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. 2022 Oct 26;17(10):e0274831. doi: 10.1371/journal.pone.0274831

Applying lead (Pb) isotopes to explore mobility in humans and animals

Jane A Evans 1,‡,*, Vanessa Pashley 1,‡, Katy Mee 2,‡, Doris Wagner 1,‡, Mike Parker Pearson 3,#, Delphine Fremondeau 4,#, Umberto Albarella 5,#, Richard Madgwick 6,‡
Editor: Peter F Biehl7
PMCID: PMC9605311  PMID: 36288369

Abstract

Lead (Pb) isotopes provide a complementary method to other provenance tools for tracking the origin and movement of humans and animals. The method is founded in the geographic distribution of Pb isotope ratios. However, unlike the Sr isotope method that is closely linked to the lithology of underlying rocks, Pb more closely reflects the tectonic regimes. This makes it particularly pertinent to use in Britain as there is major tectonic boundary (the Iapetus Suture) that runs between Berwick-upon-Tweed and the Solway Firth providing a compositional boundary in Pb isotope domains that approximates to the geographic areas of Scotland versus England and Wales. Modern pollution makes it difficult to use modern floral or faunal samples to characterize biosphere variation, and so we use geological datasets to define isoscape variation and present the first Pb isotope map of Britain. We have validated the use of these data form biosphere studies using well provenanced samples. Reference fields of diagnostic compositions, are created in μ-T space and these have been used in a test case to assess the geographic origins of Neolithic animals in Great Britain.

Introduction

Aims

Pb isotope analysis of tooth enamel is becoming an increasingly important method of analysis for constraining geographic origins in humans and animals [1–6] as it provides another dimension in the assessment of origins of populations in Britain by combining multiple isotope proxies. The primary aims of this paper are to 1) demonstrate the potential of lead (Pb) isotope analysis to provide a geographic discriminant between the two major tectonic zones of Great Britain, 2) to show that geological Pb signatures are transferred to the fauna exploiting these in the past and 3) to use the method to refine the geographic constraint of the origins of pigs found at several Neolithic henges in southern England.

Background

Reasons for using Pb isotope analysis

Sr isotopes have made a significant contribution to the understanding of human and animal movement and migration [7]. However, one problem with the Sr isotope system is that it cannot distinguish between the long-term build-up of 87Sr through time in old rocks, as opposed to the rapid build-up of 87Sr in younger, but rubidium-rich, rocks. Both cases result in the transmission of high 87Sr/86Sr ratios into the biosphere. Therefore it is not possible to discriminate between old terrains, such as the Baltic Shield in Scandinavia, and younger Rb-rich granite terrains such as those found in central Scotland, Iberia and central France, using Sr alone [8].

Pb is present in magmatic rocks, predominantly in the silicate mineral feldspar. It is found in moderate concentrations within the Earth’s crust and is recycled into metamorphic and sedimentary rocks via tectonic and erosional processes. Average Pb concentrations in igneous, sedimentary and metamorphic rocks range between 6 and 23 ppm [9]. Pb is mobile in the crustal environment and, for example, can be leached out of parent rocks and concentrated into Pb-rich sulphide ore deposits during metamorphic and hydrothermal geological events [10]. Such processes can separate, to an almost complete degree, daughter-product Pb isotopes from their radioactive parent U and Th isotopes and hence lock in the isotope signatures of the Pb from when it was mobilized and redeposited [10]. Ore deposits hence provide age-defined isotope composition for the source of Pb scavenged from the source rocks to produce the ore deposit. Such ore deposits are utilized from the Bronze Age and the recycling of the extracted Pb by human activity leads to anthropogenic Pb pollution and distribution and cultural focussing of the signal [11], blurring or obliterating a primary local Pb signature. The Neolithic material that is the focus of this study, however, predates human ore mining. Therefore, the uptake of Pb by pigs will be dominated by ingestion of locally sourced food and through the accidental ingestion of soils while rooting and grubbing. To this end, it is important to understand the isotope composition of locally derived labile Pb in soils of both silicate and carbonate composition, and their overlying biosphere. However, we cannot take direct measurements of Pb in the modern biosphere in a manner analogous to sampling Sr [12]. This is because of the ubiquitous blanket of modern pollutant Pb. The Pb used in petrol, as an anti-knock reagent, in the UK is imported Australia Pb which has a distinctive and very different isotope composition from geogenic [13] UK Pb and ore deposits.

Alternative methods of defining geospatial distribution of Pb isotope compositions therefore have to be found. In an American-based study, prehistoric wild animal data are suggested as the best proxy for biosphere-assimilated Pb [6] and there is much sense in this recommendation. However, such material is hard to come by in Britain. We have therefore opted to use geological data [14–20], and data from this study, and to validate this, where possible, against faunal data from a similar geographic location. This approach is open to further refinement and validation, but nevertheless constitutes a useful first step. We have used published isotope data from soil-forming rocks and minerals, in addition to galena ore (Pb sulphide) data, to try and define characteristic Pb isotope ranges from various lithologies of relevance to this study. We use the plotting method of Albarede et al. 2012 [21] as introduced for enamel studies [5]. This method derives the parameter of Pb model age (T) and 238U/204Pb (μ) and 232Th/238U (κ) ratios both of which are used to characterise metal sources.

Geological controls on the Pb isotope compositions of rocks of Britain

Key to this study is our ability to distinguish between northern and southern British Pb sources and this can be achieved because of differences in the underlying geology between these two parts of Great Britain. The Pb isotope composition of rocks and minerals tends to be dominated by major geological tectonic events such as mountain building, which is accompanied by metamorphism and the intrusion of granites, the heat from which drives the re-mobilisation of Pb to create ore deposits. Thus, as described by Blicher-Toft et al. 2016 [14], Pb isotope compositions across Europe reflect large-scale tectonic events. During the Caledonian Orogeny (490- 400Ma), the tectonic plates of Laurentia and Avalonia collided as the Iapetus Ocean between them closed. The junction between these two tectonic plates (Fig 1) is called the Iapetus Suture and it runs on a NE–SW line from Berwick-upon-Tweed to the Solway Firth and projects into Ireland [22]. The underlying geology to the north and south of this suture is fundamentally different; the Laurentian basement, to the north, is geologically much older (> 3000Ma–c. 1750Ma) and is depleted in uranium (U) [23] whereas the Avalonia basement in the south is geologically much younger (c. 700Ma) [22], and this means the Pb isotope compositions, related to the basements of the two areas, are different. As this geological boundary essentially defines the modern political border of Scotland with England, it provides a potential method of discriminating between Scotland and the rest of Great Britain.

Fig 1. The location of the Iapetus suture.

Fig 1

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Marine carbonates, such as Chalk and limestone, often display ‘anomalous Pb’ values [10] because they are formed as precipitates from seawater, in which the U and Pb present do not retain a proportionate parent/daughter association. This is because U and Pb have very different solubility coefficients in seawater with marine carbonates accommodating an upper limit of 3 mg/kg for uranium but only 1 μg/Kg for Pb and thorium (Th). This imbalance in uptake of U and Pb can result in modern carbonates with very high 206Pb/204Pb values, which leads to a wide range of negative model ages being calculated [24]. Such features are documented in a study of Pb isotope data from Cenozoic limestones from South America [4] and in soil leach and re-equilibrated dentine values from British Chalk sites [25].

Test case Neolithic animals

Stonehenge, and other henges in southern England were a focal point for gatherings during Neolithic times. The geographic origin of the people who created and used them is a key question to understanding their construction and use. However, Neolithic human remains are not common at many of these henges and those that are present tend to have been cremated [26], reducing the range of analytical approaches that can be employed. Consequently, studies have focused on using the feasting remains from domestic animals as a proxy for human movement. The Sr isotope values from human cremated bone are consistent with a southern British source, but bone represents a longer biological time scale and is therefore subject to biogenic averaging [27] whereas Sr isotope values suggested that Scotland could not be excluded as a source for the animals, though recent advances in mapping raise other possibilities [28]. The discovery of highly radiogenic 87Sr/86Sr values (up to 0.7172) in Neolithic animals at the Wiltshire henges has raised the issue of whether some pigs could have been brought, as livestock, from northern Britain to Wessex during the Neolithic [29, 30], a possibility that has sparked debate [31, 32]. This study explores the value of Pb isotopes to this debate.

A multi-isotope study (strontium (Sr), oxygen (O), carbon (C), nitrogen (N) and sulphur (S)) on tooth enamel from pigs observed that the animals had diverse isotope compositions (δ34SVCDT = -1.6 to 19.6, δ13C(collagen) = -23.1 to -19.7, δ15N(collagen) = 3.4 to 9.1, 87Sr/86Sr: 0.7080 to 0.7172 and δ18O(carbVSMOW) = 23.2 to 28.9). The S isotope range covers areas multiple lithologies from the coast to inland Britain and the Sr isotope values encompass virtually the full extent of the British biosphere range. There are also clear dietary differences recorded in C & N isotope data and the O isotope range (5.7‰), while not directly comparable human data, is not far from the full British human enamel range (6.4‰, n = 824) [33]. This data diversity means that there are few areas of Great Britain that can be clearly excluded for the overall source of animals found in the Neolithic henges.

Since the publication of Madgwick et al. 2019 [30] new research has uncovered new sources of highly radiogenic Sr in England. Biosphere mapping of southwest England, guided by geochemical soil profiles [34], has highlighted areas around the granite intrusions in Devon and Cornwall where Sr in plants reaches values of 87Sr/86Sr = 0.72 [28] and Roman and Medieval sheep from this region record 87Sr/86Sr ratios up to 0.717 [35, 36]. In addition, it has been shown [37, 38] that well-established forest on certain terrains, typically those with soils derived from both siliceous and carbonate bedrock, will leach out the carbonate component over time, resulting in a more radiogenic signature for the forest area. These developments mean there are now areas of England that can be considered as a possible source region as well as parts of Scotland. This paper establishes the use of Pb isotopes as a tool for discrimination between origins in northern and southern Britain.

Material and methods

Analytical details

The Pb analysis in this paper come from a number of sources:

1) new analyses of tooth enamel and bone from Durrington Walls, Cladh Hallan (CH) and Kings Gate, and from mineral samples from the Malvern Complex and the Lundy granite.

The tooth enamel selected was fully mineralized with good surface preservation and was prepared as follows: samples of enamel between 50-100mg were cut from the teeth using a rotary diamond wheel. The surfaces were fully abraded to a depth of 10–100 microns and all dentine was removed using a diamond burr. Bone samples were similarly abraded and cut using a rotary blade. The resulting sample was transferred to a clean (class 100, laminar flow) working area for further preparation. Samples were then cleaned by placing in >18 megaohm-cm resistivity Milli-Q water at 60°C for an hour and washed twice in Milli-Q water. They were then leached in 2% Teflon© distilled HNO3 for approximately 2 minutes. After a final rinse, the samples were dried and transferred into a pre-cleaned Teflon© beaker where they were dissolved in Teflon© distilled 16MHNO3, evaporated to dryness and converted to bromide form using Romil© UpA HBr.

The samples from the Malvern Complex and the Lundy Granite were drilled out from feldspar crystals exposed on the clean-cut surface of the hand specimens held in the National Geoscience Data Centre (NGDC). The resulting mineral powder was dissolved and dried down in Teflon distilled 29M HF and 16M HNO3 then 6MCl and then converted to Bromide form using Romil© UpA HBr.

2) We were unable to access many of the samples we wanted to analyse due to Covid departmental and museum closures. However, we found we could use residual aliquots of solutions previously prepared for Sr isotope analysis for pig [30] and sheep enamel [35, 36]. This had the advantage of minimizing unnecessary further sample damage of the teeth but meant we were restricted to those samples for which sufficient solution remained. These solutions were in chloride form and were dried down and converted to Bromide form using Romil© UpA HBr.

3) Previously published Pb analyse of pig enamel samples from Neolithic pig assemblage [5] is included in Table 1 and the preparation details for these samples are given in the source paper.

Table 1. Compilation of Pb and Sr isotope data from this study and previously published data.

$—previously published Pb data [5], # samples solutions [30] x- samples solutions [35, 36]. Unmarked samples are new analysis from tooth enamel and bone prepared for this study.

sample sample type ppm 87Sr/86Sr 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 207Pb/206Pb 208Pb/206Pb T μ κ source
Pig enamel from Henges
DWP04 M1 144 0.71004 18.0875 15.6150 38.0288 0.86340 2.10269 472 9.76 3.91 $
DWP05 M1 158 0.71188 18.2236 15.6237 38.1570 0.85740 2.09402 384 9.75 3.89 $
DWP07 M1 75 0.70970 18.2780 15.6247 38.1706 0.85490 2.08855 344 9.74 3.86 $
DWP10 M1 197 0.71071 18.3712 15.6498 38.2807 0.85189 2.08381 323 9.82 3.87 #
DWP13 M1 90 0.70903 18.4207 15.6338 38.3297 0.84880 2.08099 256 9.74 3.86 $
DWP15 M1 217 0.70943 18.2120 15.6295 38.1987 0.85830 2.09765 402 9.77 3.92 $
DWP20 M1 189 0.71073 18.5487 15.6566 38.3690 0.84410 2.06863 204 9.80 3.80 #
DWP22 M1 124 0.70917 18.3961 15.6312 38.3168 0.85010 2.08447 270 9.74 3.87 $
DWP24 M1 1961 0.71056 18.2378 15.6443 38.1944 0.85790 2.09469 410 9.83 3.91 $
DWP25 M1 93 0.70833 18.7106 15.6681 38.5433 0.83736 2.05995 107 9.81 3.79 #
DWP26 M1 125 0.70950 18.3790 15.6437 38.3587 0.85130 2.08753 305 9.79 3.90 $
DWP27 M1 80 0.71013 18.3942 15.6466 38.3177 0.85070 2.08359 300 9.80 3.87 $
DWP29 M1 84 0.70857 18.9283 15.6558 38.5677 0.82718 2.03801 -75 9.74 3.67 #
DWP30 M3 ND 0.70860 19.0038 15.7558 39.0624 0.82916 2.05576 61 10.10 3.90
DWP31 M1 169 0.71721 18.4283 15.6363 38.3614 0.84849 2.08165 259 9.76 3.87 #
DWP32 M1 102 0.70937 18.2833 15.6363 38.2268 0.85530 2.09126 362 9.78 3.89 $
DWP35 M1 97 0.71032 18.3465 15.6435 38.3721 0.85280 2.09198 329 9.80 3.93 $
DWP36 M1 83 0.70956 18.2885 15.6378 38.2846 0.85510 2.09343 361 9.79 3.92 $
DWP37 M1 153 0.70975 18.2465 15.6355 38.2055 0.85690 2.09391 388 9.79 3.91 $
DWP38 M2 ND 0.71146 18.8297 15.6500 38.7124 0.83116 2.05600 -20 9.72 3.80
DWP39 M1 99 0.71022 18.4926 15.6506 38.4216 0.84630 2.07773 234 9.79 3.86 $
DWP40 M1 137 0.71480 18.7752 15.6759 38.6633 0.83492 2.05931 77 9.84 3.82 #
DWP40 M2 ND 0.71919 18.5052 15.6352 38.4344 0.84491 2.07698 195 9.73 3.86
DWP45 M1 142 0.71130 18.1385 15.6223 38.0950 0.86130 2.10028 443 9.76 3.91 $
DWP46 M1 140 0.70941 18.4686 15.6467 38.3827 0.84720 2.07833 245 9.78 3.86 $
DWP54 M1 83 0.70901 18.4676 15.6638 38.2560 0.84814 2.07154 282 9.86 3.80 #
DWP55 M1 207 0.71269 18.1633 15.6323 38.1186 0.86070 2.09873 443 9.80 3.91 $
DWP56 M1 167 0.71164 18.1463 15.6306 38.1413 0.86140 2.10194 453 9.80 3.94 $
DWP57 M1 74 0.70847 18.7911 15.6584 38.5748 0.83331 2.05290 28 9.76 3.76 #
DWP60 M1 140 0.71486 18.6615 15.6542 38.5282 0.83871 2.06475 115 9.77 3.81 #
DWP60 M2 ND 0.71376 18.6383 15.6287 38.4770 0.83852 2.06444 82 9.68 3.79
DWP62 M1 98 0.70972 18.2598 15.6350 38.2061 0.85630 2.09243 377 9.78 3.90 $
DWP66 M1 ND 0.70816 18.8742 15.6604 38.6813 0.82973 2.04945 -32 9.75 3.76
DWP69 M1 128 0.71025 18.5303 15.6553 38.4834 0.84500 2.07751 215 9.80 3.87 $
DWP71 M1 92 0.70917 18.3868 15.6247 38.2877 0.84980 2.08241 263 9.71 3.85 $
DWP84 M1 ND 0.70808 18.7957 15.6532 38.5906 0.83281 2.05319 12 9.74 3.76
DWP85 M1 ND 0.70808 18.7144 15.6496 38.5562 0.83624 2.06027 67 9.74 3.79
MD124 I 243 0.71038 18.6966 15.6455 38.4824 0.83681 2.05828 72 9.73 3.76 #
MD125 M1 133 0.71042 19.0637 15.6658 38.7316 0.82177 2.03173 -165 9.74 3.67 #
MD126 M1 ND 0.71582 18.8804 15.6560 38.6343 0.82922 2.04630 -33 9.76 3.73
MD127 M3 13 0.70981 18.9309 15.6561 38.6943 0.82702 2.04401 -84 9.73 3.73 #
MD128 I ND 0.71353 18.5342 15.6077 38.4433 0.84210 2.07421 120 9.62 3.83
MD129 M1 ND 0.71404 18.8153 15.6520 38.5786 0.83187 2.05041 -4 9.73 3.74
MD130 M1 194 0.71162 19.1770 15.6814 38.5833 0.81772 2.01199 -218 9.79 3.54 #
WK108 M1 ND 0.70822 18.9383 15.6653 38.7388 0.82718 2.04555 -59 9.78 3.75
WK119 M1 263 0.71138 18.5046 15.6499 38.4170 0.84573 2.07611 224 9.79 3.85 #
WK120 M1 115 0.70849 18.7743 15.6564 38.5639 0.83393 2.05411 22 9.73 3.76 #
WK121 M1 215 0.71326 18.4821 15.6334 38.3186 0.84587 2.07331 209 9.73 3.81 #
WK122 M1 227 0.71014 18.0293 15.5994 37.8437 0.86523 2.09904 482 9.70 3.84 #
WK123 M1 98 0.70825 18.7726 15.6543 38.5304 0.83389 2.05251 32 9.75 3.74 #
Pig bone from Henges
DWRM30 bone nd 0.70768 18.8758 15.6566 38.6606 0.82945 2.04816 -41 9.74 3.75
DWRM38 bone nd 0.70857 18.8388 15.6540 38.6767 0.83094 2.05303 -18 9.74 3.78
DWRM60 bone nd 0.70950 18.7552 15.6398 38.5810 0.83390 2.05711 16 9.70 3.78
SW England Roman/Medieval sheep
EX-Med1-Ovis4 18.4321 15.6327 38.4054 0.84810 2.08365 245 9.73 3.89 x
Ex-Er-Ovis2 18.4351 15.6361 38.4203 0.84820 2.08411 249 9.75 3.90 x
Ex-Med3-Ovis9 18.4766 15.6405 38.4574 0.84650 2.08144 227 9.75 3.89 x
EX-RLF-Ovis6 18.4369 15.6476 38.4738 0.84870 2.08681 270 9.79 3.93 x
Ex-Med3-Ovis7 18.4991 15.6452 38.4843 0.84570 2.08036 219 9.77 3.89 x
EX-Med1-Ovis6 18.3998 15.6374 38.4211 0.84990 2.08815 278 9.76 3.92 x
Ex-Med4-Ovis4 18.4311 15.6243 38.4360 0.84770 2.08542 230 9.70 3.90 x
Ex-Med2-Ovis7 18.5386 15.6422 38.4992 0.84380 2.07674 184 9.75 3.87 x
EX-Med1-Ovis5 18.4398 15.6379 38.4172 0.84810 2.08342 249 9.75 3.89 x
Ex-Med2-Ovis12 18.4866 15.6421 38.4696 0.84610 2.08097 223 9.76 3.89 x
Ex-Er-Ovis11 18.4857 15.6825 38.5954 0.84836 2.08807 294 9.91 3.97 x
Ex-Med2-Ovis11 18.4106 15.6333 38.4318 0.84910 2.08751 262 9.74 3.92 x
Ex-LR-Ovis1 18.4496 15.6394 38.4381 0.84770 2.08344 245 9.76 3.90 x
Ex-Med4-Ovis3 18.4936 15.6340 38.4824 0.84540 2.08088 201 9.73 3.89 x
Ex-Med2-Ovis4 18.4629 15.6383 38.4475 0.84700 2.08244 233 9.75 3.89 x
Ex-Med2-Ovis2 18.5305 15.6510 38.5369 0.84461 2.07975 208 9.79 3.90 x
Ex-Med4-Ovis2 18.4915 15.6409 38.4785 0.84580 2.08090 220 9.76 3.89 x
Ex-Er-ovis06 18.3385 15.6251 38.3340 0.85200 2.09038 300 9.72 3.91 x
Ex-Er-ovis03 18.4051 15.6274 38.4042 0.84910 2.08664 255 9.72 3.90 x
Ex-Er-ovis04 18.4499 15.6402 38.4655 0.84770 2.08490 246 9.76 3.91 x
Malvern Precambrian Igneous Complex
MR 35599 feldspar 18.1005 15.6162 37.8133 0.86275 2.08908 460 9.75 3.78
MR 8625 feldspar 18.1043 15.6267 37.7799 0.86315 2.08679 477 9.79 3.76
Lundy Tertiary Granite
MR31924 01 feldspar 18.9863 15.6572 39.0150 0.82466 2.05490 -124 9.72 3.85
MR31924 02 feldspar 19.0036 15.6597 39.1050 0.82404 2.05777 -132 9.73 3.89
MR31924 03 feldspar 18.8821 15.6511 38.9350 0.82889 2.06201 -57 9.72 3.88
MR31924 04 feldspar 18.7627 15.6408 38.8100 0.83361 2.06847 12 9.70 3.89
MR31924 05 feldspar 18.8803 15.6500 38.9280 0.82891 2.06183 -58 9.71 3.87
MR31924 06 feldspar 18.9569 15.6586 39.10654 0.82601 2.06292 -98 9.73 3.92
MR31924 07 feldspar 18.9357 15.6586 39.08357 0.82694 2.06402 -82 9.74 3.92
MR31924 08 feldspar 18.9663 15.6600 39.11559 0.82568 2.06237 -103 9.74 3.91
MR31924 09 feldspar 18.9508 15.6591 39.09103 0.82630 2.06276 -93 9.74 3.91
MR31924 10 feldspar 19.0258 15.6613 39.11122 0.82316 2.05569 -145 9.73 3.88
MR31924 11 feldspar 18.9477 15.6596 39.133 0.82646 2.06532 -89 9.74 3.93
MR31924 12 feldspar 18.9757 15.6603 39.14144 0.82528 2.06272 -109 9.74 3.92
Scotland, Bronze Age sheep/goats
CH 412 (M1/2) 18.456 15.558 39.178 0.84298 2.12281 78 9.44 4.25
CH 412 (M1/2) 18.471 15.578 39.046 0.84338 2.11396 108 9.51 4.18
CH 595 (M1) 18.574 15.587 38.969 0.83917 2.09808 46 9.53 4.07
England, Neolithic human
63262 149 0.70887 18.8026 15.6409 38.6542 0.8318 2.05570 -2.535 9.72 3.79
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Separation of Pb from samples was undertaken using standard AG-1 X8 ion exchange techniques. In some cases, the residual solution from the AG-1 X8 column was converted to chloride and passed through an Eichrom© 50AG X8 column to separate out Sr.

Contribution of laboratory blank to analysis. The laboratory Pb blank is between 5-30pg with a fall in blank composition, measured on the clean lab windowsill outwith the laminar flow hoods, of 206Pb/204Pb = 17.72. Therefore a 50-milligram sample with 0.1ppm Pb, combined with 30pg blank contribution would provide a worst-case blank contribution of 0.6%. For a sample with a 206Pb/204Pb ratio of 18.80 this would result in a shift of -0.01 and is therefore negligible for the purposes of this study.

Samples analysed by Thermo Fisher Scientific Neptune Plus 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 (Cetac Aridus II) and normalised to NBS981 [39]. Average 2 SD reproducibility for the following ratios was 206Pb/204Pb  =  0.008%; 207Pb/204Pb  =  0.008%; 208Pb/204Pb  =  0.009%.

Sr was loaded onto a single Re Filament [40] and the isotope composition and Sr concentrations were determined by Thermal Ionisation Mass Spectrometry (TIMS) using a Thermo Triton multi-collector mass spectrometer. The international standard for 87Sr/86Sr, NBS987, gave a value of 0.710262 ± .000020 (2SD, n = 8) during the analysis of these samples. Data are corrected to an accepted value of for NBS987 of 0.710250.

Results

A new Pb iso-scape for Great Britain

A Pb isotope zonation map of Britain has been created which highlights the tectonically controlled geospatial variation in Pb isotope values across Britain using published data [14–18, 20] and data from this study. It is contoured using inverse distance weighting (IDW) with ten natural break categories for 206Pb/204Pb; and 238U/204Pb (μ) which are shown in Fig 2 alongside the sample sites (n = 633). The maps show the strong zonation of the isotope compositions across the Iapetus Suture and throughout Scotland which reflects the influence of the very old Laurentian basement that underlies much of Scotland. England is dominated by Hercynian mineralisation focussed on the Pennines, Mendips, and SW England ore fields whereas the ores in Wales have an older Avalonian signature.

Fig 2.

Fig 2

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A contoured map of A) 206Pb/204Pb isotope compositions B) 238U/204Pb (μ) values from Great Britain C) samples locations. Data for this plot are taken from the compilation of [14–16, 19, 41] and this study. Contouring is based on Inverse Distance Weighting (IDW) with ten natural break intervals. Superimposed over this contour map is the outcrop area of Chalk. This has been designated data ranges of 206Pb/204Pb 18.81 ±0.2(1SD, n = 26) and 238U/204Pb (μ) = 9.71±0.24(1SD, n = 26). Contains OS data © Crown copyright and database rights 2022.

Superimposed over this map is an outline of Chalk outcrop. This lithology is an important substrate in southern England and its domain is designated an isotope range of 206Pb/204Pb 18.815 ±0.0195(n = 26) and 238U/204Pb (μ) of 9.71±0.24 (1SD, n = 26), using data from this study and Montgomery (2002) [25].

The construction of reference data fields

We have created four reference data fields in μ- T space, representing the main geographic Pb isotope sub-divisions defined in Fig 2. The co-ordinates for the field can be obtained from J. Evans.

Ores from Scotland: The data from Scotland define a broad sloping field from c 70Ma—2400 Ma [14]. We focus on the younger end of this array which is constructed using the following data sources [14–17, 19, 41]. The boundary lines for this field encompasses 90% of these data.

Ores from Wales: The 1SD field for T vs μ, for data from Wales is taken from published studies [14, 41] and includes samples of Avonian basement from the Malverns Complex (Table 1).

Ores from England: The Pennines, Mendips and south-west England mineralization are all similar geological age. There is a large amount of data published from these areas [14, 20, 42]. The data field is based on the 1SD range of data (n = 197).

The Chalk: This is the rock that underlies much of southern Britain and is an important lithology for archaeological studies because it is both a focus of important Neolithic and Bronze Age sites and because of its preservation properties. The Chalk does not host economic Pb mineralisation so there is an absence of published Pb isotope data in the geological literature. We have derived the 1SD range for T and μ from Pb isotope analysis of Chalk soil leaches and equilibrated dentine [25], and bone samples (this study).

Testing the validity of using mineral composition as a proxy for bioavailable Pb

The transmission of these geological signals into the biosphere is critical for the application of this method to provenance studies. Fig 3 shows the reference fields in T vs μ space with archaeological faunal/human data to validate them. This exercise is limited by the paucity of biosphere data currently available. However, there is sufficient data to demonstrate the assumption that, in a situation of geogenic exposure, the faunal/human data reflect the labile Pb isotope composition of the local geological environment.

Fig 3. Validation of mineral data fields using published human and faunal sample data sources [11, 25, 43, 44] and data from this study.

Fig 3

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There are two sites where Pb isotope analyses are available from pre-mining (Neolithic) human enamel [45, 46]. The Sr isotope composition of the tooth enamel has been used to select those individuals that had a childhood origin on Chalk. These include three juveniles [45] and one adult [46]. These four samples have Pb isotope compositions that plot within the 1SD field designated to represent the Pb isotope composition of the Chalk and thus supports the transfer of the geogenic Pb composition into the tooth enamel.

The validation of the ore field in England is based on new data presented in this study from south-west England. The samples are from sheep/goat samples of Roman [36] and Medieval [35] age excavated in Exeter [47]. The Pb data from these animals plot as a cluster within 1SD of the Pb isotope field, thus showing the relationship between the Pb isotope compositions of the mineralization in England and the faunal biosphere.

Measurements of modern animals provide evidence for transmission of geogenic Pb isotope composition from ore field for Wales into the biosphere field. These samples were specifically collected from Wales in areas where Pb mining is extensive. Consequently, natural Pb levels are high and the influence of modern pollution is swamped by the geogenic component in the environment [43]. The data plot well within the 1SD data field based on mineral data from Wales.

The data used to validate the transmission Pb isotope compositions between the geogenic source and over lying biosphere for the ore field for Scotland comes from sites on the Hebrides and Orkney and both human and animal samples have been used to try and characterize this data field. All the samples have low Pb concentrations consistent with only geogenic lead exposure and/or predate anthropogenic lead release. The data scatter and human samples show considerable variation in μ and Pb model ages. Unlike the previous three data guidance fields for Chalk, England and Wales ores, the Pb isotope composition of the biosphere samples does not plot neatly within the ore field for Scotland, however, these samples show two distinguishing features. All samples have low μ values, between 9.6 and 8.9, and they also have elevated κ values >4.0 which are typical of ores from Scotland [14]. It is hoped that future studies can develop this validation of the dataset from Scotland with well provenanced samples, free from modern anthropogenic contamination, however, preservation is a problem in large parts of Scotland. We recommend that, while this method can be used to distinguish between sources in northern and southern Britain, the transmission of the high resolution Pb isotope zones seen in the mineral data (Fig 3) cannot currently be demonstrated in biosphere samples.

In summary, reference data fields for the Pb isotope composition of geographical areas of Britain, have been defined with geological data and the transfer of these signatures into the biosphere has been validated with well-provenanced faunal data. It should be noted that as most of the fields are defined at 1SD, which represent c. 66% of the reference data, plotting outside the field only excludes samples at this level of certainty. With this validation in place, the data from the pigs can be examined.

Discussion

Test case using pig enamel samples

The Pb isotope compositions of the pig enamel are presented in Table 1 and plotted in the form of μ versus T in Fig 4. The data points are colour-coded to represent 87Sr/86Sr isotope composition groupings of the data: <0.709, 0.709–0.713, and > 0.713. These Sr groups broadly represent a) carbonates terrains, b) most silicate terrains and c) Rb-rich/very old terrains, respectively.

Fig 4. The pig tooth enamel from the henges shown in relationship to the mineral reference fields.

Fig 4

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The data are colour-coded based on strontium isotope ranges of the enamel.

The Pb isotope composition from the pig enamel samples, have model ages that range between the negative values of the southern Britain Chalk field to the Palaeozoic model ages as old as 482 Ma, and all have μ values above 9.62. The μ values excludes them from the field of ore data from Scotland and hence the Pb isotope data indicate that these animals which represent a 36% subset of those in Madgwick et al. 2019 [30], are inconsistent with an origin in Scotland.

Around half the samples have model ages that plot between 195 Ma and 482 Ma, indicating Pb sources from a Palaeozoic/Late Precambrian rocks substrates and three of the samples (DW31, DW40 and DW60) with radiogenic values (> 0.713) plot in the data field for Southwest and central England and hence could be sourced from these areas. Nine of the samples with 87Sr/86Sr < 0.709 have both Pb and Sr isotope characteristics consistent with an upbringing on a Chalk terrain (Fig 4). Five samples (four from Marden [MD 124, 125, 127, and 130] and one from Durrington Walls [DW 38]) have Sr values between 0.7098–0.7116 and plot in a sub-linear array across the southern Britain Chalk field. This combination of carbonate dominated Pb values, but Sr isotope values above those generally attributable to pure Chalk, could suggest that the animals were raised on a less pure carbonate lithology such as the Jurassic Limestone or a mixed silicate -carbonate sediment.

Having established that many of the samples record combined Sr and Pb isotope compositions that can be accommodated by known tectonic and lithological domains found in Britain, we are left with a few samples for which the data are more complex. These samples record elevated radiogenic composition for both Sr and Pb. They are referred to as the “anomalous group” and are discussed below.

The relationship between the Sr and Pb isotope compositions in tooth enamel

Although both biosphere Sr and Pb are ultimately sourced from a geological origin it should not be expected that there will be a direct correlation between the two systems; they will reflect different aspects of the source biosphere. A carbonate terrain has restricted and relatively low Sr isotope compositions, due to the very low rubidium (Rb) content of limestone, whereas the Pb isotopes may range in values considerably but will tend to have elevated 204Pb/206Pb values as a consequence of high and variable time-integrated U/Pb in the limestones [4]. This will lead to high μ values and negative model ages [21].

In silicate terrains the relationships between Sr and Pb isotopes are the inverse of carbonates terrains. The variable lithologies have a wide range of Rb/Sr ratios leading to transmission of wide-ranging 87Sr/86Sr values into the biosphere whereas the Pb isotopes will reflect the overall tectonic environment.

There are six samples (three animals from Marden and two animals (3 teeth) from Durrington Walls [MD126, MD 128, MD129, DW40-M1, DW60- M1&M3] with both high Sr (87Sr/86Sr > 0.713) and Pb values (206Pb/204Pb > 18.7) which generate μ values of >9.6 and young Pb model ages between 30 Ma and 123 Ma (Fig 4). This combination of radiogenic Pb and Sr isotope features is very difficult to reconcile with a single lithological source and essentially suggests a Tertiary source with highly radiogenic Sr composition. Such a rock type does occur in the form of the small island of Lundy (4.5km2) off the north Devon coast [48]. While we cannot exclude this granite zone as a possible origin, and it is known to have been inhabited by Neolithic communities [49], it would not be parsimonious to assign origins for the anomalous pig samples to Lundy, based on the island’s small size, and relative remoteness

Several possible alternative explanations can be considered. The isotope signal could derive from an upbringing on a sedimentary rock composed of old/Rb rich silicate minerals in a carbonate matrix where the Sr was predominantly derived from the silicate component whereas the Pb was predominantly from the carbonate matrix. However, this isotope mismatch could also reflect metabolic discrimination rather than a simple foddering source. It is known that Pb can be reactivated into the blood stream during times of stress [50], so it is possible that this isotope signal could be caused by mobility, foddering or stress that disconnects the sources of Pb and Sr from different body reserves during the formation of enamel in the animals. Understanding these possible processes is beyond the remit of this paper and will involve detailed investigation on the nature and relationship of Pb and Sr uptake during enamel formation.

Conclusions

It has been frequently stated, and needs to be re-iterated, that isotope methods can only exclude possible areas of origin and, added to that, can only exclude options for which reference data are available, reliable and appropriate.

This paper develops the use of Pb isotopes as a further isotope discriminant for biosphere samples that have not been affected by anthropogenic pollution; either pre-metallurgical populations or those with enamel samples where Pb concentrations below the threshold for natural exposure in skeletal studies of 0.5 [11] or 0.7 mg/kg [51] in [human] enamel.

We have produced the first Pb isoscape map for Great Britain which provides a geographic distribution of Pb mineralization events and also characterizes the Chalk, which is an important lithology for archaeological studies. These main subdivisions are defined in T-μ space and have been validated for biosphere use with human and faunal data.

This study highlights the way in which Pb isotope domains are associated with large tectonic terranes and, because to the tectonic make-up of Great Britain, provides a powerful tool for discrimination between biosphere in Scotland (the Laurentian basement) and southern Britain (the Avalonia basement).

We have tested this application using a sample of Neolithic pig enamel from sites in southern England, some of which, because of Sr isotope composition, could not be excluded from an origin in northern Britain. Pb isotope data from the teeth excludes Scotland as a source but the diverse range of Pb isotope results, combined with other isotope proxies, are consistent with the animals being raised on a variety of lithologies of diverse age and from variable environments.

The study also highlights the fact that the uptake of both Sr and Pb may not always be consanguineous as some samples display a combination of Sr and Pb isotope signatures that are currently difficult to reconcile with a single geographic/geological origin.

This study demonstrates the diagnostic potential of Pb isotope compositions as a provenance tool in samples unaffected by anthropogenic Pb exposure and highlights the different scale of discrimination between tectonic terrains which Pb isotopes provide in comparison to the lithological scale of Sr isotope data.

Supporting information

S1 Data

(XLSX)

Click here for additional data file. (11KB, xlsx)

Acknowledgments

We thank Steve Noble for improving an earlier version of this text. We are grateful to Ros Cleal, Michele Drisse (Alexander Keiller Museum) and Lisa Brown (The Wiltshire Museum) for assisting and supporting sampling of the Neolithic pigs and Tom Cadbury (Royal Albert Memorial Museum) for assisting sampling of the Exeter archaeological samples. We are grateful to the Lundy Field Society for providing generous financial support for collecting samples from the island. Exeter samples were part of AHRC (AH/N001931/1) funded project “Exeter: A Place in Time” in collaboration with Gundula Müldner (project PI Stephen Rippon). We thank Jacqui Mulville for the sheep teeth from Cladh Hallan and Lorrain Higbee (Wessex Archaeology) for permission to published data on the Kings Gate sample. This paper is published with the Permission of the Executive Director of the British Geological Survey.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

The author(s) received no specific funding for this work.

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