Geoarchaeology and paleoenvironmental context of the Beacon Island site, an Agate Basin (Paleoindian) bison kill in northwestern North Dakota, USA Rolfe D. Mandel a, * , Laura R. Murphy a , Mark D. Mitchell b a Kansas Geological Survey & Dept. of Anthropology, University of Kansas,1930 Constant Avenue, Lawrence, KS 66047, USA b Paleocultural Research Group, P.O. Box 745309, Arvada, CO 80006, USA article info Article history: Available online xxx Keywords: Northern Great Plains Agate Basin complex Oahe Formation Paleopedology Stable carbon isotopes Phytoliths 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 16,000e15,000 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 accu- mulate 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 micro- mammal 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 d 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 Alti- thermal climatic episode. Isotope and phytolith data also point to a warmer and probably drier climate after 8000 B.P., indicated by higher d 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. © 2014 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction The Agate Basin complex was first defined more than 50 years ago in the Great Plains (Wheeler, 1954; Roberts, 1961), yet it re- mains today among the least well understood Paleoindian techno- complexes in North America because of a paucity of recorded Agate Basin components in stratified contexts. In addition, surface finds of Agate Basin projectile points are comparatively rare, particularly relative to Folsom points, which date very nearly to the same period (Stanford, 1999). Prior to our investigation at the Beacon Island site, only six sites with Agate Basin components had been excavated, all of them scattered along the far western margin of the Great Plains (Fig. 1) (see Stanford, 1999; Kornfeld et al., 2010). Among the Agate Basin components at these sites, only the work in Area 2 at the Agate Basin type-site in eastern Wyoming has been described in detail (see Frison and Stanford, 1982). Interpretations of collections * Corresponding author. E-mail address: mandel@ku.edu (R.D. Mandel). Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint http://dx.doi.org/10.1016/j.quaint.2014.06.073 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved. Quaternary International xxx (2014) 1e23 Please cite this article in press as: Mandel, R.D., et al., Geoarchaeology and paleoenvironmental context of the Beacon Island site, an Agate Basin (Paleoindian) bison kill in northwestern North Dakota, USA, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.06.073