Neodymium Isotope Analysis in Dental Enamel: A Novel Technique for Human Provenancing
Forensic Anthropology and Osteoarchaeology| Katherine H. McLean
Stable isotopic ratios are a valuable tool in both osteoarchaeology and forensic anthropology, used to provide information about the geographic origins of human remains and archaeological materials. Strontium (87Sr/86Sr) has long been the isotope of choice for provenance indication. However, promising new research indicates potential for the neodymium isotope system (143Nd/144Nd) as a novel method for human provenancing—particularly in coastal regions due to its greater correlation with bedrock values. If sufficiently accurate, the addition of a new isotopic system to our arsenal could increase the precision of geographic estimates and the power of stable isotope analysis.
Stable isotopic ratios are a valuable tool in both osteoarchaeology and forensic anthropology [1-3]. Elements such as strontium and lead can provide information about the geographic origins of human remains and archaeological materials as the ratios between the rarer isotopes and their more abundant forms pass unmodified through trophic levels [2], [4]. The isotopic composition of an organism can thus directly reflect that of the substrates upon which the organism lived, and this determination of provenance allows patterns of population mobility and migration to be studied [1]. Dental enamel is ideal for isotopic analysis as it is less vulnerable to post-mortem alteration than bone or other calcified tissues [5-7]. The lower porosity of enamel means that it is more likely to retain near in vivo isotopic concentrations and ratios [8]. The mineralisation of the permanent dentition during childhood and early adolescence thus creates a time capsule, preserving an isotopic signature of an individual’s geographic origins [2].
Strontium (87Sr/86Sr) has long been the isotope of choice for provenance indication [1], [2], [4], [9], [10]. However, an analysis that is limited to a single isotope is unreliable and fallible [11], [12]: marine strontium can overwrite local geological signatures, imported food can influence dietary isotopic values, anthropogenic disturbance can alter environmental isotopic compositions, and similar environments mimic each other’s isotopic signatures. The current provenancing of human remains is therefore often imprecise [6], [7], and a broader approach would be highly beneficial for distinguishing similar geological environments and possible interferences [11-13].
A research team based in the Netherlands has recently demonstrated promising potential for the neodymium isotope system 143Nd/144Nd as a novel method for human provenancing [6], [7], [13], [14]. If sufficiently accurate, the addition of a new isotopic system to our arsenal could increase the precision of geographic estimates and function as a proxy when trace levels of primary isotopes are too low for measurement, increasing the power of stable isotope analysis in osteoarchaeology and forensic anthropology [7].
Current Research on 143Nd/144Nd Analysis
Neodymium isotopes migrate from bedrock into the water system and vegetation [6], [7]. The element transitions into humans via dietary consumption, where it can substitute for calcium in the tooth enamel matrix [6], [7]. The comparatively heavy mass of neodymium means that it does not undergo significant alteration in both the environment or the body [15], [16]. As it is also not altered substantially by weathering [16], [17], isotope composition remains unchanged throughout the food chain, and measurements in enamel should directly reflect bioavailable levels [7].
Neodymium presents as a viable candidate for a new isotopic system for human provenancing. Neodymium isotope analysis is already widely used in geochronology and geological provenancing [15], [17-21], and has also been used to successfully provenance animal bones and archaeological artefacts [16], [22], [23]. Another promising characteristic is that neodymium has far less mobility during weathering processes than the Rb–Sr isotope system [16], [17]. Its lower mobility means that neodymium isotope composition in seawater is a more accurate reflection of bedrock composition than that of strontium [19], making it particularly promising for mobility analysis in coastal regions where sea-spray dominates strontium concentration and renders analysis unreliable [24].
Plomp et al. [7] were the first to assess whether neodymium could be effectively isolated from human tissue. They created a novel technique to analyse neodymium isotopes in human tooth enamel despite it only being present in trace amounts [7]. The researchers analysed third molars (M3) from native Dutch populations in Amsterdam and Rotterdam who were geographically stable during the time of M3 enamel development. The Amsterdam and Rotterdam areas were formed by different parts of the greater Rhine river system and so have different underlying geologies [7]. They developed a novel protocol to remove calcium and organic matter whilst retaining enough enamel to analyse 143Nd/144Nd ratios. All analyses were done using thermal ionisation mass spectrometry (TIMS) with 1013 Ω resistors fitted to the amplifier system [7]. TIMS is a highly sensitive mass spectrometry technique and is the gold standard for calculating isotopic ratios [25], [26]. A majority of Plomp et al.’s [7] results fell within the range of neodymium concentrations expected from geological data of Rhine sediments. These results indicate that the authors were successful in measuring the isotopic ratios and that neodymium analysis from tooth enamel is viable. The authors also found that the 143Nd/144Nd composition of enamel varied between the Amsterdam and Rotterdam populations [7], indicating that neodymium concentrations may indeed reflect geographic origins.
A follow-up study [6], [13] evaluated the levels of geological control on 143Nd/144Nd ratios in human enamel. Using the protocol developed by Plomp et al. [7], they analysed elemental concentrations in individuals from a variety of geological contexts (Colombia, Iceland, the Caribbean, and the Netherlands) who were geologically stable during the M3 enamel formation period. Strontium isotope values were also measured. Strontium values in the Caribbean individuals, who all came from areas with high seawater spray and North African dust blow interference, were elevated compared to local bedrock geology [6], [13]; neodymium values were significantly more correlated with bedrock values than strontium in these locations. 143Nd/144Nd ratios from most individuals were indistinguishable from the geology of the location where the individual resided when the enamel was formed, meaning that individuals from areas where the geological neodymium composition differed could be easily distinguished [6]. Plomp et al. [6] successfully linked the interpopulation variance in neodymium isotopic ratios discovered by Plomp et al. [7] to geographic variance, showing that the neodymium isotopic composition of enamel can indicate human provenance.
Viability of 143Nd/144Nd Analysis
While the overall trends from these studies appear to support the viability of neodymium isotopic ratios for human provenancing [6], [7], there were outliers. Two individuals had significantly raised neodymium elemental levels [7], and the isotopic composition of the Icelandic individual did not match geological values [6]. This suggests that local geology is not the only factor controlling neodymium in human dental enamel. The authors propose a variety of possible explanations for these outlier values that need to be investigated further: anthropogenic emissions, non-locally produced dietary contributions, or physiological differences that influence neodymium uptake [6], [7].
Various industrial processes can contaminate neodymium values [6], [7], [27], [28]. However, none of the studied individuals were known to live near industrial sites [6], [7]. Systematic pollution is also unlikely due to the overall low concentrations, as populations known definitively to be affected by neodymium pollution have significantly higher concentrations in their hair and lung tissue [27], [28]. Additionally, the stronger emission controls of the past decades have drastically reduced emissions of rare earth elements [29], rendering pollution further improbable.
Diets that contain substantial non-local components could have contributed to the variation in values [6], [7]. This explanation appears plausible for the Icelandic individual, as Iceland imports a majority of its food [30], [31]; however, it fails to explain the other outliers. Some of the Dutch isotopic compositions fell within the range expected for volcanic geology, rather than the fluvial systems underlying Amsterdam and Rotterdam [6], [7], despite the Netherlands being a primary exporter of food [32].
There could also be physiological factors at play, generating variation in neodymium uptake [6], [7]. Differences in sex, age, and activity patterns could all plausibly influence uptake—as they do in other isotopic systems [33], [34]. However, all these explanations are entirely speculative, and there is a paucity of research into neodymium’s interactions with human tissue.
Sample Sizes and Amplification Technologies
A fundamental limitation of neodymium analysis for human provenancing is the sub-nanogram sample sizes of neodymium available due to preferential exclusion of neodymium as a substitute for calcium in the enamel matrix [6], [7]. The smaller the sample, the more difficult it is to extract and analyse. Plomp et al. [6], [7] utilise 1013 Ω resistors, which greatly increase the sensitivity of TIMS [25], [35-37], as an essential part of their protocol. It is deeply problematic that replication and furthering of this analysis would thus be restricted to teams with access to the latest, highly expensive components for TIMS.
A newly developed amplification technology, however, might make neodymium isotope analysis more widely viable. Reinhard et al. [38] recently presented a study of novel ATONATM amplifiers, created in response to the push to measure increasingly smaller samples via TIMS, which have more favourable signal-to-noise ratios and increased stability and precision. Reinhard et al. [38] tested the ATONATM amplifiers’ ability to accurately and precisely measure 143Nd/144Nd ratios and found an improvement of more than a factor of 3 over previous measurements using traditional technologies. Whilst Reinhard et al. [38] did not consider archaeological or anthropological applications, novel technologies such as this could open the way for the analysis of sub-nanogram samples of neodymium in a broader range of laboratory environments.
Discussion
The unexplainability of the outlier neodymium levels and ratios is a core problem in assessing the validity of the technique for human provenancing. Variation in measurements is not in and of itself an issue, but not being able to identify the causes of that variation is a significant problem. Plomp et al.’s [6], [7] findings indicate that local geology is not the sole factor controlling neodymium concentrations in dental enamel—but what these other factors are needs to be ascertained before neodymium isotope analysis can be utilised in regular research.
Future study is needed into potential dietary controls on neodymium ratios, such as food imports, to assess the impact of the globalised food web on dietary neodymium isotope levels. Studies are also needed on potential anthropogenic effects on bioavailable neodymium levels, such as exposure to industrial processes and pollution. The use of phosphate-based fertilisers has been shown to significantly affect the environmental levels of strontium and sulphur [39-41], and so should be considered as a potential factor. To assess dietary controls and anthropogenic effects, neodymium isotope values in crops and water must be measured from a range of geographic locations and compared to substrate neodymium levels.
Modern fertilisers and long-distance trading of food should not interfere with archaeological applications, however, post-mortem alteration via chemical interaction or microbial activity (diagenesis) may instead become a notable issue [1]. There are no studies yet looking at diagenesis of neodymium in dental enamel, and the potential for neodymium isotope analysis as an archaeological, and not just forensic, tool cannot be ascertained until these are completed. If removal of large amounts of outer enamel will be required to avoid diagenetic contamination [14], [42], then, due to the low levels of neodymium in dental enamel, there may be insufficient material for analysis—even with the latest amplification technologies.
There are very few available isoscapes (maps that predict bioavailable isotopic abundance ratios) available for the neodymium system [7], [43-47]. The creation of detailed isoscapes will be necessary for the practical use of neodymium as a provenance indicator, as accurate evaluation of isotopic data from human tissue requires background data from the environment [42]. Researchers also need to develop primary datasets from a wide variety of geological contexts, with separate datasets for archaeological populations and contexts. Plomp et al. [6], [7] focus solely on modern populations for use in forensic contexts; however, datasets such as Plomp’s [13] cannot be assumed to be functional for answering archaeological research questions. The most significant limitation of neodymium isotope analysis for human provenancing is the necessity of the latest analytical techniques due to the extremely small sample sizes in enamel. The practicality of any method maintains an inverse relationship with the amount of specialised equipment required. Until technologies develop further and become more widely available, the addition of neodymium isotope analysis to multi-isotopic approaches will be very restricted.
Conclusion
Isotopic analysis is a highly complex and rapidly-evolving area of research. It appears that neodymium ratio analysis in human tooth enamel is a genuinely promising method for provenancing in osteoarchaeology and forensic anthropology—particularly for use in coastal regions with high seawater spray due to its greater correlation with bedrock values than strontium [6], [13]. However, it is not yet fit for use, as extensive work on background sampling, datasets, and pinpointing other influences on 143Nd/144Nd ratios is urgently needed, but it has strong potential to become a beneficial addition to more established isotopic systems and deserves further attention.
Glossary
Isotopes: Atoms of the same element that have differing numbers of neutrons and, as such, slightly differing masses.
Isotopic ratio: The relative abundance of different isotopes of the same element varies between geographic locales and can be measured to yield an isotopic ratio. Any given climate or ecosystem will thus have a unique isotopic fingerprint.
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Katherine is a recently graduated Honours student in Biological Anthropology. She is currently preparing for her PhD, focusing on hominoid evolutionary thanatology. Recent work includes a proposal for resurrection of Neanderthal haemoglobin in Research Ideas and Outcomes and a talk on death and genus Pan at the International Symposium on Comparative Evolutionary Thanatology.