Modern human δ¹³C isoscapes in Brazil reveal spatial structure in a C₄-dominated diet landscape

This study presents δ¹³C isoscapes for modern human tissues in Brazil, based on paired analyses of dentine collagen and fingernail keratin. Using a random forest modeling approach combined with climatic and socio-economic covariates, we reveal spatial patterns in dietary carbon source assimilation across the Northeast and Southeast regions. While δ¹³C values in dentine correlated mainly with environmental variables, those in fingernails were more closely tied to socio-economic indicators such as municipal GDP and income inequality. Partial dependence plots illustrated contrasting relationships between δ¹³C and predictor variables, suggesting that environmental factors may act as spatial proxies, while socio-economic variables more directly influence dietary intake. The difference in δ¹³C values between tissues allowed for an exploratory assessment of changes in C₄ food consumption across the life course. Despite modest model performance, the findings highlight the potential of stable isotope isoscapes as tools for dietary reconstruction and forensic applications in complex socio-geographical settings. We emphasize the need for temporally resolved data and expanded sampling to improve model robustness and support interpretations of regional and life-stage dietary dynamics.