Drinking Locally: A Water ⁸⁷Sr/⁸⁶Sr Isoscape for Geolocation of Archeological Samples in the Peruvian Andes

The analysis of ⁸⁷Sr/⁸⁶Sr has become a robust tool for identifying non-local individuals at archeological sites. The ⁸⁷Sr/⁸⁶Sr in human bioapatite reflects the geological signature of food and water consumed during tissue development. Modeling relationships between ⁸⁷Sr/⁸⁶Sr in human environments, food webs, and archeological human tissues is critical for moving from identifying non-locals to determining their likely provenience. In the Andes, obstacles to sample geolocation include overlapping ⁸⁷Sr/⁸⁶Sr of distant geographies and a poor understanding of mixed strontium sources in food and drink. Here, water is investigated as a proxy for bioavailable strontium in archeological human skeletal and dental tissues. This study develops a water ⁸⁷Sr/⁸⁶Sr isoscape from 262 samples (220 new and 42 published samples), testing the model with published archeological human skeletal ⁸⁷Sr/⁸⁶Sr trimmed of probable non-locals. Water ⁸⁷Sr/⁸⁶Sr and prediction error between the predicted and measured ⁸⁷Sr/⁸⁶Sr for the archeological test set are compared by elevation, underlying geology, and watershed size. Across the Peruvian Andes, water ⁸⁷Sr/⁸⁶Sr ranges from 0.7049 to 0.7227 (M = 0.7081, SD = 0.0027). Water ⁸⁷Sr/⁸⁶Sr is higher in the highlands, in areas overlying older bedrock, and in larger watersheds, characteristics which are geographically correlated. Spatial outliers identified are from canals, wells, and one stream, suggesting those sources could show non-representative ⁸⁷Sr/⁸⁶Sr. The best-fit water ⁸⁷Sr/⁸⁶Sr isoscape achieves prediction errors for archeological samples ranging from 0.0017 – 0.0031 (M = 0.0012, n = 493). The water isoscape explains only 7.0% of the variation in archeological skeletal ⁸⁷Sr/⁸⁶Sr (R² = 0.07), but 90.0% of archeological skeleton ⁸⁷Sr/⁸⁶Sr fall within the site isoscape prediction ± site prediction standard error. Due to lower sampling density and higher geological variability in the highlands, the water ⁸⁷Sr/⁸⁶Sr isoscape is more useful for ruling out geographic origins for lowland dwellers than for highlanders. Baseline studies are especially needed in the highlands and poorly sampled regions. Because the results demonstrate that a geostatistical water model is insufficient for fully predicting human ⁸⁷Sr/⁸⁶Sr variation, future work will incorporate additional substrates like plants, fauna, soils, and dust, aiming to eventually generate a regression and process-based mixing model for the probabilistic geolocation of Andean samples.