Diversity in plant δ⁶⁶Zn values across species and processing protocols: implications for ecological and paleodietary research

Zinc stable isotope ratios (δ⁶⁶Zn) are increasingly applied in fields such as archaeology, ecology and medicine, owing to their sensitivity to dietary variations. Despite their growing use, isotopic data for foods, particularly plants, remains limited. This has led to contradictory interpretations of animal isotopic patterns (e.g. browsers vs. grazers) due to notable gaps in the literature. To address this issue, δ⁶⁶Zn values were analyzed from 138 organic plant samples of various origins and species. Using different digestion protocols for sample preparation, we obtained reliable δ⁶⁶Zn values for 99 of these samples. We documented limited Zn isotope fractionation during incomplete digestion and during the zinc (Zn) purification and recovery using column chromatography. Notably, we observed higher δ⁶⁶Zn values in cereals, pseudo-cereals, and legumes compared to leaves, nuts, and seeds. These findings elucidate dietary isotopic differences in herbivores and highlight the utility of δ⁶⁶Zn in tracing the adoption of agriculture in archaeology, where cereals and legumes became the primary dietary sources of Zn. This study fills in critical data gaps and underscores the importance of plant δ⁶⁶Zn isotopic baselines in dietary reconstructions.