GEOARCHAEOLOGY AND PALEOENVIRONMENTAL CONTEXT OF PALEOINDIAN CULTURAL DEPOSITS AT THE BEACON ISLAND SITE (32MN234), NORTHWEST NORTH DAKOTA

The Beacon Island site is an Agate Basin (Paleoindian) bison bonebed in northwestern North Dakota. The bonebed, a product of a single kill at ca. 12.3 ka, is contained in late-Wisconsinan deposits that aggraded in a small kettle basin. The basin formed around 16-15 ka when the terminal ice front was in the region. Initially, it trapped eolian sediment, the slightly sandy loess comprising the Mallard Island Member of the Oahe Formation. Conditions probably were relatively dry when the Mallard Island Member aggraded, which accounts for the presence of loess and the absence of paludal deposits immediately above the late-Wicosnsinan glacial till that floors the basin. However, at ca. 12.3 ka paludal deposits comprising the Aggie Brown Member began to accumulate in the basin, indicating a shift to wetter conditions. It seems more than a coincidence that the presence of bison and humans at Beacon Island at ca. 12.3 ka coincides with that shift. Paludal deposits continued to aggrade at Beacon Island after 12.3 ka, resulting in deep burial of the bonebed. However, sedimentation was relatively slow, allowing soil development to keep up with deposition. This cumulization process resulted in the formation of an overthickened A horizon typical of the Leonard Paleosol. Aggradation of the Aggie Brown Member slowed around 9.7 ka and ceased by ca. 8.9 ka. The accumulation of the loess that comprises the Pick City Member was underway soon after 8.9 ka and probably marks the beginning of the Altithermal climatic episode in northwestern North Dakota.

Based on phytolith and stable carbon isotope data, cool-season C₃ prairie species dominated the site at the time of the bison kill, and only slight warming and/or drying occurred soon after 12.3 ka. The d¹³C values determined on pedogenic carbon become heavier from the bottom of the Aggie Brown Member to the top of the Pick City Member. This trend suggests increased net productivity of C₄ grasses. There is a corresponding trend in the phytolith data. Going up-profile, drought resistant Stipa species increase along with a slight increase in C₄ short grasses of the Chloridoideae family that are adapted to warm, dry climate. However, it is likely that Paleoindians associated with the Agate Basin culture at Beacon Island did not experience environmental conditions drastically different from modern conditions.