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4. Results and discussion
4.1 Copper to Neolithic southern Scandinavia
Thirteen flat axes and an arm spiral found in Denmark, typologically dating to the first part of the Earlier Neolithic (early Funnel-Beaker culture 3800–3500 BC) [52] or to LN I (2350–2100 BC), were included in the investigation. The decision to include pre-Bronze Age metalwork concurred with the aim of identifying those exchange networks that were the first to introduce copper artefacts, and to a certain extent also metallurgical practice, to southern Scandinavia. Copper in Nordic Neolithic contexts has long fascinated Scandinavian archaeology [13,14,52], and some of the results presented below are re-analyses of earlier published data [14], namely #3, #35–36, #42 and #303.
The nine copper flat axes of Bygholm type belonging to the early Funnel-Beaker period are all made of medium-arsenical copper with an average arsenic content of 0.9% and Ni = Ag = Sb around 0.02%. This copper has a characteristically low impurity signature, apart from the arsenic. The flat axe and arm spiral from the Søby Hede hoard (#42–43) both belong in this early group. Evaluation of lead isotope ratios of five reanalysed flat axes and four new analyses supports the general opinion that this so far unprovenanced Riesebusch-Mondsee copper [14,69,70] cannot be of East Alpine origin (Fig 3). Rather, the geographical distribution of such relatively pure arsenical copper suggests that it derived from south-east Europe, supported by the dagger being a new type of metal weapon in the flourishing Chalcolithic environment of the fourth millennium BC [69,71]. The dagger blade from the Danish Bygholm hoard exemplifies this. Indeed, Frank and Pernicka [69] observed that this copper type’s purity corresponds with that of south-east European Chalcolithic heavy implements (e.g. shaft-hole axes), which mainly relate to the copper deposit of Majdanpek in Serbia. The measured 206Pb/204Pb lead isotope ratio of 18.56 and the 207Pb/204Pb ratio of 15.621 from the ovoid, tongue-shaped flat axe from Viborg #36 are comparable with the known values of the Ai Bunar mine at Stara Zagora in Bulgaria [see 26]. The slightly lower values of the thick-butted flat axe from Slusegård #35 are within the range of both the Chalcolithic copper mines near Stara Zagora (Ai Bunar) and Burgas (Medni Rid), as well as the contemporary mines in the East Serbian Copper Belt [see 72]. The axe from Moesgaard #3 is probably an import via Austria’s Mondsee group, a branch of south-east European Chalcolithic groups [52]. Six other flat axes likely derive from Serbian ores, based on the comparative values from the Serbian region [72].
Fig 3.
206Pb/204Pb and 207Pb/204Pb isotope plot of the copper flat axes discussed, with comparative plot (A) indicating the difference of measurements between this study and published ones [14], obviously problematic in their 207Pb/204Pb ratios. The ore data are from: Italy AATV mining region [32,73–75], Serbian copper deposits [72,76], Bulgarian copper deposits [26,33].
doi.org/10.1371/journal.pone.0252376.g003
To sum up, the new results presented above differ from those established by previous research [14,52]. The main reason is the much-improved precision and accuracy of the measurements with MC-ICP-MS, which produce results comparing well with analyses performed with thermal ionisation mass spectrometry (TIMS) on the Bulgarian ores [26]. We therefore suggest that copper flat axes and copper trinkets made of south-east European ore travelled from or via the Mondsee culture in Austria to southern Scandinavia early in the Funnel-Beaker period. To explain the relatively higher arsenic contents, it is very likely, as Frank and Pernicka suggest [69], that a new technology had been introduced–namely the intentional addition of arsenic in the form of speiss (iron arsenides, [77]) to the already available copper. The Søby Hede hoard (#42–43) strengthens this supposition: the six times higher arsenic content of the axe (1.17%) in relation to the arm spiral (0.20%) could well be intentional. Although such a process has only been identified in Iran during the final fourth millennium BC [77,78], enrichment with arsenic may well have been invented earlier, thus giving rise to the phenomenon of arsenical copper as the dominant metal type of the rich Chalcolithic of the fourth millennium BC in south-east Europe. This hypothesis is now corroborated by the suite of Funnel-Beaker flat axes isotopically matching Bulgarian and Serbian copper ores. In conclusion, the copper from which these early items were made–including the hitherto unprovenanced Riesebusch-Mondsee copper–was most likely mined in Serbia and Bulgaria in south-eastern Europe.
In the late group of axes–tentatively dated to the LN I Bell-Beaker culture c. 2350–2100 BC [52,56]–the new metal analyses indicate an origin completely different from the axes of Riesebusch-Mondsee copper discussed above, and possibly also a wider chronological range than strictly “Bell-Beaker.” The axe from Kjelstrup in south Jutland #304 consists of high-impurity copper with Sb = Ag>As, a dilute antimonial copper especially abundant in Bosnia in the late fourth and third millennia BC [55], a region where antimony-bearing minerals are also abundant [79]. Antimonial copper with exceptionally high concentrations of antimony of more than 10% are occasionally found in the Late Bronze Age of western Hungary near Velem. St. Vid [80] and also in adjacent Lower Austria, albeit with lower antimony concentrations up to 3.5% and with lead isotope ratios consistent with the Slovakian Ore Mountains [81]. Isotopically this axe is consistent with fahlores from the Valais in Switzerland, but antimonial copper seems rare there. The other axe #302 has arsenic and silver as major impurities, a 206Pb/204Pb ratio of 19.113, and a 207Pb/204Pb ratio of 15.686, which is found in the south-east Alpine regions of the Alto Adige, Trentino and Veneto (henceforth AATV). The flat axes #44 and #45 have extreme values of 23.845/22.547 206Pb/204Pb and 1.6195/1.712 208Pb/206Pb, indicating an ore deposit with highly radiogenic lead. Similar values have been recorded in Rudna Glava and Crnajka near Majdanpek in Serbia [72], but also in eastern Bulgaria, where Chalcolithic copper smelting has been attested [82].
To sum up. The group of later flat axes are typologically much more varied and often smaller than the thick-butted trapezoidal flat axes of the Bygholm type [4]. The dataset presented here corroborates the observation that the varied group of non-Bygholm axes shows much diversity in copper composition. Earlier research uses the term “Dutch Bell-Beaker copper” (e.g. [4], which might well originate from several different sources, including south-east Alpine ores), but in fact two of the axes analysed (#44, #45) are neither an analytical match for the Riesebusch-Mondsee copper nor seem to fall within the known variation among LN I Bell-Beaker-affiliated artefacts. An explanation could be that they may have arrived in Scandinavia much earlier, sometime during the later Earlier Neolithic or the Younger Neolithic (3500–2400 BC). If so, copper flat axes in some measure reached southern Scandinavia in the long metal-poor period between the two early peaks in copper appropriation. Future typological and context studies may shed more light on this material.
4.1 Copper to Neolithic southern Scandinavia
Thirteen flat axes and an arm spiral found in Denmark, typologically dating to the first part of the Earlier Neolithic (early Funnel-Beaker culture 3800–3500 BC) [52] or to LN I (2350–2100 BC), were included in the investigation. The decision to include pre-Bronze Age metalwork concurred with the aim of identifying those exchange networks that were the first to introduce copper artefacts, and to a certain extent also metallurgical practice, to southern Scandinavia. Copper in Nordic Neolithic contexts has long fascinated Scandinavian archaeology [13,14,52], and some of the results presented below are re-analyses of earlier published data [14], namely #3, #35–36, #42 and #303.
The nine copper flat axes of Bygholm type belonging to the early Funnel-Beaker period are all made of medium-arsenical copper with an average arsenic content of 0.9% and Ni = Ag = Sb around 0.02%. This copper has a characteristically low impurity signature, apart from the arsenic. The flat axe and arm spiral from the Søby Hede hoard (#42–43) both belong in this early group. Evaluation of lead isotope ratios of five reanalysed flat axes and four new analyses supports the general opinion that this so far unprovenanced Riesebusch-Mondsee copper [14,69,70] cannot be of East Alpine origin (Fig 3). Rather, the geographical distribution of such relatively pure arsenical copper suggests that it derived from south-east Europe, supported by the dagger being a new type of metal weapon in the flourishing Chalcolithic environment of the fourth millennium BC [69,71]. The dagger blade from the Danish Bygholm hoard exemplifies this. Indeed, Frank and Pernicka [69] observed that this copper type’s purity corresponds with that of south-east European Chalcolithic heavy implements (e.g. shaft-hole axes), which mainly relate to the copper deposit of Majdanpek in Serbia. The measured 206Pb/204Pb lead isotope ratio of 18.56 and the 207Pb/204Pb ratio of 15.621 from the ovoid, tongue-shaped flat axe from Viborg #36 are comparable with the known values of the Ai Bunar mine at Stara Zagora in Bulgaria [see 26]. The slightly lower values of the thick-butted flat axe from Slusegård #35 are within the range of both the Chalcolithic copper mines near Stara Zagora (Ai Bunar) and Burgas (Medni Rid), as well as the contemporary mines in the East Serbian Copper Belt [see 72]. The axe from Moesgaard #3 is probably an import via Austria’s Mondsee group, a branch of south-east European Chalcolithic groups [52]. Six other flat axes likely derive from Serbian ores, based on the comparative values from the Serbian region [72].
Fig 3.
206Pb/204Pb and 207Pb/204Pb isotope plot of the copper flat axes discussed, with comparative plot (A) indicating the difference of measurements between this study and published ones [14], obviously problematic in their 207Pb/204Pb ratios. The ore data are from: Italy AATV mining region [32,73–75], Serbian copper deposits [72,76], Bulgarian copper deposits [26,33].
doi.org/10.1371/journal.pone.0252376.g003
To sum up, the new results presented above differ from those established by previous research [14,52]. The main reason is the much-improved precision and accuracy of the measurements with MC-ICP-MS, which produce results comparing well with analyses performed with thermal ionisation mass spectrometry (TIMS) on the Bulgarian ores [26]. We therefore suggest that copper flat axes and copper trinkets made of south-east European ore travelled from or via the Mondsee culture in Austria to southern Scandinavia early in the Funnel-Beaker period. To explain the relatively higher arsenic contents, it is very likely, as Frank and Pernicka suggest [69], that a new technology had been introduced–namely the intentional addition of arsenic in the form of speiss (iron arsenides, [77]) to the already available copper. The Søby Hede hoard (#42–43) strengthens this supposition: the six times higher arsenic content of the axe (1.17%) in relation to the arm spiral (0.20%) could well be intentional. Although such a process has only been identified in Iran during the final fourth millennium BC [77,78], enrichment with arsenic may well have been invented earlier, thus giving rise to the phenomenon of arsenical copper as the dominant metal type of the rich Chalcolithic of the fourth millennium BC in south-east Europe. This hypothesis is now corroborated by the suite of Funnel-Beaker flat axes isotopically matching Bulgarian and Serbian copper ores. In conclusion, the copper from which these early items were made–including the hitherto unprovenanced Riesebusch-Mondsee copper–was most likely mined in Serbia and Bulgaria in south-eastern Europe.
In the late group of axes–tentatively dated to the LN I Bell-Beaker culture c. 2350–2100 BC [52,56]–the new metal analyses indicate an origin completely different from the axes of Riesebusch-Mondsee copper discussed above, and possibly also a wider chronological range than strictly “Bell-Beaker.” The axe from Kjelstrup in south Jutland #304 consists of high-impurity copper with Sb = Ag>As, a dilute antimonial copper especially abundant in Bosnia in the late fourth and third millennia BC [55], a region where antimony-bearing minerals are also abundant [79]. Antimonial copper with exceptionally high concentrations of antimony of more than 10% are occasionally found in the Late Bronze Age of western Hungary near Velem. St. Vid [80] and also in adjacent Lower Austria, albeit with lower antimony concentrations up to 3.5% and with lead isotope ratios consistent with the Slovakian Ore Mountains [81]. Isotopically this axe is consistent with fahlores from the Valais in Switzerland, but antimonial copper seems rare there. The other axe #302 has arsenic and silver as major impurities, a 206Pb/204Pb ratio of 19.113, and a 207Pb/204Pb ratio of 15.686, which is found in the south-east Alpine regions of the Alto Adige, Trentino and Veneto (henceforth AATV). The flat axes #44 and #45 have extreme values of 23.845/22.547 206Pb/204Pb and 1.6195/1.712 208Pb/206Pb, indicating an ore deposit with highly radiogenic lead. Similar values have been recorded in Rudna Glava and Crnajka near Majdanpek in Serbia [72], but also in eastern Bulgaria, where Chalcolithic copper smelting has been attested [82].
To sum up. The group of later flat axes are typologically much more varied and often smaller than the thick-butted trapezoidal flat axes of the Bygholm type [4]. The dataset presented here corroborates the observation that the varied group of non-Bygholm axes shows much diversity in copper composition. Earlier research uses the term “Dutch Bell-Beaker copper” (e.g. [4], which might well originate from several different sources, including south-east Alpine ores), but in fact two of the axes analysed (#44, #45) are neither an analytical match for the Riesebusch-Mondsee copper nor seem to fall within the known variation among LN I Bell-Beaker-affiliated artefacts. An explanation could be that they may have arrived in Scandinavia much earlier, sometime during the later Earlier Neolithic or the Younger Neolithic (3500–2400 BC). If so, copper flat axes in some measure reached southern Scandinavia in the long metal-poor period between the two early peaks in copper appropriation. Future typological and context studies may shed more light on this material.