Introduction

Lead (Pb) is a poisonous element that causes diseases of the nervous, digestive and reproductive systems. Humans are exposed to Pb through natural and anthropogenic routes1. Natural exposure to Pb, exemplified by populations that predate ore extraction is, typically, through accidental hand to mouth ingestion of soil, particularly during childhood2. Anthropogenic exposure predominantly arises as a result of interaction with Pb released into the environment through the mining and use of Pb-bearing sulphide deposits. This latter exposure route is diverse and historically includes such things as (i) the use of Pb water pipes, (ii) the use of Pb as a sweetener in food and drink, (iii) Pb added to paint and makeup, and, more recently, (iv) Pb additives in petrol3,4. Exposure to anthropogenic Pb typically results in elevated blood (and thus tooth) Pb levels5. It is this elevated Pb concentration that is currently used to differentiate between individuals exposed to anthropogenic versus natural Pb sources. The upper limit for natural exposure in skeletal studies given as 0.55 or 0.7 mg/kg4,6 in enamel (equating to 0.5/0.7 μg/dL in blood)7. However, truly understanding the uptake pathways is important for prevention of exposure and, in archaeological studies, understanding human cultural development. This paper demonstrates that the different geological processes (see appendix) that control the Pb isotope composition of the silicate (natural) and sulphide (anthropogenic) Pb sources provide a more precise way of distinguishing between the two pathways.

The basic principles, and method of data display for the U-Pb isotope systems, can be found in a number of texts8,9,10. The parent isotopes 235U, 238U and 232Th decay over geological time to produce the daughter products 207Pb, 206Pb and 208Pb, respectively, with one stable isotope of Pb, 204Pb, used as the invariant reference isotope. The traditional method of data display, utilized in many archaeological studies, is to plot the 207Pb/206Pb and 208Pb/206Pb ratios and describe compositional fields within this bivariate space. However, it has been noted that this method of representation tends to compress the data and make it a relatively poor discriminant because conformable Pb ore deposits have a very restricted range on this type of plot the inclusion of the 204Pb ratios helps multiple source mixing to be identified11. While the data from this paper can be seen plotted in this conventional manner in the appendix, we have preferred to employ a recently suggested graphical method of displaying the time-integrated 238U/204Pb (μ) as a function of the Pb model age (T). A full description of this method and the equations needed to undertake the calculations are given in Albarede et al. (2012)12. This method requires the derivation of the two axis variables from the measured Pb isotope compositions and is therefore more complex that the conventional 207Pb/206Pb and 208Pb/206Pb representation. However, it has the distinct advantage in that it provides information about the geological origins of the sample without recourse to reference datasets. Ore forming events are generally related to major geological mountain building processes of which three dominate European geology13; the Alpine event of c. 60–2.5 Ma which is most evident in circum-Mediterranean geology; the Hercynian c. 280–380 Ma, which mostly affects northern continental Europe and southern Britain and the Caledonian event of c. 390–490 Ma seen in the Palaeozoic and older rocks of Britain and Scandinavia. The calculation of Pb model age gives an estimate of the age and hence geological episode to which mineralisation is associated. Mu (μ) provides evidence of the geochemical nature of the source rock of the mineralization. For example, deposits such as those in Tunisia14, source their Pb from uranium rich granite domains and hence have elevated 238U/204Pb (μ) values, whereas Pb derived from more basic/ultrabasic deposits, such as are found in Cyprus, reflect the low uranium nature of the host with low 238U/204Pb (μ) values14. The combination of the model age (T) and 238U/204Pb (μ) thus provide geological, and hence geographic, constraints on the origin of the Pb without recourse to large reference datasets.

In this study, the transfer of labile soil Pb into fauna is primarily demonstrated using Neolithic (pre-anthropogenic Pb) pigs teeth. Pigs ingest soil while grubbing for food and hence provide a simple transfer model. The animals are from the Neolithic feasting site of Durrington Walls in southern England. These data are supplemented by two human ‘natural exposure’ populations: (i) a dataset of Neolithic individuals from British archaeological sites, and (ii) 10th century individuals, all typified by very low Pb concentration levels (0.11 ± 0.18 mg/kg, 2 SD, n = 34)15,16. These are then compared with data from three Early (5–7th century) Anglo Saxon and Anglian sites in England, where elevated Pb concentrations are suggestive of anthropogenic Pb exposure. The sites, and the average Pb concentrations in the tooth enamel, are as follows: Berinsfield17 in central England, where individuals have average tooth enamel Pb concentrations of 2.5 ± 4 mg/kg (2 SD, n = 11); Eastbourne in southern England, where individuals have average tooth enamel Pb concentrations of 6 mg/kg ± 22 mg/kg (2 SD, n = 21) and West Heslerton, north eastern England, which straddles the natural/anthropogenic Pb exposure boundary (0.7 ± 2.8 mg/kg; 2 SD, n = 33)18.

Method Section

Tooth enamel samples were prepared as follows: The enamel surface of the tooth was abraded from the surface to a depth of >100 microns using a diamond coated dental bur and the removed material discarded. An enamel sample was cut from the tooth using a flexible diamond edged rotary dental saw. All surfaces were mechanically cleaned with a diamond bur to remove adhering dentine. The resulting sample was transferred to a clean (class 100, laminar flow) working area for further preparation. In a clean laboratory, the sample was cleaned ultrasonically in high purity water to remove dust. It was then rinsed twice in de-ionized water, and soaked for an hour at 60 °C, before rinsing again and then leaching for 5 minutes with Teflon distilled 0.2 M HCl,. After a final rinse, the sample was dried and transferred into a pre-cleaned Teflon beaker where it was dissoslved in Teflon distilled 8MHNO3, evaporated to dryness and converted to bromide form using Romil© UpA HBr. Soils were leached with deionised water for 24 hours, centrifuged and the supernatant decanted into clean Savillex Beakers, evaporated to dryness and converted to bromide form as before. Separation of Pb from samples was undertaken using standard ion exchange techniques. The data in this paper has been acquired over a number of years and includes lead isotope compositions that were determined by either thermal ionisation mass spectrometry (TIMS) using a Finnigan Mat 262, or multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a Nu Plasma HR or a Thermo Fisher Scientific Neptune Plus. TIMS Pb was run using rhenium filaments in a silica gel-phosphoric acid. Lead blanks were c 70 pg. Lead isotope ratios were normalised to values of NBS 98119, which gave the following reproducibility during the period of analysis: 206Pb/204Pb = 0.20%, 207Pb/204Pb = 0.29%, 208Pb/204Pb = 0.40% (2σ, n = 31). Samples analysed by MC-ICP-MS were spiked with a thallium (Tl) solution and introduced into the instrument via an ESI 50 μl/min PFA micro-concentric nebuliser attached to a de-solvating unit (Nu Instruments DSN 100 or Cetac Aridus II) and normalised to NBS98120. Average 2 SD reproducibility for the following ratios is 206Pb/204Pb = 0.008%; 207Pb/204Pb = 0.008%; 208Pb/204Pb = 0.009%. The pig enamel samples, which were analytically challenging due to low Pb yields, were run on the Neptune using a high sensitivity jet cone and reproducibility was 206Pb/204Pb = 0.027% %; 207Pb/204Pb = 0.031%; 208Pb/204Pb = 0.041%. Lead concentrations, where documented, were measured by either isotope dilution8 or solution plasma21. Details of the samples and sites and extended methodology are supplied in the supplementary information section. Data is presented in Table 1.

Table 1 The primary Pb isotope ratios of all samples discussed in this study are presented in the table.
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Bio-available Pb from modern British soils defines a broadly horizontal field of data with 238U/204Pb = 9.74 ± 0.18 (2 SD, n = 29) Fig. 1. The bio-available Pb from ancient soils, as represented by archaeological bone and dentine composition, give a comparable result of 238U/204Pb = 9.70 ± 0.08 (2 SD, n = 34). Both these results are in agreement with the average crust composition of 238U/204Pb = 9.722 and synonymous with recycled sedimentary rocks. Some samples give negative model ages which is common in samples from limestone terrains and caused by a disproportionate uptake of U compared to Pb in marine carbonates23.

Figure 1
figure 1

A comparison of the isotope composition of labile Pb in modern and ancient soils. The data from the modern soils was produced by leaching modern soil samples with deionised water. This modern data () is compared with the Pb isotope composition of bone and dentine from archaeological sites. The assumption made is that the bone and dentine re-equilibrated with the labile soil component close to the time of burial and thus provide a measure of labile Pb that predates modern pollutants ().

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The transfer of the bio-available soil Pb into fauna is shown in Fig. 2. Data from the Neolithic pigs tooth enamel range between model ages (T) of 209 and 471 Ma with 238U/204Pb = 9.78 ± 0.05 (2 SD, n = 23). Human tooth enamel data yields a similar 238U/204Pb = 9.73 ± 0.06 (2 SD, n = 56). The coincidence of the soil and faunal data fields provides firm evidence that natural Pb exposure is consistent with the ingestion of the bioavailable component of Pb in silicate based soil.

Figure 2
figure 2

Natural Pb exposure. A 238U/204Pb (μ) vs T(model age) for Late Neolithic pigs () and humans () and post Neolithic low Pb exposure individuals ().

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Figure 3 shows the Pb isotope composition of tooth enamel from 5–7th century individuals whose elevated Pb concentrations is taken as evidence of anthropogenic Pb exposure. The figure includes data from galena (PbS) in British deposits of the Mendips, Pennines and central Wales for comparison. The most obvious aspect of the diagram is the steeply sloping data fields created by both the tooth data and the galena compositions. The central Wales data best highlights the highly correlated nature of the ore composition, created during the process of mineralization. Similar arrays can be seen in many galena datasets14. The tooth enamel samples from West Heslerton and some of the Eastbourne samples plot close to those of the English galena compositions suggesting the Pb exposure of these individuals was dominated by British ore. However, six of the Eastbourne samples, and most of the Berinsfield data, extend beyond the range of the British deposits suggesting that some individuals carry a component of non-British Pb.

Figure 3
figure 3

238U/204Pb (μ) vs T(model age) for Anthropogenic Pb exposure. Anglo Saxon and Anglian human tooth enamel from Berinsfield (), Eastbourne () and West Heslerton () data fields compared with English () and Welsh () Pb isotope data from Galena. The extent of 238U/204Pb (μ) attributable to natural exposure is given as the 2SD range derived from the data used in Fig. 1.

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The Pb is locked into tooth enamel during mineralization, which for the M2 teeth of this study, occurs between two and eight years age24. There are a number of options for the Pb source and ingestion route. The main route for modern children’s Pb exposure is though hand to mouth soil ingestion25. However non-local Pb isotope signatures can arise from a number of routes: (1) The individual was exposed to Pb somewhere other than where they were found ie they are not of local origin (2) They were exposed to Pb from a non-local source1, and (3) They inherited a non-local Pb composition from their mother via placental26 or lacational27 transfer that was available or re-mobilized during tooth mineralization.

Strontium and oxygen isotope analysis has also been undertaken on these samples but does not support a non-British childhood for the majority of individuals from the Berinsfield and Eastbourne sites17,28 and so we rule out an immigrant population.

Thus the most likely exposure routes would appear to be either from imported goods or, that these are first generation arrivals whose mothers carried and transferred a Pb isotope signature from her homeland29; or it may be a combination of both. Grave goods from Berinsfield highlight continental connections17.

Some constraints can be placed on the geological origin of the Pb these people were exposed to: the Berinsfield array indicates an end-member of a geologically young, low-U Pb terrain, whereas the Eastbourne upper end-member indicates a U-rich terrain that is c. 600 Ma old. Lead and Pb-bearing silver deposits with isotope compositions similar to those seen in the Eastbourne and Berinsfield populations can be found in Europe14 and hence this signature could have been introduced to England either by early Anglo-Saxon groups arriving in England or through trade and exchange of coins, ornaments or weaponry with continental populations.

This study shows that naturally derived, bio-available Pb from ingested soil is characterized by a horizontal data array in 238U/204Pb-T space, which is mimicked by fauna exposed to this type of Pb. In contrast, sulphide ore deposits define steeply dipping data arrays, a trend that is also reflected in the tooth enamel of people who have been exposed to anthropogenic Pb. This difference in the orientations of the fields can thus be used to distinguish between natural and anthropogenic exposure.

It is proposed that this approach to characterizing the origin of human Pb exposure provides an alternative method to examining Pb sources, regardless of exposure levels, and allows new insights into the rise of mining during the Anthropocene30, development of metal working and trade in the ancient world and its impact on human health.