Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 https://doi.org/10.1186/s40623-019-1060-4

TECHNICAL REPORT Open Access The Iceland Palaeomagnetism Database (ICEPMAG) Justin A. D. Tonti‑Filippini1,2* and Maxwell C. Brown1

Abstract The extensive lava piles of Iceland contain a unique record of geomagnetic feld variations spanning the past ~ 16 Ma. Since the 1950s, palaeomagnetic data have been obtained from over 9400 Icelandic lavas. We have compiled all palaeomagnetic data currently available and developed a publicly accessible database (http://www.icepm​ag.org). The data within the database are primarily palaeodirections, with a relatively smaller number of palaeointensity data (8936 entries contain direction only, 218 intensity only, and 337 both direction and intensity). In addition, the database contains a wide range of metadata, including geochronological information, site details, and laboratory methods. The search interface of the database allows users to search for data using a range of customisable flters (e.g. by publica‑ tion, geological age, location, laboratory method, palaeomagnetic statistics) and to quickly visualise and download the matching search results. The data within the database have signifcant potential for understanding long-term palaeomagnetic feld variations at high latitude, the behaviour of excursions and reversals, and geological mapping on Iceland. Keywords: Palaeomagnetism, Geomagnetism, Database, Iceland, ICEPMAG

Introduction the reliability of the data. Modelling of the geomagnetic Igneous rocks, sediments, and archaeological materials feld on diferent timescales, for example, relies on such have the capacity to preserve a record of Earth’s palaeo- databases, and models based on updated global datasets magnetic feld. Compilations of palaeomagnetic data are are likely to surpass their predecessors and ofer new vital for understanding the global behaviour of the pal- insights into deep Earth processes (Johnson and McFad- aeomagnetic feld and its long-term evolution. Te value den 2015). of palaeomagnetic databases has been recognised since Te issue of careful storage and selection of data is the 1960s, as researchers began to compile databases for particularly relevant for Iceland, where lavas suitable for diferent purposes, driven by specifc scientifc questions palaeomagnetic studies are abundant. Lavas provide our or a general desire to catalogue all available data. Mod- best opportunity to recover the past behaviour of the ern databases, such as the GPMDB (McElhinny and Lock magnetic feld beyond the timescales regularly covered 1996; Pisarevsky et al. 2018), GEOMAGIA50 (Korhonen by marine sedimentary cores. Iceland, due to its proxim- et al. 2008; Brown et al. 2015), and MagIC (Jarboe et al. ity to a mantle plume and an actively spreading tectonic 2012), continue to compile legacy and new data. Tese plate boundary (Fig. 1), has erupted prolifc stacks of databases have the potential to improve reconstructions lava fows over the past 16 million years (Ma) (McDou- and global models of the geomagnetic feld, but only if gall et al. 1984) and experienced minimal continental < 10◦ the data are accompanied by detailed information that drift and tectonic rotation or tilting, with generally allows researchers to make informed assessments about of regional tilt (Kristjánsson 2002). In many places, the lava piles have been uplifted and exposed in deep verti- cal sequences as a result of glacial and hydraulic action. *Correspondence: [email protected] Iceland’s unique geological setting, combined with low 1 Institute of Earth Sciences, University of Iceland, Sturlugata 7, vegetation cover and a sparse population, provides excel- 101 Reykjavík, Iceland Full list of author information is available at the end of the article lent opportunities to study long sequences of lavas for

© The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 2 of 19

Fig. 1 Map of ICEPMAG sites with bedrock geology and regional boundaries. All ICEPMAG sampling site locations. Regional boundaries defned by country subdivision code ISO 3166-2:2013. Tectonic geology map adapted from Jóhannesson and Sæmundsson (2009) based on data publicly available from The Icelandic Institute of Natural History (open license: http://www.ni.is/ranns​oknir​/landu​pplys​ingar​/skilm​alar)

mapping purposes and for revealing the evolution of the and consistent format. Te construction of ICEPMAG is palaeomagnetic feld through statistical analyses (e.g. important for preserving the legacy of Iceland’s unique Kristjánsson 2013; Suttie et al. 2015). Iceland is the only and extensive palaeomagnetic dataset and is in line with location with an almost continuous history of volcanism other recent eforts to produce large regional compila- over 0–16 Ma, particularly beyond 5 Ma, and can resolve tions by combining new palaeomagnetic data with legacy long time series of palaeomagnetic variations. Te coun- data, such as those for the North and South West USA, ◦ ◦ try’s position, between 63 and 67 latitude, also makes it Mexico, Hawaii, South Pacifc, and Reunion (see Johnson one of very few high-latitude locations where lavas suit- and McFadden 2015). able for palaeomagnetic study are exposed, and is impor- ICEPMAG was designed to be a useful tool for pal- tant for understanding geomagnetic feld behaviour at aeomagnetists to carry out research on variations in the high latitudes. palaeomagnetic feld over the past 16 Ma, including long- In this paper, we describe the construction and func- term trends in palaeosecular variation, the behaviour of tionality of a newly developed palaeomagnetic database reversals and excursions, and the variations of natural for Iceland, ICEPMAG, which compiles all palaeomag- remanent magnetisation (NRM) as a measure of rela- netic data published from Icelandic lavas. Vital informa- tive intensity through time or with virtual geomagnetic tion can be lost as researchers retire and studies age, so pole (VGP) latitude. Te database will also be useful for an emphasis was placed on capturing all available meta- geologists working on Iceland, e.g. for identifying rever- data and enriching existing compilations where possible. sal boundaries which can be used in magnetostratigraphy Tere are a signifcant amount of data from Iceland which mapping allowing correlation of lavas across hillsides and have not previously been compiled into a comprehensive valleys. ICEPMAG will be an important reference and Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 3 of 19

repository for future work carried out in the University of and transitional palaeomagnetic directions, illustrating Iceland’s recently refurbished palaeomagnetic laboratory. the ‘R3–N3’ reversal recorded by the lavas in Hvalförður, Tis paper contains summaries of the history of palaeo- western Iceland (Sigurgeirsson 1957). magnetism in Iceland, global palaeomagnetic databases, A number of studies published in the 1970s and early and the past and present uses of Icelandic palaeomag- 1980s produced very large datasets from thousands of netic data; descriptions of the design principles and Icelandic lava fows, e.g. Watkins and Walker (1977) in methodology behind the construction of the ICEPMAG eastern Iceland, Watkins et al. (1977) in Borgarförður database; as well as an overview of the data in the data- (western Iceland), and McDougall et al. (1984) in the base and its functionality, and issues, potential uses and Westfords. Data from these large studies contributed future work related to Icelandic data. to some promising developments in the applications of Iceland palaeomagnetism (e.g. Kristjánsson and McDou- Background gall 1982). Te collection from eastern Iceland, for which Palaeomagnetism in Iceland feldwork was carried out over 1964–1965, generated sev- Palaeomagnetic research in Iceland has contributed to eral papers and theses and was, at the time, probably the a number of geoscientifc discoveries. We briefy note largest palaeomagnetic collection from anywhere in the the most signifcant of these and refer the reader to world and ‘the most comprehensive source of data from Kristjánsson (1982, 1993, 2008) for more details. Discov- a single region on long-term (0.1–10 Myr) variations of ery of reversely magnetised rocks early in the twentieth the geomagnetic feld’ (Kristjánsson 1985c). Te results century fuelled much speculation about the past nature from the campaign included confrmation of more than of the geomagnetic feld and drew signifcant attention to sixty reversals between 2–13 Ma (Dagley et al. 1967), sig- the feld of palaeomagnetism. nifcantly more than the number derived from marine Te frst detailed study on Icelandic lavas to investi- sediment cores, and the application of reversals in stratig- gate geomagnetic feld reversals was carried out by Jan raphy across 10 km or more (Kristjánsson 2008). Dagley Hospers (1951), a PhD candidate from the University and Wilson (1971), and also Wilson et al. (1972), high- of Cambridge at the time. Hospers published his results lighted the benefts of sampling a large number of lavas in with the assistance of Sir Ronald Fisher, who had devised a single region and statistically analysing their remanence a method of spherical statistics (Fisher 1953) that enabled intensities as a function of VGP latitude. Kristjánsson Hospers to better display and analyse his results through and Jóhannesson (1989) analysed directions from ~ 4000 the use of VGPs. Data from Hospers’s Icelandic samples, Icelandic lava fows and found that the magnitude of total which Fisher used as an example when publishing his VGP dispersion has decreased signifcantly over the past method, contributed to many accepting the legitimacy 15 Ma, and that the geomagnetic feld’s polarity ‘had no of geomagnetic feld reversals (Frankel 1987). Hospers discernible efect on its mean confguration or directional (1953) interpreted his own measurements to suggest dispersion’. that ‘taken over periods of several thousands of years the By far, the most signifcant individual contributor to magnetic pole centres on the geographic pole’—an early palaeomagnetism in Iceland since the 1960s has been Leó formulation of the geocentric axial dipole (GAD) hypoth- Kristjánsson, whose body of work spans over 50 years esis (see Merrill and McElhinny 1983). (Kristjánsson 1968; Kristjánsson and Jónsson 2017)—see Palaeomagnetic work was continued by a number of Kristjánsson (2013) for a general summary of his work. Icelandic scientists who contributed greatly to the feld. Palaeomagnetism on Icelandic lavas has been primar- Trausti Einarsson and Þorbjörn Sigurgeirsson used a ily used as a stratigraphic mapping aid. Hospers initially feld compass (Einarsson 1957) to measure the polarity came to Iceland with the hope of using diferences in the of thousands of lava fows across Iceland while building strength of magnetisation to correlate the lavas (Frankel up maps of polarity zones, intending to use the results 1987). Generally, Icelandic lavas are strongly magnet- to assist in geological age determination (Kristjánsson ised with an easily isolated component of primary origin, 1982). Tis technique is similar to the geochronological containing negligible or low viscous remanent magneti- approach later used to construct the geomagnetic polar- sation (VRM), and are not commonly afected by chemi- ity time scale (GPTS) (see Ogg 2012). Sigurgeirsson col- cal weathering or lightning impacts (Kristjánsson 2013). laborated with Ari Brynjólfsson (1957) to use alternating Tese characteristics, combined with good exposure, feld (AF) demagnetisation to remove viscous overprints mean that sections of tens of lavas or more can be cor- in Icelandic lavas, probably the frst demonstration that related across hillsides to make composites of hundreds a stable primary remanence direction could be isolated of lavas. Frequent reversal boundaries provide impor- with this technique (Kristjánsson 1993). Te innovative tant markers for geological mapping and allow lavas to technique enabled the frst detailed study of intermediate be traced across several kilometres (see Kristjánsson Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 4 of 19

2009). Samples across individual lavas separated by tens information (e.g. Khramov 1971, 1976; Hicken 1972; of metres to several kilometres can also yield consist- Irving et al. 1976; McElhinny and Cowley 1977). Pole ent directions (e.g. Kristjánsson and Sigurgeirsson 1993; catalogues facilitated the correlation of global data and Kristjánsson and Audunsson 2007; Kristjánsson 2016). enabled testing of ideas about the past behaviour of the Icelandic lavas have continued to be used to improve geomagnetic feld, e.g. the GAD hypothesis, polar wan- controls for local stratigraphic mapping and the resolu- der, and geomagnetic polarity reversals. tion of the GPTS (e.g. Kristjánsson et al. 1980; Kristjáns- From the late 1980s onwards, with the advent of mod- son and Jóhannesson 1996; Helgason and Duncan 2001; ern computing, digital relational databases were devised Kristjánsson et al. 2004), as well as to further character- to link palaeomagnetic data with other geoscientifc ise palaeomagnetic feld reversals (e.g. Kristjánsson and data (e.g. Pesonen and Torsvik 1989; Barton 1991; McEl- Sigurgeirsson 1993; Goguitchaichvili et al. 1999a; Brown hinny and Lock 1991; Tanaka and Kono 1994). Prior to et al. 2006) and excursions (e.g. Levi et al. 1990; Kristjáns- the International Association of Geomagnetism and son 1999; Udagawa et al. 1999; Jicha et al. 2011). Aeronomy (IAGA) providing downloadable compila- Palaeomagnetic techniques have also been used to tions, there were no formal archives for palaeomagnetic understand the tectonic history of the island (Jancin data outside of internal library databases (see Tauxe et al. 2010; Horst et al. 2018; Titus et al. 2018; Young et al. 2016). IAGA’s Global Paleomagnetic Database (GPMDB), 2018), e.g. palaeomagnetic directions combined with originally created by McElhinny and Lock (1996), is con- structural data have been used to quantify varying clock- tinually updated (see Pisarevsky 2005; Pisarevsky et al. wise rotation with distance from the Husavík–Flatey fault 2018). in northern Iceland (Titus et al. 2018). Over the past 30–40 years there have been advances Comparatively, few studies have investigated absolute in the way palaeomagnetic data are measured and ana- palaeointensity of Icelandic lavas, which have gener- lysed (see Johnson and McFadden 2015). In 2003, the ally targeted the younger lavas/neo-volcanic zone in the Magnetics Information Consortium (MagIC) identifed a proximity of Iceland’s central spreading ridge (this is dis- need to integrate existing databases, supplemented with cussed further in “Palaeointensity data” section). How- more detailed information and data (i.e. the measure- ever, several studies sought to investigate the absolute ments themselves), ‘into a single unifed data structure’ intensity of the feld during reversals (e.g. Shaw 1975; (Tauxe et al. 2003). MagIC developed an online database Tanaka et al. 1995; Goguitchaichvili et al. 1999a, b; Brown for rock and palaeomagnetic data which incorporated et al. 2006). fexible metadata structures and an adaptable web portal. Despite access to abundant material suitable for palaeo- Te MagIC database enables researchers to make their magnetism, there are some caveats to research in Iceland. data quickly and more widely accessible, and available in

Volcanism can be sporadic,4 with an average time inter- greater detail than is permitted within the space limita- val between fows of ~ 10 years, and there are often5 sig- tions of scientifc journal articles. MagIC also sought to nifcant hiatuses within sections, perhaps up to 10 years, compile and archive legacy data into a consistent format meaning that major excursions and transitional events (Koppers et al. 2005). are not always recorded (Kristjánsson 2002). In some areas, stratigraphy is complicated by lavas coming from Icelandic data in global analyses diferent sources, i.e. rift zones and central volcanoes. Palaeomagnetic data from Iceland have played a signif- < Recent (i.e. 2 Ma) stratigraphy is also complicated by cant role in various global compilations which sought glaciation and erosion, and the efects of local anoma- to understand the evolution of the palaeomagnetic feld. lies, deformation, and tectonic tilting can be difcult to For example, in the compilation of Johnson and Con- detect, possibly infuencing the reliability of palaeodi- stable (1995) for studies of palaeosecular variation, Ice- rectional determinations (see Kristjánsson and Jónsson land contributed ~ 15% of the global data. Icelandic data 2007). Alteration and low potassium and argon contents have been particularly important for studying the global in Icelandic lavas can impact the accuracy of radiometric behaviour of geomagnetic feld reversals and transitional dating eforts which may have uncertainties in the order feld states: in the compilation of Love (1998), which of 5% (Kristjánsson 2002). investigated transitional pole paths during geomagnetic reversals, Icelandic lavas provided ~ 75% of the data Global palaeomagnetic databases from transitional events and were the ‘most signifcant Researchers have been compiling global palaeomag- contributors’ to identifcation of preferred longitudes for netic databases since the late 1960s, when early ‘pole transitional poles (Constable 2001). In a compilation of catalogues’ were developed to combine palaeomag- palaeointensity data by Perrin and Schnepp (2004), Ice- netic data (i.e. palaeodirections) with radiometric dating land contributed the ‘largest regional dataset’ and most Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 5 of 19

of the data from locations in the Atlantic Ocean. A more and Jónsson 2007). Te demagnetising step which results α recent analysis of compilations from Cenozoic large igne- in the smallest 95 value is then selected for the site’s ous provinces (Suttie et al. 2015) included data from 390 mean direction (e.g. Kristjánsson 2016). Justifcation for α Icelandic fows, the largest regional compilation in the the use of the ‘ 95-minimising’ method is discussed in study and ~ 37% of the total dataset. Kristjánsson and Jónsson (2007), which considers that Icelandic data have the potential to be useful in palae- ‘...there is no worthwhile gain in quality by applying fur- osecular variation (PSV) and time-averaged feld (TAF) ther demagnetization’ and fnds no signifcant diference studies. Te PSVRL database, described by McElhinny between studies of the same lavas where this method and and McFadden (1997), has been one of the main sources more detailed treatment/analysis (i.e. PCA) have been for PSV and TAF studies over the past two decades applied. Similarly, Kristjánsson (2013) suggests, for Ice- (Johnson and McFadden 2015) and contains data from landic samples, a ‘remanence direction isolated at 10 mT 3719 sites in the age range 0–5 Ma. Ten studies from Ice- AF treatment is generally almost unchanged after treat- land were included in PSVRL, with data from 748 sites ment at 15, 20, 25 and 30 mT’, and the ‘use of extensive (~ 20% of the dataset) meeting their applied quality cri- demagnetising procedures such as those that have been teria. In PSVRL and similar databases, such as those developed for paleomagnetic materials of inferior qual- described by Quidelleur et al. (1994) and Johnson and ity from elsewhere, therefore serves no practical purpose’. Constable (1996), Icelandic data dominate the latitude Te issue of magnetic cleaning and removal of secondary band 60°–69.9° and sites with VGPs of reverse polarity overprints was also addressed during the compilation of (Kelly and Gubbins 1997). the PSVRL database: McElhinny and McFadden (1997) Johnson and McFadden (2015) advocate for more strin- listed a number of studies recommended for replace- gent quality criteria and collection of new data for studies ment, and it is worth noting that no Icelandic studies of PSV, as it is considered that early (i.e. pre-1980s) pal- were included in this list. aeomagnetic studies, which generally aimed to test fun- Te paucity of PSV10 data for 5–10 Ma (only 166 sites) damental ideas such as magnetic polarity and the GAD could be rectifed somewhat if Icelandic data were con- hypothesis, produced data with a coverage and qual- sidered to be of adequate quality. For example, the study ity inadequate for what is required from datasets today. detailed in Kristjánsson et al. (2004) includes measure- Tis view is exemplifed by the recent PSV10 compila- ments from 319 lavas across the 5–9 Ma age range. It is tion (Cromwell et al. 2018), which sought to include ‘all intended that searching for and retrieving information high-quality paleodirectional data’ from volcanic units in with specifc metadata (e.g. by specimen demagnetisa- the 0–10 Ma age range. PSV10 includes 2401 sites glob- tion method), to address questions like this, will be made ally, with the vast majority of sites (93%) falling in the age signifcantly easier by the creation of the ICEPMAG range 0–5 Ma and most (1753 sites or 73%) being of nor- database. mal polarity. Iceland has produced many data from 5–10 Ma (and of reverse polarity), but they appear to have Previous Icelandic compilations been excluded from PSV10 due to the requirements for Te largest (previously existing) compilation of Icelandic the application of modern magnetic cleaning methods data is described by Kristjánsson (2013) and consists of or, more specifcally, that ‘principal component analysis’ information from over 5800 Icelandic lava fows: ~ 5200 (PCA) was applied to determine specimen directions, were considered suitable for statistical analyses, with an i.e. the ‘DC-4’ or ‘DC-5’ protocol, using the classifcation additional ~ 600 not used in all analyses because of their of McElhinny and McFadden (2000). Studies were also lack of NRM intensity information or large directional α ◦ excluded from the compilation if ‘demagnetization meth- uncertainties (i.e. 95% confdence radii 95 > 12 ). Te ods were not clearly stated’ or they specifcally targeted results from ~ 80 sites in this compilation were never for- transitional events (Cromwell et al. 2018). Only two Ice- mally published. Te dataset described by Kristjánsson landic studies (Udagawa et al. 1999; Døssing et al. 2016), (2013) contains ten data felds and some limited meta- with samples from 62 sites within a narrow age win- data in an unpublished notes fle. dow (0.6–3.1 Ma), were included in the PSV10 dataset although available data span back to 16 Ma (Kristjánsson Construction of the database 2013). Design principles Since the early 1980s, studies in Iceland have largely While ICEPMAG is intended to be used by palaeomag- utilised a magnetic cleaning method which is not cap- netists assessing Icelandic data and building their own tured in the ‘DC’ classifcation codes, where specimens compilations, it was constructed with the needs of the are only demagnetised ‘as far as necessary to confrm the wider Earth science community in mind, e.g. geologists presence of a stable remanence direction’ (Kristjánsson and other Earth scientists working in Iceland, to aid in Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 6 of 19

geological mapping projects or other volcanological Table 1 Relational table for Icelandic region names applications. We wanted ICEPMAG users to be able to ID Region (English) Region (Icelandic) quickly search for and visualise data that may be rele- vant to their own work (e.g. viewing study locations by 1 Capital Region Höfuðborgarsvæði age) and make their own judgements about the quality 2 Southern Peninsula Suðurnes and usefulness of those data. It was important that the 3 Western Region Vesturland database could be easily accessed and understood by 4 Westfords Vestfrðir frst-time users. 5 Northwestern Region Norðurland vestra To achieve this, ICEPMAG follows the design prin- 6 Northeastern Region Norðurland eystra ciples of GEOMAGIA50 (Korhonen et al. 2008; Brown 7 Eastern Region Austurland et al. 2015), which allows users to search for and access 8 Southern Region Suðurland data across multiple studies. Te database is stored on Only the IDs in the frst column are stored in the master table a MySQL server which allows for the database to be queried with specifc ‘statements’ and returns a fexible for lists of things that are recurrent, e.g. the laboratory output that can be adapted and displayed by external method codes defned in MagIC’s data model. scripting (i.e. the search results). Users access the data- base through a web query form (http://icepm​ag.org/ Methodology query​.php), which sends user queries to the SQL server. Te frst step in the construction of the database was to Te results from the query are sent back to the web- build a source library from an extensive literature search, site and printed to a results page through interpretive utilising websites such as Google Scholar (https​://schol​ scripting which also generates downloadable fles and ar.googl​e.com/), ISI Web of Science (http://webof​knowl​ graphics. No software needs to be installed, so updates edge.com/), and MagIC (https​://www2.earth​ref.org/ can be made regularly without requiring users to down- MagIC​/). Tis process was aided signifcantly by an online load new software. paper archive (https​://noten​dur.hi.is/leo/) compiled by ICEPMAG was designed to allow transfer of data Leó Kristjánsson, who also provided hard copies of several to the MagIC database to remedy the lack of Icelan- papers unavailable on the Internet. Tis process identifed dic studies there. At the time of writing, MagIC con- 79 papers containing original site-level palaeomagnetic tains only 27 studies from Iceland with data and some data (averaged site-mean direction and/or intensity from limited metadata from 2634 sites. Wherever possible, a lava fow) from 9491 Icelandic sites (as of March 2019). ICEPMAG has incorporated the vocabulary of Mag- Most of the papers were published in peer-reviewed jour- IC’s data model (v3.0) to enable a seamless transfer. nal articles, with two contributions from academic theses. ICEPMAG follows the site–sample–specimen hier- Of these papers, Leó Kristjánsson is the lead author of archy as utilised by MagIC and described by Butler 25 and co-author of a further 9, contributing palaeomag- (1992), where a site represents an individual unit or bed netic directions from over 6339 lavas. Not all of the papers of an igneous complex (e.g. a lava fow or dike) from were widely available; 19 do not possess permanent digital which samples were taken and cut into individual spec- object identifer (DOI) links and several were only avail- imens for palaeomagnetic measurements. ICEPMAG able in hard copies which are not globally distributed. stores data at the site level; this approach required a While building up the source library, data and meta- trade-of between accessibility and the level of detail data types common to all papers, and potentially use- stored in the database. ICEPMAG cannot store data ful to future users, were identifed. Tese were used to to the level of detail that MagIC can, but retrieval and select appropriate felds for the ICEPMAG database (see visualisation of Icelandic data are straightforward and “Data felds” section). Relevant felds were also drawn tailored towards Iceland. from the GEOMAGIA50 and MagIC databases, with Another key principle in the construction of future uploads to the MagIC database in mind. Selected ICEPMAG is the use of ‘master’ and ‘relational’ tables data felds were then used to set up master and relational (Additional fle 1: Tables S1–S16). Instead of storing long tables using the MySQL database, the structure of which text strings in the master table, some of the data felds list was based on GEOMAGIA50. Te master table contains integer IDs which reference separate relational tables, e.g. 70 data felds, where each row represents an individual region names with special Icelandic characters (Table 1). site. Fifteen of the data felds in the master table do not Tis structure allows for more detailed descriptions and contain numerical data or text, but instead list integer ID complex text to be stored in relational tables, rather than codes which reference relational tables (discussed above). in the master table. Tis approach is particularly useful Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 7 of 19

Te next step involved progressing through the source or reverse), as well as statistical parameters such as 95% α θ library study by study and populating the master and confdence limit ( 95 ), angular standard deviation ( 63 ), relational tables with data and metadata. Tis was resultant Fisher vector (R), dispersion parameter (k), par- achieved by: (1) copying data directly from published allel latitude uncertainty (DP) and meridian uncertainty spreadsheets, the MagIC database, or the compilation (DM). Also included are various metadata relating to the described by Kristjánsson (2013), if available; (2) translat- direction calculation such as demagnetisation type (e.g. ing tables from a published PDF documents; or (3) typ- AF or thermal) and peak demagnetisation step (for AF), ing out data manually if a PDF could not be read digitally. number of samples/specimens used, type of specimen If copied from existing compilations, data were cross- direction calculation (e.g. principal component analy- checked against the original source material. sis), sample direction averaging method (e.g. averaged Papers were read through, and key metadata, if not declination and inclination), site direction averaging copied directly from existing entries in MagIC, were method (e.g. mean calculated with Fisherian statistics) extracted from the text and/or fgures in each paper, e.g. and magnitude of tilt correction (if applied). To describe location and region names, sampling and orientation the method used by Leó Kristjánsson for many entries methods, laboratory protocols and statistical techniques in ICEPMAG (as discussed in “Icelandic data in global used. Geographic coordinates (i.e. site latitude and lon- analyses” section), we have listed the specimen direction gitude) are occasionally given in papers, but often the calculation type as ‘LP-DC2’ (with respect to the method coordinates needed to be inferred from published loca- codes in MagIC’s vocabulary) and the site direction aver- tion maps. aging method as ‘a95 min’ (i.e. the mean direction at the α To allow the user access to the database, a web query AF step which produces the smallest 95 is selected). form was designed for ICEPMAG’s front-end. Tis query form was based on the form used for GEOMA- Palaeointensity data and metadata GIA50. Fields considered useful for searching and flter- Tese felds include data obtained from palaeointen- ing data were used to construct the form, which utilises sity determinations, such as site mean palaeointensity both HTML and PHP scripting. A number of output and and uncertainty (usually standard deviation), and cor- download options were included in the form (see “Query responding dipole moment calculations for VDM and form” section). Interpretive PHP scripting, also adapted VADM and their uncertainties. Also included are vari- from GEOMAGIA50, used to transfer user queries from ous metadata such as palaeointensity method used (see the query form to the server side MySQL database and “Palaeointensity data” section for an overview of the pal- return data back to a results page, so users can visualise aeointensity methods included in the database), num- and download data (see “Results page” section). ber of samples/specimens used, and type of alteration Te fnal step in the construction process was to design check. Fields for anisotropy and cooling rate corrections additional functionality and webpages for the website, were not added at this time as no studies reported this including a searchable list of included studies, news information. and updates related to the database, statistics on data- base usage, as well as relevant credits and links to other Magnetisation and susceptibility data and metadata databases. Tese felds include data such as mass or volume normal- ised average magnetisation (often listed as JNRM or J100 Data felds in publications, denoting the NRM intensity or intensity Tis section describes the types of information stored after 10 mT AF demagnetisation) and associated uncer- across ICEPMAG’s 70 data felds (Additional fle 1: tainty, Koenigsberger ratio and average bulk susceptibil- Table S1). As with GEOMAGIA50, all data are provided ity, as well as metadata for the demagnetisation step at at the site level, i.e. data are from individual lava fows (as which remanence is reported (Leó Kristjánsson com- noted in “Design principles” section). monly reported remanence intensity at J100).

Palaeomagnetic data and metadata Sampling metadata Palaeomagnetic data are split into directional and pal- Tese felds include site name (the ID of an individual aeointensity data and associated uncertainties. Site fow), sampling type (e.g. portable drill or hand samples), directional data are reported in both geographic and height (above arbitrary point, e.g. base of profle) and/or tilt-corrected coordinates. Tese felds include declina- elevation (above sea level), number of samples collected tion, inclination, corresponding VGP latitude and lon- and sample names (if listed), as well as specimen type gitude (calculated from tilt-corrected and uncorrected (e.g. 2.5 cm diameter cores cut into 2–2.2 cm lengths), directions, if provided), directional polarity (e.g. normal Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 8 of 19

number of measured specimens per sample and speci- Geographic data and metadata men names (if listed). Tese data felds include coordinates for site latitude and longitude, and metadata such as location/area name and Age data and metadata wider region name. Tese felds include estimated age and associated error, and/or upper and lower bounds on age estimates, as well Study information as metadata about the employed dating method, such as Te reference ID feld links to a relational table contain- geological estimate or radiometric dating (see “Locations ing study metadata such as authors, title, year, publica- and ages of palaeomagnetic sites” section for an overview tion name, as well as DOI and MagIC links (if they exist). of dating methods used on Iceland). User interface Geological data and metadata Query form Tese felds contain information about geology of the site ICEPMAG’s database can be accessed through an online including bedding orientation (dip and dip direction), query form which allows users to search for and retrieve geologic class (e.g. igneous), and geologic type (e.g. lava data based on a range of customisable constraints (Fig. 2): or volcanic dike).

Fig. 2 ICEPMAG query form. Screenshot of the query form for ICEPMAG’s website. Additional search options can be viewed by expanding the ‘Other constraints’ box Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 9 of 19

Fig. 3 ICEPMAG query form—other constraints. Screenshot of the expandable ‘Other constraints’ options for ICEPMAG’s query form

• Optional outputs, including the level of detail in the • Publication constraints, which allow for specifc ref- HTML results table, an interactive site location map, erences (by author and year) to be selected, or to as well as a downloadable VGP plot and a CSV fle search by year of publication; containing data for the search results; • Other constraints (Fig. 3), which allow the user to • A flter for the results to select only sites which report search for results using various metadata such as directional data, VGP data and/or palaeointensity geologic type, sampling type, specimen type, palae- measurements; ointensity method, dating method, site statistics, and • Geographic constraints, such as a selected region or directional polarity. location within Iceland (can select multiple options), or between specifed coordinates; Te advanced search options allow the user to quickly • Age constraints, which allow the user to select data set up search parameters and return diferent datasets, from sites estimated to be older than, younger than, or search for studies containing particular types of data or in between certain ages; or methods, e.g. the user may want to remove studies with a small number of samples per fow, such as the Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 10 of 19

Fig. 4 Example site location map. Screenshot of an interactive location map generated by an ICEPMAG query (all sites in the Westfords region)

Fig. 5 Example of VGP plot. Screenshot of a VGP plot generated by an ICEPMAG query (all published VGP data from the Westfords)

Watkins and Walker (1977) study in the Eastern Region et al. 1977; Kristjánsson 1985a; Eriksson et al. 2014), or in which only two samples per fow were collected, or only retrieve sites with normal, reverse or transitional specifcally target or exclude studies of dikes (e.g. Piper VGPs. Researchers can get a quick overview of the data, Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 11 of 19

Fig. 6 Histogram of sites by publication year. Distribution of ICEPMAG sites across year of publication, coloured by geographic region (legend inset) which studies may be relevant, and where the data come Te ‘VGP plot’ tab displays a plot of all VGP positions from, without having to fnd and compile each dataset contained in the search results. Te results are plot- individually. ted on a Mollweide projection of Earth, centred on the Greenwich meridian (e.g. Fig. 5). Te plot is generated Results page by parsing the results to a Python script which generates After selecting the constraints and clicking on ‘Perform downloadable PNG and SVG fles and allows the user to Query’, the results page opens in a new window and dis- quickly view VGP data matching the search criteria. plays four tabs. Te ‘Download data’ tab provides a link to a down- Te ‘Palaeomagnetic data’ tab contains a summary of loadable CSV fle which contains the full results of the the query parameters, the main results table, and several search. Te CSV fle contains 70 data felds and allows to relational tables which provide descriptions relevant to user to retrieve and use and the full dataset contained in the integer ID felds stored in the master table. Tis tab ICEPMAG. allows for the user to quickly view the main felds from the data returned in the search. Te master table is a Database overview truncated version of the full results table which can be Locations and ages of palaeomagnetic sites downloaded as a CSV fle (discussed below). Te out- In general, palaeomagnetic studies in Iceland have tar- put to the webpage is limited because of the potential to geted areas away from the central spreading ridge and return and display thousands of site results. recent glacial and volcanic activity (Fig. 1). Data in Te ‘Location map’ tab produces an interactive map ICEPMAG are dominated by a number of large studies which plots all sites matching the search terms (e.g. published in the 1970s and 1980s (Fig. 6); these stud- Fig. 4). Te map is generated by parsing the results from ies targeted older lavas in the Westfords and Eastern PHP to the Google Maps Javascript application program Region, with some reporting results from over 1000 interface (API) and allows the user to zoom and navigate lavas each (Watkins and Walker 1977; McDougall et al. around the map to view the site locations. Matching sites 1984). Relatively, few studies have been carried out in are shown as markers over the default map, with options the Southern Region, which is prone to frequent seismic to display underlying terrain or satellite imagery. Hov- and volcanic activity, as well as glacial fooding, and long ering over a marker will display the reference and loca- sequences of lavas are not easily accessible. Recently, a tion IDs, which correspond to relational tables displayed number of studies have targeted Holocene lavas closer to underneath the map. the central ridge and volcanic centres (e.g. Stanton et al. Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 12 of 19

Fig. 7 Histogram of sites by age. Distribution of ICEPMAG sites across the 0–16 Ma age range, coloured by geographic region (legend inset). Sites with negative values (< 0 Ma) indicate lavas erupted since the standard year AD 1950

2011; Tanaka et al. 2012; Cromwell et al. 2015; Pinton et al. 2018). Sites in ICEPMAG are reasonably distributed across the 0–16 Ma age range (Fig. 7), although the distribu- tion of estimated ages has some peaks due to a number of large studies reporting hundreds of sites each:

• 2–3 Ma (Doell 1972; Kristjánsson et al. 1980, 1991; Helgason and Duncan 2001; Tanaka and Yamamoto 2016) • 8–10 Ma (Saemundsson et al. 1980; Helgason 1982; McDougall et al. 1984; Kristjánsson and Jóhannesson 1999; Kristjánsson et al. 2006) • 12–14 Ma (Piper et al. 1977; McDougall et al. 1984; Fig. 8 Pie charts of dating methods. Distribution of ICEPMAG sites by dating method. NB: 74 sites (with no age or dating method specifed) Kristjánsson et al. 1995; Kristjánsson and Jóhannes- have been excluded son 1996; Kristjánsson 2009).

Radiometric (K–Ar and Ar/Ar) dating of lavas is difcult in Iceland, due to a low potassium content (e.g. McDou- (e.g. Renne 2000; Kuiper et al. 2008; Renne et al. 2010; gall et al. 1984; Guillou et al. 2010; Jicha et al. 2011), so Rivera et al. 2011), so there is significant potential most studies have relied on broad magnetostratigraphic to refine the age estimates in ICEPMAG, especially controls to constrain age estimates. Relatively, few sites with respect to the dating of reversals and excursions (only 230 or ~ 2%) in ICEPMAG contain precise dating (which are the clearest stratigraphic markers), and this information (Fig. 8), and most of the geological age esti- may help refine ages in the GPTS. mates are published with age uncertainties in the range of Directional data ± 0.5 Ma. Dating with modern methods (e.g. Camps et al. 2011; Typically, the number of samples collected per site has Singer 2014) is required to revise and improve pre- been low (Fig. 9), with 78% of sites reporting results cise age estimates, as much of the radiometric work from four samples or fewer. It has been considered that was carried out using the K-Ar method prior to the ‘...collecting larger numbers of samples per fow is not mid-1980s (e.g. Moorbath et al. 1968; Saemundsson necessary because of the very good stability of original and Noll 1974; McDougall et al. 1976; Saemundsson remanence in Icelandic lavas...’ (McDougall et al. 1984). α95 et al. 1980; Jancin et al. 1985). Methods, standards, Tis view is refected in the distribution of values and decay constants have changed since that time (Fig. 10): nearly half (49%) of the site mean directions in Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 13 of 19

Fig. 9 Histogram of samples collected per site. Distribution of number of samples collected per site, coloured by geographic region Fig. 11 Histogram of k values. Distribution of ICEPMAG sites by (legend inset) reported k value (limited to < 4000)

Fig. 10 Histogram of α95 values. Distribution of ICEPMAG sites by ◦ reported α95 value (limited to < 45 ) Fig. 12 Histogram of sites by VGP latitude. Distribution of ICEPMAG sites by VGP latitude, coloured by geographic region (legend inset)

α ◦ α ◦ ICEPMAG have 95 ≤ 5 , and 72% with 95 ≤ 10 . Direc- α 5◦ tions with an 95 ≤ are generally considered ‘good’, total), 42% and 43% are designated as normal and reverse although no set criteria exist and, depending on the polarity, respectively, with 15% reporting transitional 15◦ < ◦ research purpose, values up to could be considered (i.e. VGP latitude 40 ) poles. Te asymmetry in very ◦ ◦ acceptable (see Butler 1992). low latitudes (i.e. between 10 S and 10 N) appears to be McDougall’s view is also refected in the distribution of a product of extensive sampling of the R3–N3 reversal k, the Fisherian dispersion parameter (Fig. 11). For sites in southwest Iceland (e.g. Shaw 1975; Kristjánsson and in ICEPMAG, k is much less frequently reported than Sigurgeirsson 1993; Goguitchaichvili et al. 1999a) which α 95 ; only 1224 sites report k values (while 7479 report reports transitional directions biased towards very low α ◦ 95 values). k > 50 has been taken as a parameter for northern latitudes (i.e. 0–10 N). some statistical studies (e.g. Johnson et al. 2008) and it appears that nearly all the Icelandic data would fulfl this; Palaeointensity data of ICEPMAG sites reporting k values, 94% (1153 sites) Figure 13 displays the number of sites in ICEPMAG by have k > 50 (Fig. 11). Butler (1992) suggests a minimum palaeointensity method. While Tellier–Tellier-type acceptable value of k > 30, and expects ‘good’ values of k > methods (see below) have been mostly used for palae- 100 for fresh volcanic rocks. ointensity studies on Iceland, Shaw-type methods have Published VGPs, calculated from site mean direc- contributed the largest number of palaeointensity results tions, are reasonably well distributed globally (Fig. 12). to ICEPMAG, primarily from two large studies (Shaw Of the sites in ICEPMAG reporting VGP data (8561 in et al. 1982; Roberts and Shaw 1984). Studies contributing Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 14 of 19

Other methods Microwave excitation for demagnetisation, as proposed by Walton et al. (1993), has seen limited use in Iceland by Brown et al. (2006) and Stanton et al. (2011). Te ‘MSP- DB’ (Dekkers and Böhnel 2006) method has been trialled on Icelandic lavas by Michalk et al. (2008) and Muxwor- thy and Taylor (2011). Muxworthy (2010) also trialled the ‘Wilson’ method (Wilson 1961), as Schweitzer and Sofel (1980) had done previously.

Discussion Selection criteria Tere were no reliability criteria enforced in the con- Fig. 13 Histogram of sites by palaeointensity method. Distribution struction of ICEPMAG. Our knowledge on what of ICEPMAG sites by palaeointensity method, coloured by geographic constitutes reliable data is always evolving and being re- region (legend inset) evaluated. Te primary aim of the ICEPMAG database is to preserve data that have already been collected and make them available to researchers for whatever appli- cation is being pursued. Tis approach maximises the palaeointensity results to ICEPMAG are listed below amount of data available to researchers and allows indi- without reservation. However, some of the studies viduals to refne the dataset based on their own criteria, contain limited or questionable results, e.g. Kristjáns- specifc research requirements, and previous observa- son (1985b) demonstrated that several data in the two tions. A number of reliability criteria have been suggested large Shaw-type studies (cited above) are internally for various purposes (e.g. McElhinny and McFadden inconsistent. 2000; Johnson et al. 2008; Cromwell et al. 2018), and we encourage the reader to think about the applicability of Thellier–Thellier‑type methods these criteria when using Icelandic data. Tere may be Te Tellier–Tellier method in its original form uses for the data in ICEPMAG that have not been envi- (Tellier and Tellier 1959) has been used by Sch- sioned and blanket application of such criteria may hin- weitzer and Sofel (1980), Goguitchaichvili et al. (1999a, der research in some direction. b, c), Camps et al. (2011), Stanton et al. (2011), and Tan- Guidelines for data usage aka and Yamamoto (2016). Te Coe-Tellier (or ‘ZI’) variation (Coe 1967) has been used by Senanayake et al. It is suggested that users of ICEPMAG think critically (1982), Marshall et al. (1988), Levi et al. (1990), Tan- about the data in the database and how to use them. Te aka et al. (2012, 1995), and Vérard et al. (2012). More query form has been designed for researchers to quickly recently, the ‘IZZI’ variant (Tauxe and Staudigel 2004) return data that ft various statistical parameters. Te has been used by Michalk et al. (2008) and Cromwell form enables users to get an overview of the content as a et al. (2015). Other studies on Iceland, e.g. Linder and whole and remove data from studies that may not ft their Leonhardt (2009) and Ferk and Leonhardt (2009), have needs. Palaeomagnetic data in the database have been included additional alteration and additivity checks as collected for diferent purposes. It would not be advised specifed by the ‘MT4’ method (Leonhardt et al. 2004). to include data collected to understand, e.g. the infuence of tectonic rotation in northern Iceland (Jancin 2010; Horst et al. 2018; Titus et al. 2018; Young et al. 2018) Shaw‑type methods in statistical analyses of palaeosecular variation. Tis Te use of AF demagnetisation in palaeointensity is a compromise in the construction of the ICEPMAG experiments was initially proposed by van Zijl et al. database. (1962) and used on Icelandic lavas in this original form Before analysing the data, researchers should con- by Smith (1967a, b) and Lawley (1970), and partially by sider the precision of the data and the level of precision required to solve the problem in question, e.g. palaeo- Schweitzer and Sofel (1980). Te method was revised ±1◦ and popularised by Shaw (1974) and subsequently used magnetic directions are not more precise than in on Icelandic lavas by Shaw (1975), Shaw et al. (1982), declination and inclination, due to the errors inherent in Senanayake et al. (1982), and Roberts and Shaw (1984), sampling and measurement processes. Furthermore, the and recently to a limited extent by Tanaka et al. (2012). kind of metadata required to ensure the data are ft for Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 15 of 19

purpose should also be considered, e.g. whether or not a northwest Iceland. Tere is perhaps a greater potential tilt correction has been applied to the data, how reliable to fnd unaltered lavas at higher elevations in the West- this correction is, and if a regional rather than a local tilt fords. Palaeointensity studies on Iceland could also be has been applied. Tese are important to consider when more fruitful if rocks with more favourable rock mag- building a dataset for long term feld analysis. netic properties are sought, e.g. volcanic glass (Cromwell et al. 2015), rather than the massive parts of lava fows. Serial correlation Radiometric dating of lavas on Iceland has received Another issue with Icelandic data is serial correlation of little attention since the mid-1980s (see “Locations and palaeodirections, or ‘oversampling’ of the feld, where ages of palaeomagnetic sites” section). Renewed40 39 interest multiple lava sequences have erupted in short-time inter- using more modern approaches to, e.g. Ar/ Ar dating, vals, potentially leading to inaccurate estimates of PSV could signifcantly improve our understanding of the age and biasing of mean directions (Johnson and McFadden of palaeomagnetic variations on Iceland, including the 2015). Tis is discussed in several papers (e.g. Kristjáns- age of reversal boundaries, which could aid in refning son 2002, 2013). Johnson and Constable (1997) suggested the GPTS. thinning of datasets to address this problem, a method which has been applied to Icelandic data in a number Conclusion of cases (e.g. Kristjánsson et al. 1980, 2003, 2006). Tests Summary of outcomes such as a non-random-ordering (NRO) factor can also be Research in Iceland has produced an extensive collection performed (e.g. Biggin et al. 2008; Brown et al. 2009) to of palaeomagnetic data which has been compiled into identify serial correlation. ICEPMAG. Tis new database accompanies other recent eforts to compile large regional datasets and provides a Future applications of Icelandic data useful tool for palaeomagnetists and other Earth scien- Kristjánsson (2008, 2013) advocates for further research tists carrying out research in Iceland or looking to utilise on large collections of Icelandic lava fows to improve Icelandic data. Palaeomagnetic data from 9491 sites with our understanding of the nature of the geomagnetic feld. palaeodirections and/or palaeointensity measurements, Despite such a large existing collection from the region, published across 79 studies between 1953 and 2018, can still only a small percentage of the exposed sections in be easily accessed and downloaded. 8936 contain direc- Iceland have been sampled for palaeomagnetic purposes. tion only, 218 intensity only, and 337 both direction and Statistical analyses of data from Icelandic lavas appear intensity. Each entry in the database contains a signif- to yield several results in contrast to some of those from cant amount of metadata. global datasets and further research may help to address Te ICEPMAG database is accessible through a pub- a number of fundamental and long-standing questions in licly available website (http://www.icepm​ag.org), which palaeomagnetism, such as whether or not secular vari- features a simple query form to enable rapid search- ation and polarity reversal rates are related to the mag- ing and visualisation of Icelandic data. Te results from nitude of the dipole moment, reversals follow preferred queries are displayed on a separate results page which longitudinal paths and/or are signifcantly infuenced by contains summary tables of the data and calls additional non-dipolar felds during transitions, and how mean feld scripting to produce an interactive location map, a VGP intensity varies as a function of VGP latitude. Analyses of plot, and a downloadable spreadsheet containing the full Icelandic data could also be used to investigate the pres- results table. ence of asymmetries in the normal and reversed palaeo- magnetic feld (e.g. epoch lengths, feld intensities, mean VGPs). Path forward Tere is signifcant potential to explore the long-term ICEPMAG will be uploaded to MagIC, contributing an evolution of geomagnetic feld intensity on Iceland. Tere additional 52 studies and 6857 sites to the global palaeo- have been a small number of palaeointensity studies rela- magnetic database. tive to directional studies. Although there are a signif- Subsequent revisions to ICEPMAG are expected to cant number of palaeodirections from the past ~ 2 Ma, include: there are not many data from the 500 or so post-glacial lavas on Iceland (directions or intensities) and absolute • Addition of new data from the Iceland Palaeomag- palaeointensity data in general (although compared with netism Laboratory as they are published, as well as other locations, there are more). To date, no palaeoin- new studies from other laboratories. tensity studies have been carried out in the older lavas of • Addition of new age data as new geochronological studies are published. Tonti‑Filippini and Brown Earth, Planets and Space (2019) 71:83 Page 16 of 19

• Addition of other studies containing data that were Brown MC, Singer BS, Knudsen MF, Jicha BR, Finnes E, Feinberg JM (2009) No evidence for Brunhes age excursions, Santo Antão, Cape Verde. not available in tabular form in the published litera- Earth Planet Sci Lett 287(1–2):100–115. https​://doi.org/10.1016/j. ture. epsl.2009.07.039 • Continued long-term maintenance of the database Brown MC, Donadini F, Korte M, Nilsson A, Korhonen K, Lodge A, Lengyel SN, Constable CG (2015) GEOMAGIA50.v3: 1. general structure and modif‑ and adaptations based upon the needs of the palaeo- cations to the archeological and volcanic database. Earth Planets Space magnetic community. 67:83. https​://doi.org/10.1186/s4062​3-015-0232-0 Brynjólfsson A (1957) Studies of remanent magnetism and viscous magnetism in the basalts of Iceland. Adv Phys 6(23):247–254 Butler RF (1992) Paleomagnetism—magnetic domains to geologic terranes. Additional fle Blackwell, Boston, p 319 Camps P, Singer BS, Carvallo C, Goguitchaichvili AT, Fanjat G, Allen B (2011) The Kamikatsura event and the Matuyama-Brunhes reversal recorded Additional fle 1. Additional Tables. in lavas from Tjörnes Peninsula, Northern Iceland. Earth Planet Sci Lett 310(1–2):33–44. https​://doi.org/10.1016/j.epsl.2011.07.026 Coe RS (1967) The determination of paleo-intensities of the Earth’s magnetic Acknowledgements feld with emphasis on mechanisms which could cause non-ideal We acknowledge Leó Kristjánsson’s immense body of work on the palaeo‑ behavior in Thellier’s method. J Geomagn Geoelectr 19(3):157–179. magnetism of Icelandic lavas (described in the main text). Construction of the https​://doi.org/10.5636/jgg.19.157 database was aided signifcantly by Leó’s digital compilation, which provided Constable C (2001) Geomagnetic reversals: rates, timescales, preferred paths, data from ~ 5800 sites (Kristjánsson 2013). We thank Leó for providing helpful statistical models and simulations. In: Jones CA, Soward AM, Zhang K comments on an earlier draft of the manuscript. (eds) Earth’s core low. Taylor and Francis, London We acknowledge the work of Fabio Donadini, Kimmo Korhonen and Lauri Cromwell G, Tauxe L, Halldórsson SA (2015) New paleointensity results from Pesonen who initiated the GEOMAGIA project and developed the principles rapidly cooled Icelandic lavas: implications for Arctic geomagnetic and coding that this database borrows from heavily. We thank Nick Jarboe feld strength. J Geophys Res Solid Earth 120:2913–2934. https​://doi. for helping us understand the inner workings of MagIC. We thank the two org/10.1002/2014J​B0118​28 anonymous reviewers whose comments/suggestions helped improve and Cromwell G, Johnson CL, Tauxe L, Constable CG, Jarboe NA (2018) PSV10: a clarify this manuscript. global data set for 0–10 Ma time-averaged feld and paleosecular varia‑ tion studies. Geochem Geophys Geosystems 19(5):1533–1558. https​:// Authors’ contributions doi.org/10.1002/2017G​C0073​18 MCB conceived the project and provided the GEOMAGIA code used to Dagley P, Wilson RL (1971) Geomagnetic feld reversals-a link between develop ICEPMAG. JTF compiled all palaeomagnetic data and developed the strength and orientation of a dipole source. Nat Phys Sci 232:16–19 structure and functionality of the database under the guidance of MCB. The Dagley P, Wilson RL, Ade-Hall JM, Walker GPL, Haggerty SE, Sigurgeirsson T, manuscript was written by JTF with contributions from MCB. Both authors Watkins ND, Smith PJ, Edwards J, Grasty RL (1967) Geomagnetic polarity read and approved the fnal manuscript. zones for Icelandic lavas. Nature 216:25–29 Dekkers MJ, Böhnel HN (2006) Reliable absolute palaeointensities independ‑ Funding ent of magnetic domain state. Earth Planet Sci Lett 248(1–2):507–516. This project was completed as part of a Master’s programme at the University https​://doi.org/10.1016/j.epsl.2006.05.040 of Iceland Faculty of Earth Sciences and no funding was received. Doell RR (1972) Paleomagnetic studies of Icelandic lava fows. Geophys J R Astron Soc 26:459–479. https​://doi.org/10.1111/j.1365-246X.1972.tb057​ Availability and requirements 63.x Project name: ICEPMAG. Project home page: http://icepm​ag.org/. Operating Døssing A, Muxworthy AR, Supakulopas R, Riishuus MS, Niocaill CM (2016) system(s): Platform and browser independent. Programming language: PHP, High northern geomagnetic feld behavior and new constraints on the SQL, HTML, JavaScript, Python. Other requirements: none. License: none. Any Gilsá event: Paleomagnetic and 40Ar/39Ar results of 0.5-3.1 Ma basalts restrictions to use by non-academics: none. from Jökuldalur, Iceland. Earth Planet Sci Lett 456:98–111. https​://doi. org/10.1016/j.epsl.2016.09.022 Competing interests Einarsson T (1957) Magneto-geological mapping in Iceland with the use of The authors declare that they have no competing interests. a compass. Adv Phys 6:232–239. https​://doi.org/10.1080/00018​73970​ 01015​91 Author details Eriksson PI, Riishuus MS, Elming S-Å (2014) Magma fow and palaeo-stress 1 Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, deduced from magnetic fabric analysis of the Alftaforthur dyke swarm: Iceland. 2 Centre for Earth Evolution and Dynamics, University of Oslo, Sem implications for shallow crustal magma transport in Icelandic volcanic Sælands vei 2A, 0371 Oslo, Norway. systems. Geol Soc London Spec Publ 396:1–26. https​://doi.org/10.1144/ SP396​.6 Received: 22 May 2019 Accepted: 15 July 2019 Ferk A, Leonhardt R (2009) The Laschamp geomagnetic feld excursion recorded in Icelandic lavas. 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