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Home , Frederick Soddy, Heritage science, Isotopes of copper, Isotope geochemistry

Nord and Billström Herit Sci (2018) 6:25 https://doi.org/10.1186/s40494-018-0192-3

REVIEW Open Access in : present and future possibilities Anders G. Nord1* and Kjell Billström2

Abstract This paper is focused on methodology and scientifc interpretations by use of isotopes in heritage —what can be done today, and what may be accomplished in the near future? Generally, isotopic compositions could be used to set constraints on processes and manufacturing of objects (e.g. the 14C technique). Furthermore, isotopic compositions (e.g. Sr and Pb isotopes) are useful for tracing the origin of a component or a metal. The concepts and isotopic fractionation are explained, and the use of stable respectively radioactive isotopes is exempli- fed. Elements which today have a large potential in heritage research are reviewed, and some recent and less known applications from the literature are summarized. Useful types of mass spectrometers are briefy described, and the need for reliable standards as well as accurate measurements and corrections is stressed. In future, further chemical elements may be utilized for isotope studies in heritage science, and possible candidates are discussed. The paper may in particular be valuable to readers less acquainted with the use of isotopic measurements. The many examples from referenced papers and also results from the authors’ studies in this feld may inspire imaginative and inquisitive scientists to try new applications utilizing isotope data in heritage science. Keywords: Isotopes, Standards, Dating, Measurement, Mass spectrometer, , , Art

Brief history and introduction kinds of , having atomic weights around 20 respec- Some 100 years ago, the knowledge concerning the com- tively 22. Te diferent atoms were named “isotopes” after position of atoms was elucidated by brilliant scientists the Greek name which means “the same place” (i.e. in the such as , J. J. Tomson, , Periodic Table of Elements). Many of the scientists were Henri Bequerel, Niels Bohr, F. W. Aston, Frederick Soddy rewarded with a in or : J. J. and many others [1]. Much of the early work was concen- Tomson (1906), Ernest Rutherford (1908), Marie Curie trated on radioactivity. During studies on the radioactive (1911), Niels Bohr (1922), Frederick Soddy (1921), Fran- decay of and , a confusing discovery was cis Aston (1922), (1934) and James Chad- that there seemed to be several kinds of thorium atoms wick (1935). which decayed at diferent rates. Te American chem- Today, the concept isotope is well known to the scien- ist T. W. Richards showed in 1913 that produced tifc world, and numerous applications have for almost a from the decay of uranium had a diferent atomic weight century been used in science. were among the as compared with “ordinary” lead from lead . Soddy frst to use this technique when attempting to date rocks. showed that a radioactive element may have diferent Among the earliest works is a isotope study [2] atomic masses although with identical chemical proper- where uranium minerals were dated, which a few years ties. Francis W. Aston showed with his so-called mass later was followed by a radiogenic Pb study aimed to date spectrograph that gas is composed of two diferent the mineral uraninite [3]. Some decades later, isotopes were used also in other felds, e.g. medicine and archae- ology. In 1943, Georges de Hevesy was awarded a Nobel *Correspondence: [email protected] Prize for his isotopic methods to trace chemical reactions 1 National Heritage Board of Sweden, Kevingeringen 10, and processes in the human body. Still most important 182 50 Danderyd, Sweden Full list of author information is available at the end of the article in archaeology is the well-known

© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/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. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Nord and Billström Herit Sci (2018) 6:25 Page 2 of 13

technique [4], for which Willard F. Libby in 1960 received isotopes allow evaluations of both the . Today’s archaeologists can and diet pattern. A brief summary of isotope systems beneft from a number of scientifc techniques to inves- and their possible use in heritage science is presented in tigate excavated objects [e.g. 5–8], among them vari- Table 1. ous isotopic techniques. Isotope measurements are also applied in many other felds such as forensics, food and Stable isotopes and isotopic fractionation beverage industry, environmental , history, her- Local variations in the proportions of stable isotopes of itage science, and also utilized to detect forgeries in art. an element are brought about by isotopic fractionation. Te present paper gives examples of factors controlling Each element’s isotopes have identical chemical prop- isotopic compositions and gives prominence to chemical erties, whereas physical properties such as melting and elements which are apt to applications in heritage science boiling points display very slight diferences. Tis is the research, today and in the future. A brief description of basis for isotopes of diferent elements to behave difer- useful measuring instruments is included. ently during natural processes, and therefore the ratio of these isotopes changes or “fractionates”. As early as 1931, Fundamentals of isotopes Harold Urey discovered the deuterium isotope (2H or Each element is built of atoms. However, for most ele- “D”) and demonstrated discrepancies in vapor pressure ments their atoms may have diferent masses, and the among the isotopes of gaseous elements. Accordingly, variants are called isotopes. Te isotopic distribution for a evaporation, condensation, rain, snow, melting, crystal- specifc element is almost uniform in the lithosphere and lization, adsorption, difusion etcetera may give rise to atmosphere. However, small variations of the isotopic a small but (sometimes) measurable isotopic diferen- compositions of certain elements in nature do occur, and tiation [9]. For example, “ordinary” , ­H2O, boils at isotope studies are concerned with the interpretations of 100.00 °C at 1 bar, while “heavy” water, ­D2O, has a boil- such variations. Tese variations are due to two mecha- ing point of 101.42 °C. Complicated biochemical pro- nisms: (i) isotopic fractionation—which essentially is due cesses may also induce a slight isotopic fractionation, for to the breaking of physical bonds of diferent strengths; example in , during the synthesis of amino and (ii) —whereby one unstable (par- acids or lipids, and processes in bacteria [10–16]. In these ent) isotope spontaneously disintegrates and eventually processes, the isotopic fractionation is due to physical results in a stable (daughter) isotope. Te frst mechanism processes such as difusion. Occasionally, so called mass- concerns stable (non-radiogenic) isotopes and is above all independent fractionation (MIF) can be important in e.g. important for elements, as will be explained below. biochemical reactions as shown for a number of light Most chemical elements in nature exist with two or elements [17]. Generally, a light element induces larger more isotopes. Te various isotopes of each element have isotopic fractionation because the relative diference in the same number of (Z), but this is not true for mass between its isotopes is greater than for a heavy ele- the number of (N) or the mass (M Z N). For ment. For instance, deuterium (D, 2H) is twice as heavy = + 1 181 example, carbon (always with Z = 6) has three isotopes: as H, i.e. 100% heavier, while Ta is only 0.56% heavier the two stable isotopes 12C (with six neutrons) and 13C than 180Ta. Te light elements, , carbon, nitro- (seven neutrons), and (with only about one part per tril- gen and to mention a few, are therefore especially lion) the radioactive (non-stable) isotope 14C (with eight susceptible to natural isotopic fractionation, while much neutrons), often denoted C-14. is outstanding with less physical diferences are observed for the heavier ele- its ten stable isotopes. On the other hand, 20 elements ments. Te reason that the heavy elements and exist in nature with only one stable isotope. Among these lead have large and easily measurable diferences among elements are and , which is unfortunate their isotopes is principally due to radioactive decay pro- since both are important to archaeologists and cesses (cf. below). . It should also be added that no element with an Z > 83 is stable, i.e. containing only sta- Measuring techniques ble isotopes. Tere are basically two ways of applying iso- Fundamentally, there are two methods to determine the topic techniques: one to track the background of objects amount of various isotopes in a special material. Te frst or human remains (origin, provenance, diet, migration), is to directly measure the radioactivity of the isotope in and another for dating of investigated objects or sites. question, by counting particles (of α, β, or γ type) that Some isotope systems have been utilized for many dec- are produced by a specifc decay. Tis method has been ades, while others have only recently been tested or used used when applying the Carbon-14 dating technique for certain applications. Te stable and for other systems such as U-He and -3He, have a unique archaeological potential, since analyses of and have been described to detail in the literature [18]. Nord and Billström Herit Sci (2018) 6:25 Page 3 of 13

Table 1 Isotope systems which have been used in archaeology and cultural heritage Element Dating Provenance Diet for humans Migration among humans Other

H X B X C XX (C-14) XX N X O (X) X (X) Paleothermometry Mg (X) Si (X) S (X) X Ore prospecting, pollution Cl (X) Ar X Ca (X) Fe (X) Cu (X) Zn (X) Sr XX X Ag (X) Sn (X) Nd X Hg (X) Pb XX Th X U X The most commonly used systems are marked XX, those often used X, and those recently tested or used (X)

Te prevailing way of analyzing isotopes is to make use of some kind of mass spectrometer that is designed to determine the distribution of an element’s isotopes. Te working principle behind this technique is that diferent types of atoms or molecules can be separated accord- ing to their masses, and the ratios between the isotopes of interest can be subsequently measured. Most mass spectrometers are designed for the of organic compounds. However, several types of mass spectrom- eters have been developed to determine isotope ratios and the basic features of a few commonly used types are presented here. Te frst generation of magnetic sector instruments in the 1940:s are often named “Nier spec- trometer” after its inventor Alfred Nier [19], cf. Fig. 1. Tis type is sometimes abbreviated Isotope Ratio Mass Spectrometer (IRMS) and another acronym is Analysis (SIRA), often used for analysis of gaseous compounds of light stable elements. During the latest decades, more advanced mass spectrometers have been developed, producing increasingly better precision for sample sizes becoming smaller and smaller (nano- gram to picogram levels) [e.g. 20–23]. Te amount of Fig. 1 Schematic picture of a Nier-type isotope ratio mass spectrom- sample is therefore rarely a problem, but their purity is eter using a gas source analytical medium (in this example carbon of the greatest importance. For solid samples, a Termal dioxide, ­CO2). Reproduced after chemical sciences: a manual for CSIR- Ionization Mass Spectrometer (TIMS) is often used, e.g. UGC National Eligibility Test, http://wikibooks.org Nord and Billström Herit Sci (2018) 6:25 Page 4 of 13

for strontium and specimens, and generates Standards and data for H, C, N, O, S very precise results. It utilizes a minute solid sample, pre- Te lightest of all elements, hydrogen (Z = 1), has two viously isolated in a clean laboratory following standard stable isotopes: 1H (major part) and 2H (deuterium; often ion exchange procedures. Certain spectrometers, like a denoted D), which constitutes only 0.015% of all hydro- 12 MC-ICP-MS spectrometer (Multiple Collector—Induc- gen atoms. Carbon (Z = 6) has two stable isotopes, C (ca tively Coupled Plasma—Mass Spectrometer; cf. Fig. 2) 98.9%) and 13C (ca 1.1%). Te ratio 13C/12C, here denoted are equipped with a laser system. Such laser-based sys- ­R13, varies within a small range in an organism mainly tems remove (ablate) particles from the surface of a sam- due to consumed food (“you are what you eat”), refer- ple which are led to an analyzer. Tis technique as well ring to the stable isotope signature of the body [24]. Te as the ion microprobe (SIMS; secondary ion mass spec- value of the sample (s) is usually given relative to a stand- 13 trometry) are capable of analyzing samples in situ, which ard (std) by the expression δ C = [R13s/R13std − 1] × 1000. eliminates the need for tedious laboratory treatment. Te Tis gives a number (in ‰), easier to handle, which is latter techniques are versatile and can be used for a range either positive or negative relative to a standard. Te of elements, allowing both stable and radioactive iso- international standard was for many years PDB, a belem- topes to be analyzed in very small samples; besides, the nite from the Cretaceous Pee-Dee formation in South sample through-put is large compared to e.g. the TIMS Carolina. It is now exhausted, and therefore a V-prefx is technique. Te precision obtained with in situ tech- used (e.g. V-PDB), following the nomenclature suggested niques is, however, not as good as for TIMS-analyzed at a conference held in Vienna in 1993 [25]. samples. Irrespective of the instrumentation used, sam- (Z = 7) exists in nature with two stable iso- ples must be handled with care and thorough analytical topes, 14N (ca 99.64%) and 15N (0.36%). Te 15N/14N protocols need to be followed in order to obtain precise isotopic ratio is usually given as δ15N (in ‰) relative and accurate results. A feature in common for isotopic to a standard, commonly nitrogen in the air. Oxygen 16 measurements is the need for standardization, and this is (Z = 8) exists with three stable isotopes: O (99.76%), accomplished in diferent ways and is method-specifc. It 17O (0.04%) and 18O (0.20%). Usually the 18O/16O ratio is common practice to run samples in duplicate to ensure is determined, and its value is conventionally given as a good reproducibility. δ18O relative to an international standard (V-SMOW or V-PDB). (Z = 16) exists in nature with four stable Light stable isotope applications isotopes with mass numbers 32, 33, 34 and 36. Teir rela- Stable isotopes of light elements are utilized in many tive abundances are around 95.0, 0.76, 4.2 and 0.015%, felds. Te archaeological applications dominate, for respectively. Usually the 34S/32S ratio is determined instance to investigate changes in habitat, food, animal which is expressed as a δ34S value versus the Canyon Dia- herding, dietary tendencies and migration patterns for blo troilite (CDT), an sulfde standard. humans. A commonly used division is between “tradi- tional” stable isotopes of the light elements of primar- Common light elements—archaeology ily H, C, O, N and S, which are further discussed below. Among the light elements, carbon has the largest impact Heavier elements such as lead and strontium, as well as of modern . Organic carbon is iso- “non-traditional” elements (such as Mg, Cl, Fe and Cu) topically “light” (i.e. with a comparatively large propor- are considered in a subsequent paragraph. tion of 12C) with strongly negative δ13C values, for pit coal as low as − 25‰ and for methane even lower. In archaeology, δ13C values can usually give information on a human’s diet [e.g. 26, 27]. δ13C changes with the trophic level; lowest for green plants (ca. − 27‰), less nega- tive for terrestrial animals (ca. − 22‰), and highest for marine animals (ca. − 16‰). Even degraded lipids pre- served in ancient pottery may be analyzed [28]. Nitrogen isotopes are useful in archaeology to establish a probable diet pattern. An organism can integrate nitrogen from nitrates, ammonium or (rarely) free nitrogen in the air. Te δ15N value of green plants is around 2–4‰, while herbivorous animals have values around 6‰, i.e., a higher Fig. 2 An MC-ICP-MS spectrometer (Nu Plasma II) at the Department trophic level gives a higher value [29]. Marine organisms of Geological Sciences, Swedish Museum of Natural History, Stock- may have still higher values, and Inuit’s whose main food holm. Photo: Melanie Schmitt Nord and Billström Herit Sci (2018) 6:25 Page 5 of 13

is marine protein can have δ15N values as high as 18‰ of great interest in heritage science. It is a metamorphic [30]. product of limestone (a carbonate rock), whose dominant Oxygen is mainly integrated in humans through inges- mode of formation is from various organisms living in tion of water, the isotopes of which depend on altitude water. Accordingly, its carbon and oxygen isotope com- and latitude, and its isotopes may thus be used to verify positions have been used to determine the provenance a hypothetical geographic origin. In humans and animals, of marble in buildings and monuments in the Mediterra- sulfur is found in collagen and keratin. Tese proteins are nean area. An early study was published in 1972 by Craig relatively resistant towards acids and can be retrieved & Craig and was later followed by others [49–53]. even in prehistoric graves in regions now sufering from Oxygen isotopes may also be utilized in studying the acidifcation [31]. Marine, freshwater and terrestrial origin of e.g. . For instance, δ18O for meteor- ecosystems often display distinctly diferent sulfur iso- ological precipitations shows seasonal variations—those tope compositions, and accordingly sulfur isotope data deposited during the summer are less negative than the in human and faunal tissues can infer the consumption winter values [54, 55]. Accordingly, δ18O data can be used pattern of foods [32–34]. Te proportion of deuterium for paleo-thermometry [56]. An interesting study has in bone collagen of humans and animals shows a trophic been made by Dansgaard et al. [57]. Cores drilled from level efect, with increasing values from herbivores to Greenland ice were cut into small fractions, which were omnivores to humans [35]. A pioneering study of remains analyzed for δ18O. In this way, “age rings” similar to tree of humans, animals and plants from archaeological sites rings were obtained. Te cores could thus be dated, and by means of hydrogen isotopes has been published by the age could often be verifed from deposits of ashes Yang and Leng [36]. As hydrogen and oxygen are con- emitted from known volcanic eruptions. Inversely, earlier stituents in water, a combined O–H isotope systematics unknown eruptions could be identifed. Similar applica- may be useful to trace the origin of water found in e.g. tions may as well be used in heritage science where e.g. plants and living organisms. Studies involving hydrogen ash layers are present at excavation sites. Oxygen may isotopes in hair have been published [37–39]. also be extracted from silicates, which defne the pre- By combining diferent isotope systems of light ele- dominant mineral in nature, by means of a power- ments, such as carbon, nitrogen, oxygen, sulfur and ful chemical reagent such as pentafuoride [58], hydrogen, a detailed understanding can often be reached and such isotopic compositions can be used to relate regarding the origin and diets of e.g. archeological human an archaeological object to a specifc environment, for remains. In particular, sulfur and hydrogen isotope data instance in gemmological applications [59] and poten- can be a valuable complement to other stable isotope tially to determine the provenience of silica-rich artefacts (C and N) data obtained on human bone. Many stud- (e.g. obsidian tools) to a specifc rock quarry. Sulfur iso- ies involving human diets, nutritional ecology, extrac- topes have been used in environmental studies of dam- tion methods and also sheep’s wool have been published aged outdoor cultural monuments and buildings [60–65]. [40–46], and also a study combining stable isotopes and In many cases the origin of sulfur in the gaseous oxides ancient DNA analyses [47]. Extensive review concerning was due to the impact of fossil fuels. mammalian proteins has been published by Wilkinson [42]. An interesting multi-isotope study of the remains Provenancing objects using heavy isotopes (Pb, Sr, of King Richard III has been published by Lamb et al. Nd) [48]. For instance, there was a signifcant shift noted in Numerous examples demonstrate that Pb and Sr (and the nitrogen, but not carbon, isotope values towards the also e.g. Nd) isotopes may be used to set constraints on end of his life, which may be explained by an increase of the source or provenance of cultural objects. Such appli- luxury food and drink such as game birds, freshwater fsh cations are based on the long-lived decay series exempli- and wine. Tis is the frst suggestion of wine afecting the fed above for age dating. Te diference is that instead of isotope composition of an individual and thus has wider measuring a ratio between parent and daughter isotopes, implications for isotope-based palaeodietary and migra- appropriate isotope ratios including only the daugh- tion reconstructions. ter element are used for the provenance. For strontium and neodymium, the radioactive decays of interest are Common light elements—other applications 87Rb to 87Sr and 147Sm to 143Nd, and the relevant isotope Te study of light stable elements is also used in many ratios used are 87*Sr/86Sr and 143*Nd/144Nd (the asterixis other felds of heritage science. Some of these applica- denotes an isotope formed from radiogenic decay). tions have a geological background. For instance, geolo- Tese elements, along with lead, are advantageous gists measure carbon isotopes while prospecting for fossil because their isotopic variations are comparatively large fuel and to interpret the origin of carbonates. Marble is Nord and Billström Herit Sci (2018) 6:25 Page 6 of 13

and easily measured, and their isotope systems are well Mediterranean area [68–74]. Chinese glazes have been characterized. examined with strontium isotopes [75]. Te results on Lead is typically a trace element in bronze, coins and some of these studies also have relevance for manufactur- many other metal objects, and a major constituent of ing techniques. crystal glass and lead pigments. Tis enables lead iso- Lead pigments have also been analyzed for their iso- topes to be used to determine the provenance of these topic composition. In this way ancient Chinese and cen- objects. Lead exists with four stable isotopes having tral Asian pigments have been examined [76]. Fortunato mass numbers 204, 206, 207 and 208. Te three latter et al. [77] have determined lead isotope abundance ratios isotopes are successively added as a result of decay of U in lead white, a common constituent in seventeenth cen- and T isotopes, and their relative isotopic abundance tury oil paintings. Works of art by Rubens, van Dyck is often given by the ratios 206Pb/204Pb, 207Pb/204Pb and and other Flemish masters gave very similar lead iso- 208Pb/204Pb, using 204Pb as a reference isotope [54, 55]. tope distributions, which indicates a distinct origin of Since these decays are extremely slow, the geological age raw materials. Also Fleming [78] has described how lead of a mineral deposit is an important factor for its lead pigments in oil paintings by old masters may be used to isotopic composition. For Pb-rich no signifcant settle doubts on genuineness. With respect to Swedish in situ decay of U and T takes place after their forma- material, studies of mediaeval lead pigments from mural tion, thus implying that their Pb isotope ratios carry a paintings, church portals and baptismal stone fonts have “fossil record” which represents their time of formation. been performed with the aim to determine their origin Te isotopic variations in the lithosphere may therefore [79–81]. Most lead pigments were found to originate be considerable, which is favorable when attempting to from the Harz and Erzgebirge regions in Germany, but use Pb isotopes to reveal the provenance of the lead in also lead pigments from Russia and Sweden (the Bergsla- an object. For example, during the Middle Ages several gen ore district) were identifed. lead-bearing ore deposits were mined in south Sweden in A large number of archaeological bronze artefacts exca- the so called Bergslagen area west and north-west of Lake vated in Scandinavia have also been analyzed [82, 83]. In Mälaren. Tis region is geologically very old, around these studies, it was defnitely shown that the lead in the 1900-1800 Ma (million years), which gives quite difer- bronzes did not originate from Swedish ores, but rather ent lead isotope distributions (206Pb/204Pb values cluster from ore regions mainly in Austria, Spain and Sardinia. around 15.70) as compared with ores from the European Ancient coins have been studied elsewhere, as well continent exhibiting much younger Palaeozoic to Meso- as ancient bronze artefacts, and their origin and authen- zoic (545-65 Ma) terrains, where 206Pb/204Pb ore values ticity examined with the aid of lead isotopes [78, 84–87]. usually are in the range 17.5–19.0. Te ancient plumbing system in Pompeii has been exam- When using lead isotopes to provenance an object, ined for lead isotopes, indicating multiple sources for it is often customary to display data in 206Pb/204Pb ver- the lead [88]. Te ship Batavia, belonging to the Dutch sus 207Pb/206Pb or 208Pb/206Pb diagrams, respectively. East India Company, was shipwrecked outside Australia Numerous reference data from ancient mining districts in 1628. Some of their objects have been analyzed are reported in this way, and the isotopic compositions of for lead isotopes [89], showing that the metal originated discrete ores or regions may provide characteristic “fn- from various sources, among them the Swedish Berg- gerprints”. A disagreement between data for sample and slagen region with its very distinct . A ore deposit, respectively, defnitely shows that the ana- fnal example concerns the murder of the Swedish war- lyzed lead cannot originate from the deposit in question. rior king Karl XII in 1718. He was shot in Norway with a An agreement (“isotopic match”) indicates that the lead bullet containing lead to make it heavier. Te lead isotope may originate from that specifc deposit. It must also be study gave no clear evidence for the lead provenance— remembered that an overlap of isotopic fngerprints may however, it was certainly not lead from the Swedish Berg- exist between diferent ore districts, and that lead isotope slagen region [90]. data are not available for all conceivable mines. How- As regards strontium, the 87Sr/86Sr ratio difers for ever, lead isotope abundances in combination with other various geological environments, and the local variations observations such as trace elements can often help pin- can be appreciable. Since strontium is chemically related pointing a possible origin of the lead. Eforts to compile to , it is partly integrated in teeth and bone in an extensive database for lead isotopes have been made humans and animals. Dental enamel is mainly formed by a group in Oxford (the OXALID database) [66], and during the childhood and this part does not exchange currently work is in progress for further published data its strontium subsequent after its formation, while other [67]. Strontium, neodymium and oxygen isotopes have parts of the body are afected later in life. Strontium been used to provenance ancient glass, for instance in the and other isotopes can therefore be used to study the Nord and Billström Herit Sci (2018) 6:25 Page 7 of 13

migration of humans or animals during their life-time calcium isotope ratios were not substantially afected by [91–94]. Famous examples are given by various isotopic diagenetic changes. Even a preliminary study of analyses of the remains of “Ötzi the Ice-man” [95, 96]. (Z = 34) isotopes in organic Se-bearing species has been Tere are also examples of multi-isotopic approach in published [107]. Swedish archaeology, such as “Te man from Granham- Some isotope studies of metallic elements will be mar”, who seems to have travelled a lot before he was considered. Isotopes of copper and iron (having two fnally murdered [97], and there are many further exam- respectively four stable isotopes) have been measured in ples [e.g. 98, 99]. artefacts [108–114]. A great challenge is that both met- als are ubiquitous in the lithosphere, so the task to prov- Provenancing with less common stable isotope enance from their isotopic abundance may be anywhere systems from difcult to hopeless. A more promising candidate With the advent of new generations of ICP-MS and SIMS within the isotope feld may be with its fve stable instruments, it is today possible to detect and measure isotopes. Copper and zinc isotope ratios in human bone natural isotope variations among numerous “non-tra- and enamel have been examined by Jaouen et al. [115, ditional” elements such as , , , 116]. Tin, with ten stable isotopes, is interesting being a , iron, , copper, zinc, tin and many others. constituent in many bronze artefacts, but the analysis is Tese have recently attracted a large interest in the scien- indeed complicated. Nevertheless, some interesting stud- tifc community [17, 100, 101]. However, several factors ies have been published by Pernicka et al. [e.g. 117–119] obstruct a general usage in heritage science. Apart from and Balliana et al. and others [120, 121]. (with problems with infnitesimal fractionations induced in seven stable isotopes) is also likely to be difcult to use nature for the heavier elements, as well as uncertainties for isotope measurements because of minor fractiona- about the nature of processes leading to measurable frac- tion in nature, but it may have a potential as a tracer of tionations, reference data are still relatively few, and the organic matter [17]. Analyses of cinnabar pigments (HgS) analytical challenges are large and their common usage in have actually been undertaken [122], but in this study cultural sciences remains to be proven. the provenance was based on the pigments’ sulfur iso- Be that as it may, the recent decade has witnessed a topes, and fortunately the number of cinnabar sources steadily increasing interest for isotope studies regard- in Europe is limited. However, this necessitates that the ing the above-mentioned elements. Among these boron pigment certainly is the mineral minimum, and not a is the lightest element, with an atomic number Z = 5, synthetic product (called vermilion) made from mercury which is advantageous since relatively large isotopic and sulfur. In the latter case the sulfur may originate from diferences may be expected. Boron is a trace element anywhere in Europe. With the aim to trace the prov- present in natron glass, and a study of Roman glass has enance of coinage through variations in isotopic abun- recently been published and proven boron to be a prov- dances, silver, copper, and lead isotopes were measured enance indicator for glass [102]. In that paper it could be in 91 coins from the East Mediterranean Antiquity and concluded that the Greco-Roman glasses showed a rather Roman world, medieval western Europe, sixteenth to homogeneous isotopic composition, expressed as δ11B. eighteenth century Spain, Mexico, and the Andes [123]. Magnesium (Z = 12) has three stable isotopes, and in Te isotope measurements demonstrate that also silver a recent study Mg isotopes were measured in mammal isotopes has a large potential for provenance studies. Pre- tooth enamel [103]. It was found that δ26Mg increases 1492 European silver can be distinguished from Mexican from herbivores to higher-level consumers, discrimi- and Andean metal. European silver dominated Spanish nating most of the trophic steps. Tis, combined with coinage until Philip III, but had, 80 years later after the values for Ba/Ca, δ13C and δ34Ca may prove useful in reign of Philip V, been fushed from the monetary mass paleodietary studies. Also, silicon (Z = 14) isotopes have and replaced by Mexican silver. been studied, usually with a geological focus [104] but has also been used as a proxy for environmental change Radioactive isotopes used for age dating Radiocarbon dating [105]. Calcium (Z = 20) is a main constituent of bone. Pilot studies have been undertaken by Reynard et al. Tere are separate classes of radioactive decay systems [106], using bones from humans and animals from three on which diferent isotopic dating methods are based; archaeological sites. Tere was a signifcant diference in cosmogenic (e.g. C-14, tritium and Cl-36), the the 44Ca/42Ca isotope ratios between humans and ani- uranium/thorium-series disequilibria systems (e.g. the mals, which was attributed to diferences in metabolic 230T-232T and 210Pb methods), long-lived systems (e.g. processes rather than due to dairy consumption among the U–Pb, K–Ar and Ar–Ar methods), and radiation the adult humans. It was also concluded that the bone damage methods (e.g. thermoluminescence and fssion Nord and Billström Herit Sci (2018) 6:25 Page 8 of 13

track methods). Te systematics behind these techniques yield a date [124–128]. Another correction is are very diferent and, with the exception of the car- necessary to compensate for the so-called Suess efect. bon-14 method, the reader is referred to text for Tis is a change in the ratio of the atmospheric concen- more detailed information. Te most frequently used trations of 13C and 14C by the admixture of fossil-fuel archaeological dating technique is the well-known C-14 derived , which is depleted in 13C and con- “radiocarbon” method [4, 6, 18, 124–127]. Successful tains no 14C. Te efect is named after the Austrian chem- applications include dating of e.g. human and animal ist Hans Suess, who noted the infuence of this efect on remains, mummies, wood artefacts, parchment, textiles, the accuracy of radiocarbon dating [129, 130]. cereals, charcoal, peat and slag. Te method is based on Numerous famous and successful C-14 age dating’s the fact that cosmic neutrons react with atmospheric have been published during the last half-century on 14N to form radioactive 14C, which decays by β-emission materials such as the Dead Sea scrolls, the Shroud of with a half-life ­(T1/2) of 5730 ± 40 years. (After a half-life, Turin, Egyptian mummies, and Ötzi the Iceman. It must 50% of the radioactive isotopes have decayed). Almost also be remembered that the C-14 technique is only all living matter is constantly exchanging 14C with their applicable for organic material. However, iron artefacts environment, mainly by food or atmospheric ­CO2. Tis may be dated because they contain small amounts of the exchange expires when the organism dies, and the 14C carbon (charcoal) once used for reduction of the iron decay can consequently be used as an “archaeologi- oxide ore. Bone material can often be dated from its col- cal chronometer”. Te age is determined by measuring lagen, which is remarkably stable, even in soil sufering the proportion of 14C in relation to 12C and comparing from atmospheric acidifcation [31]. A number of correc- this with corresponding theoretical value for “modern” tions are necessary for a reliable result (cf. below). carbon. Initially, the organic material to be dated was burned Dating with other isotope systems to CO­ 2 in oxygen, whereupon its radioactivity was deter- Dating with the C-14 techniques has its limitations when mined in a relatively tedious process where the 14C activ- older artefacts are to be dated. After ten half-lives (about ity was estimated from the measurements of β-particles 60,000 years) only one per mil of the original amount of emitted during decay of 14C to 14N. Te procedure also 14C atoms remains, which is challenging for a high-qual- involved a conventional gas mass spectrometer to obtain ity analysis. In this case, radioactive isotopes with longer the 13C/12C ratio needed for corrections (cf. below). half-life’s can be considered. For instance, 36Cl with a Accordingly, radiocarbon ages determined some decades half-life of 301,000 years may be utilized for very old ago may sufer from low accuracy. Today the number of materials in specifc cases [131]. Tritium (3H), decaying 14C atoms in relation to the stable carbon isotopes can be to 3He with a half-life of about 12 years, is produced from directly measured with an Accelerator Mass Spectrom- cosmic-ray neutrons interacting with 14N. Tritium is a eter (AMS) in samples with as little as 100 μg of carbon, minor constituent of water molecules on the surface of and yet producing precise ages of around ± 40 years for the earth due to fall-out from meteoric precipitation. Te samples being a few 1000 years old. However, there are tritium component in water can be used to determine the several sources of error to be considered. For instance, age of a water mass (e.g. groundwater). Tritium analyses, it is imperative to avoid contamination from modern based on the measurement of emitted β-particles, com- material, which would otherwise give a much too low bined with analyses of 3He (tritium—3He method) have (“young”) age. Likewise, an organism having absorbed been applied on problems in archeology [132]. However, carbon dioxide from would cause a much too because of atmospheric testing of nuclear weapons, the high age. If trees growing near a high-way would be radi- amount of tritium in the atmosphere rose and this may ocarbon dated, their “age” would erroneously amount to obstruct the interpretation of data. In this context also several million years. Furthermore, a number of correc- the 90Sr isotope may be mentioned which originates in a tions are necessary for a reliable result. Te 14C/12C ratio similar manner as tritium. Nuclear accidents like that in in the atmosphere has shown small fuctuations with Chernobyl (1986) have infuenced the concentration of time and locality, and certain calibration curves should 90Sr in the atmosphere. therefore be employed. Furthermore, the ­CO2 uptake Te efect of so called uranium/thorium-series disequi- in an organism is afected by isotopic fractionation. It is libria give rise to a number of dating methods. Briefy, assumed that in relation to C-12, all organisms discrimi- these disequilibria occur when biologic and inorganic nate against C-14 about twice as much as against C-13, reactions break the complex chains which defne the and this can be accounted for in a procedure involving decay of U and T parent isotopes. Tis, in turn, to the determination of the 13C/12C ratio. Terefore, many an interplay between various processes and for instance corrections are necessary to the raw radiocarbon age to the application of the 238U/234U—230T series methods Nord and Billström Herit Sci (2018) 6:25 Page 9 of 13

enable events up to 500,000 years to be dated. Examples below. An example worth considering is the dating of the include dating of lake sediments, corals, dropstones and Turin shroud, discussed by Meacham [141]. early human species, archeological sites [55; and refer- Many isotope studies are focused on archaeological ences therein]. Obviously, this may be applied in studies examinations of human remains in order to ascertain of anthropology and ancient mining. In , where information on diet or diseases. A body may have rested long time scales often are considered, rocks are dated by in the ground for centuries or thousands of years and using decay series like A (parent isotope) → B (daugh- may have been afected by the surrounding soil. In par- ter isotope), where element A has a very long half-life. ticular, diagenetic efects on bone and teeth have been Te common principle of these systems is that time can studied. Te problem is important because bone apa- be determined by measuring the ratio between newly tite yields dietary information about various life-stages. formed daughters and remaining parent isotopes. For Most authors agree that diagenesis is a problem for these instance, 238U decays to stable 206Pb with a half-life of ca. materials, and that the conditions at archaeological sites 4.5 billion years, but despite this slow decay it has been must be evaluated in each case [142–145]. Also a study shown that the technique is capable of dating carbonate of modern and ancient buried wood has been published, samples as young as 250,000 years [133, 134]. Te age of showing a linear correlation between carbohydrate con- fossil enamel has been successfully determined with U– tent and the stable carbon isotope composition, because Pb dating techniques [135]. Another useful decay series, the carbohydrates are preferentially degraded during with applications to archeological fnds, is K–Ar (and a early diagenesis [146]. Usually an inorganic compound variant known as the Ar–Ar method which is based on is more stable than organic and biological material, so the same decay of 40K to 40Ar). For instance, the fossil the problem of diagenesis is likely to be smaller for metal remains of “Lucy” from Awash Valley in Ethiopia could artefacts, ceramics etc. However, also weathering may be dated to 3.2 million years by use of K–Ar and Ar–Ar potentially alter the original isotopic composition of dating of a surrounding layer of volcanic ashes [136]. an object, and it may be essential to compare analytical Another well-known example utilizing the Ar–Ar isotope results from both inner and outer parts of the artefacts. technique is the dating of the Vesuvius eruption (AD 79). Furthermore, contamination from modern material that Despite the slow decay of 40K to 40Ar (half-life 1250 mil- accidently got mixed with an object of some kind, and lion years), which therefore entails large analytical chal- also conservation treatments, may jeopardize a judicial lenges to date young events, Renne et al. [137] were able interpretation of the results obtained. Occasionally, it is to obtain an Ar–Ar age indistinguishable from the histor- of outermost importance to avoid the destruction of an ical age of the eruption. object which may imply that a technique like SIMS (in Finally, fssion track dating and thermoluminescence situ technique afecting only a very tiny surfcial area) is are two dating methods which have been applied on glass recommendable. objects and on dating ceramics [138, 139]. Te former Te provenance of an artefact can be determined or method relies on counting tracks caused by spontaneous at least suggested from the of elements fssion of 238U atoms, whereas the latter is based on elec- like lead, strontium and neodymium. Many other ele- tromagnetic emission from non-conducting crystalline ments have also been used, like boron, copper, tin etc. solids. Dating of fossil bones and teeth has been tested by Te technical developments during the last decades means of fssion tracks [140]. allows extremely small sample volumes to be investigated which, however, make tough demands regarding the ana- Limitations lytical protocols used at isotope laboratories. In particu- Tis paper has exemplifed how certain isotope systems lar, it is crucial to maintain very low levels of laboratory can be used to reveal a human’s diet, migration pat- contamination and consider appropriate standardization terns or age. However, there are often limitations which procedures to master instrumental isotopic fractionation can make the results uncertain. A determined age may and use precautions not to alter the isotopic composition be if not erroneous so at least questionable. Te prob- during pre-analysis sample treatment. For those heavy lems with the C-14 dating technique has already been elements, for which isotopic diferences among samples discussed. Following the frst enthusiasm over this are notoriously small, optimized laboratory procedures remarkable method, it has been shown that a number are especially important. It may be difcult to convinc- of corrections are necessary to get a reliable result, and ingly prove, based on e.g. neodymium isotopes, that cer- that older dating’s might be questioned. It is of the great- tain samples have a diferent provenance than others, est importance that no trace of “modern” carbon has pol- due to the minor isotopic diferences that are developed. luted the sample used for the dating. Uncertainty how Yet, it may still be possible to group samples by claiming to validate other experimental isotope data is discussed that one set of samples has an isotopic signature that is Nord and Billström Herit Sci (2018) 6:25 Page 10 of 13

distinct from that of another set. Lead isotope prove- mass spectrometers induce a much better accuracy, the nancing of objects is governed by relative large isotopic future use of isotopes in heritage science is certainly diferences developed among potential source areas, but likely to grow with time. a complication is that there might be an isotopic overlap Authors’ contributions between geographical regions. Tere is a problem with AN and KB have together written the manuscript. Both authors read and isotopic overlap also for e.g. Cu and Zn and a further approved the fnal manuscript. hinder for a successful provenancing when using these Author details systems is that there are typically numerous potential 1 National Heritage Board of Sweden, Kevingeringen 10, 182 50 Danderyd, regions sharing a specifc type of metal deposit. At the Sweden. 2 Dept. of Geosciences, Swedish Museum of Natural History, P.O. Box 50007, 104 50 Stockholm, Sweden. best, some conceivable deposits can be dismissed. Authors’ information Possible future applications Anders G. Nord (born in 1942). At the University of Stockholm he acquired a sound knowledge of chemistry, physics, , and mineralogy. Isotope measurements are now routinely used in many He graduated in chemistry (1974) and was later associate professor. During felds such as geology, medicine, heritage science, archae- employment at the Swedish Museum of Natural History, a system for analysis ology, forensics, environmental studies and in the food of light stable elements was built in cooperation with Kjell Billström. Other felds were carbon-14 dating and lead isotope analyses. For 20 years he was and beverage industry. At present about ten elements, fnally employed at the National Heritage Board of Sweden as a conserva- or isotope systems, are currently used on a routine basis tion scientist, with focus on the degradation of archaeological artefacts and for investigations in the feld of heritage science (Table 1). museum objects, as well as analysis of medieval murals. Helping colleagues regarding isotope data for provenance and other interpretations was another Te technical development of spectrometers has now important task. Anders Nord is still active as a research scientist and has so far reached a where so called compound-specifc type published about 200 scientifc papers and contributed to eight books. of analysis is possible. Data from this approach is nor- Kjell Billström (born in 1954) studied geology at the University of Stockholm, where he graduated. Since 1978 he has been employed at the mally obtained by using a gas chromatograph (GC) which Department of Geosciences at the Swedish Museum of Natural History. He allows a controlled introduction of a certain molecular frst worked with light stable isotopes, but has later concentrated on ore species into a MC-ICP-MS system and then e.g. analyzes formation, development of the earth crust, and geological dating using heavy radioactive elements. He has also a wide experience in problems where lead the isotopic composition of carbon in organic species, or isotopes have been of certain importance, and of testing and running various chlorine contained in e.g. pesticides. Another interesting models of mass spectrometers. technique utilizing specifc isotopic properties is NMR, Acknowledgements Nuclear Magnetic Resonance, today commonly used The authors will express their sincere gratitude to the staf at the Swedish in medicine to study anomalies in human tissues. Te Museum of Natural History for many valuable discussions and useful improve- method is based on the weak magnetic feld of the nuclei ments of the manuscript. We are also grateful to Inga Ullén (Swedish Museum 1 13 of National Antiquities), Kerstin Lidén (Archaeological Research Laboratory, of certain isotopes such as H and C, which in a strong University of Stockholm), and Adrian Boyce (SUERC, Glasgow) for useful magnetic feld may cause resonance with low-frequency examples concerning isotope studies in archaeology. Finally we want to thank radio waves. Te technique has also been used to investi- the Torsten Söderberg Foundation, which has provided economic support for many of our earlier investigations where isotope data have been of crucial gate old human remains [147]. importance. In the future, probably many more elements’ isotope systems may be used. Hypothetically, for instance ele- Competing interests The authors declare that they have no competing interests. ments such as , , , manga- nese, nickel and the metals group are of interest Availability of data and materials in the study of cultural objects, preferably as components Available upon request. in multi-isotope studies. Moreover, isotope studies may Consent for publication successfully be combined with analytical data for trace Not applicable. elements. Although not based on isotopes, the hydra- Ethics approval and consent to participate tion method is also worth mentioning in this context. Not applicable. Tis method allows dating of obsidian, a dense volcanic rock often used for producing stone tools [148]. Dating Funding There is no funding for this summarizing paper. of microfossils in fint artefacts with the aid of palyno- logical techniques is also possible at many instances. Publisher’s Note Understanding isotope systematics has also proven useful Springer Nature remains neutral with regard to jurisdictional claims in pub- when investigating the efect of human activities on the lished maps and institutional afliations. environment. Considering the constantly growing feld Received: 10 January 2018 Accepted: 16 April 2018 of applications and the fact that each new generation of Nord and Billström Herit Sci (2018) 6:25 Page 11 of 13

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