87Sr/86Sr Isotope Ratio as a Tool in Archaeological Investigation: Limits and Risks
Abstract
:1. Introduction
2. Strontium and Rubidium Isotopic Abundance, and Decay of
3. The Strontium and Rubidium in Minerals
3.1. General
3.2. Strontium Isotopes in Minerals and Whole Rock
3.3. Minerals as Constituents of the Rock
3.3.1. Monomineralic Consolidated/Unconsolidated Rock
3.3.2. Polymineralic Consolidated/Unconsolidated Rock
3.4. Variation in Strontium Isotopes in Different Minerals (i) and Strontium Isotopes in the Total Rock (Tot)
3.4.1. Different Decay Speed of in the Different Minerals
3.4.2. Relation of the Isotope Data for Minerals and for Whole Rock
3.5. Selective Mineral Dissolution and Its Important Role on the Strontium Isotopes Values in the Water Solution
3.6. Sorption/Desorption and Minerals
4. Strontium Isoscapes and Their Use in Archaeology
4.1. General
- (i)
- Isoscapes generally refer to large-scale grid sampling with cells in the order of tens or hundreds of km2, and in nature, variation in the strontium isotope ratio does not necessarily merge continuously from one value to another, but it may be sharp, even between neighboring sites when they are located on the boundary between different geological formations. A good example of this condition is reported by Montgomery et al. [5]. Moreover, frequently, sampling for isoscapes is not randomly distributed in the area of interest. For instance, in Italy, covering an area of 302.073 km2, Lugli et al. [28] used 1920 data of the 87Sr/86Sr ratio: on average, one sample for 157 km2; in France, Willmes et al. [29] used data referring to 840 sites for an area of 551.626 km2: one sampling grid for 657 km2! The criteria for sampling, including the sampled materials, is another important point. In the isoscape of Italy, Lugli et al. [28] include data related to ‘plant’, ‘water’, ‘biomineral’ (i.e., bones, teeth, and bio-calcareous shells), ‘food’, ‘soil’ (including both exchangeable soil fractions and bulk soils) and ‘rock’ (mainly evaporites, metamorphic and magmatic rocks, and a few sedimentary bulk rocks). Thus, at best, the most common isoscapes can only give generic indications for wide areas.
- (ii)
- As a general rule, the sampling grid used to perform isoscapes and local investigations should be the same. This, of course, is practically impossible to obtain because isoscapes are usually made at large scale, as stated above. Thus, for local investigation, scientists should use isoscapes with great caution and integrate them with values obtained from more detailed random sampling.
4.2. Archaeological Investigation and Present-Day Environmental Condition
5. Investigating and Planning Strontium Isotope Research
- (i)
- Sampling should be random to avoid bias of the data obtained on the statistical population sample. Homogenized sampling, where several samples are collected in a defined small area and then reduced to only one homogenized sample for analysis [7], in our opinion, is not a good method, because in this way, the variance in the data population for the area of interest is reduced. This could make definition of allogenous samples and comparison with other areas impossible.
- (ii)
- What does “same strontium isotope ratio” mean? Modern technology furnishes strontium isotope data with analytical uncertainty on the fifth or even on the sixth decimal digit, whereas, also for small areas (up to one km2 as an order of magnitude), isotope data, at best, may exhibit variation in the fourth decimal digit (see, for instance, [24]). If that is so, two samples could be considered as having approximately the same value if they do not differ on the fourth digit. Thus, in principle, the identification of exotic samples not belonging to the population of data related to materials coming from a defined area should consider the variability in isotope data for the area of interest.
- (iii)
- Are the sampled biological remains from the same site? The answer depends on (a) the spatial definition of “same site” and (b) how we define the belonging of biological remains to the area of interest.
- (a)
- Spatial definition depends, of course, on the aim of the investigation. In other words, we go back to the investigation scale. For instance, Cavazzuti et al. [30], in studying human settlements located in the Po plain (Northern Italy), assume that the settlements, although far, occur in a very “homogeneous” (it is not clear what they mean: isotopically homogeneous, mineralogically homogeneous, or both?) flat area without geographical barriers. They define three different areas around each settlement with a radius of 5 km (“site catchment area”), from 5 to 20 km (“immediate hinterland”), and from 20 to 50 km (“broader hinterland”) and they compare the strontium isotope values to the background of these three areal categories.
- (b)
- The belonging of human or, in general, animal remains to the area of interest depends on how we operationally define this belonging. Operatively, the minimum time of belonging may be evaluated trough the mean residence time of calcium or strontium in bones. The mean residence time, however, depends on the bone type and on the age of the individuals. For instance, the turnover for femur is about 25–30 years, whereas for ribs is about 5–10 years. Therefore, using the ribs of an individual from another area, the individual will be found to belong to the area of interest approximately 5–10 years after its arrival. Instead, using the femur, it will appear to belong to that area after 25–30 years. Thus, in a defined area, using contemporaneously data from femur and rib, the variability in the strontium data may increase. This is not sufficiently considered in the scientific papers.
- (iv)
- Before sampling, the geology and mineralogy of the area should be carefully considered to give an idea of the dominant mineral sources.
- (a)
- An accurate geological analysis suggests that, reasonably, the soil of the area of interest did not change its mineralogical and geochemical characteristics from the time of the settlement to the present. Under this condition, the isotopic prospecting of the available strontium of the present-day soil and plants is the most elementary way for determining the isotope reference background which the biological remains of the area of interest may be compared to.
- (b)
- There is evidence or suspect that the mineralogy and geochemistry of the area are not preserved; obviously, present day material cannot be used to define the geochemical background of the area. In this case, different biological remains (teeth, bones, shells, seeds, et cetera) may be used. In the event of an area of interest having no archaeological evidence (different types of burial, funerary objects, behaviour of the different animal species, etc.), suggesting a different provenance of human or animal remains, all the last ones must be considered as potentially belonging to the settlement of interest. It is evident that in this way, the variability in the isotope data could be significantly expanded, and some external individuals could be attributed to the settlement.
- (v)
- If possible, the variability in the data should be defined using a high number of analyses obtained from different individuals (indicatively, more than 15; the use of too few samples may be misleading). For each area, in case the data have normal distribution, data far from the prevalent distribution values could be identified with some statistical method. For instance:
- (vi)
- In general, we can only establish if the analyzed individuals may belong to the same group, not that they do belong to the same group. In fact, samples settled on different areas with similar geological formations exhibit the same isotopic values, even if the areas are far from one another. This happens, for example, if the individuals come from areas located on carbonate formations of a very similar geological age and with a similar genetic and diagenetic history. This is an important limit for the use of strontium isotopes alone. For example, in the Illasi valley, Lessini mountains, NE of Verona (Italy), plants grown on hydrothermalised carbonate formations from the Late Carnian to Liassic ages have very similar isotopic values (about 0.7083 ± 0.0003, our unpublished data), even if they are located many kilometers away from one another. On the contrary, samples coming from the same hypothetical locality straddling Cretaceous and Late Carnian–Liassic formations exhibit significant isotopic differences already on the fourth digit (Cretaceous carbonate, 0.7077 against Late Carnian–Liassic carbonates, 0.7083) (see also [5]).
6. Summary
- (i)
- We cannot assume that the current geochemical, mineralogical, and geological conditions of the investigated area are the same as in the past because variation in the surface conditions is frequent also during a short time.
- (ii)
- The use of large-scale isoscapes is risky because local investigation is usually performed on a smaller scale.
- (iii)
- Before studying human, animal, and plant remains, an accurate control of their diagenetic condition is essential because pollution of the samples by environmental strontium-bearing material with different isotope ratios is very easy (for instance, diagenesis with dissolution/deposition of carbonate).
- (iv)
- The samples (soil, human/animal remains, plants, etc.) should be selected randomly. Usually, this is not considered in the literature.
- (v)
- To reach a reliable scientific conclusion, the investigation of a large number of remains and related measurements is necessary. Without a large number of data, comparison between different areas is risky (statistically insignificant).
- (vi)
- If samples fall outside the prevalent distribution interval, we can state that they do not belong to the same group. The individuals falling in the prevailing distribution interval do not necessarily belong to the same group; we can only state that it is not excluded they belong to the same group.
Supplementary Materials
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A
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Minerals | Chemical Formula | Sr (ppm wt) | Rb (ppm wt) |
---|---|---|---|
Gypsum | ·O | >1000 | ≈0 |
Anhidrite | >1000 | ≈0 | |
Plagioclase | 200–1000 (1) | 5–40 (1) | |
K-felspar | 50–800 (1) | 200–800 (1) | |
Calcite | 100–700 | ≈0 | |
Aragonite | 100–1000 | ≈0 | |
Dolomite | 100–500 | ≈0 | |
Phyllosilicates (*) | Largely variable | <100 (1) | 100–2000 (1) |
Calcite (Cc) | 0.30 | 0.0800 | 0.0952 | 0.7060 | 0.7061 |
Muscovite (Mu) | 0.70 | 0.0070 | 39.11 | 0.7060 | 0.7333 |
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Rossi, M.; Iacumin, P.; Venturelli, G. 87Sr/86Sr Isotope Ratio as a Tool in Archaeological Investigation: Limits and Risks. Quaternary 2024, 7, 6. https://doi.org/10.3390/quat7010006
Rossi M, Iacumin P, Venturelli G. 87Sr/86Sr Isotope Ratio as a Tool in Archaeological Investigation: Limits and Risks. Quaternary. 2024; 7(1):6. https://doi.org/10.3390/quat7010006
Chicago/Turabian StyleRossi, Mattia, Paola Iacumin, and Gianpiero Venturelli. 2024. "87Sr/86Sr Isotope Ratio as a Tool in Archaeological Investigation: Limits and Risks" Quaternary 7, no. 1: 6. https://doi.org/10.3390/quat7010006
APA StyleRossi, M., Iacumin, P., & Venturelli, G. (2024). 87Sr/86Sr Isotope Ratio as a Tool in Archaeological Investigation: Limits and Risks. Quaternary, 7(1), 6. https://doi.org/10.3390/quat7010006