Geoarchaeology and paleoenvironmental context of the Beacon Island site, an Agate Basin (Paleoindian) bison kill in northwestern North Dakota, USA

Abstract A geoarchaeological investigation that included soil-stratigraphic and paleoecological analyses was conducted at Beacon Island, located in the flood pool of Lake Sakakawea in northwestern North Dakota. Beacon Island is a multi-component archaeological site that includes an Agate Basin (Paleoindian) component consisting of the butchered remains of at least 29 Bison antiquus , along with projectile point fragments and other artifacts. The bison bonebed is a product of a single kill at ca. 10,300 B.P. and is buried in a shallow kettle basin. The basin formed around 16,000–15,000 B.P. and initially trapped loess comprising the Mallard Island Member of the Oahe Formation; other members of the Oahe Formation subsequently filled the basin. Conditions probably were relatively dry when the Mallard Island Member aggraded, but by ca. 10,300 B.P. paludal deposits comprising the Aggie Brown Member began to accumulate in the basin, indicating a shift to wetter conditions. Based on phytolith and stable carbon isotope data, cool-season C 3 prairie species dominated the site at the time of the bison kill, and the micromammal faunal assemblage suggests that this boreal grassland may have been punctuated by stands of shrubby vegetation. Slight warming and drying occurred soon after ca. 10,300 B.P., indicated by relatively higher δ 13 C values determined on soil organic matter and a significant increase in concentrations of microscopic particulate charcoal and drought-resistant Stipa-type phytoliths, but paludal deposits continued to aggrade, resulting in deep burial of the bonebed. Sedimentation was relatively slow in the basin, allowing soil development to keep up with deposition. This cumulization process resulted in the formation of an overthickened A horizon in the Aggie Brown Member, typical of the Leonard Paleosol. Aggradation of the Aggie Brown Member ceased by ca. 8000 B.P., and loess comprising the Pick City Member began accumulating soon after that time, likely marking the initiation of the warm, dry Altithermal climatic episode. Isotope and phytolith data also point to a warmer and probably drier climate after 8000 B.P., indicated by higher δ 13 C values and the appearance of warm-season C 4 chloridoids at the site. Hence the Agate Basin occupation coincided with the coolest and perhaps the wettest climatic episode recorded in the sampled deposits. However, it is likely that the people associated with the Agate Basin culture at Beacon Island did not experience environmental conditions dramatically different from the modern conditions at the site.