Spatial and temporal variations in the isotopic composition of bison tooth enamel from the Early Holocene Hudson–Meng Bone Bed, Nebraska

The Hudson–Meng bone bed in northwestern Nebraska is an early Holocene deposit of hundreds of bison of differing ages that were killed in a catastrophic event of unknown origin. The M1, M2, and M3 molars of individuals, ranging from ca. 1 to 7 years of age at the time of their death, were examined to determine inter- and intra-tooth isotopic variability, and to link this variability to time in order to better understand the environments that existed prior to the mass death event. The δ13C value of molar structural carbonate increases by 2–3‰ from the M1 to M3 molars, reflecting increasing direct forage signals, and decreasing maternal influences. The δ18O value of molar structural carbonate in a given individual shows no consistent trend with time from birth, indicating a fairly direct linkage to ‘dietary water’ regardless of tooth ontogeny. Detailed ‘down-tooth’ isotopic measurements indicate a small seasonal signal in the δ13C value of forage/maternal milk, which appeared to have been largely dominated by C4 flora. In contrast, there was almost a 10‰ range in the isotopic composition of dietary water, a range that is within present-day summer versus winter precipitation in the region. The δ18O value of bulk tooth enamel, arranged by approximate time prior to the death event, indicate a consistent increase in the δ18O value of body water (2–3‰), suggesting a long-term decrease in winter/spring precipitation and/or drought. The δ18O trend with time, coupled with the high C4 grass abundance, suggest environmental stresses on the herd preceding the catastrophic death event.