Strontium isotopes (87 Sr/86 Sr) in terrestrial ecological and palaeoecological research: empirical efforts and recent advances in continental-scale models

Wildlife trade investigations benefit from multivariate stable isotope analyses

The investigation of wildlife trade and crime has benefitted from advances in technology and scientific development in a variety of fields. Stable isotope analysis (SIA) represents one rapidly developing approach that has considerable potential to contribute to wildlife trade investigation, especially in complementing other methods including morphological, genetic, and elemental approaches. Here, we review recent progress in the application of SIA in wildlife trade research to highlight strengths, shortcomings, and areas for development in the future. SIA has shown success in species identification, determination of geographic provenance, and differentiating between captive-bred and wild individuals. There are also emerging applications of SIA in wildlife trade research including the use of labelling for traceability, more in-depth analyses such as compound specific isotope analysis (CSIA), the use of trace metal isotopes, and monitoring the health of individuals (e.g. dietary history and nutritional status). While these applications have shown the utility of SIA in wildlife trade investigations, there are a number of limitations and issues where standardisation of analytical procedures would improve the comparability and interpretation of results. First, there is high variation within many stable isotopes geographically and within tissues - this variation presents opportunities for tracking and monitoring but can also challenge detection of patterns when variation is high. Second, the choice of isotopes and tissues within an organism (and ideally, multiple isotopes and tissues) should be considered carefully as different isotopes and tissue types have variable strengths and weaknesses depending on the research question. Third, validation of SIA methods remains underutilised in the field but is critical for applying SIA broadly to wildlife trade investigations and, particularly, for applications in forensics and in court. Fourth, standards are essential for comparisons across studies. Fifth, while some reference databases exist for the use of SIA in wildlife trade research (e.g. ivory), there are still few comprehensive reference databases available. Development of robust reference databases should be a priority for advancing the use of SIA in wildlife trade research, and ecological study more broadly. Ultimately, further recognition of these primary challenges (and development of solutions) within wildlife SIA research will improve the potential for this technique in tackling the threat of overexploitation to global biodiversity - particularly in concert with the application of other investigative techniques such as genetics and elemental analysis.

3000 yr-old patterns of mobile pastoralism revealed by multiple isotopes and radiocarbon dating of ancient horses from the Mongolian Altai

Pastoral nomadism is of great cultural and economic importance in several regions of the world today. However, documenting ancient patterns of mobility in societies where pastoralism was central is challenging and requires tailored approaches and methodologies. Here we use strontium, oxygen and carbon isotopic analyses of dental enamel, together with a local strontium isoscape, to reconstruct the mobility patterns of seven domestic horses deposited in a Late Bronze Age grave from western Mongolia. Radiocarbon indicates that the animals were deposited within a short period of time, 3000 years ago. The isotope time series obtained from tooth enamel shows that four of the seven horses exhibited a common pattern characterized by a high frequency of mobility, suggesting that in this area (1) cyclical pastoral mobility dates back at least to the Late Bronze Age and (2) the animals belonged to the same herding family, implying that only a small community was involved in the funerary rite of this structure. The data show that the other three horse individuals had a distinct mobility pattern and that one was not from the local area, pointing to flexibility in mobility patterns over time or circulation of animals between herding groups. These results illustrate the power of the isotopic approach to reconstruct animal biographies and effectively address the archaeology of pastoral nomadism and mobility.

Strontium isotope mapping of elephant enamel supports an integrated microsampling-modeling workflow to reconstruct herbivore migrations

Strontium isotope ratios (87Sr/86Sr) in dental tissues are widely used to study animal and human migration. However, questions remain regarding how different biological processes and sampling methods influence measured 87Sr/86Sr and subsequent interpretations. We present a unique experiment with the known relocation history of a zoo elephant to evaluate the influence of biological turnover, tissue type, and sampling methods. We collected 87Sr/86Sr data from the elephant's tusk and molar enamel using in-situ laser ablation microsampling, conventional drilling, and micromilling techniques. Our data comparisons show that the innermost enamel best records the primary 87Sr/86Sr turnover history, while enamel maturation affects outer enamel 87Sr/86Sr. Conventional drilling and micromilling show attenuated 87Sr/86Sr records due to enamel maturation and sample averaging. To effectively account for 87Sr/86Sr signal attenuation due to turnover, we demonstrated a microsampling-modeling workflow using our tusk and enamel data. This recommended workflow can facilitate geospatially explicit interpretations of seasonal migration.

Strontium isotope and trajectory method elucidating overseas migration of Mythimna separata to Japan

The oriental armyworm, Mythimna separata, generally migrates from eastern to northeastern China in early summer, and some individuals are believed to migrate overseas to Japan depending on meteorological conditions. This potential migration was investigated with the immigrants' strontium radiogenic isotope ratio 87Sr/86Sr and backward flight trajectories from Japanese trapping sites. The results showed that the 87Sr/86Sr ratios of Chinese reared M. separata were significantly higher than those of reared insects of Japanese immigration areas. As some individuals trapped in western Japan had 87Sr/86Sr ratios higher than a statistical discriminating ratio, they likely originated in China. Trajectory analysis also indicated those individuals might have originated from the East Asian continent, such as the first-generation outbreak region in China and their migration waypoint regions. Our analysis, thus, suggests direct or multistep overseas migration of individual M. separata from the East Asian continent to Japan, giving insight into migration pathways and population dynamics.

Strontium isotopes track female dispersal in Taï chimpanzees

OBJECTIVES

Chimpanzees (Pan troglodytes) are patrilocal, with males remaining in their natal community and females dispersing when they reach sexual maturity. However, the details of female chimpanzee dispersal, such as their possible origin, are difficult to assess, even in habituated communities. This study investigates the utility of 87Sr/86Sr analysis for (1) assessing Sr baseline differences between chimpanzee territories and (2) identifying the status (immigrant or natal) of females of unknown origin within the territories of five neighboring communities in Taï National Park (Côte d'Ivoire).

MATERIALS AND METHODS

To create a local Sr isoscape for the Taï Chimpanzee Project (TCP) study area, we sampled environmental samples from TCP-established territories (n = 35). To assess dispersal patterns, 34 tooth enamel samples (one per individual) were selected from the Taï chimpanzee skeletal collection. 87Sr/86Sr analysis was performed on all 69 samples at the W.M. Keck Lab. The theoretical density and overlap of chimpanzee communities as well as generalized linear mixed models (GLMMs) were used to test each question.

RESULTS

87Sr/86Sr ratios for natal male chimpanzees ranged from 0.71662 to 0.72187, which is well within the corresponding environmental baseline range of 0.70774-0.73460. The local Sr isoscapes fit was estimated with the root-mean-square error value, which was 0.0048 (22% of the whole 87Sr/86Sr data range). GLMMs identified significant differences in 87Sr/86Sr ratios between natal and unknown North community origin groups, suggesting that after 1980, females of unknown origin could be immigrants to North community (n = 7, z-ratio = -4.08, p = 0.0001, power = 0.94).

DISCUSSION

This study indicates that 87Sr/86This study indicates that 87Sr/86Sr analysis can successfully identify immigrant females in skeletal collections obtained from wild chimpanzee communities, enabling the tracking of female dispersal patterns historically. There are, however, significant limitations within the scope of this study, such as (1) the absence of reliable maps for the TCP study area, (2) limited capacity for environmental sampling, (3) small sample sizes, and (4) tooth formation in wild chimpanzees.

Study on the material source and enrichment mechanism of REE-rich phosphorite in Zhijin, Guizhou

Rare earth element (REE)-rich phosphorite in the Guizhou region mainly exists in the Doushantuo Formation and Gezhongwu Formation in early Cambrian strata, which are some of the important strata containing phosphorite resources in China. The early Cambrian Zhijin phosphorite in Guizhou Province, China, has high rare earth element and yttrium (REY) contents of up to 2500 ppm, with heavy REY (HREY) contents accounting for ~ 30% of the total REY contents. However, the specific controlling source and environment of phosphorite (especially the REEs in Zhijin phosphorite) are still unsolved. Through field geological investigations; mineralogical, geochemical, Sr-Nd isotope analyses; and tectonic characteristics, the material source, sedimentary environment and seawater dynamics of REEs in phosphorite are studied. It is considered that the REEs enriched in the Zhijin phosphorite are mainly affected by precipitation from hydrothermal fluid. Moreover, from the late Ediacaran to the early Cambrian, the depositional environment from the bottom to the top of the water tended to be hypoxic, and the activity of hot water fluid tended to be strong. The change in redox conditions is closely related to the rise and fall of sea level. Combined with the tectonic background, these results show that the weakly oxidized environment may be an important factor controlling the enrichment of REEs. The enrichment of REEs may be closely related to volcanic hydrothermal activity, later diagenesis and seawater dynamics.

Investigating strontium isotope linkage between biominerals (uroliths), drinking water and environmental matrices

This study presents the mineralogy and strontium isotope ratio (87Sr/86Sr) of 21 pathological biominerals (bladder and kidney stones) collected from patients admitted between 2018 and 2020 at the Department of Urology of the San Pio Hospital (Benevento, southern Italy). Urinary stones belong to the calcium oxalate, purine or calcium phosphate mineralogy types. Their corresponding 87Sr/86Sr range from 0.707607 for an uricite sample to 0.709970 for a weddellite one, and seem to be partly discriminated based on the mineralogy. The comparison with the isotope characteristics of 38 representative Italian bottled and tap drinking waters show a general overlap in 87Sr/86Sr with the biominerals. However, on a smaller geographic area (Campania Region), we observe small 87Sr/86Sr differences between the biominerals and local waters. This may be explained by external Sr inputs for example from agriculture practices, inhaled aerosols (i.e., particulate matter), animal manure and sewage, non-regional foods. Nevertheless, biominerals of patients that stated to drink and eat local water/wines and foods every day exhibited a narrower 87Sr/86Sr range roughly matching the typical isotope ratios of local geological materials and waters, as well as those of archaeological biominerals from the same area. Finally, we conclude that the strontium isotope signature of urinary stones may reflect that of the environmental matrices surrounding patients, but future investigations are recommended to ultimately establish the potential for pathological biominerals as reliable biomonitoring proxies, taking into the account the contribution of the external sources of Sr.

Comparison between strip sampling and laser ablation methods to infer seasonal movements from intra-tooth strontium isotopes profiles in migratory caribou

Strontium isotopes analysis is a powerful tool in the study of past animal movements, notably the sequential analysis of tooth enamel to reconstruct individual movements in a time-series. Compared to traditional solution analysis, high resolution sampling using laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has the potential to reflect fine scale mobility. However, the averaging of the ⁸⁷Sr/⁸⁶Sr intake during the enamel mineralization process may limit fine scale inferences. We compared solution and LA-MC-ICP-MS ⁸⁷Sr/⁸⁶Sr intra-tooth profiles from the second and third molars of 5 caribou from the Western Arctic herd, Alaska. Profiles from both methods showed similar trends, reflecting the seasonal migratory movements, but LA-MC-ICP-MS profiles showed a less damped ⁸⁷Sr/⁸⁶Sr signal than solution profiles. Geographic assignments of the profile endmembers to the known summer and winter ranges were consistent between methods and with the expected timing of enamel formation but showed discrepancy at a finer scale. Variations on LA-MC-ICP-MS profiles, consistent with expected seasonal movements, suggested more than an admixture of the endmember values. However, more work in understanding enamel formation in Rangifer, and other ungulates, and how ⁸⁷Sr/⁸⁶Sr daily intake translates into enamel are needed to assess the real resolution that can be achieved with LA-MC-ICP-MS.

Male mastodon landscape use changed with maturation (late Pleistocene, North America)

Fossil remains usually reveal little about lifetime landscape use beyond place of death, but ever-growing tusks of American mastodons (Mammut americanum) record this fundamental aspect of paleobiology. Using oxygen and strontium isotopes from a serially sampled male mastodon tusk, we reconstruct changing patterns of landscape use during his life. We find clear shifts in landscape use during adolescence and following maturation to adulthood, including increased monthly movements and development of a summer-only range and mating ground. The mastodon died in his inferred summer mating ground, far from landscapes used during other seasons. Mastodons had long gestation times, and late Pleistocene populations lived in harsh, rapidly changing environments. Seasonal landscape use and migration were likely critical for maximizing mastodon reproductive success.

Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use were likely critical for reproductive success of mastodons (Mammut americanum) and other megafauna. However, little is known about how their geographic ranges and mobility fluctuated seasonally or changed with sexual maturity. We used a spatially explicit movement model that coupled strontium and oxygen isotopes from two serially sampled intervals (5+ adolescent years and 3+ adult years) in a male mastodon tusk to test for changes in landscape use associated with maturation and reproductive phenology. The mastodon’s early adolescent home range was geographically restricted, with no evidence of seasonal preferences. Following inferred separation from the matriarchal herd (starting age 12 y), the adolescent male’s mobility increased as landscape use expanded away from his natal home range (likely central Indiana). As an adult, the mastodon’s monthly movements increased further. Landscape use also became seasonally structured, with some areas, including northeast Indiana, used only during the inferred mastodon mating season (spring/summer). The mastodon died in this area (>150 km from his core, nonsummer range) after sustaining a craniofacial injury consistent with a fatal blow from a competing male’s tusk during a battle over access to mates. Northeast Indiana was likely a preferred mating area for this individual and may have been regionally significant for late Pleistocene mastodons. Similarities between mammutids and elephantids in herd structure, tusk dimorphism, tusk function, and the geographic component of male maturation indicate that these traits were likely inherited from a common ancestor.

A bioavailable strontium (87Sr/86Sr) isoscape for Aotearoa New Zealand: Implications for food forensics and biosecurity

As people, animals and materials are transported across increasingly large distances in a globalized world, threats to our biosecurity and food security are rising. Aotearoa New Zealand is an island nation with many endemic species, a strong local agricultural industry, and a need to protect these from pest threats, as well as the economy from fraudulent commodities. Mitigation of such threats is much more effective if their origins and pathways for entry are understood. We propose that this may be addressed in Aotearoa using strontium isotope analysis of both pests and products. Bioavailable radiogenic isotopes of strontium are ubiquitous markers of provenance that are increasingly used to trace the origin of animals and plants as well as products, but currently a baseline map across Aotearoa is lacking, preventing use of this technique. Here, we have improved an existing methodology to develop a regional bioavailable strontium isoscape using the best available geospatial datasets for Aotearoa. The isoscape explains 53% of the variation (R² = 0.53 and RMSE = 0.00098) across the region, for which the primary drivers are the underlying geology, soil pH, and aerosol deposition (dust and sea salt). We tested the potential of this model to determine the origin of cow milk produced across Aotearoa. Predictions for cow milk (n = 33) highlighted all potential origin locations that share similar ⁸⁷Sr/⁸⁶Sr values, with the closest predictions averaging 7.05 km away from their true place of origin. These results demonstrate that this bioavailable strontium isoscape is effective for tracing locally produced agricultural products in Aotearoa. Accordingly, it could be used to certify the origin of Aotearoa’s products, while also helping to determine if new pest detections were of locally breeding populations or not, or to raise awareness of imported illegal agricultural products.

Determination of Sr–Nd–Pb Isotopic Ratios of Rock Reference Materials Using Column Separation Techniques and TIMS

Thermal ionization mass spectrometry (TIMS) can provide highly accurate strontium (Sr), neodymium (Nd), and lead (Pb) isotope measurements for geological and environmental samples. Traces of these isotopes are useful for understanding crustal reworking and growth. In this study, we conducted a sequential separation of Sr, Nd, and Pb and subsequently measured the 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 13 widely used rock certified reference materials (CRMs), namely BCR-2, BHVO-2, GSP-2, JG-1a, HISS-1, JLk-1, JSd-1, JSd-2, JSd-3, LKSD-1, MAG-1, SGR-1, and 4353A, using TIMS. In particular, we reported the first isotopic ratios of Sr, Nd, and Pb in 4353A, Sr and Nd in HISS-1 and SGR-1, and Sr in JLk-1, JSd-2, JSd-3, and LKSD-1. The Sr–Nd–Pb isotopic compositions of most in-house CRMs were indistinguishable from previously reported values, although the Sr and Pb isotopic ratios of GSP-2, JSd-2, JSd-3, and LKSD-1 obtained in different aliquots and/or batches varied slightly. Hence, these rock reference materials can be used for monitoring the sample accuracy and assessing the quality of Sr–Nd–Pb isotope analyses.

Silver Linings at the Dawn of a “Golden Age”

Nearly four decades after the first applications of strontium isotope analyses in archaeology and paleoecology research, it could be said that we are entering a “Golden Age”. Here, we reflect on major past developments and current strengths in strontium isotope research, as well as speculate on future directions. We review (1) the currently limited number of (but much needed) controlled feeding experiments, (2) recent advances in isoscape mapping and spatial assignment, and (3) the strength of multi-proxy approaches (including both the integration of strontium isotopes with other isotope systems and complementary techniques such as ancient DNA analyses). We also explore the integration of strontium isotope research with other types of paleoecological or archaeology data, as well as with evidence and interpretative frameworks from other fields (such as conservation ecology, conservation paleobiology or history). This blending is critical as we seek to advance the field beyond simply distinguishing local or relatively sedentary individuals from those that were non-local or highly mobile. We finish with a call for future research centered on balancing methodological developments and novel applications with critical self-reflection, deeper theoretical considerations and cross-disciplinarity.

Triple sulfur-oxygen-strontium isotopes probabilistic geographic assignment of archaeological remains using a novel sulfur isoscape of western Europe

Sulfur isotope composition of organic tissues is a commonly used tool for gathering information about provenance and diet in archaeology and paleoecology. However, the lack of maps predicting sulfur isotope variations on the landscape limits the possibility to use this isotopic system in quantitative geographic assignments. We compiled a database of 2,680 sulfur isotope analyses in the collagen of archaeological human and animal teeth from 221 individual locations across Western Europe. We used this isotopic compilation and remote sensing data to apply a multivariate machine-learning regression, and to predict sulfur isotope variations across Western Europe. The resulting model shows that sulfur isotope patterns are highly predictable, with 65% of sulfur isotope variations explained using only 4 variables representing marine sulfate deposition and local geological conditions. We used this novel sulfur isoscape and existing strontium and oxygen isoscapes of Western Europe to apply triple isotopes continuous-surface probabilistic geographic assignments to assess the origin of a series of teeth from local animals and humans from Brittany. We accurately and precisely constrained the origin of these individuals to limited regions of Brittany. This approach is broadly transferable to studies in archaeology and paleoecology as illustrated in a companion paper (Colleter et al. 2021).

A strontium isoscape for the Conchucos region of highland Peru and its application to Andean archaeology

Strontium isotope (⁸⁷Sr/⁸⁶Sr) analysis of human skeletal remains is an important method in archaeology to examine past human mobility and landscape use. ⁸⁷Sr/⁸⁶Sr signatures of a given location are largely determined by the underlying bedrock, and these geology specific isotope signatures are incorporated into skeletal tissue through food and water, often permitting the differentiation of local and non-local individuals in past human populations. This study presents the results of a systematic survey of modern flora and fauna (n = 100) from 14 locations to map the bioavailable ⁸⁷Sr/⁸⁶Sr signatures of the Conchucos region, an area where the extent of geologic variability was previously unknown. We illustrate the necessity to examine the variation in ⁸⁷Sr/⁸⁶Sr values of the different geological formations available to human land use to document the range of possible local ⁸⁷Sr/⁸⁶Sr values. Within the Conchucos region we found significant variation in environmental ⁸⁷Sr/⁸⁶Sr values (0.7078–0.7214). The resulting isoscape represents the largest regionally specific bioavailable ⁸⁷Sr/⁸⁶Sr map (3,840 km²) to date for the Andes, and will serve as a baseline for future archaeological studies of human mobility in this part of the Peruvian highlands.

Re-evaluating Scythian lifeways: Isotopic analysis of diet and mobility in Iron Age Ukraine

The Scythians are frequently presented, in popular and academic thought alike, as highly mobile warrior nomads who posed a great economic risk to growing Mediterranean empires from the Iron Age into the Classical period. Archaeological studies provide evidence of first millennium BCE urbanism in the steppe while historical texts reference steppe agriculture, challenging traditional characterizations of Scythians as nomads. However, there have been few direct studies of the diet and mobility of populations living in the Pontic steppe and forest-steppe during the Scythian era. Here, we analyse strontium, oxygen, and carbon isotope data from human tooth enamel samples, as well as nitrogen and carbon isotope data of bone collagen, at several Iron Age sites across Ukraine commonly associated with ‘Scythian’ era communities. Our multi-isotopic approach demonstrates generally low levels of human mobility in the vicinity of urban locales, where populations engaged in agro-pastoralism focused primarily on millet agriculture. Some individuals show evidence for long-distance mobility, likely associated with significant inter-regional connections. We argue that this pattern supports economic diversity of urban locales and complex trading networks, rather than a homogeneous nomadic population.

Monthly mobility inferred from isoscapes and laser ablation strontium isotope ratios in caprine tooth enamel

Strontium isotopic analysis of sequentially formed tissues, such as tooth enamel, is commonly used to study provenance and mobility of humans and animals. However, the potential of ⁸⁷Sr/⁸⁶Sr in tooth enamel to track high-frequency movements has not yet been established, in part due to the lack of data on modern animals of known movement and predictive model of isotope variation across the landscape. To tackle this issue, we measured the ⁸⁷Sr/⁸⁶Sr in plant samples taken from a 2000 km² area in the Altai Mountains (Mongolia), and the ⁸⁷Sr/⁸⁶Sr in tooth enamel of domestic caprines whose mobility was monitored using GPS tracking. We show that high-resolution, sequential profiles of strontium isotope composition of tooth enamel reliably reflect the high-frequency mobility of domestic livestock and that short-term residency of about 45 days can be resolved. This offers new perspectives in various disciplines, including forensics, ecology, palaeoanthropology, and bioarchaeology.

Historical Landscape Use of Migratory Caribou: New Insights From Old Antlers

Accumulations of shed caribou antlers (Rangifer tarandus) are valuable resources for expanding the temporal scope with which we evaluate seasonal landscape use of herds. Female caribou shed their antlers within days of giving birth, thus marking calving ground locations. Antler geochemistry (87Sr/86Sr) reflects the isotopic signature of regions used during antler growth, thereby providing data on a second component of seasonal landscape use. Here, we evaluate shed caribou antlers from the Coastal Plain of the Arctic National Wildlife Refuge, Alaska. The Central and Eastern regions of the Coastal Plain are calving grounds for the Porcupine Caribou Herd, while the Western Coastal Plain supports calving by the Central Arctic Herd. We found that antler 87Sr/86Sr from the Central and Eastern Coastal Plain were isotopically indistinguishable, while antler 87Sr/86Sr from the Western Coastal Plain was significantly smaller. For each region, we compared isotopic data for “recent” antlers, which overlap the bulk of standardized state and federal caribou monitoring (early 1980s and younger), with “historical” antlers shed in years predating these records (from the 1300s to the 1970s). For Porcupine Herd females calving in the Arctic Refuge, comparisons of antler 87Sr/86Sr through time indicate that summer ranges have been consistent since at least the 1960s. However, changes between historical and recent antler 87Sr/86Sr for the Central Arctic Herd indicate a shift in summer landscape use after the late 1970s. The timing of this shift is coincident with multiple factors including increased infrastructural development in their range related to hydrocarbon extraction. Accumulations of shed caribou antlers and their isotope geochemistry extend modern datasets by decades to centuries and provide valuable baseline data for evaluating potential anthropogenic and other influences on caribou migration and landscape use.

Sr isotope composition of Golden Delicious apples in Northern Italy reflects the soil 87 Sr/86 Sr ratio of the cultivation area

BACKGROUND

Apples have a leading role in the Italian fruit sector, and high-quality apples, including the Golden Delicious variety, are cultivated mainly in the Northern mountain districts. In the present study, Golden Delicious apples from PDO (Protected Designation of Origin) and PGI (Protected Geographical Indication) cultivation districts were characterized according to their Sr isotope composition and compared with apples from other Northern Italian districts.

RESULTS

Apples collected in two consecutive years (2017 and 2018) confirmed the low annual variability of the ⁸⁷ Sr/⁸⁶ Sr ratio. The isotope ratio of apples was highly correlated with that of the soil extracts of the respective orchards. Statistical differences were highlighted between cultivation districts. However, because similar geological features characterized some areas, their ratios overlapped and a complete separation of the districts was not possible.

CONCLUSION

The ⁸⁷ Sr/⁸⁶ Sr ratio is an excellent marker for studies of food traceability because it retains the information about the place of origin. However, its strength is limited when comparing products from cultivation areas sharing similar geological features. In the perspective of geographical traceability, a multichemical characterization can overcome the limits of single-parameter approach. © 2020 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A strontium isotope baseline of Cyprus. Assessing the use of soil leachates, plants, groundwater and surface water as proxies for the local range of bioavailable strontium isotope composition

In this study, a strontium isotope baseline for Cyprus is presented. The aim of the study was two-fold; first to provide an environmental multi-proxy-based baseline (water/plants/soil leachates) suitable for archaeological provenance and mobility studies, food source authentication, and forensic investigations; and second, to contribute to the debate around which proxy (or combination of proxies) might be most suitable to define bioavailable fractions of strontium in geologically complex areas also exposed to sea-spray and other Sr-bearing aerosols. Lowest bioavailable strontium isotope signatures range is found within terranes dominated by ophiolites, where ⁸⁷Sr/⁸⁶Sr ratios range from 0.7055 to 0.7081, however, results reveal a high degree of variability in bioavailable ⁸⁷Sr/⁸⁶Sr ratios, both spatially, along depth profiles and amongst the different proxies. A narrower range of bioavailable Sr isotope signatures is observed within the Circum Troodos Sedimentary Successions (C.T.S.S.), both in spatial distribution and between different proxies. Observed range is ⁸⁷Sr/⁸⁶Sr = 0.7079 to 0.7089 in areas dominated by pre-Quaternary C.T.S.S., and ⁸⁷Sr/⁸⁶Sr ratios = 0.7076 to 0.7086 in areas covered by Quaternary C.T.S.S., revealing the lithologies to be very homogenous with respect to bioavailable strontium ratios. Intra-site variations in three archaeological sites (multiple samples from each site from within a 500 m radius) within the pre-Quaternary and Quaternary C.T.S.S. are smaller than inter-site variations, suggesting that tracing studies inferred from baselines sampled within a limited spatial area could lead to erroneous conclusions regarding provenance. The study points to the necessity for conducting multi-proxy, spatially extensive sampling to adequately characterize complex geological areas, if these should serve as reliable reference areas in provenance studies.

Distribution of Trace Elements, Sr-C Isotopes, and Sedimentary Characteristics as Paleoenvironmental Indicator of the Late Permian Linxi Formation in the Linxi Area, Eastern Inner Mongolia

The Late Permian on the periphery of the Songliao Basin, eastern Inner Mongolia, is an important hydrocarbon source rock system. Its sedimentary environment plays an important role in the evaluation of hydrocarbon prospects in the area. Unfortunately, until now, the interpretation of the sedimentary environment of this area has been controversial. We investigated the Late Permian sedimentary environment by studying the sedimentary characteristics and geochemistry. Based on these investigations, we conclude that the Linxi Formation is mainly composed of clastic sediments, interbedded with limestone lenses, with bioherm limestone at the top of the formation. Inner-layer marine fossils (calcium algae, bryozoans, and sponges) and freshwater and blackish water microfossils (bivalves) are all present, indicative of a typical shallow water sedimentary environment with an open and concussion background. In terms of geochemistry, the formation is relatively light rare Earth enriched, with significant positive Eu anomaly, slight positive La and Y anomaly, weak positive Gd anomaly, and lack of Ce anomaly. The average B/Ga ratio of the mudstone is greater than 3.3, and the average Sr/Ba ratio of the limestone is greater than 1.0. The range of the 87Sr/86Sr ratio is from 0.707285 to 0.707953. The range of δ13C values is from −4.0‰ to 2.4‰. The sediment assemblages, rare Earth elements, trace elements, and 87Sr/86Sr and C isotopes of the formation indicate that the Linxi Formation formed in a marine sedimentary environment and occasional marine-terrestrial transitional facies. The formation can be further divided into littoral facies, neritic facies, bathyal facies, and delta front.

Does faecal matter reflect location? An initial assessment of isotopic variability between consumed prey remains and faecal matter for wild jaguars. ISOTOPES IN ENVIRONMENTAL AND HEALTH STUDIES 2019;55:478-498. [PMID: 31402710 DOI: 10.1080/10256016.2019.1648263]

Faecal isotopic analysis may complement other non-invasive wildlife survey tools for monitoring landscape use by carnivores, such as motion-detecting cameras and non-invasive genetic sampling. We analysed carbon, nitrogen, and strontium isotopes in faecal matter produced by jaguars (Panthera onca) as well as bones from consumed prey at the Mountain Pine Ridge Forest Reserve (MPR) in Belize, Central America. The MPR is ideally suited for a spatial isotope study as vegetation and geology both vary considerably. The isotopic composition of faecal matter should reflect the habitat and geology where consumed prey lived. We used bone from consumed prey recovered from jaguar scats as a proxy for diet. Faecal matter and bone showed comparable spatial isotopic trends, suggesting that the isotopic composition of jaguar faeces can be used to detect foraging in different habitats (pine forest versus broadleaf forest) or on different geologies (Mesozoic carbonates; Palaeozoic granite, contact metamorphics, and metasediments). This result is reassuring as bones are not always present in carnivore scats. Studying landscape use by cryptic and wide-ranging carnivore species like jaguars remains challenging. Isotopic analysis of faecal matter complements the existing array of non-invasive spatial monitoring tools.

Intra- and Intertree Variability of the 87Sr/86Sr Ratio in Apple Orchards and Its Correlation with the Soil 87Sr/86Sr Ratio

The ⁸⁷Sr/⁸⁶Sr ratio of horticultural products mostly derives from that of the bioavailable Sr fraction of the soil where they grow and, therefore, varies according to the local geolithological features. This study investigated the intra- and intertree variability of the ⁸⁷Sr/⁸⁶Sr ratio in two apple orchards in South Tyrol and its relation with the soil ⁸⁷Sr/⁸⁶Sr ratio. In both orchards, a moderate homogeneity of the ⁸⁷Sr/⁸⁶Sr ratio was observed among subsamples of the same tree part (shoot axes, leaves, apple peels, and pulps). Moreover, the ⁸⁷Sr/⁸⁶Sr ratio homogeneity among tree parts was high intratree and low intertree. The variability of the ⁸⁷Sr/⁸⁶Sr ratio within the tree and within the orchard is explained in light of the ⁸⁷Sr/⁸⁶Sr ratios of the soil. This ⁸⁷Sr/⁸⁶Sr variability within orchards does not preclude its use as a geographical tracer; however, this aspect should be evaluated to correctly design a sampling campaign or to generalize the results.

Recent applications of isotope analysis to forensic anthropology

Isotope analysis has become an increasingly valuable tool in forensic anthropology casework over the past decade. Modern-day isotopic investigations on human remains have integrated the use of multi-isotope profiles (e.g. C, N, O, H, S, Sr, and Pb) as well as isotopic landscapes (“isoscapes”) from multiple body tissues (e.g. teeth, bone, hair, and nails) to predict possible region-of-origin of unidentified human remains. Together, data from various isotope analyses provide additional lines of evidence for human identification, including a decedent’s possible region-of-birth, long-term adult residence, recent travel history, and dietary choices. Here, we present the basic principles of isotope analysis and provide a brief overview of instrumentation, analytical standards, sample selection, and sample quality measures. Finally, we present case studies that reflect the diverse applications of isotope analysis to the medicolegal system before describing some future research directions. As shown herein, isotope analysis is a flexible and powerful geolocation tool that can provide new investigative leads for unidentified human remains cases.

Intra- and inter-individual variability of stable strontium isotope ratios in hard and soft body tissues of pigs

RATIONALE

Stable isotope analysis has become an indispensable tool for the identification of the flow of matter in ecological sciences. Stable strontium isotope ratios are used in several different scientific fields as markers for provenance, mobility, and migration. A crucial prerequisite for such studies is a reliable definition of local ranges and an estimation of expected isotopic variability.

METHODS

To study the local variability of stable strontium isotope ratios at a specific place, a farrow of ecologically and locally grown pigs was investigated and strontium was extracted from their hard tissues (tooth enamel, bones and bristles) as well as soft tissues (meat, organs and blood) via dry- and wet-ashing procedures and subsequent column separation. The ⁸⁷ Sr/⁸⁶ Sr ratio was then measured via TIMS (thermal ionization mass spectrometry).

RESULTS

Reliable ⁸⁷ Sr/⁸⁶ Sr data was generated from all tissue types. Both intra- and inter-individual variability was high and tissue-specific patterns could be identified. High ranges occurred in sample types which were analyzed for animals of different age (pigs as well as piglets): teeth, parietal, mandible, femur, and leg muscle as well as in jaw muscle samples, which were only analyzed in adult pigs. Tooth enamel data showed a tooth-type-dependent pattern.

CONCLUSIONS

We conclude that a time factor played a role for the observed variability, e.g. a shift in food supply. Furthermore, we presume that local range estimates from the literature only work for tissues which integrate all consumed components over a sufficient amount of time and average fluctuations which can be found in tissues with higher turnover rates.

Modelling strontium isotopes in past biospheres - Assessment of bioavailable 87Sr/86Sr ratios in local archaeological vertebrates based on environmental signatures

⁸⁷Sr/⁸⁶Sr isotopic ratios in skeletal remains of archaeological vertebrates are used for provenance analysis since long. However, the definition of the past bioavailable isotopic ratio at the site of recovery is not known beforehand and geological maps can provide no more than gross expectations. Therefore, the assessment of the "local Sr isotopic signature" is still of crucial importance. In this study, we present a tool for the prediction of such local isotopic signatures by creating a concentration weighted mixing model that links lithospheric, biospheric, and atmospheric strontium per site. The major strontium sources and their input into an animal's body were assessed by choosing elemental strontium and its isotopic signature in groundwater, soil, vegetation, and precipitation as components for the mixing model, augmented by literature values. The model was applied to 24 sites located in the alpine transect of the Inn-Eisack-Adige-Brenner passage across the European Alps, a passage used since the Mesolithic. Predicted local bioavailable ⁸⁷Sr/⁸⁶Sr ratios were compared with measured values from locally excavated archaeozoological bone samples from three taxa of large and mainly residential vertebrates (cattle, pig, red deer) to verify the models' accuracy. With regard to the fact that the environmental samples predict the past local bioavailable ⁸⁷Sr/⁸⁶Sr at a specific site while the vertebrates had different and species-specific home ranges, thereby integrating strontium from a region of primarily unknown size, the model is capable of assigning reasonable expectation values. For 11 sites, up to 100% of the vertebrate isotopic signatures were correctly predicted. Mismatches at the remaining sites are explainable by special environmental factors, and also the fact that some import of animals can never be excluded beforehand. Suggestions for site-specific adjustments of the model are made.

Multi-isotope provenancing of archaeological skeletons including cremations in a reference area of the European Alps

RATIONALE

Due to the spatial heterogeneity of stable isotope ratios of single elements measured in attempts to georeference bioarchaeological finds, multi-isotope fingerprints are frequently employed under the assumption that similar isotopic signatures are indicative of similar shared environments by the individuals studied. The extraction of the spatial information from multi-isotope datasets, however, is challenging.

METHODS

Gaussian mixture clustering of six- to seven-dimensional isotopic fingerprints measured in archaeological animal and human bones was performed. Uncremated animal bones served for an isotopic mapping of a specific reference area of eminent archaeological importance, namely the Inn-Eisack-Adige passage across the European Alps. The fingerprints consist of ⁸⁷ Sr/⁸⁶ Sr, ²⁰⁸ Pb/²⁰⁴ Pb, ²⁰⁷ Pb/²⁰⁴ Pb, ²⁰⁶ Pb/²⁰⁴ Pb, ²⁰⁸ Pb/²⁰⁷ Pb, and ²⁰⁶ Pb/²⁰⁷ Pb ratios, and δ¹⁸ O_phosphate values in uncremated bone apatite, while the thermally unstable δ¹⁸ O values of human cremations from this region were discarded.

RESULTS

The bone finds were successfully decontaminated. Animal and human isotope clusters not only reflect individual similarities in the multi-isotopic fingerprints, but also permit a spatial allocation of the finds. This holds also for cremated finds where the δ¹⁸ O_phosphate value is no longer informative. To our knowledge, for the first time Pb stable isotopes have been systematically studied in cremated skeletal remains and proved significant in a region that was sought after for its ore deposits in prehistory.

CONCLUSIONS

Gaussian mixture clustering is a promising method for the interpretation of multi-isotopic fingerprints aiming at detecting and quantifying migration and trade.

Isotope values of the bioavailable strontium in inland southwestern Sweden-A baseline for mobility studies

The inland area of southwestern Sweden is well known for its well-preserved archaeological animal and human remains dating back to the Mesolithic and Neolithic (10000–4000 and 4000–1700 BC). They allow application of multiple bioarchaeological methods, giving insights into various and complementary aspects of prehistoric human life, as well as economic and social structures. One important aspect concerns human mobility and its relation to social networks and to circulation of objects. Here, strontium isotope analysis plays a crucial role. The present study aims to construct a strontium isotope baseline of southwestern Sweden with considerably greater coverage and higher resolution than previously published data. As the region has been affected by glacial events, the relation between bedrock geology and isotope signals of the bioavailable strontium in such areas is given special attention. We determined strontium isotope ratios for 61 water and five archaeological animal samples, and combined the data with previous measurements of two water and 21 non-domestic faunal samples. The results reveal a complex pattern. Several areas with distinct baseline ranges can be distinguished, although with overlaps between some of them. Overall, the bioavailable strontium isotope signals mirror the basement geology of the region. The highest ratios occur in the geologically oldest eastern parts of the Precambrian terrain, while lower ratios are found in the western part, and the lowest ratios occur in the youngest Paleozoic areas. At the same time, there are minor deviations compared to the underlying bedrock, due to glacial transport, overlying sediments, and local intrusions of younger rocks. The background data set now available allows for more nuanced and detailed interpretations of human and animal mobility in the region, in particular by identification of subregions with differing strontium isotope ratios within the Precambrian province. Also, we can now identify long distance mobility with greater confidence.

Applying the principles of isotope analysis in plant and animal ecology to forensic science in the Americas

The heart of forensic science is application of the scientific method and analytical approaches to answer questions central to solving a crime: Who, What, When, Where, and How. Forensic practitioners use fundamentals of chemistry and physics to examine evidence and infer its origin. In this regard, ecological researchers have had a significant impact on forensic science through the development and application of a specialized measurement technique-isotope analysis-for examining evidence. Here, we review the utility of isotope analysis in forensic settings from an ecological perspective, concentrating on work from the Americas completed within the last three decades. Our primary focus is on combining plant and animal physiological models with isotope analyses for source inference. Examples of the forensic application of isotopes-including stable isotopes, radiogenic isotopes, and radioisotopes-span from cotton used in counterfeit bills to anthrax shipped through the U.S. Postal Service and from beer adulterated with cheap adjuncts to human remains discovered in shallow graves. Recent methodological developments and the generation of isotope landscapes, or isoscapes, for data interpretation promise that isotope analysis will be a useful tool in ecological and forensic studies for decades to come.

A bioavailable strontium isoscape for Western Europe: A machine learning approach

Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) are gaining considerable interest as a geolocation tool and are now widely applied in archaeology, ecology, and forensic research. However, their application for provenance requires the development of baseline models predicting surficial ⁸⁷Sr/⁸⁶Sr variations (“isoscapes”). A variety of empirically-based and process-based models have been proposed to build terrestrial ⁸⁷Sr/⁸⁶Sr isoscapes but, in their current forms, those models are not mature enough to be integrated with continuous-probability surface models used in geographic assignment. In this study, we aim to overcome those limitations and to predict ⁸⁷Sr/⁸⁶Sr variations across Western Europe by combining process-based models and a series of remote-sensing geospatial products into a regression framework. We find that random forest regression significantly outperforms other commonly used regression and interpolation methods, and efficiently predicts the multi-scale patterning of ⁸⁷Sr/⁸⁶Sr variations by accounting for geological, geomorphological and atmospheric controls. Random forest regression also provides an easily interpretable and flexible framework to integrate different types of environmental auxiliary variables required to model the multi-scale patterning of ⁸⁷Sr/⁸⁶Sr variability. The method is transferable to different scales and resolutions and can be applied to the large collection of geospatial data available at local and global levels. The isoscape generated in this study provides the most accurate ⁸⁷Sr/⁸⁶Sr predictions in bioavailable strontium for Western Europe (R² = 0.58 and RMSE = 0.0023) to date, as well as a conservative estimate of spatial uncertainty by applying quantile regression forest. We anticipate that the method presented in this study combined with the growing numbers of bioavailable ⁸⁷Sr/⁸⁶Sr data and satellite geospatial products will extend the applicability of the ⁸⁷Sr/⁸⁶Sr geo-profiling tool in provenance applications.

Strontium isotopic aspects of Paranthropus robustus teeth; implications for habitat, residence, and growth

The strontium isotopic ratio ⁸⁷Sr/⁸⁶Sr has been studied in the Sterkfontein Valley of South Africa to infer both habitat usage and residence for a number of early hominins. This paper examines the existing ⁸⁷Sr/⁸⁶Sr data collectively derived from three studies of Paranthropus robustus teeth with the aim of exploring whether the dataset as a whole may provide deeper insight into habitat, mobility, and growth for this species. ⁸⁷Sr/⁸⁶Sr from seven Swartkrans Member I third molars varies in a well defined narrow range, and while some canines were consistent with this range, a number of P. robustus canines and first and second molars were not, and therefore represent individuals who had arrived from other localities. A first and third molar ⁸⁷Sr/⁸⁶Sr was found to differ in TM1517c, the holotype P. robustus specimen from Kromdraai, suggesting this individual had moved to the locality sometime after the first molar and before the third molar had completely mineralized. While early forming teeth vary widely, the relatively low variation and absence of exogenous ⁸⁷Sr/⁸⁶Sr in third molars suggest that these teeth mineralized relatively late when compared to life history events bearing on higher primate residence patterns. The implications for further study of habitat, residence, and growth are discussed.

Using Carbon, Oxygen, Strontium, and Lead Isotopes in Modern Human Teeth for Forensic Investigations: A Critical Overview Based on Data from Bulgaria

Isotopic data obtained from human remains can provide information about an individual's origin, migration, and diet. We evaluate the usefulness of carbon, oxygen, strontium, and lead isotopes for forensic investigations by comparing data from Bulgarian teeth with data from other regions. Geo-referencing based on oxygen or strontium isotopes can be misleading due to overlap with other countries in Europe and other continents. Carbon and lead isotopes, in combination with oxygen and strontium isotopes, provide the most useful information for identification of local vs foreigner status. In particular, high-precision Pb isotopes show a distinct "Bulgarian" range; however, it is possible that individuals from other countries in Eastern Europe and/or central to western Asia could have overlapping isotopic values. Additional high-precision multi-isotope data from modern humans from different regions in the world are required to transition from speculative to more quantitative estimation of a geographical place of origin for unidentified human remains.

Investigating human geographic origins using dual-isotope (87Sr/86Sr, δ18O) assignment approaches

Substantial progress in the application of multiple isotope analyses has greatly improved the ability to identify nonlocal individuals amongst archaeological populations over the past decades. More recently the development of large scale models of spatial isotopic variation (isoscapes) has contributed to improved geographic assignments of human and animal origins. Persistent challenges remain, however, in the accurate identification of individual geographic origins from skeletal isotope data in studies of human (and animal) migration and provenance. In an attempt to develop and test more standardized and quantitative approaches to geographic assignment of individual origins using isotopic data two methods, combining ⁸⁷Sr/⁸⁶Sr and δ¹⁸O isoscapes, are examined for the Circum-Caribbean region: 1) an Interval approach using a defined range of fixed isotopic variation per location; and 2) a Likelihood assignment approach using univariate and bivariate probability density functions. These two methods are tested with enamel isotope data from a modern sample of known origin from Caracas, Venezuela and further explored with two archaeological samples of unknown origin recovered from Cuba and Trinidad. The results emphasize both the potential and limitation of the different approaches. Validation tests on the known origin sample exclude most areas of the Circum-Caribbean region and correctly highlight Caracas as a possible place of origin with both approaches. The positive validation results clearly demonstrate the overall efficacy of a dual-isotope approach to geoprovenance. The accuracy and precision of geographic assignments may be further improved by better understanding of the relationships between environmental and biological isotope variation; continued development and refinement of relevant isoscapes; and the eventual incorporation of a broader array of isotope proxy data.