Reassessing prehistoric fish consumption in Southern Brazil through compound specific stable isotope analysis of amino acids: implications for bulk collagen isotope studies

Bulk stable isotope analysis (SIA) of human bone collagen indicates that the shellmound cultures, locally known as sambaquis, which flourished between 7,000 and 1,000 years ago along the southern coast of Brazil were heavily reliant on marine resources. However, further examination of dietary variability between these groups  is hampered by complexity in the aquatic isoscape. Here, we explored the utility of compound specific stable isotope analysis (CSIA) of individual amino acids in bone collagen to provide greater dietary resolution without reliance on an extensive survey of baseline fauna at each locale. We measured the δ13C and δ15N values of amino acids from 28 human individuals from 4 sites and undertook statistical analysis (fingerprinting, Bayesian modelling) to determine the most likely autotrophic nutrient source and to provide trophic level estimates. We were able to discern differences in sambaquis diet with greater confidence compared to bulk collagen SIA, highlighting substantial dietary variation between the sites investigated. Importantly, our results expose the inherent fragility of using bulk collagen isotope analysis for estimating human trophic level and marine consumption which we contend is overestimated due to the differences in the metabolic behaviour of glycine, the dominant amino acid in collagen, between terrestrial and aquatic organisms. The results therefore will therefore have wider implications for assessing the degree to which humans relied on coastal resources, a question that has been a goal of stable isotope analysis of archaeological human remains for decades.