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

Abstract

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 M 1, M 2, and M 3 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 δ 13C value of molar structural carbonate increases by 2–3‰ from the M 1 to M 3 molars, reflecting increasing direct forage signals, and decreasing maternal influences. The δ 18O 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 δ 13C value of forage/maternal milk, which appeared to have been largely dominated by C 4 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 δ 18O value of bulk tooth enamel, arranged by approximate time prior to the death event, indicate a consistent increase in the δ 18O value of body water (2–3‰), suggesting a long-term decrease in winter/spring precipitation and/or drought. The δ 18O trend with time, coupled with the high C 4 grass abundance, suggest environmental stresses on the herd preceding the catastrophic death event.