Cover photograph: Mt. Pico, Pico Island, , October 2012 (photo: Hans Johansson)

Abstract

The Azores is situated in the North Atlantic Ocean and is one of the most active volcanic regions in the Northern Hemisphere. The volcanic history of the islands is fairly well known and several explosive trachytic eruptions have been reported but the geo‐ chemical compositions of the glass component of the as well as the dispersal of to distal areas are less well known. The purpose of this study is twofold: (1) to present major element geochemistry of the glass component from several historic as well as prehistoric tephras, including the trachybasaltic AD 1957‐58 eruption on the island of Faial, and the trachytic explosive eruptions of Sete Cidades (c. AD 1440), Fogo A (c. 5000 BP), Fogo AD 1563 and Furnas AD 1630 on the island of São Miguel; (2) to present a refined tephrostratigraphy for the island of Pico. Analyses of major element geochemistry suggest that tephras from the three active stratovolcanoes on São Miguel can be separated in biplots showing e.g. FeOtot vs.TiO2 and FeOtot vs. CaO.

The tephrostratigraphy of Caveiro bog on the island of Pico is based on a radiocarbon dated core with eight tephra layers extending back to c. 7000 BP. All tephras are of trachybasaltic/basaltic trachyandesitic composition except the oldest layer, which is of basanitic composition. An attempt was made to correlate the tephra record of Caveiro bog with the previously investigated Caveiro. A tephra‐based correlation between the Caveiro bog and Lake Caveiro is not straightforward and only three tephras in Caveiro bog can possibly be correlated with tephras found in the sediments of Lake Caveiro.

Proximal glass data from the Furnas volcano on São Miguel suggest that distal crypto‐ tephras found in Ireland may have an origin in the Azores and not on Jan Mayen as previously has been suggested. The similarity of the proximal tephras on São Miguel and distal tephras in Ireland is demonstrated by high similarity coefficients (>0.95) and biplots showing major element composition also support a correlation between Azorean eruptions and distal tephras in Ireland. Thus, trachytic tephras erupted from explosive eruptions on the island of São Miguel may have a potential to contribute to the construction of a European‐wide tephrochronology framework.

Trachytic tephras erupted from explosive eruptions on the Azores may be more widely dispersed than previously thought and may provide useful isochrones for correlation of paleoclimate archives in the north‐central Atlantic, North Africa and the Iberian regions. The trachybasaltic/basaltic trachyandesitic tephras erupted from cinder cones on Pico Island are probably only useful for a local tephrochronology in the Azores region, and not for a wider Atlantic or European framework.

1 The licentiate thesis consists of a summary and two papers:

Paper 1 Johansson, H., Lind, E. M., Wastegård, S., in review. Proximal tephra glass geochemistry from eruptions in the Azores archipelago and links with distal sites in Ireland. The Holocene.

Paper 2 Johansson, H., Lind, E. M., Wastegård, S., manuscript. Tephrostratigraphy of the Caveiro bog, Pico Island, Azores (Portugal)

For Paper 1, I carried out all fieldwork on the islands of São Miguel and Faial. The field‐ work on São Miguel was carried out by me and Stefan Wastegård.. I led the writing of the text, made the maps, figures and tables in collaboration with Stefan Wastegård and Ewa Lind. The Electron Microprobe Analyses (EMPA) were performed by me and Stefan Wastegård. For Paper 2, Stefan Wastegård and I carried out the fieldwork. I am responsible for analyses of the lithology, tephra identification and measurements of carbon content. I did the sampling and identification of the plant macrofossils with help of Mats Regnell and Simon Larsson. The EMPA were also done together with Stefan Wastegård. I constructed the age‐depth model in collaboration with Ewa Lind. The text, maps, figures and tables were made by me in collaboration with Stefan Wastegård and Ewa Lind.

2 Introduction

The thesis of Sigurdur Thorarinsson, “Tefrokronologiska studier på Island” was the first doctoral thesis in geography defended at Stockholm University College (Thorarinsson, 1944). This was the starting point of modern tephrochronology in Europe. Thorarinsson worked with proximal tephra layers on visible to the naked eye, but he also predicted that it should be possible to find Icelandic tephras in distal locations (Thorarinsson, 1944; 1981). This prediction was later confirmed by findings of tephra in bogs in Scandinavia and the British Isles (e.g. Persson, 1966; Dugmore, 1989; Hall and Pilcher, 2002; Lind et al., 2013). During a volcanic eruption airborne fragmental material of different sizes are ejected. Tephra, the Greek word for ash, is commonly used as an umbrella term for ejected material. has a grain size smaller than 2 mm, which means that all distal tephras are in the ash fraction. Tephrochronology focuses on the bulding of chronologies using the erupted volcanic glass component. Volcanic glass is the uncrystallised product of a rapidly cooling (Fig. 1).

Fig. 1. Electron microscopy image of a trachytic tephra shard from the Fogo A eruption (c. 5000 BP).

During large and explosive eruptions tephra can be ejected above the troposphere and deposited over large areas, far away from the volcano. Explosive volcanic eruptions often last only for relatively short periods of time from days to months, and the deposition of volcanic ash is almost immediate (Robock, 2000; Durant et al., 2010; Rose and Durant, 2011). For this reason tephra layers are possible to use as isochrons in the fallout area of the tephra. As such, tephras can be used to link archives from different geological and archaeological settings (e.g. Dugmore et al., 2000; Casely and Dugmore, 2004; Church et al., 2007; Lowe, 2011). In many cases they can also provide high‐ resolution dating of paleoclimate archives (Lowe, 2011, Davies et al., 2012, Wastegård and Boygle, 2012, Abbott et al., 2012). The volcanic glasses found in distal areas are often not visible to the naked eye. Previously these non‐visible tephras were called microtephras (Turney et al., 1997). Today, they are more frequently referred to as cryptotephras meaning hidden tephras from the Greek word crypto (=Hidden) (Lowe, 2011). The size of cryptotephra particles is in the range of c. 20‐100 μm. The technique to detect and analyse cryptotephras took an important step forward in the 1990s (e.g Dugmore et al., 1992; Turney et al., 1997; Lane et al., 2011; Lowe, 2011;

3 Davies, 2015), partly due to new techniques for extracting tephra from minerogenic deposits (Turney, 1998; Mackie et al., 2002; Blockley et al., 2005). Today it is possible to use Electron Microprobe Analyses (EMPA) for major element analysis on tephra shards less than 3μm across (Hayward 2012). It is also possible to analyse c. 30 trace elements from one tephra shard with laser ablation inductively coupled plasma mass spectro‐ metry technique (LA‐ICP‐MS) (Pearce et al., 2011). These technical developments have opened opportunities to improve existing tephra frameworks (Dugmore, 1989; Turney et al., 1997; Lane et al., 2011; Lowe, 2011; Blockley et al., 2014). Today, a growing number of European sites can be linked and synchronised using tephra layers from Icelandic, Central European and Mediterranean sources (e.g. Lane et al., 2011, 2015). There are also recent findings of even more far‐travelled tephras sourced to North America in the Greenland ice‐cores and in European bogs, which indicates the possibility of distribution of tephras over larger areas than previously thought (Fig. 2) (Coulter et al., 2012; Jensen et al., 2014). An additional volcanic source is the Azores, which has so far been less investigated compared to other European volcanic centres, especially regarding geochemical analyses of the glass component of the tephra (Table 1).

Fig. 2. Volcanic centres and volcanoes in the Northern hemisphere mentioned in the text. Highlighted centres and volcanoes: 1. Iceland, 2. Jan Mayen, 3. Icelandic plateau, 4 Massif Central, 5. Campania, 6. Eifel region, 7. Azores, 8. Katmai. Map by Lind (2014), modified after Abbott & Davies. (2012).

4 Table 1. Summary of tephras with published major element geochemistry from eruptions on the Azores, or tephras that have tentatively been attributed to eruptions in the Azores archipelago. Proximal tephra glass geochemistry from eruptions on the islands of São Miguel, Faial and Pico (Björck et al., 2006, Johansson et al., in review). The tentatively sourced tephras are found in Morocco and SW Ireland (Hall and Pilcher, 2002, Chambers et al., 2004, Barton et al., 2014; Reilly and Mitchell, 2015,). Several other eruptions during the last 5000 years have been reported from São Miguel, but no detailed glass geochemical data exist (Sete Cidades P1‐16, Furnas H/I c. AD 1440, Furnas G c. 800 cal. BP, Furnas F c. 1000 cal. BP, Furnas C c. 1900 cal. BP and Furnas B c. 2500 cal. BP and finally Fogo B, C, and D) (Booth et al., 1978; Cole et al., 1999; 2008; Guest et al., 1999).

Name of Tephra Date Island or Site Geochemistry Reference Fogo A c. 5000 BP São Miguel, Azores Trachytic Johansson et al.,in review Fogo AD 1563 AD 1563 São Miguel, Azores Trachytic Johansson et al.,in review Furnas AD 1630 AD 1630 São Miguel, Azores Trachytic Johansson et al.,in review Sete Cidades c. AD 1440 c. AD 1440 São Miguel, Azores Trachytic Johansson et al.,in review Capelinhos AD 1957‐58 AD 1957‐58 Faial, Azores Basaltic Johansson et al.,in review Cav A‐O c. 5.8 ‐1 kyr cal. BP Pico, Azores Basaltic Björck et al.,2006 TAF_S1_R2 c. 30‐18 kyr cal. BP Taforalt, Morocco Trachytic Barton et al., 2014 PMG‐5 c. AD 1600 W Ireland Trachytic Hall & Pilcher, 2002 MOR‐T2 c. AD 1400 W Ireland Trachytic Chambers et al,.2004 MOR‐T7 c 1670 cal. BP W Ireland Trachytic Chambers et al,.2004 MOR‐T8 c. 1800 cal. BP W Ireland Trachytic Chambers et al,.2004 MOR‐T9 c. 1915 cal. BP W Ireland Trachytic Chambers et al,.2004 DCSH‐2 c. AD 1600? W Ireland Trachytic Reilly and Mitchell, 2015

The overarching aim of the present project is to evaluate whether tephra from Azorean eruptions can contribute to the growing tephra frameworks of Europe. More specifically, the thesis aims to: (A) Provide major element geochemistry of glass shards from five previously known eruption on the islands of Faial and São Miguel (Capelinhos AD 1957‐58, Sete Cidades c. AD 1440, Fogo A c. 5000 BP, Fogo AD 1563 and Furnas AD 1630); (B) establish a tephro‐ stratigraphy for the Caveiro bog on Pico Island; (C) investigate whether proximal tephras from Azorean eruptions can be linked to tephras in distal areas, and evaluate if tephras from Pico can be used in a wider European tephra framework.

5 Study area and geological background

The Azores Islands are nine inhabited islands in the north central Atlantic Ocean between 39°43’ N‐36°55’N and 31°16’W‐24°46’ W (Fig. 3.).

Fig. 3. (A.) The position of Azores in the North Central Atlantic Ocean. (B.) Azores archipelago highlighted.

The Azores archipelago, discovered by Europeans c. AD 1430, is situated on the triple junction between the Euro‐Asian, African and the North American plates. The eastern part of the archipelago is oldest, dated with K‐Ar to c. 5.2‐4.6 Ma. (Feraud et al., 1981). São Miguel has a late Pliocene K‐Ar age of c. 4 Ma. (Abdel‐Monem et al., 1975). The oldest parts of Faial are c. 0.8 Ma (Moore, 1990; Beier et al., 2012; Hildenbrand et al., 2012), and finally the Island of Pico is the youngest island in the archipelago with an age of c. 0.3 Ma (Chovelon, 1982). There is probably a mantle plume connected to the triple junction, and the in the Azores is still under discussion (Montelli et al., 2004). Five of the islands have had historical eruptions (São Miguel, Pico, Faial, Terceira and São Jorge) and in total c. 30 eruptions have occurred in the Azores archipelago during the last 600 years (Björck et al., 2006). All three volcanic centres on São Miguel are characterised by trachytic subplinian/plinian phreatomagmatic eruptions (Cole et al., 1995; Guest et al., 1999; Cole et al., 1999; Queiroz et al., 2008; Cole et al., 2008). The Capelinhos AD 1957‐58 basaltic eruption on the Island of Faial was characterised by Strombolian phreatomagmatic activities (Machado et al., 1962; de Lima et al., 2011).

6 Volcanologists have worked extensively on the distribution and nature of deposits from eruptions in the Azores region, including the field character of tephras, style of eruptions and bulk geochemistry based on whole rock samples (Walker and Croasdale, 1971; Moore, 1990; Cole et al., 1995; Guest et al., 1999; Queiroz et al., 2008; França, 2009). However, analyses of the glass component of the tephras are less common. The Electron Microprobe and X‐Ray Fluorescence analyses on tephras and whole rock samples on the island of São Miguel show that trachytic tephras dominate (Guest et al., 1999; Johansson et al., in review.). Analyses of the glass component and volcanic rocks from the islands of Faial and Pico have shown a basaltic‐trachybasaltic geochemistry (Björck et al., 2006; Camacho et al., 2007; de Lima et al., 2011; Johansson et al., in review.). To this date, only one study focused on tephra deposited in lake sediments. Björck et al. (2006) analysed tephras from a lake on Pico in a project focusing on the Holocene paleoclimate evolution in the central‐north Atlantic. All identified tephras were basaltic, probably originating from eruptions on Pico.

Climatic setting

The Azores is an isolated archipelago in the temperate zone located in the north‐central Atlantic Ocean. The archipelago is situated on the border between the Azores current, a branch of the North Atlantic drift and the Canary current. The subtropical gyre also influences the Azores archipelago. Azores has also an important climatological position in relation to the North Atlantic Oscillation (NAO) (Wanner et al., 2001; Rimbu et al., 2003). The Azores is one pole of the NAO, the other being the Icelandic low, and the fluctuations in the difference of atmospheric pressure at sea level between Azores and Iceland during the year has an important impact on the climate in the North Atlantic region (Hurrell, 1995). During winter the south west winds are dominating over the Azores region, with relatively high wind speeds. High pressure conditions are dominating during summers and the wind direction is towards northeast (Fig. 4). Both the polar and subtropical jetstream located in the tropopause are possible pathways for tephra transportation from eruptions in the Azores to the other regions. However, to our knowledge there are no simulation models of tephra distribution and transport of ash from eruptions in the Azores region. Climatological studies on the islands of Azores have a potential to improve the overall picture of paleoclimatic conditions in the North Atlantic region (Connor et al., 2013).

7

Fig. 4. Sketch map of the North Atlantic Sea with Azores and Jan Mayen islands. An Loch Mór, Inis Oírr, (MOR‐T tephras) and Portmagee (PMG‐5 tephra) also marked on the map. The pattern of the wind currents in North Atlantic Sea and the Isobars of pressure in mb during winter (January) (upper) and summer (July) (lower). PF Polar Front, AF Arctic Front. Sketch modified after Trujillo Alan (2008) and Barry (2014)

8 Methods

Fieldwork Fieldwork was conducted on three of the Azores Islands in 2012 and 2013. Four sites on the Island of São Miguel were sampled for proximal tephra deposits from the reported fallout areas of the Sete Cidades AD c. 1440, Fogo A c. 5000 BP, Fogo AD 1563 and Furnas AD 1630 eruptions. On the island of Faial two samples were taken from the surface of the proximal ash/lapilli deposit of the Capelinhos AD 1957‐58 eruption (Paper 1). The tephra samples were collected with a small shovel and were stored in plastic bags until analyses. The Caveiro bog on the Island of Pico was cored with a 1 m Russian peat corer with a chamber diameter of 5 cm. A total length of 3.30 m sediment was taken from the central part of the bog (Paper 2). All cores were immediately wrapped in plastic. After transportation back to Sweden the cores were stored in the cold room in the Geosciences house at Stockholm University.

Tephra extraction and preparation for microprobe analysis

All samples from São Miguel and Faial were initially treated with 10% H2O2 to remove organic matter. The tephra samples were sieved through 80μm and 25μm meshes in order to retrieve shards of suitable size for geochemical analyses. The size fraction between 25‐80μm was mounted in epoxy on glass slides. The samples were grounded to a thickness of c. 38 μm and then polished. The methodology for the cores from Caveiro bog was somewhat different and the cores were subsampled for visible tephra layers; these tephra layers are also confirmed by low organic carbon levels (Paper 2). The samples had a volume of approximately 1.5 cm3 and they were treated with a standard acid digestion method to remove organic matter (Dugmore, 1989). Preparation for microprobe analysis analytic procedures followed standard methods (Dugmore, 1989; Dugmore and Newton, 1995).

Electron Microprobe Analyses Only the glass component of tephra samples was analysed for this study. Minerals were observed during the analyses but these were not further analysed since the main focus was to provide glass data in order to enable correlations with distal areas. The Electron Microprobe Analysis (EPMA) data were obtained on a Cameca SX100TM electron microprobe at the University of Edinburgh, with an accelerating voltage of 15 KeV, and a beam current of 2 nA for major elements and 80 nA for the minor elements and a beam diameter of 5 μm. The X‐ray spectrum was analysed using a Wavelength Dispersive

Spectrometer (WDS) technique and 10 major oxides were measured (SiO2, TiO2, Al2O3,

FeOtot, MnO, MgO, CaO, Na2O, K2O and P2O5). Na2O was counted in the first and last counting to monitor sodium loss to avoid drift in results of the measurements. In the

9 calibration of the WDS five calibration blocks were used. During the analysis two secondary glass standards, BCR2g and Lipari obsidian were used to monitor drift in the analyses (Hunt and Hill, 1996; Kuehn et al., 2011; Kuehn, 2012; Hayward, 2012). Backscatter electron imaging was used to avoid mineral inclusions in the glass and the tephra shards were randomly selected. Totals major oxides below 95% are excluded from the totals.

Similarity Coefficient The Similarity Coefficient method (SC) can rapidly characterise large data sets, and give an indication of the similarity between tephras (Borchardt et al., 1972; Hunt and Hill, 1993). A SC value of two identical tephras would be 1. Toms et al. (2004) has worked with the problem to characterise the geochemical fingerprint, and the correlation of tephras in alluvial sequences in the central western Argentina. They suggested that a SC of ≥0.95 between two tephra samplesis a reasonable geochemical correlation and an indication that the tephras derive from the same eruption. The calculation of SC can be used to combine several comparative data sets, such as X‐Ray fluorescence (XRF) and Electron Microprobe (EMPA) data (Froggatt, 1992). SC calculations have also been used to correlate distal cryptotephras in loess in Washington, NW USA(Busacca et al., 1992). The principle formula of the method is:

. 1/ d (A.B) is the Similarity Coefficient for comparison between sample A and B i is the element oxide number. n is the numbers of elements in calculation.

Ri is XiA/XiB (if XiB˃ XiA), or XiB/ XiA (if XiA˃XiB).

XiA is the concentration of elements i in sample A, and XiB is the concentration of elements i in sample B. In this study Similarity Coefficients were calculated as follows: 1. All geochemical analyses were normalised to 100% and means for each element (oxide)were calculated 2. All oxides with means ≥ 1% were selected. 3. The concentration of each element in consecutive data set was dived by each other:

XiA/Xi B, X2A/X2B, X3A/X3B,…….XnA/XnB 4. The ratios (R) are summed for all. 5. The sum is divided by the number of elements (n).

10 Organic Carbon The sediment and peat samples from Caveiro bog were dried in crucibles at 105°C for c. 12 hours and afterwards ashed at 550°C for 4 hours. All samples were weighed before and after ashing and the organic carbon content was calculated.

Radiocarbon dating and age‐depth model Plant macrofossil remains from the Caveiro bog core were collected from layers with high organic carbon content and were identified using a stereo binocular. The dried organic materials were sent to the Tandem Laboratory at Uppsala University for radio‐ carbon analyses. The samples were treated with an acid‐base to remove from possible contamination of carbonate and humic acids. The organic material was combusted to carbon dioxide and converted to graphite by an iron‐catalytic reaction. The age‐depth model was constructed using the OxCal online version 4.2 P_sequence function (Bronk Ramsey et al., 2010; Bronk Ramsey and Lee, 2013) applying the radiocarbon atmo‐ spheric curve IntCal13 (Reimer et al., 2013). The total length of the core is 3.30 m and the core contains c. 0.68 m of visual tephra layers and rock fragments. These coarse layers represent a large portion of the total sediment but were likely deposited during short periods of time (hours to months), and were therefore excluded from the depth‐ age model. The ages of the tephra layers are estimated from the age‐depth model, although we are aware that all ages in the lower part are uncertain due to a low sediment accumulation rate and fewer radiocarbon dates compared with the upper part. For calibration from 14C years to calendar years elsewhere in the text the online program: www.calpal‐online.de/ was used. The tephras erupted from the cinder cones on the Island of Pico are dominantly of lapilli size and the fallout of these tephras occurs relatively close to the eruption vent. Depending on the magnitude of the eruption and the direction of the wind, the accumulation of tephras can have a huge variability in space and time (Bursik et al., 1992). However, it is important to eliminate the tephras from the age‐depth model to get an accurate rate of sedimentation in proximal areas; otherwise the chronology is likely to be erroneous.

11 Summary of results

Paper 1 The Azores archipelago is one of the most active volcanic areas in the Northern Atlantic region, with approximately 30 eruptions during last 600 years. This study presents tephra glass geochemistry of five major Holocene eruptions: the Capelinhos AD 1957 trachybasaltic eruption on the island of Faial and four trachytic eruptions on the island of São Miguel; Sete Cidades c. AD1440; Fogo A 5000 BP; Fogo AD1563 and the Furnas AD 1630. Geochemical analyses were done with new improved Electron Microprobe Analyses (EMPA) methodology to avoid sodium and potassium migration, and to get a better accuracy of the measurements. The geochemical results are presented in terms of Total Alkali versus Silica diagrams (TAS) (Fig. 5.), major element biplots and calculation of similarity coefficients of the volcanic glass. Major element geochemistry can be used to distinguish products from different volcanoes. Tephras found in Irish and bogs that have previously been attributed to volcanic eruptions on Jan Mayen also have a strong correlation with tephras to eruptions from Azores and especially from the Furnas volcano (Fig.4). We tentatively suggest that the Irish MOR‐T2, T7, T8 and T9 and PMG‐5 tephras were erupted from the Furnas volcano, and that these tephras are the first evidence of tephra dispersal from Azores to Europe.

14,5

14,0

13,5

13,0 O wt% 2 Furnas 1. AD 1630 12,5 Furnas 2. AD 1630 O +K

2 Fogo AD 1563

Na Fogo A 12,0 Sete Cidades AD c.1440 MOR-T2 MOR-T7 11,5 MOR-T8 MOR-T9 PMG-5 11,0 62 63 64 65 66 67

SiO2 wt%

Fig. 5. TAS diagram comparing proximal tephras from São Miguel (Sete Cidades AD c. 1440, Fogo 5000 BP, Fogo AD 1563 and Furnas AD 1630) and distal tephras from Ireland (MOR‐T2‐T9 and PMG‐5). All data normalised to 100%. (Hall and Pilcher, 2002; Chambers et al., 2004).

12 Paper 2 A 3.30 m core spanning 7000 years from the Caveiro bog, Pico Island was surveyed for tephra. Eight basaltic tephra layers from Caveiro bog contained a suitable amount of shards to provide material for robust geochemical analyses. Results show that seven of these tephras have a trachybasaltic/basaltic trachyandesitic composition and one has a basanitic composition. The geochemical results are presented in terms of Total Alkali versus Silica diagrams (TAS), major element biplots and calculation of similarity coefficients of the volcanic glass. Major element geochemistry can be used to distinguish products from different cinder cone eruptions. Similarity coefficient calculations and major element glass geochemistry confirm the correlation between some of the tephras from the Caveiro bog and the nearby lake Caveiro, but no trachytic layers were encountered in any of the sites which make regional correlations (i.e. between islands) more difficult.

Discussion

The overarching aim of this thesis is to evaluate whether tephra from eruptions in the Azores archipelago can contribute to the growing tephrochronology frameworks of Europe. Several explosive eruptions producing trachytic airfall deposits have occurred on the island of São Miguel during the last 5000 years (Booth et al., 1978). Major element glass geochemistry of five potentially widespread tephras (Capelinhos AD 1957‐58, Sete Cidades c. AD 1440, Fogo A c. 5000 BP, Fogo AD 1563 and Furnas AD 1630) has been analysed and a tephra‐based correlation between Azores (e.g. Furnas AD 1630) and Ireland has been proposed (Fig.4). A majority of cryptotephras found in NW Europe (British Isles & Scandinavia) are of

Icelandic origin and usually of dacitic and rhyolitic composition with SiO2 contents above c. 66 wt% and K2O + Na2O between c. 4 and 10 wt% (e.g. Dugmore and Newton, 1995;, Davies et al., 2003;, Wastegård and Davies, 2009). Tephras with trachytic composition (SiO2 between 60‐65 wt% and K2O +Na2O˃ c. 10 wt%) are less common but some records have been described from late Holocene lake and peat deposits on Ireland (Hall and Pilcher, 2002; Chambers et al., 2004). Several volcanic centres are known to produce trachytic tephra, including Mediterranean and North Atlantic sources. The trachytic tephras found in the An Loch Mór lake sediment, W Ireland have previously been attributed to eruptions on Jan Mayen in the Northern Atlantic Ocean (Chambers et al., 2004). A recent study of soil sections on Jan Mayen, however, has not shown any evidence for trachytic eruptions there during the late Holocene (Gjerløw et al., 2015) and the suggested correlation between Jan Mayen and Ireland remains inconclusive due to lack of data.

13 All tephras that were analysed from São Miguel have SiO2 contents between c. 63‐66.5% and alkali contents between 12‐14 wt% (normalised values; Fig. 5). Our results show that differences occur in major element glass geochemistry performed on tephra from the three main volcanic centres on São Miguel (Fig. 4.). Especially differences in TiO2,

CaO and FeOtot have a potential to separate tephras erupted from Sete Cidades, Fogo and Furnas (Paper 1). Our results, however, only provide data from three historic eruptions and the mid Holocene Fogo A eruption. Tephra from several Holocene eruptions remain to be analysed for their geochemical composition, including explosive eruptions from the Furnas, Fogo and Sete Cidades volcanoes (Booth et al., 1978; Cole et al., 1999; Guest et al., 1999; Cole et al., 2008). It remains to be seen if major element geochemistry also can separate older, prehistoric tephras that were not a part of this study.

Fig. 6. Differences in proximal glass geochemistry between tephras erupted on the Island of São Miguel (Sete Cidades c. AD 1440; Fogo A c. 5000 BP; Fogo AD 1563 and Furnas AD 1630). All data normalised to 100%. (A) Biplot Fetot vs. TiO2; (B.) Biplot Fetot vs. CaO.

Results from this study show that proximal tephras erupted from the Furnas volcano on Sao Miguel can be correlated with distal cryptotephras found in Irish lakes and bogs (Fig. 4) (Hall and Pilcher, 2002; Chambers et al., 2004). Trachytic tephras from the Azores may be more widely dispersed than previously thought and may provide useful isochrons for correlation of paleoclimate archives in the north‐central Atlantic, North Africa and the Iberian regions. Some of the Azorean eruptions are historically dated (Sete Cidades c. AD 1440, Fogo AD 1563, Furnas c. AD 1440 and Furnas AD 1630) and might be especially useful as isochrons in paleoclimate archives from the last Millen‐ nium. These tephras have a potential to improve the European tephrostratigraphy/ tephrochronology framework, especially in sites in south and central Europe where tephras from more than one European volcanic system have been found (e.g. Lane et al., 2011; 2013).

14 The original analyses of the Irish cryptotephras which were compared here were made approximately 10‐15 years ago (Hall and Pilcher, 2002; Chambers et al., 2004) using microprobe settings, which were not designed to avoid migration of sodium and potassium as successfully as current systems (Kuehn et al., 2011; Kuehn, 2012; Lilja et al., 2013). The result of the migration of alkali elements suggests a change in the ratio of the chemical composition in the measurements, hence the counts of aluminium and silicon increases. The Tephra Analytical Unit at the University of Edinburgh has developed a method to reduce beam‐induced migration of sodium and potassium (Hayward 2012). Results from a recent study by Lilja et al. (2013) using these new settings suggests that analyses made c. 10 years earlier may suffer from an alkali migration in the size of 0.5‐1%. If the EMPA data of the Irish MOR‐T2, T7, T8 and T9 and the PMG‐5 tephras generated an alkali migration of the size that Lilja et al. (2013) estimated, the correlation between the Irish tephras and the Furnas volcano is even stronger (Paper 1). Therefore it is important to be cautious when comparing microprobe analyses performed in 1990 or earlier with new results. Tephras erupted from the island of Pico have a trachybasaltic or basanitic glass geochemistry which indicates that the style of eruption was less explosive compared to those on São Miguel (Paper 2). Tephras from Pico are therefore likely to have a limited distribution and are probably not useful in an Atlantic tephra framework. Even to correlate between the Caveiro bog and Lake Caveiro (Björck et al, 2006), situated only c. 500 m from each other is not straightforward and indicates that some tephras probably have an extremely limited distribution. Only three tephras in Caveiro bog out of ten (Caveiro‐135, Caveiro‐230 and Caveiro‐280) can possibly be correlated with tephras found in Lake Caveiro (Cav A, Cav F and Cav O tephras). Some of the eruptions may; however, have produced tephras with a wider distribution and these might be possible to use in a local Azorean tephrochronology framework. Also the trachybasaltic tephras from the Capelinhos 1957‐58 on the island of Faial was probably not distributed over vast areas, and thus not useful for correlations outside the Azores archipelago (Paper 1). Finally, one should be aware that radiocarbon dating can be problematic in volcanic areas since there is a high possibility of contamination of old volcanic carbon dioxide in the plant macrofossils, which may result in too high radiocarbon ages due to old radio‐ carbon effects (Sulerzhitzky, 1971; Pasquier‐Cardin et al., 1999; Viveiros et al., 2010). Hence it is important to conduct as many as possible radiocarbon analyses in order to get a robust age‐depth model.

15 Conclusions

The trachytic tephras erupted on the Island of São Miguel are possible to distinguish from each other using major element biplots (e.g. FeOtot vs. TiO2 and FeOtot vs. CaO). Statistical correlations of geochemical compositions of tephras from trachytic eruptions in the Azorean archipelago suggest that Azorean tephras are important additions to the growing tephra frameworks of Europe and Africa. Especially sites on the British Isles, the Iberian Peninsula and NW North Africa are within the potential dispersal areas of tephras from eruptions in the Azores. The trachybasaltic tephras erupted from the cinder cones on the Island of Pico, and from the Capelinhos AD 1957‐58 eruptions on the Island of Faial are probably only valuable for a local tephrochronology in the Azores region, and not for a broader European framework.

Recommendations for further studies on the Azores islands

The present study has provided new data on trachytic eruptions on Sao Miguel and a tephra‐based correlation of geochemical composition with sites on Ireland has been established. On the contrary, analyses of the core from Pico Island confirm previous work by Björck et al (2006) that most, if not all tephras from there are locally derived and probably not useful in a wider context. One important question that has arisen is whether the geochemistry of the São Miguel volcanoes is stable or has changed over time. Several prehistoric eruptions dated by radiocarbon have been described, but there is very little glass shard data available. A field campaign to collect tephras from prehistoric eruptions from the Furnas, Fogo and Sete Cidades volcanoes would provide important information and would make these tephras possible to use in the development of the European and North Atlatic tephra frameworks. Some tephras would be especially important to analyse for their geochemical composition. Five eruptions from the Furnas volcano have been radiocarbon dated (Cole et al., 1999) and one of these, the Furnas C tephra (c. 1900 cal. BP; Cole et al., 1999) is especially important , because it has the same age as trachytic tephras found on Ireland (e.g. MOR‐T8 and MOR‐T9)(Chambers et al., 2004). Sete Cidades is one of the most active volcanic centres and 17 have been identified during the last 5000 years (Cole et al., 2008). Only two of these have been radiocarbon dated, in order to assess a possible trend in major element geochemistry of the eruptions from Sete Cidades (Cole et al., 2008).

16 Five major eruptions have occurred in the Fogo volcanic centre during the last c. 5000 years (Storey, 1982). The Fogo B eruption has an estimated age of c. 3200 BP and it would be important to analyse the geochemistry of the glass component of the Fogo B tephra in order to assess a possible trend in major element geochemistry of the eruptions from the Fogo volcano. It would also be valuable to analyse minor, trace element and stable isotopes on the samples that we already have analysed for major elements. Trace elements have been used to distinguish tephra horizons that are inseparable on the basis of major elements (Pearce et al., 2004,2007). An ultimate goal for future projects would be to evaluate if prehistoric proximal Azores tephras can be linked to tephras found in distal locations.

Acknowledgements

First of all I would like to thank my supervisor Stefan Wastegård and my co‐supervisor Ewa M Lind for all the assistance and fruitful discussions from tiny details to the overall picture of tephrochronology in the Northern Hemisphere. I also thank my additional co‐ supervisor Otto Hermelin at the Department of Geological Sciences Stockholm University for his help with logistical issues during the fieldwork on Azores and for the inspiring lectures in volcanology. Thanks to Alasdair Skelton, Department of Geological Sciences, Stockholm University, and the head of the research school focusing on natural hazards. I would also like to thank Dr. J.L Gaspar and Dr. Ana Gomes, University of Azores for excellent help with finding suitable sampling locations at São Miguel. I thank Dr. Zilda França at the University of Azores for her help with references and fruitful discussions. Dr. Chris Hayward at the tephra Analysis Unit, School of Geosciences, University of Edinburgh for his helpfulness with the electron microprobe analysis. Dan Zetterberg at the Department of Geological Sciences Stockholm University for help with grinding the tephra glass slides. Dr. Samantha Engwell, University of Pisa for her helps with references and fruitful discussions, and Karin Persson Stockholm University for the assistance with GIS cartography. I acknowledge Hildred Crill at the Department of Geological Sciences, Stockholm University and Anders Moberg at the Department of Physical Geography, Stockholm University for their comments that improved the manuscripts. Thanks also to Eirik Gjerløv, Nordic Volcanological Centre University of Iceland, for his contribution of tephra geochemistry data from Jan Mayen. The Swedish Research Council and Upplands‐Väsby municipality are gratefully acknowledged for financial support. Finally, but not least I would like to thanks my family and all my friends for all the support that I got through these fantastic years at the research school for teachers.

17 References

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