Fossil packrat middens and the tandem accelerator mass spectrometer

Analyses of well-preserved plant remains from ancient packrat (Neotoma) middens have yielded much information on the history of vegetation, fauna and climate from the more arid portions of North America over the past 40,000 years^1,2. In most of the modern deserts, woodland or forest communities were present during the last glacial age, the Wisconsin. Radiocarbon dating of packrat midden assemblages from a single cave or from several nearby sites in a homogeneous local area can yield a detailed, local vegetation chronology for many thousands of years from within 30-50 m on the rocky slopes in front of the dry rock shelters^3,4. The development of radiocarbon dating using the tandem accelerator mass spectrometer (TAMS) allows direct dating of very small samples. Here we present 20 examples of TAMS radiocarbon dates on packrat midden materials to illustrate their usefulness in testing anomalous mixtures of species in Ice Age communities, understanding the details of plant migrations, and refining Wisconsin and Holocene time boundaries based on range changes of important plant species (Table 1, Fig. 1).