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Since prehistoric times, the island of Sardinia—in the western Mediterranean—has played a leading role in the dynamics of human population and mobility, in the circulation of raw materials and artefacts, idioms and customs, of technologies and ideas that have enriched the biological, linguistic and cultural heritage of local groups. For the Phoenician and Punic periods (from the 9th to the 3rd centuries BCE), the ancient site of Nora—in southern Sardinia—represents an emblematic case in the study of migratory phenomena that occurred on the Island from the Iron Age until the Roman conquest. Despite the importance of exploring (and characterising) such movements from a wider bio-cultural perspective, the application of bio-geochemical tools for geographical provenance to the ancient skeletal populations of Sardinia is yet scarce. The present work is the first step towards filling this gap with the development of the first isoscape of southern Sardinia using new bioavailable Sr isotope data and a machine-learning approach. From a geolithological point of view, Sardinia is rather heterogeneous and requires detailed studies to correctly assess the distribution of the isotopic signature of bioavailable Sr. The random forest model employed here to construct the Sr isoscape uses several external environmental and geological variables. The most important predictors are related to age and bedrock type, with additional input from local soil properties. A 10-fold cross-validation gives a mean square error of 0.0008 and an R-squared of 0.81, so the model correctly predicts the 87Sr/86Sr ratio of unknown areas. By using a Bayesian provenance assignment workflow, we tested the isoscape here produced to determine the geographic origin and the mobility of archaeological and modern fauna collected from the Phoenician-Punic site of Nora and the surrounding Pula Plain. Our results indicate that archaeological sheep and goats (87Sr/86Sr 87Sr/86Sr > 0.7090) show compatibility with several natural and anthropogenic locations in southern Sardinia, as expected based on modern species distribution data. Finally, we discuss the large Sr isotopic variability of the Nora baseline, where human mobility studies of human cremated and inhumed individuals are currently underway.</div

Sr-isotope ratios predicted by using a Random Forest algorithm.

Left panel, modelled 87Sr/86Sr isotope ratio of South Sardinia by using a Random Forest algorithm with n = 5 external predictors. The 10-fold cross-validation resulted in RMSE = 0.0008 and R2 = 0.81; right panel, associated spatial uncertainties were obtained from a quantile Random Forest regression (ranger R package) and calculated as half of the (quantiles) q0.84—q0.15 difference. Both maps are given with a spatial resolution of 1 km.</p

Descriptive statistics of the Sr isotope data measured in this study.

Summary isotope data for each sample type (i.e., mammal, plant, rock, soil leachate, and water). For each sample category, the main statistic parameters are provided.</p

Sr isotope results of the samples considered in this study.

List of the environmental samples analysed in this work and the 87Sr/86Sr isotope ratio results for each sample. (DOCX)</p

Sr isotope baseline at Nora and surroundings (i.e., Pula Plain, up to 35 km from the Nora site).

Coloured dots are environmental samples from Nora and the Pula Plain. Violin plots represent the Sr isotope distributions of the RF isoscape, extrapolated at different radial distances from Nora. The blue dashed line represents the Sr isotope composition of modern seawater (0.70917).</p

Geographical and geological framework of this study (Southern Sardinia).

The left panel depicts the sampling spots, including the location of the Nora archaeological site and relevant places mentioned in the manuscript; the satellite map is provided by Google through the QGIS QuickMapServices plug-in. The central panel and the right panel report the geolithological features of the area, with the main formations classified by rock ‘type’ and age. These maps are based on the geolithological map of Italy available at the Geoportale Nazionale (http://wms.pcn.minambiente.it/ogc?map=/ms_ogc/wfs/Carta_geolitologica.map).</p

Bayesian probability provenance assignment of the six fauna samples analysed in this study.

Modern (n = 4; in Fig 6, black silhouette) and archaeological (n = 2; in Fig 6, dark orange silhouette) samples collected at the Nora site and surroundings. The red dot in the first panel depicts the location of the Nora site. The yellowish areas represent those with a higher probability of origin. Animal silhouettes are from phylopic.org.</p

Spatial uncertainty of the Southern Sardinia Isoscape as GEOTIFF.

Spatial uncertainty of the Southern Sardinia Isoscape as GEOTIFF.</p

Performance of the Random Forest model.

(A, B) Variable importance for the Random Forest model used in this work. The six variables (r.srsrq1, r.bulk, r.clay, r.cec, r.minage_geol and r.fert) were selected by using the VSURF algorithm. See also Bataille et al. [97]. The ‘%IncMSE’ (A) represents the relative increase of the cross-validation mean squared error, randomly permuting values of that specific variable (the higher the value, the more important the variable is in the model prediction). The ‘IncNodePurity’ (B) expresses how much a specific variable impacts the tree-split; (C, H) partial dependence plot for the VSURF-selected variables, showing their nonlinear behaviour: r.srsrq1 is the predicted first quartile of the global 87Sr/86Sr model reported in Bataille et al. [98]; r.bulk is the soil bulk density (kg/m3); r.clay is the clay soil content (weight %); r.cec is the soil cation exchange capacity; r.minage_geol is the log of the minimum geological age from GLiM; r.fert is the global nitrogen fertilization. (I) Values predicted by the RF model (isoscape 87Sr/86Sr) plotted vs. their respective spatial uncertainties obtained through the quantile RF; the blue line is a linear regression fit (R2 = 0.17, p < 0.01).</p

The ancient site of Nora (Cagliari, South Sardinia).

On the left is the archaeological site of Nora. On the right are some ancient monuments of the Imperial Roman period (a., Terme a mare–Bath by the sea; b., Tempio Romano–Roman Temple; c., Casa dell’Atrio tetrastilo–House of the Tetrasyle Atrium. These images are the propriety of the Department of Cultural Heritage–at the University of Padua. The photographer Gianni Alvito (Teravista, CA) provided us with courtesy of the Ministero della Cultura–Italian Ministry for Culture (former Ministero per i Beni e le Attività Culturali e il Turismo), Soprintendenza Archeologia, Belle Arti e Paesaggio per l’area metropolitana di Cagliari e le province di Oristano e Sud Sardegna. (TIF)</p