Exploring the acid-insoluble shell organic matrix of freshwater mussels (Unionoida) as a proxy for palaeodietary and paleoenvironmental studies

The shells of freshwater mussels, commonly found at archaeological sites associated with the Kodjadermen-Gumelnița-Karanovo VI human groups (5th millennium BCE, Romania and Bulgaria), were a significant dietary component for this population. The organic matrix of these ancient shells is often well-preserved and can be analysed using stable isotopes.This study establishes the empirical relationship between the isotopic composition of the acid-insoluble shell organic matrix (AIM) and the soft tissues of modern Unio tumidus. A quality criterion for AIM (C:N ratio 3.6 (3.3–<3.8)) was defined, and isotopic offsets (Δ15NAIM-defatted body = +0.95‰, Δ13CAIM-defatted body = +0.93‰ for soft tissues; Δ15NAIM-muscle = +1.7‰, Δ13CAIM-muscle = +0.3‰ for the adductor muscle) were determined.Our data suggest that the Gumelnița mussels were omnivorous, with a similar trophic level to carp and pond turtles, sourced from diverse water bodies (rivers, lakes …) based on δ13C values.