Abstract

The uncertainties regarding future climate change have put considerable

notice to the climate variability following the Late Glacial-Holocene

transition (ca. 13-9 ka BP) in the North Atlantic region as well as the forcing

mechanisms behind climate changes. Much attention has focused on short

climate events in order to understand the mechanisms that drove these

changes but also to identify the leads and lags in the climate system.

Chronological uncertainties for these events remain but an accurate

chronological framework for the North Atlantic region would enhance

possibilities to solve some of the chronological questions.

Tephrochronology uses volcanic ash from a volcanic eruption which

creates a marker horizon in marine and lake sediments, peat bogs and

glacier ice as the ash is spread over large areas. These time-parallel markers

allow precise correlations between archives. The purpose of this thesis is to

improve and refine the Early Holocene tephrochronological framework with

focus on dating and identification of new and previously known tephra

horizons on the Faroe Islands, especially around the climatic events of the

Preboreal Oscillation (11,300-11,100 cal. yr BP), the Erdalen events

(10,100-10,050 and 10,000-9800 cal. yr BP) and the 9.3 ka BP event. A

second goal is to develop the methodology of tephrochronology for finding

cryptotephra (not visible by the eye) horizons in lacustrine sediments.

The findings of eight tephra horizons spanning ca. 11,350 to 9700 cal yr

BP where three are known from other locations in Europe show the

potential of tephrochronology for linking records across the North Atlantic

region. Refined ages for the Askja-S and Hässeldalen tephra were obtained

from an age model built on eight AMS radiocarbon ages with the

Saksunarvatn ash as a constrained age. The results from using XRF ITRAX

core scanner to locate tephra in lacustrine sediments illustrate that high

concentrations of basaltic tephra could be captured but not lower

concentrations of rhyolitic shards.

2

This licentiate thesis consists of a summary and two papers:

Paper I

Lind, E.M. and Wastegård, S. in press (2011). Tephra horizons contemporary to short

Early Holocene climate fluctuations: New results from the Faroe Islands. Quaternary

International, doi:10.1016/j.quaint.2011.05.014.

Paper II

Kylander, M.E., Lind, E.M., Wastegård, S. and Löwemark, L. in press (2012).

Experiences from XRF Core Scanning of Gyttja Rich Sediments for Tephrochronology.

The Holocene 22 (3).

In Paper I Stefan Wastegård, Liselott Wilin and I carried out field-work on the Faroe

Islands. Analyses of the lithology, the organic carbon content, picking and identifying

of macrofossils for radiocarbon dating were done by me and Liselott Wilin with help

from Mats Regnell. Analysis of the main element of the tephra shards, the interpretation

of the results and the construction of the age-depth model were all done in collaboration

with Stefan Wastegård. The extraction of tephra, the counting of shards as well as the

writing and drawing of tables, maps and figures were done by me.

Paper II is also based on sediment from the fieldwork on the Faroe Islands. Malin

Kylander made the XRF core scanning and the geochemical analyses and most of the

writing. I did the tephra extraction, the shard counting and the geochemical analyses. I

draw the tables, the map and helped out with other figures together with Malin

Kylander. All co-authors participated in the interpretation of the results.

3

Towards an Early Holocene Tephrochronology for the Faroe Islands:

Methodology and first results

Introduction

The uncertainties regarding future climate change have put considerable notice to the

climate variability following the Late Glacial-Holocene transition (ca. 13-9 ka BP) in

the North Atlantic region as well as the forcing mechanisms behind climate changes.

Environmental response to Early Holocene warming was complicated and did not reveal

a consistent pattern over the area. Much attention has focused on short climate events in

order to understand the mechanisms that drove these changes but also to identify the

leads and lags in the climate system. Chronological uncertainties for these events

remain but an accurate chronological framework for the North Atlantic region would

enhance possibilities to solve some of the chronological questions.

The general warming of the Early Holocene was interrupted by a number of climate

fluctuations (Dansgaard et al., 1971; Rundgren, 1995; Bond et al., 1997; Mayewski et

al., 2004; Nesje, 2009). Several short cooling events like the Preboreal oscillation

(PBO), the Erdalen events, the 9.3 ka BP event and the abrupt cold 8.2 ka BP event

(Table 1) occurred before the warmest period of the Holocene, the Holocene thermal

maximum (HTM), began (Anderson, 1909; von Post, 1924; 1930; Rosén et al., 2001;

Mayewski et al., 2004; Vinther et al., 2009).

Table 1.Short climate events during Early Holocene recorded in the North Atlantic region.

Age (cal. yr BP) Climate event References

11,300-11,100 Preboreal Oscillation Rundgren (1995); Björck et al. (1996;

1997)

10,100-10,050 and

10,000-9800 Erdalen events

Björck et al. (1996); Dahl et al. (2002);

Bakke et al. (2005)

9300 9.3 ka BP event Rasmussen et al. (2007); Yu et al.

(2010)

8200 8.2 ka BP event

(Finse event)

von Grafenstein et al. (1999); Nesje and

Dahl (2001); Tinner and Lotter (2001);

Alley et al. (2003); Rasmussen et

al.(2007)

4

The PBO and the 9.3 ka BP events are registered as fluctuations in the δ18

O Greenland

ice-core record (Rasmussen et al., 2007) but only the PBO is detectable in lithological

and palynological records from around the Nordic seas (Lowe et al., 1994; Björck et al.,

1996; 1997). None of these events nor the 8.2 ka BP event have yet been registered in

terrestrial records on the Faroe Islands (Jessen et al., 2008; Olsen et al., 2010) but all

above mentioned climate shifts are registered as ice rafted debris (IRD) events in the

North Atlantic (Bond et al., 1997). The Erdalen events are recorded as terminal

moraines locally in Erdalen, Norway (Nesje, 1984) but also from other records around

the North Atlantic (Björck et al., 2001, [10.3 ka BP cooling event]). The most extreme

event in the period in the North Atlantic region was the 8.2 ka BP event which is

recorded in marine and lacustrine proxies and as well as in the ice-core records

(Klitgaard-Kristensen et al., 1998; von Grafenstein et al., 1999; Rasmussen et al.,

2007).

Also the HTM warming has been recorded in different archives in many places in

Europe (Snowball et al., 2004; Bakke et al., 2005; Andresen et al., 2006; Renssen et al.,

2009) but at different periods and with different duration. Although similarities in the

palaeoclimatic reconstructions from different parts of the North Atlantic region are

present, there are still uncertainties not only to the degree of synchroneity or

asynchroneity between sites, but also as to the chronologies. These uncertainties are one

of the reasons behind the rising demands in palaeoclimate research for precise dating

techniques and of studies with high temporal resolution. The ultimate chronological

framework would be one that ties the ice-core records together with the marine, the

lacustrine and the terrestrial archives.

Tephrochronology can help to address the demands for better age control by linking

and dating palaeoclimatic sequences or events (e.g. Lowe, 2001). The deposition of

tephra from a volcanic eruption is nearly instantaneous and creates a marker horizon in

marine and lake sediments, peat bogs and glacier ice as the ash is spread over large

areas. These time-parallel markers allow precise correlations between archives (Turney

and Lowe, 2001) and the method relies firstly on stratigraphy (Feibel, 1999). Few

geochronological methods have the capacity of tephrochronology that offers both

temporal and spatial precision (Lowe, 2011).

The proximity to Iceland with its volcanic activity as well as the Faroe Islands

location in the main branch of the North Atlantic Current make the islands an ideal

place to search for tephra but also to conduct climate research. Sediment sequences

reaching back to Early Holocene are sparse on Iceland and for that reason the Faroe

Islands is probably a better place to find tephra. The climate conditions of the Faroe

Islands are controlled by its location in the pathway of the westerly cyclones and by the

North Atlantic Current.

5

Objectives

If the identification of known or new tephra horizons in close connection to some of the

Early Holocene climate events could be developed and their spatial distribution be

better known, it could provide opportunities to test hypothesis regarding synchronous or

non-synchronous response to climate forcing. The very best outcome of this PhD

project if the terrestrial tephrochronological records from the Faroe Islands and Norway

would be connected to the ice-core records from Greenland. This would greatly improve

the usefulness of the North Atlantic tephrochronological framework. The overall aim of

this PhD study is to i) improve the Early Holocene (ca. 11.6-8 ka BP)

tephrochronological frameworks for the north Atlantic region with focus on dating and

identification of new and previously known tephra horizons; ii) increase the knowledge

of the Early Holocene climate on the Faroe Islands with special focus on short climate

events; iii) try to synchronise terrestrial and ice-core records with special emphasis on

the Preboreal oscillation, the Erdalen events, the 9.3 ka BP event and the 8.2 ka BP

event; iv) apply and evaluate new methods for identification of discrete cryptotephra

horizons.

More specific, the main question addressed in Paper I is:

To improve and refine the Early Holocene tephrochronological framework with

focus on dating and identification of new and previously known tephra horizons

on the Faroe Islands.

The main questions addressed in Paper II are:

To assess the potential of XRF core scanning for locating visible tephra and

cryptotephra layers.

Compare the elemental XRF core scanning data with the electron microprobe

(WDS) analysis.

Study area

The Faroe Islands are an island group consisting of 18 islands with an area of 1399 km2

(Fig. 1). The islands have a maritime climate with cold summers and warm winters. The

mean August temperature is 10.5° C and for January 3.2°C (Rasmussen and Noe-

Nygaard, 1970). The dominating winds come from SW and precipitation increases from

west to northeast from a mean annual of 900 to 2000-2500 mm (Søgaard, 1996). The

Islands are remnants of large low relief plateau of basaltic Paleogene bedrock deriving

from volcanic eruptions prior to the opening of the North Atlantic basin. The landscape

consists of large flat plateaus with steep slopes down to the ocean (Rasmussen and Noe-

6

Nygaard, 1970). The highest peak Slættaratindur lies on the island of Streymoy and has

an altitude of 882 m a.s.l.

During the last glaciation an ice-cap up to 700 m a.s.l. covered the Faroe Islands and

extended out on the shelf. No erratics from outside the Faroe Islands have been found

on the islands and the geomorphic tracks are in the form of striae, Roches Moutonnées

and U-shaped valleys while depositional landforms are sparse (Jørgensen and

Rasmussen, 1986). Moraine systems connected with two glacial stages, one older and

one younger associated with Younger Dryas, has been found (Humlum et al., 1996).

Studies of the deglaciation of the Faroe Islands suggest that they were deglaciated

between 11,400-11,000 cal. yr BP, and that valleys at higher elevation were deglaciated

first (Jóhansen, 1975; Jessen et al., 2008; Olsen et al., 2010). Present vegetation is

strongly affected by sheep grazing and the dominating vegetation consists of sedges,

grasses and perennial herbaceous plants. No trees grow except for a few fenced areas,

but natural tree growth has been identified during earlier periods of the Holocene

(Hannon et al., 2001, 2005; Hannon and Bradshaw, 2008).

The investigated sites are both located on the Faroe Islands in the North Atlantic. The

study site of Paper I is the Høvdarhagi bog on the island of Sandoy (Fig. 1-2). The

second study site (Paper II) is from Havnardalsmyren (Fig. 1) on the island of

Streymoy. Høvdarhagi bog (61°54´N, 06°55´W, 5 m a.s.l) is an overgrown lake located

ca. 1.5 km northwest of the small town Skopun and Havnardalsmyren (62°01’N,

06°84’W; 28 m a.s.l) is a bog in the valley of Havnardalur ca. 4 km northwest of

Tórshavn.

Fig. 1. The North Atlantic region with the

Faroe Islands highlighted and the two study

sites, Høvdarhagi bog and Havnardalsmyren.

Fig. 2. The study site from Paper I, Høvdarhagi

bog, on the island of Sandoy 1.5 km northwest

of the village of Skopun.

7

Methodology

Tephrochronology

The word tephra is of Greek (τέφρα) origin and means ash, and it includes all airborne

material ejected from a volcano during an eruption. The Icelander Sigurdur

Thorarinsson was the first to recognize the possibilities for tephrochronology in

Northern Europe by his research in southwestern Iceland. He defended his thesis at

Stockholm University College in 1944 (Thorarinsson, 1944). Thorarinsson also coined

the term tephrochronology (Thorarinsson, 1944, 1974, 1951, 1981). In the 1960s and

1970s Christer Persson continued Thorarinsson´s tephrochronological work in

Scandinavia with the aim to make a tephrochronological connection between Iceland

and Scandinavia (Persson, 1966, 1968, 1971).

Volcanic ash from an eruption is usually deposited within hours or days (Mills, 2000;

Robock, 2002; Rose and Durant, 2009) but the grain size of tephra shards decreases

with the distance from the volcano and in most parts of northwestern Europe only

tephra shards that cannot be seen by the naked eye (cryptotephra, 10-100 µm) are found

(Einarsson, 1986; Wastegård, 2005). The first use of tephra as time markers was

established in New Zealand and in Tierra del Fuego in the 1920s for Holocene and Late

Pleistocene deposits (Einarsson, 1986 and references therein). The technique of using

far-travelling glass shards of microtephra or cryptotephra is today a well-used method in

Quaternary palaeoenvironmental research (Turney et al., 1998, 2001, 2006; Lowe,

2001; Davies et al., 2002).

Tephrochronology of the north Atlantic region

The tephrochronological framework for Europe derives from volcanic areas in Germany

(Eifel), France (Massif Central), Italy (Campanian and Aeolian Islands plus Etna) (e.g.

van den Boogard and Schmincke, 1984; Juvigné et al., 1996) and from the Norwegian

Jan Mayen (Imsland, 1984), however, in northwestern Europe the majority of identified

tephra horizons derive from Iceland (Persson, 1966, 1968, 1971; Dugmore, 1989, 1995;

Boygle, 1998, 2004). The frequent Icelandic explosive activity back in time produced

numerous tephra layers all deriving from a number of volcanic zones. During the

Holocene these zones are comprised of approximately 30 volcanic systems. The number

of basaltic tephra layers (SiO2 ~50%) is four times that of silicic (SiO2 ~>70%) layers in

historical times, but probably also during the Holocene. The large basaltic production

was favored by the wet environments after the deglaciation but also due to the fact of

ice caps that cover parts of the volcanic zones on Iceland (Larsen and Eiríksson, 2008).

Tephra studies of Icelandic provenance include not only the terrestrial realm

(Wastegård et al., 2001; Davies et al., 2003) but also marine (Rasmussen et al., 2003;

Wastegård et al., 2006; Gudmundsdóttir, 2011; Thornalley et al., 2011) and ice

(Grönvold et al., 1995; Zielinski et al., 1997; Mortensen et al., 2005; Abbott et al., in

8

press). Tephrochronological frameworks have been established on the British Isles,

Scotland and Ireland (e.g. Dugmore, 1989; Bennett et al., 1992; Pilcher and Hall,

1992,1996; Dugmore et al., 1995; Turney et al., 1997; Hall and Pilcher, 2002; Ranner et

al., 2005; Pyne-O´Donnell, 2007), in northern Germany (Merkt et al., 1993; van den

Bogaard and Schmincke, 2002; Lane et al., in press), in Scandinavia (Mangerud et al.,

1984, Birks et al., 1996; Wastegård et al., 1998; Björck and Wastegård., 1999; Boygle,

2004; Pilcher et al., 2005; Davies et al., 2007; Vorren et al., 2007) and on the Faroe

Islands (Persson, 1968, 1971; Mangerud et al., 1986; Dugmore and Newton, 1998;

Wastegård et al., 2001; Wastegård, 2002).

Some tephra horizons more than others are thought to be key horizons. These are

horizons pinpointed by the group: INTegrating Ice-cores, MArine and TErrestrial

records (INTIMATE) and are believed to be important for palaeoclimatic records for the

Late-glacial Holocene transition. An ideal tephra horizon should have a widespread

distribution in close time to a rapid climate event; it must be possible to separate the

geochemical signature from other tephras and a very precise age should be associated

with the layer (Davies et al., in press). One of these Early Holocene tephras is the

Askja-S or Askja 10 ka that has until now been found in Sweden, in Norway, Northern

Ireland and on the Icelandic shelf but most recently also in Germany and on the Faroe

Islands (Davies et al., 2003; Pilcher et al., 2005; Turney et al., 2006; Gudmundsdóttir et

al., 2011; Lane et al., in press; Lind and Wastegård, in press). Askja-S has been 14

C

dated to ca 10.8 ka BP (Wohlfarth et al., 2006) and its age fall between the PBO (11.3-

11.1 ka BP; Rundgren, 1995; Björck et al., 1996; 1997) and the Erdalen events (10.1-

10.0 and 10.0-9.8 ka BP; Dahl et al., 2005; Bakke et al., 2005).

Another widely spread tephra horizon in close time to the Askja-S tephra is the

Saksunarvatn tephra. It has been found on the Faroe Islands, Norway, Germany, the

Orkney Islands, the Shetland Islands (e.g. Waagstein and Jóhansen, 1968; Jóhansen,

1975, 1981, 1985; Mangerud et al., 1984, 1986; Bennett et al., 1992; Merkt et al., 1993;

Bunting, 1994; Birks et al., 1996; Wastegård et al., 2001) and in the NGRIP record

(Mortensen et al., 2005) but still not from mainland Britain and Sweden. The basaltic

Saksunarvatn tephra has been dated to 10,347 ± 89 cal. yr BP (Rasmussen et al., 2006).

The Hässeldalen tephra, previously found in Sweden, has recently also been discovered

in Denmark (Larsen et al. in prep), in Germany (Lane et al., in press) but now also on

the Faroe Islands (Lind and Wastegård, in press). The approximate age of 11,300 cal yr

BP (Davies et al., 2003; Wohlfarth et al., 2006; Lind and Wastegård, (in press) makes it

a really important marker horizon for the Early Holocene warming and the PBO.

Taphonomic considerations or reworking of tephras

Tephra deposition sometimes occurs as patchy patterns in sediments and this is thought

to be caused by redeposition by wind and snow-melt (Boygle, 1999; Bergman et al.,

2004). Tephra can also be trapped in snow, on sea-ice or in icebergs during cold

conditions. A lag time from the initial deposition on snow or ice to the thawing and

9

eventual deposition in lake and ocean occurs (Austin et al., 2004; Davies et al., 2007;

Brendryen et al., 2010; Pyne-O´Donnell, 2011). Weather conditions during the eruption

and atmospheric circulation can have an impact on the deposition pattern but also

variables as stream inlets and outlets of the catchment (Lacasse, 2001; Davies et al.,

2010; Pyne-O´Donnell, 2011). Tephra shards can also migrate down in soft lacustrine

sediment, a process not only subscribed to glass shards but also for microfossils. Other

taphonomic processes are bioturbation and root penetration that pushes the shards

downwards and a long trail of shards is the result (Davies et al., 2001, 2005, 2007;

Pyne-O´Donnell, 2011 and references therein). A reworked part of a tephra layer can be

used as an isochrone but firstly the accurate date and extent of the reworked tephra has

to be identified (Lowe, 2011).

Fingerprinting of tephra

By identifying and characterizing the geochemical signature of each tephra layer the

volcanic source volcano and eruption can often be established and fingerprinted.

Methods used include mineralogical (petrographic) examination by optical microscopy

or geochemical analysis of glass shards or loose mineral grains (free crystals) using the

electron microprobe and other analytical instruments (Lowe, 2011). The chemical

composition of a single glass shard can be analysed with the electron microprobe

technique (EMPA) equipped with wavelength-dispersive spectrometers (WDS), which

has an analytical precision of ±1% and 10 major oxides are usually measured (Hunt et

al., 1998). High quality data can be produced by analysing tephra shards with areas only

3 µm in diameter (Hayward, in press). Another possibility for more precise geochemical

identifications is to measure trace- and rare earth elements in single shards. In one glass

shard with the size of ~10 µm around 30 trace elements can be determined with the

laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) technique

(Pearce et al., 1999, 2007, in press). The optimal use of an ash layer is achieved when

the tephra horizon has a clear geochemical fingerprint and has been dated with one or

several dating methods such as ice-varves, radiocarbon dating or fission-track dating. A

time correlation between sites can then be accomplished and used in this way

tephrochronology is an age-equivalent method.

Methods applied in this study

Methods of detecting tephra in sediments

Locating tephra in sediment is a quite time-consuming method, performed by removing

the organic content, sieving and applying the density separation technique for

minerogenic sediments (Turney, 1998; Davies et al., 2005). Other methods used are

magnetic susceptibility measurements, reflectance variations or screening for tephra

with X-ray diffraction (Pilcher and Hall, 1992; Caseldine et al., 1999; Blockley et al.,

10

2005; Andrews et al., 2006). The XRF core scanner is a non-destructive tool that

acquires in situ high-resolution data from sediment with a large number of geochemical

elements where no sample preparation is needed (Jansen et al., 1998; Francus et al.,

2009). In a number of studies the XRF core scanner has been used successfully for

locating tephra layers (Moreno et al., 2007; Francus et al., 2009; Langdon et al., 2010).

Two different methods were used to detect cryptotephra in the lake sediment. In

Paper I the core was subsampled and searched for tephra while in Paper II a new non-

destructive method (XRF ITRAX) was applied for finding tephra shards. The results of

the non-destructive method were also compared to the subsampling method. The

wavelength dispersive spectroscopy (WDS) geochemical measurements were also

compared with the XRF ITRAX geochemical data. Otherwise the identification and

definitions of the tephra layers were identical in both papers. Major element chemistry

was used to confirm the correlations to a source volcano and/or eruption event.

Counting of shards for every cm was applied to identify long shard tails that could be

indicative of reworked sediments. The age model in Paper I is based on 14

C dated

terrestrial macrofossils and the age of the Saksunarvatn ash of 10,347 ± 89 cal. yr BP

(Rasmussen et al., 2006) as a constrained age.

Tephra extraction

5*1 cm subsamples of sediment were cut out from the Høvdarhagi bog (Paper I) core

and at heated (550°C for 4.5 h) to remove organic matter. After cooling the samples

were homogenized and stored overnight in 10% HCl to remove any carbonates present

in the sediment. The samples were sieved through a 25 and 80 µm mesh with distilled

water and two fractions (2.3-2.5 [rhyolitic tephra] and >2.5 [basaltic tephra] g/cm3)

were extracted by using different densities of heavy liquid (sodium polytungstate;

Na6[H2W12O40]) following the technique described by Turney (1998). The two fractions

were mounted and tephra shards were identified under a polarizing microscope (Fig. 3).

At depths with identified cryptotephra new subsamples were taken out in 1 cm3 cubes

and exposed to the same method as described above and counted.

Fig. 3. A and C. Rhyolitic Askja-S tephra shard. B. Basaltic Saksunarvatn ash shard.

11

Microprobe analyses

Preparation for microprobe analysis and analytical procedures follows Dugmore et al.

(1995) where 1-2 cm3 of sediment containing tephra is exposed to acid digestion

(sulphuric [H2SO4] and nitric [HNO3] acid) and then exposed to the same method of

sieving and heavy liquid separation as described by Turney (1998). For the microprobe

analyses tephra shards were mounted in epoxy on glass slides and polished. Major

element geochemistry of volcanic shards were performed on a WDS electron

microprobe (Cameca SX100) with an accelerating voltage of 15 kV, a 2 nA beam

current, and a beam diameter of 8.8 µm, at the Department of Geology, University of

Edinburgh in November 2009 and October 2010. Sodium oxide (Na2O) was measured

in the first and last counting interval to monitor the degree of mobilization. The other

oxides measured were; silica (SiO2), titanium (TiO2), iron (FeOtot) aluminium (Al2O3),

manganese (MnO), magnesium (MgO), calcium (CaO), potassium (K2O) and

phosphorus (P2O5). The WDS system was calibrated using standard calibration blocks.

Two secondary glass standards BHVO2g and Lipari obsidian (Hunt and Hill, 1996)

were analysed at regular intervals to monitor for instrumental drift and to assess the

precision during analyses. Point analyses were randomly selected from the polished

slide and embedded minerals were avoided. Totals of major oxides below 93% were

omitted.

XRF core scanning

In the search for a faster method to detect cryptotephra in sediment an XRF core

scanner were used. The particular XRF core scanner used, namely the ITRAX from Cox

Analytical Systems (Gothenburg, Sweden) located at the Department of Geological

Sciences at Stockholm University measures a 1 m long core usually within one day and

no sample preparation is needed, thus the sediments are left undamaged. The cores were

first cleaned and covered with a thin plastic film that protects the sediments from

drying. An ITRAX scan produces an optical RGB image, a radiographic signal and µ –

XRF elemental profile. A Mo tub set at 30 kV and 25 mA with a step size of 600 µm

and a dwell time of 50 s were used for scanning as well as a Cr tube set at 30 kV, 40mA

with a step size of 600 µm and a dwell time of 50 s. The Cr tube was used to improve

data for the lighter elements, particularly silica (Si). Reliable (i.e., with sufficient

counts) geochemical data was acquired for Si, potassium (K), calcium (Ca), magnesium

(Mg), iron (Fe), strontium (Sr) and zirconium (Zr). A dispersive energy spectrum is

measured for every point on the core surface and the goodness of fit of each measured

spectra with the theoretical spectra is characterized by the mean standard error (MSE);

the peak fitting parameters are adjusted to reduce the MSE for each measured spectra.

In that there could be problems with scattering or excitation if the sediments have large

differences in composition, water content and porosity or grain sizes all data are

normalized to the (incoherent + coherent) scattering. The MSE and elemental data were

used to try and identify target depths for high-resolution sub-sampling and the method

12

was tested by comparing with shard concentrations. The geochemistry of the tephra

layers as determined using the ITRAX XRF core scanner were also compared to the

WDS systems data.

Radiocarbon dating

Macrofossils for radiometric measurements were picked out from the Høvdarhagi core

at layers with high organic content in order to date the new tephra layers found. 1 cm

samples of sediment were cut out and rinsed in water to clean the plant material from

contaminants. The samples were sent to the AMS facility at the Ångström Laboratory,

Tandem Laboratory, at Uppsala University. The material was there subjected to a

standard acid-base treatment to remove possible contaminants as carbonates and

infiltrated humics. The dried organic material (pH 4) was combusted to CO2 gas and

transformed to solid graphite by a Fe catalytic reaction. Calibrated dates and age-to-

depth modeling was obtained by using OxCal online version 4.1.6 (Ramsey et al., 2010)

and applying the radiocarbon atmospheric calibration curve IntCal09 (Reimer et al.,

2009).

13

Summary of papers

Paper I

Lind, E.M. and Wastegård, S. (2011) in press. Tephra horizons contemporary with short

Early Holocene climate fluctuations: New results from the Faroe Islands. Quaternary

International.

The purpose of this paper was to improve and refine the Early Holocene

tephrochronological framework with focus on dating and identification of new and

previously known tephra horizons on the Faroe Islands, especially around the climatic

events of the Preboreal Oscillation (11,300-11,100 cal. yr BP) and the Erdalen events

(10,100-10.050 and 10,000-9800 cal. yr BP). In a palaeolake core from the island of

Sandoy we found seven tephra horizons with Icelandic provenances.

A depth-age model was based on with eight AMS radiocarbon ages spanning ca.

2000 years from ca. 11,700 to 9750 cal. yr BP. The only visible tephra in the core was

the well-known Saksunarvatn Ash dated to ca. 10,300 cal. yr BP. Three other tephra

horizons were identified above and three below of it (Fig. 4). The rhyolitic Hässeldalen

Tephra (HDT) dates to ca. 11,350 cal. yr BP previously only found in Sweden, a

double-peaked basaltic tephra Sandoy A and B from the Veidivötn-Bárdarbunga

volcanic system at the same depth and another rhyolitic tephra from the Askja volcanic

system dated to ca. 10,400 cal. yr BP, tentatively correlated with the Askja-S/10 ka

Tephra. Askja-S has earlier been found in large parts of the terrestrial North Atlantic

region but not on the Faroe Islands. The three tephra layers above the Saksunarvatn Ash

are all previously unreported tephras. A rhyolitic tephra, L274, with an age of ca.

10,200 cal. yr BP, which origin is still unidentified, one silicic layer (SILK) from the

Katla volcano and a tephra correlated with the Torfajökull volcanic system, both with an

estimated age of ca. 9700 cal. yr BP.

These new tephra horizons show the potential of tephrochronology as a method for

linking and dating climate archives. Tephra from at least seven Icelandic eruptions

reached the Faroe Islands in ca. 2000 years. These tephra horizons have the potential to

be a part of a high-resolution framework for the North Atlantic region with the

possibility to link not only the terrestrial records from NW Europe but also to correlate

them to ice-cores from Greenland. The identification of the Askja-S Tephra and

Hässeldalen Tephra on the Faroe Islands confirms that these tephras are more

widespread than previously thought. Both Askja-S and the HDT are deposited in close

vicinity to the PBO and could be marker horizons for it. The unidentified tephra, L-274,

was deposited ca. 10,200 cal. yr BP and could become a marker horizon for the short

cold Erdalen events but the usefulness remains to be seen until its geochemistry is

identified.

14

Fig. 4. The age model is constrained by eight 14

C dates and the Saksunarvatn Ash

(10.347 ±89; Rasmussen et al., 2006) at peak shard concentration. The climate events

“Erdalen event” refers to ages in Dahl et al. (2002) and the Preboreal Oscillation (PBO)

to ages from Björck et al. (1997). The model is constructed with the online OxCal 4.1.6

software (Bronk Ramsey, 2008) and the INTCal09 calibration curve

(Reimer et al., 2009). All radiocarbon dates were used, although three (Ua-39724, Ua-

39722 and Ua-39725) lowered the agreement index.

15

Paper II

Kylander, M.E., Lind, E.M., Wastegård, S., Löwemark, L. (2012) in press.

Recommendations for Using XRF Core Scanning as a Tool in Tephrochronology. The

Holocene 22 (3).

In this paper we assess the potential of XRF core scanning for tephrochronology but

also to compare the elemental data generated for the detected tephra layers to element

data from individual shard analysis by electron microprobe (WDS). Three cores from

the Faroe Islands, Havnardalsmyren, were scanned with an ITRAX XRF core scanner.

Two of the cores had visible tephra layers, the Saksunarvatn ash (ca. 10,300 cal yr BP),

while the Askja-S tephra (ca. 10,400 cal yr BP) was present as a cryptotephra in all

three cores.

We used a combination of optical and radiographic images, spectral fits and

elemental profiles (Si, K, Ca, Ti, Mn, Fe, Sr and Zr) to locate layers of tephra. The cores

were then subsampled for tephra analysis by removing the organic and carbonate

content, washing and sieving the sediment also the density separation technique

(Turney, 1998) for collecting the two fractions of tephra (rhyolitic and basaltic). Tephra

shards were counted and prepared for microprobe WDS analyses where ten major

oxides were measured (SiO2, TiO2, Al2O3, FeOtot, MnO, MgO, CaO, Na2O, K2O and

P2O5). The results show that the XRF positively captured the higher concentrations of

basaltic tephra shards (SiO2 ~50%) but not the lower rhyolitic (SiO2 ~>70%)

concentrations (Table 2). Possibly there might be a critical value of shard concentration

for detection but no consistent value was found. The XRF core scanning systems also

have more difficulties to acquire Si and therefore it could be a problem to find the Si-

rich rhyolitic shards.

The elemental data from the XRF core scan was not suitable to be used for

geochemical identifications and did not show similarities in the spatial relationship

between the tephra layers (Fig. 5).

16

Table 2. Data for the three cores from Havnardalsmyren,confirmed depths of tephra, depths of visible

tephra, Itrax detection of tephra, tephra found with the subsampling method, type of tephra and maximum

of shard at that level.

Core, depth

(m)

Location of

tephra (m)

Visible

tephra

ITRAX

detection of

tephra

Tephra detection

conventional

method

Tephra

type

Maximum of

shards/ cm3 wet

sediment

3A: 3.00-4.00 3.20-3.22 Yes Yes Yes B c. 40,000

3.85-3.89 No No Yes R c. 850

2D: 3.13-4.13 3.13-3.32 3.24-

3.27 Yes Yes B 12-41,000

3.35-3.45 No Yes Yes B c. 60

3.85-3.87 No No Yes R c. 90

5A: 3.40-4.40 3.60-3.65 No No Yes B c. 100

4.10-4.14 No No Yes R c. 160

B refers to basaltic tephra while R is for rhyolitic tephra.

Fig. 5. Bi-plot of normalized FeOtot vs. TiO2 from tephra layers H1-H4 in core 2D as

analysed by electron microprobe on individual shards (A) and Fe and Ti scattering

normalized peak areas from bulk sediment using XRF core sanner data (B).

17

Ongoing and future work

During the second half of the PhD project I will continue to improve the tephro-

chronological network for the North Atlantic region by searching for tephra in a new

field area in central Norway. I will be involved in the search for tephra in the NEEM

ice-core from Greenland. The study of short Early Holocene climate fluctuations on the

Faroe Islands will also be finalised. More specifically, the future work will include:

i. A study with emphasis on synchronization of the Early Holocene North Atlantic

tephrochronological framework by connecting the tephra records from the Faroe

Islands with tephra horizons from the new field area; central Norway but also with

records from some of the Greenland ice-cores; NEEM, NGRIP or GRIP.

During spring 2011 ~100 m of Early Holocene ice samples from the Greenland NEEM

ice-core have been sampled for tephra. In August 2011 I went with colleagues from

Stockholm University for fieldwork on Fosen, northwest of Trondheim, central Norway

and brought 20 m of Late Glacial-Early Holocene sediment back to Stockholm for

analyses. The analyses of these sediment sequences for tephra will start in October this

year.

ii. A multiproxy study where I focus on the short climate events during early

Holocene, the Preboreal Oscillation, the Erdalen events and the short cold period

around 9.3 and 8.2 ka BP on the Faroe Islands.

The study will be performed on a palaeolake core from Høvdarhagi bog, Faroe Islands.

An age-depth model has already been constructed; pollen- and organic carbon content

analyses are already conducted (Wilin, 2010) as well as XRF ITRAX core scanning of

the cores. In September the results from the stable isotope analyses (bulk δ13

C) will be

ready and in December 2011 I will start with macrofossil analyses on the core.

iii. There are some considerations about the Saksunarvatn tephra deriving from three

or four eruptions instead of one and during my work with the Faroese sediments I

have come across several layers in one core closely spaced but with the same

main element geochemistry. Laser ablation inductively coupled plasma mass

spectrometry (LA-ICP-MS) have been performed on the tephra shards from these

different layers and some preliminary results show that the layers might be able to

distinguish from each other.

18

Acknowledgements

First and foremost I would like to thank my family for putting up with me although I am

often out of town and have little time to be an ordinary mom who bakes cakes instead of

dashing about in different geological contexts. But I am pretty certain this makes me a

nicer person to be around the days I am at home.

I would also to thank my dear supervisor Stefan Wastegård for his big patience,

interesting discussions and for always having time for me and my ideas. Furthermore I

wish to thank my other supervisors; Margareta Hansson and Britta Sannel for the help

I've received so far, in time I will probably test your patience too. The Department of

Physical Geography and Quaternary Geology is a nice place to work at and the floorball

gang makes it even nicer. Thanks also to all my friends, my roomy and other PhD

students for all the laughs, funny moments and planning meetings. I would also like to

thank my co-authors for good teamwork and interesting discussions and Frank Preusser

for the help I received to improve my thesis. Last but not least I wish to thank my field

assistant Liselott Wilin for making my fantastic field trips even more fantastic. My

work has been funded by the Swedish Research Council, Ahlmann´s Foundation,

Swedish Society for Anthropology and Geography (SSAG) and Bert Bolin Centre for

Climate Research (BBCC).

19

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