Environmentally adjusted δ ¹³C thresholds for accurate detection of C₄ plant consumption in Europe

Detecting C₄ plants consumption is central to investigating animal ecology, agriculture, dietary transitions, and socio-environmental adaptations, and can be done using carbon isotope analysis. The conventional δ¹³C threshold used to identify C₄ plant intake does not consider substantial ecological variability across Europe. By analyzing over 4,000 δ ¹³C values from archaeological C₃ and C₄ grains, we present a European-wide C₃ grain δ ¹³C baseline and establish adjusted δ ¹³C threshold estimations for C₄ consumption from the site to the ecozone scale using multicomponent environmental models and ecozone cluster analysis. We show that a fixed threshold lead to under- or overestimation of C₄ plant consumption, particularly in northern/humid and southern/arid regions, where the threshold needs to be revised downwards or upwards by up to 2‰. This refined framework offers a more accurate baseline for interpreting human and animal diet and enhances our understanding of the spread, adoption and consumption of C₄ crops across Europe.