Provenance analysis of cremated skeletal remains by stable isotopes.

Cremated human remains are a rather neglected research substrate in physical anthropology; its investigation is still mainly restricted to the osteological level. The application of archaeometric methods to cremations is limited because the organic skeletal components are fully combusted at high temperatures. Stable isotope ratios of heavy elements such as strontium and lead, however, are thermally stable and permit research targeting questions of mobility, migration, and trade. In many cremations, neither dental remains nor the petrous bone are preserved. In such case, no skeletal element that retains the isotopic signature of childhood is available and compact bone has to be chosen instead. This raises interpretive problems, since due to its slow remodeling rate, compact bone integrates the element uptake over many years prior to death. This can generate a mixed isotope ratio in migrants. Such mixed ratios are no longer compatible with the place of origin, and not yet with the place of recovery. Provenance analysis with a single isotope ratio (mostly 87Sr/86Sr) therefore has its limits. A combination of strontium and lead stable isotopes in cremations generates a multi-dimensional isotopic fingerprint that is however more difficult to interpret. Data mining methods that permit a similarity search are a promising approach. In this paper, possibilities and limitations of stable isotope analysis of cremated finds are discussed together with the substrate-specific methodological and interpretive problems. The research potential is demonstrated by use of selected examples.

Which isotopes should we choose? Entropy-based feature ranking enables evaluation of the information content of stable isotopes in archaeofaunal material.

RATIONALE: Methods for multi-isotope analyses are gaining in importance in anthropological, archaeological, and ecological studies. However, when material is limited (i.e., archaeological remains), it is obligatory to decide a priori which isotopic system(s) could be omitted without losing information. METHODS: We introduce a method that enables feature ranking of isotopic systems on the basis of distance-based entropy. The feature ranking method is evaluated using Gaussian Mixture Model (GMM) clustering as well as a cluster validation index ("trace index"). RESULTS: Combinations of features resulting in high entropy values are less important than those resulting in low entropy values structuring the dataset into more distinct clusters. Therefore, this method allows us to rank isotopic systems. The isotope ranking depends on the analyzed dataset, for example, consisting of terrestrial mammals or fish. The feature ranking results were verified by cluster analysis. CONCLUSIONS: Entropy-based feature ranking can be used to a priori select the isotopic systems that should be analyzed. Consequently, we strongly suggest that this method should be applied if only limited material is available.

Evidence for sea spray effect on oxygen stable isotopes in bone phosphate - Approximation and correction using Gaussian Mixture Model clustering.

Palaeobiodiversity research based on stable isotope analysis in coastal environments can be severely hampered by the so-called "sea spray" effect. This effect shifts the isotopic signal of terrestrial individuals towards too marine values. It is commonly agreed upon that sea spray influences sulphur stable isotopes. However, we were able to approximate a remarkable sea spray effect also in carbon and oxygen stable isotopes of bone carbonate previously. In the present study we could approximate a minimum sea spray effect of about 13.9% even present in oxygen isotope values of bone phosphate, which was validated by Gaussian Mixture Model (GMM) clustering. This approximated value is by some magnitudes smaller than the minimum sea spray effect approximated for both δ13Ccarb and δ18Ocarb, and quite close to the sea spray detected for δ34Scoll in a previous study. It may therefore be interpreted as purer minimum sea spray signal compared to the approximation in bone carbonate. Furthermore, detection of sea spray in δ18Ophos can serve as additional validation of the effect present in bone carbonate, which is more prone to diagenetic alteration compared to bone phosphate. Moreover, the presence of the sea spray effect in both δ18Ocarb and δ18Ophos demonstrates that sea spray can be taken up by terrestrial mammals not only via food (δ18Ocarb) but also via drinking water (δ18Ophos). Finally, this study once more confirmed that calculation of δ18Ophos from δ18Ocarb values using a fixed oxygen isotope spacing (Δδ18O) can be highly misleading, especially in coastal environments affected by sea spray.

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 Ophosphate 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 Ophosphate 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.

Using Gaussian Mixture Model clustering for multi-isotope analysis of archaeological fish bones for palaeobiodiversity studies.

RATIONALE: Modern methods in mass spectrometry permit fast accumulation of a huge amount of data. The analysis of multi-isotope data sets of archaeological remains is of increasing importance for the study of palaeobiodiversity. However, common bivariate isotopic data analysis fails to detect certain patterns in a multi-dimensional data set. This problem can be solved by cluster analysis. METHODS: Gaussian Mixture Model (GMM) clustering was applied to a multi-isotope data set including 184 individual mass spectrometric measurements (δ(13) Ccollagen , δ(15) Ncollagen , δ(13) Ccarbonate , and δ(18) Ocarbonate values) of archaeological fish bones (n = 46) from the Viking Haithabu and medieval Schleswig sites in northern Germany. The number of components was first restricted to the expected number of three (freshwater, brackish, and marine environment). Subsequently, classification was conducted with respect to an optimal Bayesian Information Criterion (BIC). RESULTS: Restriction of the number of components to three clusters leads to the expected clustering results according to the gross ecological niches (freshwater, brackish, marine). The isotopic data of fish bone were, however, optimally clustered into four clearly separated, reasonable groups, namely a freshwater, a brackish, and two marine groups. The two marine clusters differ in their oxygen isotope ratios, indicating different water temperature and therefore probably imported fish. Restriction of the number of clusters resulted in better training and test results. CONCLUSIONS: The GMM clustering method is applicable to complex multi-dimensional stable isotope data sets established by isotope ratio mass spectrometry (IRMS). This exemplary application resulted in an identification of habitat preferences and non-local individuals. Depending on the scientific question to be solved, restriction of the cluster size could lead to a better reproducibility; however, with loss of dissolution. Copyright © 2016 John Wiley & Sons, Ltd.