Geological constraints on the use of lead isotopes for provenance in archaeometallurgy

The first part of this paper is a brief critical history of the use of Pb isotopes for inferring the geological provenance of archaeological materials, with an emphasis on non-ferrous metals. The second part examines variation in the Pb isotopic ratios of oxide and sulphide ore minerals in selected regions of the world, and relates these to the geological histories of ore formation in each region. This exercise shows that in regions where most ore deposits are of similar geological age—as in the Andes, Europe and the circum-Mediterranean—provenance analysis with Pb isotopes is inherently difficult because geographically distant sources often exhibit similar isotopic ratios. Conversely, regions with many periods of ore formation—such as southern Africa—appear to be very promising regions for future studies of provenance with Pb isotopes. The wider implication of this exploratory survey is that archaeologists should carefully consider the range and clustering of geological Pb isotopic ratios in their regions of interest before investing large sums of money into Pb isotopic analysis of artefacts.     

Lead isotope analysis of intra‐skeletal variation in a 19th century mental asylum cemetery: diagenesis versus migration

Living bone sequesters environmental lead (Pb) from both inhalation and ingestion, providing a record of Pb exposure over a lifetime. Questions about the effects of diagenesis and how to remove them have hampered most isotopic and elemental determinations. As a result, researchers often restrict their analyses to tooth enamel, despite its limitations. We report Pb isotopes in teeth and bones in a frontier population of 15 individuals from a late 19th century mental hospital graveyard in Pueblo, Colorado, a town active at that time in the smelting of ores. Analysis of lead isotopes sequestered in healing bone from rib fractures gives an isotopic fingerprint from the last few months of individual's lives. When bone tissues or teeth from different stages of life are analysed, life history trajectories such as migration routes can be developed which are partially self‐correcting for diagenesis. Copyright © 2005 John Wiley & Sons, Ltd.     

A GEOLOGICAL PERSPECTIVE ON THE USE OF Pb ISOTOPES IN ARCHAEOMETRY

Lead isotope ratios in ore bodies and magmatic rocks depend in a complex way on several a priori independent parameters, including the geological age of the tectonic province in which the ores and magmas formed and its U/Pb (μ) and Th/U (κ) ratios, two very sensitive parameters characteristic of metal sources. All these parameters are entangled in hard‐to‐read Pb isotopic ratios. With respect to the commonly used fingerprinting techniques, which rely on the comparison of raw isotope ratios, the main motivation for the present work is to provide a method for making geologically and geochemically educated guesses about metal provenance even in the absence of isotopic data on reference ores. It shows how to unscramble a geological model age and μ and κ information from isotopic measurements. This approach brings to light a new organization of the Pb isotope database and an untapped wealth of information that can be used for provenance studies and other archaeometric purposes. We provide expressions with which to calculate these parameters and, using literature data, demonstrate how Pb isotopes in ores and magmas define clear zones in the silver‐rich provinces of the Central Andes. We further show how the geological model age and μ and κ values fingerprint production areas in 16th–18th century silver coins minted in Mexico and South America. Finally, we use Pb isotopes to illustrate how the Reconquista of the Emirate of Granada (1482–91) and the seizure of the Betic silver mines are reflected in the silver coins of the Catholic Monarchs.     

High precision isotopic analyses of lead ores from New Mexico by MC-ICP-MS: implications for tracing the production and exchange of Pueblo IV glaze-decorated pottery

Between ca. 1275 and 1700 CE, Pueblo groups in the northern Southwest United States produced and exchanged ceramic bowls decorated with lead-based glaze paints. Previous studies of these glaze-decorated bowls have used lead isotopic analysis by ICP-MS to identify the sources of lead used by Pueblo potters, and investigate how social or economic factors may have influenced resource use among different Pueblo communities (e.g. 13 and 14; Huntley et al., 2007; Huntley, 2008). However, interpretations of much of this isotopic data have remained provisional because of overlap among the isotopic ratios of potential sources and because the isotopic composition of many glaze paints do not clearly match any known source. Here, we use multi-collector ICP-MS to re-measure the lead isotopic composition of 48 samples of lead sulfide (galena) and lead carbonate (cerussite) from sources in New Mexico that were potentially utilized by Pueblo potters, including mines within the Cerrillos Hills, Magdalena, Hansonburg, and Joyita Hills mining districts. These results define the isotopic composition of lead ores from these districts with greater precision and accuracy than achieved in previous studies and better distinguish among these mining districts in lead isotope space. Most significantly, we find that galena mineralization within the Cerrillos Hills only has a modest degree of isotopic variation, with ²⁰⁶Pb/²⁰⁴Pb ratios from 18.508 to 18.753, ²⁰⁷Pb/²⁰⁴Pb ratios from 15.580 to 15.607, and ²⁰⁸Pb/²⁰⁴Pb ratios from 38.388 to 38.560. These ranges are far narrower than previously reported, and should supersede previously published values for this district. In total, we conclude that isotopic measurements of both ores and glaze paints made by MC-ICP-MS will provide new information about the provenance of lead in glaze paints and allow for more detailed interpretations about resource procurement and exchange in the Pueblo world.     

TRACING THE RESOURCES OF IRON WORKING AT ANCIENT SAGALASSOS (SOUTH‐WEST TURKEY): A COMBINED LEAD AND STRONTIUM ISOTOPE STUDY ON IRON ARTEFACTS AND ORES*

Lead and strontium isotope analyses were performed by thermal ionization mass spectrometry (TIMS) on Roman to Byzantine iron artefacts and iron ores from the territory of ancient Sagalassos (south‐west Turkey), to evaluate Pb and Sr isotopes for provenance determination of ores for local iron production. It can be demonstrated that for early Roman artefacts and hematite iron ore processed in early Roman times from Sagalassos proper, as well as for magnetite placer sands and early Byzantine raw iron from the territory of the city, Sr isotopes are much less ambiguous than Pb isotopes in providing clearly coherent signatures for ore and related iron objects. Late Roman iron objects were produced from iron ores that as yet remain unidentified. Early Byzantine iron artefacts display more scatter in both their Pb and Sr isotope signatures, indicating that many different ore sources may have been used. Our study demonstrates that iron objects can be precisely analysed for their Sr isotopic composition, which, compared to Pb isotopes, appears to be a much more powerful tool for distinguishing between chronological groups and determining the provenance of raw materials.     

Combined Pb–Sr isotopic analysis in provenancing late Roman iron raw materials in the territory of Sagalassos (SW Turkey)

In early Roman times, iron was likely supplied to the city of Sagalassos through the smelting of close-by hematite ores. In the early Byzantine period, magnetite–titanite placer sands in some instances could have been exploited for its iron. For the intermediate late Roman period, however, the source of the locally used iron was unknown. Pb and Sr isotopic analyses of iron ores from the area of Camoluk, just south of the territory of Sagalassos, and of late Roman iron artefacts from the antique city itself, reveal a very close resemblance. This makes the use of the Camoluk ores to supply Sagalassos with raw iron in the late Roman period likely. It is also shown that combined Pb and Sr isotopic analyses provides a powerful tool to distinguish chronological groups of iron provenance and a technique that can determine the nature and source of iron raw materials used.     

COMPOSITIONAL VARIATION IN ROMAN COLOURLESS GLASS OBJECTS FROM THE BOCHOLTZ BURIAL (THE NETHERLANDS)*

We investigated the major and trace element composition and Pb and Sr isotope characteristics of a series of about 20 colourless glass objects from a single high‐status Roman burial from the Netherlands (Bocholtz). The major elements show a relatively homogeneous group, with one outlier. This is corroborated by the Sr isotopes. Based on the Sb and Pb content, three major groups can be discerned, with two other outliers. This grouping is corroborated by the contents of the trace elements Bi, Sn, Ag, As and Mo, and by variations in lead isotopic ratios. On the basis of these results, we conclude that the glass of all objects was probably made with sand and lime from the same source. The variation in trace elements and lead isotope composition is most likely the result of variations in the composition of the sulphidic antimony ore(s) that were used to decolourize the glass. The composition of the Bocholtz glass is compared with that of other Roman glass, and implications for production models, trade and use of colourless glass objects are discussed. On the basis of isotopic and major element variation, we conclude that the antimony ore presumably originated from different mines.     

Micro-invasive method for studying lead isotopes in paintings*

A micro-invasive technique is presented that enables pigment sampling from individual layers of a painting cross-section by obtaining a furrow 10–50 μm wide of chosen length. Combined with increased sensitivity of lead (Pb) isotope analysis using multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) equipped with a 10¹³ Ω resistor or a Pb double-spike technique, the amount of Pb needed for isotopic analysis is drastically reduced, while maintaining a relative 2 SD precision for ²⁰⁶Pb/²⁰⁴Pb of < 0.02%. The methodology proved able to characterize Pb isotope differences within paint layers.     

Significance and Limitations of Stable Oxygen Isotope Ratios in the Apatite Phosphate of Archaeological Vertebrate Finds for Provenance Analysis in an Alpine Reference Region

A multi‐isotope fingerprint consisting of δ¹⁸O_phosphate, ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁷Pb was established in the bioapatite of 219 individual archaeofaunal remains (cattle, pig, red deer) excavated from sites located along a specific transect of the European Alps, namely the Inn–Eisack–Adige–Brenner Passage, that has been of eminent importance since European prehistory. This reference area is vertically stratified, and since δ¹⁸O in the skeleton is influenced by climate, water source, physiology and even culture, we tested the relative contribution and importance of δ¹⁸O as a component of the multi‐isotope fingerprint for provenance analysis in this alpine region by a novel mathematical approach. In particular, we adapted a supervised learning approach through expectation–maximization (EM) clustering for fingerprint extraction and evaluated the contribution of each isotopic ratio to the data structure. While an altitude effect was evident in δ¹⁸O, its overall structural importance in the complete isotopic fingerprint was rather low. Therefore, provenance analysis of bioarchaeological finds in this region is possible by measuring stable Sr and Pb ratios alone, which is of considerable importance when δ¹⁸O values are not available, e.g., in cremated finds, although some information is lost. Whether this is tolerable depends on the scientific question to be solved.     

“Biblical” bronze coins: new insights into their timing and attribution using copper and lead isotopes

Ancient Judaean bronze coins are important for reconstructing the timing of events, for their cultural information, and for understanding alliances in the Eastern Mediterranean as Roman influence replaced the Hellenic Empires. Isotopic analyses of metals important in archaeology have become increasingly routine using multiple collector–inductively coupled plasma–mass spectrometry. A relatively nondestructive acid swab method for the isotopic analysis of lead (Pb) and copper (Cu) in bronze is described and tested for various isotopic sampling biases. The method is used to examine changes over time in fiduciary metal sources in the Levant during the first centuries bce and ce. The metal sources, timing, and attribution of Judaean “widow's mites” (ca. 80–40?bce) and the bronze coins of Roman governors Valerius Gratus and Pontius Pilate under Tiberius (14–37?ce) are used to illustrate how changes in the isotopes of Pb (widow's mites) and Cu (Roman governors) can be used to elucidate the compositional chronology of archaeological bronzes from this region and period.     

Multi-isotope fingerprints (O,Sr and Pb) in archaeological animal bone bioapatite: Similarity search and the suitability for provenance analysis in a geologically complex Alpine region

Multi-isotope fingerprints in the bioapatite of archaeological skeletons are mostly superior over single isotope analyses for provenance studies. Gaussian mixture model (GMM) clustering is a novel tool for a similarity search among multidimensional data sets and at the same time permits the evaluation of the structural importance of particular isotopic ratios in the data set. We applied three GMM clustering experiments on multi-isotope fingerprints—stable strontium (Sr), lead (Pb) and oxygen (O) isotopic ratios—established in 217 archaeological animal bones excavated along a specific transect across the European Alps. This reference region had been in use since prehistoric times by humans who crossed the Alps from north to south, and vice versa. The resulting clusters permit a spatial assignment of the specimens with a very high probability, in particular with regard to the geological complexity of the region. A combination of Sr with Pb stable isotopes led to an optimal differentiation between the southern and northern Alpine forelands that cannot be distinguished from each other by ⁸⁷Sr/⁸⁶Sr ratios alone, while the contribution of δ¹⁸O is not particularly high. The isotopic mapping and subsequent cluster analysis is suitable for the analysis of archaeological human finds and the reconstruction of the direction of transalpine mobility and trade.     

A LEAD ISOTOPE DATABASE: THE LOS PEDROCHES – ALCUDIA AREA (SPAIN); IMPLICATIONS FOR ARCHAEOMETALLURGICAL CONNECTIONS ACROSS SOUTHWESTERN AND SOUTHEASTERN IBERIA*

One hundred and twenty‐five new lead isotopic analyses on galena, mainly from the Los Pedroches – Alcudia Valley area of southern Iberia, are presented. These data allow us to define four compositional groups in the ²⁰⁷Pb/²⁰⁶Pb versus ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb versus ²⁰⁶Pb/²⁰⁴Pb diagrams. Group I compositions are intermediate between those previously reported for southeastern and southwestern Iberian ore mineral locations. The compositions of Group II, the largest group, overlap with ore data from southwestern Spain. Groups III and IV have compositions that have never before been identified in this region and are comparable to those from the Sardinian Cu ores, which might explain the previously assumed exotic origin for local Bronze Age artefacts.     

PROVENANCING FLINT ARTEFACTS WITH ICP–MS USING REE SIGNATURES AND Pb ISOTOPES AS DISCRIMINANTS: PRELIMINARY RESULTS OF A CASE STUDY FROM NORTHERN SWEDEN

Archaeological flint artefacts from the late Mesolithic/early Neolithic site of Vuollerim, northern Sweden, have been geochemically investigated with ICP–SFMS and MC–ICP–MS in search for the geological/geographical origin of the non‐local flint. The Vuollerim flints were compared with reference samples from Denmark (Cretaceous/Tertiary flint) and Russia (Carboniferous flint). Elemental concentrations as well as elemental ratios for REEs and isotopic ratios for Pb and Sr are presented. Significant differences were found between different geological/geographical contexts. Two of the Vuollerim samples can be ascribed a South Scandinavian origin. Possibly also eastern flint is present, although the results are not conclusive in this case.     

THERMI ON LESBOS: A CASE STUDY OF CHANGING TRADE PATTERNS

Summary Provenance studies on archaeological metal objects rely on their trace element abundance pattern and lead isotopic composition. A comparison of these features in artifacts from different archaeological sites often requires a comparison of results reported by different laboratories. In this case reasonable conclusions can only be expected if it is assured that the respective laboratories produce compatible results. A comparison of some twenty copper-based artifacts from Thermi I and II shows this compatibility still to be less than perfect between Oxford and Heidelberg — Mainz. We find the lead isotopic compositions reported by the two laboratories to agree within the stated uncertainties of the abundance ratios of 0.1 percent. The same is also true, to within 50 percent or better, for the contents of As, Sb, Ag, Au and possibly Ni. Serious discrepancies exist, however, for Co, Pb and Zn that we argue to be due to analytical problems at Oxford. Discriminant analysis of the data from all metal-bearing occupational levels at Thermi reveals a remarkably good correspondence between trace element signature and occupational level which suggests that at the beginning of the third millennium BC intercourse between Lesbians and Aegean metal traders was rare. Lead isotope data show that the change in metal, at least that upon the transition from Towns I + II to Towns III–IV, was indeed due to a change in source region of the metal and not one caused by a change in technology.     

Numerical modelling of 3D fluid flow and oxygen isotope exchange in fractured media: spatial distribution of isotope patterns

An understanding of fluid flow, mass transport and isotopic exchange in fractured rock is required to understand the origin of several geological processes including hydrothermal mineral deposits. The numerical model HydroGeoSphere simulates 3D advection, molecular diffusion, mechanical dispersion and isotopic exchange in a discretely fractured porous media, and can be used to better understand the processes of mass transport and isotopic exchange in fractured rocks. Study of ¹⁸O isopleth patterns for different types of fractures and fracture networks with a range of structural complexity and hydraulic properties shows that fracture properties and geometry control mass transport and isotopic exchange. The hydraulic properties, as well as the density, spacing, and connectivity of fractures determine the isotopic patterns. Asymmetries in the geometry of oxygen isotope patterns could be used to determine the direction of hydrothermal fluid flow.     

How Mineralogy and Geochemistry Can Improve the Significance of Pb Isotopes in Metal Provenance Studies

Lead isotopes combined with trace element data represent a powerful tool for non‐ferrous metal provenance studies. Nevertheless, unconsidered geological factors and archaeological data, as well as ignored analytical procedures, may substantially modify the interpretation of the isotopic and trace element signature obtained as a potential ore candidate. Three archaeological examples, accompanied by high‐resolution lead isotopic measurements (MC–ICP–MS), are presented here to discuss the above‐mentioned criticisms and to propose some solutions. The first example deals with prehistoric/historical gold/silver‐mining activity from Romania (the Baia Bor?a and Ro?ia Montan? ore deposits). The second one regards the lead/silver metallurgical activity from the Mont‐Lozère massif (France) during medieval times. The third example focuses on the comparison between two batches of lead isotope data gathered on Roman lead ingots from Saintes‐Maries‐de‐la‐Mer, using different SRM 981 Pb values.     

Isotopic Comparison of Hair,Nail and Bone: Modern Analyses

This paper presents a comprison of the isotopic values of eight pairings of hair keratin and bone collagen and 12 pairings of hair keratin and nail keratin taken from living humans resident in the U.K., with the aim of examining whether modern human isotopic data can be directly compared to archaeological isotopic data.Results showed that bone collagen was enriched relative to hair keratin from the same individual by +1·4‰ in δ¹³C and +0·86‰ in δ¹⁵N, with some small degree of variability. Isotopic comparison of hair keratin and nail keratin from the same individual showed that there is no significant difference between hair and nail keratin in δ¹³C, but that nail keratin is on average +0·65‰ more enriched in δ¹⁵N than hair.Differences in amino acid composition between hair keratin and bone collagen may account for the carbon isotopic differences between the two proteins, and there is no significant overall carbon isotopic difference between hair and nail. However there are significant isotopic differences for nitrogen in the two pairings, that differences in amino acid composition and turnover times cannot explain. We suggest that these results indicate that constancy of isotopic values between tissues, even for similar proteins, cannot be taken for granted.     

Considerations on the provenance determination of plant ash glasses using strontium isotopes

Strontium isotopic analysis has been proposed as a suitable method to determine the primary production location of ancient plant ash glasses. The technique is based upon the assumption that Sr enters this glass type with the plant ash used as a flux material, and that the ⁸⁷Sr/⁸⁶Sr ratio of the resulting glass reflects the geological provenance of that flux. In such case, the bulk Sr isotopic composition of the bedrock should be inherited unchanged in the plants growing on that bedrock. Different types of plant ash glasses have been shown to have widely differing ⁸⁷Sr/⁸⁶Sr compositions. In this study, the ⁸⁷Sr/⁸⁶Sr composition of several plant species growing on different bedrock types is measured, and compared to the bulk Sr isotopic composition and petrology of that bedrock. The paper shows that the ⁸⁷Sr/⁸⁶Sr ratio of these plants is a function not only, or even mostly, of the local geology, but also of the Sr isotopic composition of the total water consumed by that plant. This is highly likely to be both plant species dependent and dependent on the small-scale hydrology of the area immediately surrounding the plant. In this way, no definite relation between the isotopic composition of a geological outcrop and the plants growing on this bedrock can be inferred. Hence, the isotopic composition of a plant ash made from such plants is uncertain and moreover species dependent. Though groups of plant ash glasses can certainly be compared in time and space using Sr isotopes, it may prove difficult to ascertain a plant ash glass type to a specific geographical-geological region.     

Lead isotopic analyses of copper ores in the Early Bronze Age central Hexi Corridor,north-west China

This paper explores the possible provenance of ores employed for metallurgical production during the Early Bronze Age in the central Hexi Corridor of north-west China. In total, 78 pieces of copper (Cu) ore samples were collected from five Early Bronze Age sites and one Cu deposit site (the Beishantang Cu deposit) in the Heihe River region of the central corridor. These sites were dated to the late Machang (4100–4000 bp ), Xichengyi (4000–3700 bp ), Qijia (4000–3600 bp ) and Siba (3700–3400 bp ) cultures. After comparing with published lead (Pb) isotopic data from other possible Cu deposits in north-west China, the results show that the Cu ores collected from the Early Bronze Age sites were most likely derived from the adjacent Beishan Cu deposit. More intriguingly, for the first time in Hexi Corridor, a dozen Cu ores were discovered containing highly radiogenic Pb. Though fundamentally different from those in the Central Plains, they illustrate a possible new type of Cu used in Bronze Age western China, and the first-hand materials are significant for further understanding the provenance of raw metals for metallurgical production in the prehistoric Hexi Corridor.     

Reconstructing faunal migrations using intra-tooth sampling and strontium and oxygen isotope analyses: a case study of modern caribou (Rangifer tarandus granti)

Isotopic analyses of incrementally developed dental tissues can be used to reveal patterns of movement and diet in animals. However, the suitability of these methods for the reconstruction of herd movements has not yet been demonstrated. Inter-individual behavioural and isotopic variability at the herd scale, and the implications for archaeological and palaeocological applications, can only be demonstrated through the testing of modern animals. In this pilot study, dual-element isotopic profiles were created from incrementally developed dental tissues of five individuals selected from a modern herd of migratory Alaskan caribou (Rangifer tarandus granti). Enamel from second and third molars from the individuals was sequentially sampled in order to reconstruct time-series isotopic profiles. Variation in the strontium (⁸⁷Sr/⁸⁶Sr) and oxygen (δ¹⁸O_CARB) isotope ratios of sequentially sampled enamel were compared to documented herd movement patterns and local geological and environmental conditions. Four individuals displayed the same general trends, although absolute isotopic values varied. One individual displays a very different trend and may represent a behavioural outlier or an immigrant from a semi-domesticated reindeer herd. The implications of this study to herd movement reconstruction in the past are discussed.