for REE content in water soluble, acid soluble, and lithogenic mineral phases, could be conducted ahead. The results would indicative of which phases hosts the REE, if the dominant phases varies throughout the sequence, and if there are ratio changes between the different phases. This could enable further understanding of REE behavior in peat profiles, and what governs their variability throughout peat profiles.

Grain-size analysis The finding that the size of the bog is important for the magnitude of deposited minerals (Paper II), together with the results from Davidsmosse (Paper III) showing similarities with bog storm records reconstructed from counting of sand sized quartz grains in bogs, raises questions about the size of the deposited minerals in the bogs studied here. An attempt was made to conduct grain size analysis on the Davidsmosse sequence in a Malvern Mastersizer 2000, but due to the low sample mass, even after combining up to six samples, insufficient obscuration values were achieved, prevent- ing analysis. However, using a Mastersizer equipped with a small volume sample unit should enable analysis ahead. Another study goal could be to sample across a bog surface to analyze variations in grain size. This would enable further understanding if and how grain sizes varies across the bog surface, which would enable further understanding of the main transport mechanisms, i.e. if the minerals deposited on the bog were deposited from suspension in the atmosphere, saltation, or creep. In turn this will help inform what climate information can be inferred from past grain-size changes in a particular record.

GPR, depth measurements and carbon content In addition to the work described within this thesis, several field campaigns to Draft- inge Mosse and Store Mosse to measure basal depths and within peat stratigraphy with ground penetrating radar (GPR) were conducted. The aim of this ongoing work is to both to get a better understanding of bog basin topography, as well as to explore if within-peat stratigraphy can be captured by GPR analysis with the goal to calculate regional C budgets based on physically anchored measurements. Previous work has shown that physical basal depth measurements, both by probing the peat column, as well as GPR measurements can cause over- or underestimation of total depths (Parry et al., 2014)(Parry et al., 2014). These biases can be caused by hitting resistant material or layers, such as fossil tree parts or sand layers during physical probing, while GPR can give incorrect basal depths based on that the radar frequency will go through parts of the peat column at different velocities, related to differences in water content 38

(Parry et al., 2014). Combining GPR measurements with depth probing is therefore recommended. Some early results of these measurements are displayed below (Fig. 16 and 17). Figure 16 shows the result of across bog probing at Draftinge Mosse, conducted during outreach projects with high school students in autumn 2017 and 2019. Each dot in the figure represent a probe depth measurement (color indicates depth), whereas the lines represents interpolated depths (data processing and map design by R. Gyllen- creutz).

Figure 16. Map showing the result of depth probe measurements (coloured dots) at Draftinge Mosse,and contour lines.

The radar analysis was conducted using a Malå GPR equipment, equipped with a 100 MHz rough terrain antenna In addition to the GPR measurements and depth probing, a west to east transect was also cored (using a Russian corer) to obtain information about the vertical location of the fen to bog transition across Draftinge Mosse (Fig 17.). This data was then used to guide interpretation of the GPR data, indicating that the results from the GPR measurements reliably capture within peat stratigraphy changes such as the fen to bog transition (Fig. 17). These studies are still ongoing and further data-processing, data collection, and interpretation are still under development. The study design was outlined by M. Kylander with study design modifications, fieldwork, and data-processing by B. Reinardy, R. Gyllencreutz, B. Chandler, and the author.

Figure 17. A west to east GPR transect across Draftinge Mosse, including the results of nine depth measurements (yellow) and fen bog transitions (green horizontal line) across a west to east transect. These early results indicate that GPR measurements can be used to depict within peat stratigraphy changes. Processing of data was conducted by B. Chandler in Reflexw and Matlab using the following processing steps: 1) dewowing, 2) static correction, 3) bandpass filtering (Ormsby filter with corner frequencies of 40–80–160– 320 MHz), 4) Trace regularisation to 0.25 m using a 2D linear interpolation algorithm (Matlab), 5) background removal, 6) Automatic gain control (AGC) with a window length of 200 ns to boost amplitudes.

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Acknowledgements

First and foremost, I want to thank my three supervisors. Dr Malin Kylander: thank you for your relentless support, all the comments and feedback, for believing in me, for your friendship, and last but not least – all the laughs! I feel so lucky and grateful to have received so much support – thank you! I also want to thank Prof. Rich Bindler for all the support and feedback during these years which helped condense my work, take a step back, and move forward when I felt stuck. Prof. Carl-Magnus Mörth: thank you for all the support during these years: ICP analysis; data interpretation; and work- life balance discussions. I also want to thank Prof. Antonio Martínez Cortizas for in- troducing me to the world of statistics, all the scientific discussions, science camps, and for your friendship. Dr. Olalla López-Costas is thanked for all the collaboration, the glance into the world of paleopathology, and for all the fun times in Stockholm, Santiago de Compostela, and Dublin. I am also grateful to all of my co-workers at the Department of Geological Sci- ences: - thank you for your friendship and support. A special thanks to Carina Johans- son, Richard, Gyllencreutz, Heike Siegmund, Per Hjelmqvist, Benedict Reinardy, Frederik Schenk, and Mikaela Holm for all the help in the lab, scientific discussions, fieldwork, and for your friendship. I am also grateful to Eve Arnold for all the support during the early days this project, and for always keeping an open door. I am also grateful to Therese Granlund and Jenny Söderlind for the help during the early days of the project. Hildred Crill is thanked for helping me become a better writer, and for giving me thoughtful and valuable feedback during presentation rehearsals. Thanks also to Björn Eriksson, Arne Lif and Krister Junghahn for IT support, logistics, and various tasks around the office. I also want to thank all my friends in the PhD group for support, “fika” breaks, and scientific discussions. I owe a special thanks to Alba Legarda, Elin Tollefson, Eleonor Ryberg, Daniel Ellerton, Petter Hällberg, Sandra Gdaniec, Henrik Swärd, Natalia Barrientos Macho, Hagen Bender, Reuben Hansman, Alexandre Peillod, and Susanne Sjöberg – your friendship helped me endure the hard times as well as to enjoy the best. I am also endlessly grateful to my family. Daniel, Emil, and Ingrid: you are the world to me, and I love you so much. Thank you for all your support and love. Mamma och pappa, Carola, Malin, Anders och Zacharias: tack för att ni finns, allt ert stöd, och att ni gjort för mig. Thanks also to all my dear friends, Emma, Lina, Louisa, and Marcela, for being there, supporting me, and for your loyal friendship. I am also grateful to Maria Zachario for all the support during these years. I also want to extend gratitude towards everyone that I forgot to mention here – you know who you are!

Picture of early peat explorers. Bjurulf (left) and assistant (right), during a SGU peat mapping campaign in Sweden in the 1920s. Coring equipment attached to bikes. Source: Von Post and Granlund (1926).