Spatiotemporal Variation in Suidae–Cervidae Bone Collagen Stable-Isotope Offsets During the Middle and Late Holocene in China: An Exploratory Analysis Using Data-Driven Boundaries

Stable-isotope data are widely used to reconstruct ancient diets, but cross-regional comparisons of raw δ13C and δ15N values can be confounded by ecological baseline variation. Using published bone collagen data, we calculated mean isotope values for human, pig, and deer samples within each site–period unit and derived Hominidae–Suidae, Hominidae–Cervidae, and Suidae–Cervidae offsets. Data-driven breakpoint analysis was conducted along chronological, longitudinal, and latitudinal axes, with breakpoint robustness assessed through bootstrap stability and site dominance. Suidae–Cervidae Δδ13C and Δδ15N produced more stable breakpoints than Hominidae–Suidae and Hominidae–Cervidae offsets. Suidae–Cervidae offsets were substantially larger west of 115.66°E than to the east, north of 33.60°N than to the south, and increased after approximately 5.3 ka BP. Joint temporal, longitudinal, and latitudinal analysis showed that the largest offsets occurred among samples located west of 115.66°E and north of 33.59°N. This growing divergence likely reflects increasing differentiation between the diets of settlement-associated pigs and the natural herbivore baseline represented by deer. Potential causes include C4-derived resources, settlement refuse, manured foods, and deliberate feeding. These findings demonstrate that inter-animal isotope offsets can reveal spatiotemporal structures that are less apparent in raw isotope values and provide an additional perspective on changes in animal management and subsistence practices.