Abstract The rich and long-lasting Nordic Bronze Age was dependent throughout on incoming flows of copper and tin. The crucial turning point for the development of the NBA can be pinpointed as the second phase of the Late Neolithic (LN II, c. 2000–1700 BC) precisely because the availability and use of metal increased markedly at this time. But the precise provenance of copper reaching Scandinavia in the early second millennium is still unclear and our knowledge about the driving force leading to the establishment of the Bronze Age in southern Scandinavia is fragmentary and incomplete. This study, drawing on a large data set of 210 samples representing almost 50% of all existing metal objects known from this period in Denmark, uses trace element (EDXRF) and isotope analyses (MC-ICP-MS) of copper-based artifacts in combination with substantial typological knowledge to profoundly illuminate the contact directions, networks and routes of the earliest metal supplies. It also presents the first investigation of local recycling or mixing of metals originating from different ore regions. Both continuity and change emerge clearly in the metal-trading networks of the Late Neolithic to the first Bronze Age period. Artifacts in LN II consist mainly of high-impurity copper (so-called fahlore type copper), with the clear exception of British imports. Targeted reuse of foreign artifacts in local production is demonstrated by the presence of British metal in local-style axes. The much smaller range of lead isotope ratios among locally crafted compared to imported artifacts is also likely due to mixing. In the latter half of Nordic LN II (1800–1700 BC), the first signs emerge of a new and distinct type of copper with low impurity levels, which gains enormously in importance later in NBA IA.

Citation: Nørgaard HW, Pernicka E, Vandkilde H (2019) On the trail of Scandinavia’s early metallurgy: Provenance, transfer and mixing. PLoS ONE 14(7): e0219574. https://doi.org/10.1371/journal.pone.0219574 Editor: Peter F. Biehl, University at Buffalo - The State University of New York, UNITED STATES Received: March 22, 2019; Accepted: June 26, 2019; Published: July 24, 2019 Copyright: © 2019 Nørgaard et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the manuscript and its Supporting Information files. The complete repository information is given in S1 Table. The analytical data is given in S2 Table. Funding: The research leading to these results received funding from the Independent Research Fund Denmark under grant agreement DFF – 6107–00030 and the Research Fund Sapere Aude program (DFF –IP 6107-00030B). We acknowledge financial support by Deutsche Forschungsgemeinschaft within the funding programme Open Access Publishing, by the Baden-Württemberg Ministry of Science, Research and the Arts and by Ruprecht-Karls-Universität Heidelberg. Competing interests: The authors have declared that no competing interests exist.

Introduction This article throws new light on the early phases of metallurgy in southern Scandinavia. It does so by tracking the incoming flows of copper to the region, which ultimately led to the breakthrough of the Nordic Bronze Age c. 1600 BC–a golden epoch boasting such highly sophisticated bronzework as the Trundholm sun chariot. Scandinavia for a long time was marginal to the metallurgical evolution that had begun in the ninth millennium BC in the ancient Near East with the use of beads, pendants and sheet ornaments made from annealed native copper [1, 2]. In central and Mediterranean Europe, the first indications of copper-based technologies date to the later fifth millennium [3–5], initiated through the import of metal ornaments and tools mainly from the Balkan region [6]. According to currently available evidence, however, a regional metallurgy in central Europe does not appear until the beginning of the fourth millennium, with depositions of copper objects such as the Stollhoff hoard [4, 7], the early phases of the Mondsee cultural group [8], and even traces of copper smelting at Brixlegg in Austria around 4000 BC [9]. As early as c. 4400 BC, there are signs of a faint awareness of copper technologies in Scandinavia in the form of rare imports of copper axes into the region’s Late Mesolithic communities [4, 10]. A thousand years later, local metallurgy was likely practiced in the Middle Neolithic Funnelbeaker culture [10–12], only to disappear again subsequently. During most of the third millennium, metallurgy seems absent from the region, even if experiments with casting copper axes and hammering sheet ornaments reappear in Bell Beaker environments in Jutland, 2400–2100 BC. Typology-based studies suggest that the incipient production and import of copper objects in the Funnelbeaker culture around 3500 BC was due to Scandinavia’s involvement in prestige good trading that extended as far as the copper-producing hubs of central and southeastern Europe [8, 13, 14]. A similar explanation may fit the reappearance of copper axes and ornaments c. 2400 BC in the Jutlandish Bell Beaker culture during the first phase of the Late Neolithic (LN I) [12, 15, 16]. At 2000 BC, however, a copper-based technology begins to achieve full economic and social integration in Scandinavia simultaneously with the spread of bronze, or copper with similar properties, across Europe and large tracts of Afro-Eurasia [15, 17–19]. The co-occurrence of these two phenomena is highly significant. Prior to this threshold, metal objects and knowledge of their production had appeared and disappeared several times over the millennia, indicated by, for instance, metallurgical experiments documented at early Funnelbeaker settlement sites [20]. The lack of metal resources can be identified as one major reason for the relatively late enduring involvement with metallurgy in Scandinavia. Although copper ores were discovered and exploited in central and northern Scandinavia from the Middle Ages onwards, these indigenous sources were most likely unknown to Bronze Age people and were not exploited [21]. Nordic Bronze Age (NBA) societies were, therefore, completely dependent on exogenous sources from the beginning [15, 21–25]. The second phase of the Late Neolithic (LN II, c. 2000–1700 BC) can be pinpointed as the crucial turning point, for the very reason that the availability and use of metal increased markedly and grew substantially in the following centuries. With the Scanian Pile hoard as an early highlight showcasing local metalworking activities as early as c. 2000 BC, conclusive evidence now exists for indigenous production of metal objects in southern Scandinavia, more precisely in eastern Jutland, Funen, Zealand and Scania. The high degree of fragmentation in the Pile hoard shows that objects, including rings, were hacked into pieces that would fit into a small crucible and were recast as axes. The next four hundred years laid the foundation for the final breakthrough and subsequent rise of the NBA c. 1600–500 BC with advances in the repertoire and refinement of artifacts, while the consumption of bronze spread northward to central Scandinavia [15, 19] (Fig 1). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. Increase in metal consumption c. 2400–1500 BC. The histogram is based on standard values of metal weight calculated for each main artefact type (i.e. flanged axe, chisel, shafthole axe, and so on [15, 19, 26–44] per 100 year hence compensating for the differing length of the periods. LN II (2000–1700 BC) emerges as the first period of growth. NBA IA (1700–1600 BC) has slightly more metal in circulation. NBA IB (1600–1500 BC) is the breakthrough period, with plentiful metal. The two interpolation maps illustrate the development from LN II (B) to NBA IA (C) on a geographical scale, with orange areas denoting the highest densities. The total number of objects, on which Fig 1 is based, is 1879 from Denmark, Sweden and Norway. Reprinted from [19] based on map images provide by Natural Earth (public domain) under a CC BY 4.0 license, with permission from ArkIT and Helle Vandkilde, original copyright [2017]. https://doi.org/10.1371/journal.pone.0219574.g001 The metal supply to southern Scandinavia held the key to creative cultural achievement in the far northwest of the Bronze Age world. Vandkilde’s recent publication [19] has provided a good overview of scalar connectivity 2000–1700 BCE (Fig 2). Interpretations of recent results in trace element and isotope science have identified a diverse and geographically shifting network of long-distance trading routes over the entire duration of the Bronze Age up to c. 500 BC [21, 25, 45, 46] although many of the claimed relationships between ores and artefacts were contested [47, 48] and were shown to be valid only in much later phases of the Bronze Age [45]. This interpretation was necessarily painted with a broad brush as it covered 1,500 years, and the crucial early part of this period was underrepresented in the data used in these publications (only seven data sets from LN II–NBA IA apart from the project presented here [46]), leaving the precise provenance of copper reaching Scandinavia in the early second millennium still as an open question. By comparison, the present study draws on a much larger number of both trace element and isotope analyses of copper-based artifacts, amounting to 210 samples and representing 50% of all existing metal objects known from this period in Denmark. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 2. Model of multi-scalar connectivity in northern Europe around 2000 BC from local to super-regional levels and consisting of four overlapping spheres of interaction. The Baltic coast of Mecklenburg-Vorpommern was the pick-up zone for metals to Scandinavia, but the three nodal hub regions–southern Britain with Wessex, eastern Denmark/Scania and the Circum-Harz region with Halle–were directly linked. Remarkably, the main resources on which these hubs relied–namely copper, tin and amber–were located outside their geographical precinct. Map images are provided by Natural Earth (public domain) under a CC BY 4.0 license. The map, first published by H. V. [19], is with permission reworked by L. H. and H. V. using the software Adobe Illustrator. Contains data from [15, 49–59]. https://doi.org/10.1371/journal.pone.0219574.g002 Because the objects investigated in this project date to 2000–1600 BC comprising Late Neolithic (LN II) and earliest Bronze Age (NBA IA), it has extraordinary potential to answer questions regarding the final breakthrough of the NBA c. 1600 BC–not only about the specific sources of the imported copper, but also the ways this metal was used locally. The result therefore offers an improved understanding of long-term developments and also of the degree to which shifts in metal supplies coincided with culture-historical watersheds. The article thus substantially improves our understanding of the hitherto vaguely understood metal trading networks and metallurgical developments that led to the NBA. It also investigates, for the first time, the question of local recycling, or mixing, of metals originating from different ore regions [47].

Methods Sampling and analyses were performed with a view to limit as much as possible destructive interventions on Bronze Age artifacts. Archived samples collected within the Stuttgarter Metallanalysen project (SAM) in the 1970s were therefore used [17, 60, 61]. Sample preparation included tracing and sorting the samples taken from C. Cullberg for the SAM project [62] between 1959 and 1962 of Danish artifacts and aligning specimens and artifacts (S1 Table). For minor and trace element analysis corrosion material mixed with the drill shavings was carefully removed under a microscope, if necessary. The analysis of minor and trace elements was accomplished with an energy-dispersive X-ray fluorescence spectrometer (EDXRF). Concentrations of the elements Mn, Fe, Co, Ni, Zn, As, Se, Ag, Cd, Sn, Sb, Te, Au, Pb, and Bi were measured at the CEZA in Mannheim, Germany, with an ARL Quant X (Thermo Scientific) instrument. The samples were placed on a 20-position sample changer, which is especially useful for drilled samples. The samples were measured in two exposures of 600 seconds each following a modified version of the procedure by Lutz and Pernicka [63]. Eighteen samples could thus be measured in one run within less than eight hours. Two reference materials obtained from the Bundesanstalt für Materialprüfung in Berlin (BAM211 and BAM376) were included in each run. As indicated in the database (S2 Table) the detection limits are 0.05% for Fe, around 0.01% for Co, Ni, and As, and around 0.005 for Ag, Sb, Sn, Au, Pb, and Bi. Mn, Cd, Se, and Te were also measured but were below 0.005% in all samples. Zn was below the detection limit of 0.1% in all samples. The calculations for the amount of metal in circulation in the Late Neolithic and NBA IA are based on studies of a large number of inventories performed in Denmark, northern Germany [15, 26–40, 64], and Sweden [15, 41]. In all, 210 analyses were accomplished within the present project, of which 155 are from artifacts dating to LN II. The remaining data illustrating general tendencies are taken from the SAM and SMAP projects [17, 61, 65] and small-scale local studies [19, 66] with different devices. The precision of the SAM data is with ± 30% relatively low but they are quite accurate [67]. However, some trace elements need to be considered with caution: high As values are significantly lower in the SAM dataset. Sb and Ag are only comparable at concentrations above 0.35%, and Ni has a good comparability at concentrations between 0.12 and 1.2%. Outside this range larger deviations occur. The determination of the lead isotope ratios was performed with a multiple-collector inductively-coupled plasma mass spectrometer (MC–ICP–MS) at the Curt–Engelhorn Center for Archaeometry (CEZA) in Mannheim, Germany. The instrument used was a Thermo Scientific Neptune Plus mass spectrometer. For the measurements solutions with a lead concentration of 100 ng/ml were prepared after dissolution of the solid samples and chemical separation of the lead. For the separation of lead the samples were rinsed with dilute HNO 3 to remove surface contamination and then dissolved in half-concentrated HNO 3 in an ultrasonic bath (70°C) for several hours. Insoluble residues were removed by decantation from the resulting solution, which was then diluted with deionised water [68]. Columns were prepared with PRE filter resin and Sr resin, and preconditioned with 500μl 3N HNO 3 before the solution was added. The matrix was eluted in four steps, using HNO 3 , and then the Pb was eluted using HCl. After drying (48h), thallium was added to the sample solutions to monitor and correct the internal mass fractionation [69]. Standard solutions were measured after every fourth sample and intensive cleaning of the tubing was conducted after every sample. The isotope ratios measured were 208Pb/206Pb, 207Pb/206Pb, and 206Pb/204Pb, with relative uncertainties of less than 0.01 for the first two ratios and 0.03% for the last. The ratios 208Pb/204Pb and 207Pb/204Pb were calculated from the other ratios.

Material Right from the beginning, the number of locally produced objects exceeded those of imported objects in southern Scandinavia. Axes–functioning as weapons as well as tools–constitute by far the most common category of metal artifact produced in Scandinavia during the four hundred years of the earliest Bronze Age (Fig 3), and these therefore provide the critical archaeological data for archaeometallurgical analysis. The acquired samples are partly legacies of the Stuttgarter Metallanalysenprojekt [17, 60–62, 65], while in a number of cases new samples were obtained from objects in the National Museum of Denmark, Copenhagen, in order to obtain a balance between different object types: in particular, between imported axes and the predominant local group of axes. The majority of samples (141 counts) derive from LN II (2000–1700 BC) and a further 50 samples from NBA IA (1700–1600 BC). Although fifty samples may appear as a small fraction, they nevertheless represents 28% of the known artifacts; 62% of known axes from the relatively brief NBA IA period were also available for analysis, in addition to several spearheads. All samples stem from distinct morphology-defined types, which are easy to divide into locally made and imported [15]: this division is a key component of the archaeometallurgical study described below. Some local axe types clearly express their localness through style, while others retain foreign traits due to having direct foreign antecedents or inspirations (e.g. pseudo-British axes are local versions of high-tin British axe imports, and the large Virring type axes are distinctly local versions of central European Langquaid type axes; see S1 Fig). Such artifact groups often have a confined geographical distribution, as do technical and ornamental traditions [70]. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 3. Metalwork in LN II (Fig 3A) and NBA IA (Fig 3B) quantitatively arranged according to the main types, local production or foreign import. Although local production is by far the most common, it includes a category of hybrids. This indicates that imports were routinely remelted and some were recast in an ‘in-between’ style. This hints at copper mixing. Regional inventory studies were used to calculate the total number of objects in circulation [15, 26–41, 64]. https://doi.org/10.1371/journal.pone.0219574.g003 In consequence, artifacts of a specific characteristic style or technological feature can be provenanced to particular regions and cultural groups. Larger-scale geographical patterns thus help to distinguish local products from foreign imports. For instance, axes of British type and Langquaid type found in Denmark are classified as imported because of their distinct morphology and their main distribution area in the British Isles and central Europe respectively [51, 71]. In contrast, local axes such as the Gallemose and Torsted-Virring types are characteristically Nordic in their style and geographical distribution. The scientific provenancing of copper through geochemistry and isotopy is thus profoundly enriched by the substantial typological knowledge that is a cornerstone of this study.

Conclusions: Scandinavia’s early metallurgy in a wider Bronze Age world Trading the local resource of amber enabled Scandinavia to enter the international marketplace in metals and metallurgy around 2000 BC by expanding in two directions (Fig 2). One route led down across the Baltic Sea toward the rich Únĕtice hubs in the Middle Elbe–Saale region. This connection stands out clearly in the archaeological data. In all likelihood it was maintained by frequent maritime and riverine travel, with the Mecklenburgian coastlands used as an easily accessible pick-up zone. The other route, also maritime but perhaps less routinely frequented, led to the British Isles, from where exotic axes were brought back to Scandinavia–indeed this Nordic collection boasts the largest number of British type axes outside the British Isles. These two major routes recur in the LN II data presented above, from minute typological details to micro-traces of the metals. Overall, analytical results bolster and nuance our current knowledge of Early Bronze Age Europe as tightly interconnected on scalar levels from micro to macro. New knowledge has emerged in the following areas: Firstly, the transport of British axes to Scandinavia from England, possibly without the involvement of Ireland, figures much more clearly than previously. This indeed makes sense because of the relatively short distances involved when opting for routes along safe coastal waters (Fig 2). This result is visible in the combinations of data regarding elevated tin levels, low-impurity copper, and isotopic signatures in western-style axes. It also tallies with amber concentrations found mainly in the Wessex region [53, 54, 56, 112]. It is moreover possible that British bronze, and the western connection more broadly, is underestimated in the data. Secondly, the geochemical characteristics of Scandinavian metal during LN II are strikingly similar to those found in the Únĕtice region. Minor but clear deviations between the two regions in elemental compositions can be accounted for by local mixing. The prevalent fahlore copper in all probability derived from the Inn Valley of the eastern Alps (Ni-free Ösenring copper) and from the Slovakian Ore Mountains (Ni copper groups). The lead role of Slovakia is unexpected and new and corroborates earlier studies in central Europe [113]. These two mining areas were vital players in the immense Únĕtician metal inventory. There is little in the archaeological data to suggest direct Scandinavian contact with the mining regions; rather, coveted metals reached Scandinavia through the agency of Únĕtice mediators, and in amounts that almost certainly exceeded shipments from Britain. Thirdly, the investigations have thrown new light on the NBA IA period, the age that transitions to the final breakthrough of the Nordic Bronze Age c. 1600 BC. Though the seventeenth century BC is still shadowy, the NBA IA data patterns reveal both continuity and change compared to LN II. The same two-pronged trading connections, Britain and Únĕtice, continue to feed Scandinavian metallurgy. Local Nordic metalwork production on the one hand emerges as a cohesive autonomous tradition, yet on the other as clearly gravitating toward international standards of weaponry styles and techniques. Bronze is now fully implemented, as elsewhere in temperate Europe. Importantly, Ni-fahlore copper groups are still in use, but along with a new minor-impurity copper of cluster 4 (chalcopyrite) that has now rapidly become the prevailing copper type following its feeble introduction in LN II. Slovakia is still the lead provenance area, while the Inn Valley Ni-free fahlore has vanished. The Únĕtice region can probably still be attributed the role of mediator of Slovakian copper to Scandinavia up to c. 1600 BC, when the Únĕtician collapse paves the way for the emergence of the truly international Middle Bronze Age bringing Scandinavia into contact with the Mediterranean palace-based polities. The continued use of fahlore in NBA IA is reminiscent of the Earliest Bronze Age, while the prevalence of copper produced from ore that consist mainly of chalcopyrite forecasts new times. In the Middle Bronze Age, beginning around 1600 BC, chalcopyrite copper becomes utterly dominant, in many cases directly associated with industrial mining at Mitterberg in the eastern Alps. Fourthly, the initial hypothesis of a local artifact production based on the smelting of raw copper and tin has been proven wrong. Compositional similarities between imports and local products in Scandinavia that imply interdependency hint strongly at the repeated remelting of imported artifacts (hacked into suitable pieces) as the direct source for local Nordic products, themselves also repeatedly recast. Since even the imports have a high variance in compositional patterns, the inference is plausible that these were also subject to mixing of the few primary copper types available as appropriate. This also suggests that ingots in the traditional meaning did not yet exist. Consistent with this is the evidence that the Bronze Age smith differentiated between axe metal (Ni fahlore) and ring metal (Ni-free fahlore) in dealing with two copper types with different properties. The British axes should be seen in the same light as quasi-ingots capable of providing tin. It is still unclear whether this system of mixing artifact metal, rather than properly alloying copper with tin, continued from LN II into NBA IA. To this can be added that early Scandinavian metal technology expresses individual characteristics at the same time as it relies on European role models.

Acknowledgments We are grateful to the Independent Research Fund Denmark and the Research Fund Sapere Aude program for supporting this research. We acknowledge the achieved support by Deutsche Forschungsgemeinschaft within the funding programme Open Access Publishing, by the Baden-Württemberg Ministry of Science, Research and the Arts and by Ruprecht-Karls-Universität Heidelberg. We are grateful to the National Museum Copenhagen for the permission for additional sampling and to Bernd Höppner, Daniel Berger, Gerhard Brügmann, Sigrid Klaus and Joachim Lutz at the Curt–Engelhorn Zentrum for Archaeometry who provided analytical assistance. Louise Hilmar, graphic department Moesgaard Museum, and Casper Skaaning Andersen, Archaeological IT department, Aarhus University, improved the graphics. We also wish to thank the two anonymous reviewer for their constructive suggestions of improvements in the manuscript.