Using Stable Isotope Chemistry to Monitor Changes in Climate and Prey Choice among the Parowan Fremont

University Of Utah, Salt Lake City UT

Investigators

Abstract

Archaeologists working in the American Southwest are regularly confronted with collections of artifacts, animal bone, ceramic materials and stone tools, that have accumulated in stratified archaeological deposits over long periods of time. Residential sites may have been occupied for decades and/or repeatedly reoccupied over several centuries. Over time, the composition of archaeological assemblages from stratified deposits often varies in patterned ways. Such variation is thought to reflect temporally correlated changes in human behavior and thus becomes a valuable diagnostic tool, if patterning in assemblage composition can be accurately linked to changes in behavior. In this regard, the differential representation of small versus large animal prey in archaeological collections of butchered animal bone has long been used to inform reconstructions of prehistoric hunting strategies. Changes over time in species representation have been thought to reflect the rate at which large versus small animals were encountered within a given foraging environment. A decline in the abundance of large animals is commonly thought to indicate resource depression, that the largest, most profitable prey types, such as deer, antelope and bighorn sheep in the Southwest, have been hunted intensively and their numbers have declined as a consequence. Changes over time in the abundance of large prey are apparent at two sequentially occupied sites in the Parowan Valley of southwestern Utah. Median Village (AD 900-1050) and Evans Mound (AD 1050-1175) are large, stratified residential sites that were occupied by maize agriculturalists known as the Fremont. The decline in large animals at Evans Mound has been attributed to over-hunting associated with human population growth, a commonly invoked explanation for an apparent general decline in large game over the Fremont period (AD 600-1350). In a reanalysis of faunal collections from these sites, Ugan identified a striking correlation in several aspects of the animal bone assemblage with precipitation, reconstructed from a local tree ring sequence. For example, the relative frequency of large animals and adult versus juvenile deer co-varies with tree ring widths suggesting climate change, (i.e., effective moisture) not over-hunting, may have been the primary causal mechanism for variation in the abundance of large animals taken by Fremont hunters. The ratios of carbon-13 to carbon-12 (d13C) and nitrogen-15 to nitrogen-14 (d15N) in forage consumed by both large and small animals in these collections are very sensitive to changes in effective moisture and can serve as proxies for past climatic conditions. However, prehistoric plants are rarely preserved in the archaeological record. Fortunately, animal bone is often well preserved and the collagen, or protein component, of such bone records the stable carbon (13C/12C) and nitrogen (15N/14N) isotope ratios of the plants consumed. Analyzing the stable isotope ratios of animal bone from these sites will provide a record of climate change intimately associated with the hunting behavior that produced the assemblages. This is the first opportunity to use isotope techniques to track prehistoric climate change on a well controlled but limited geographic and temporal scale. Jackrabbits (Lepus cf. californicus) were chosen for analysis because they were abundant at both sites and were apt to forage on local plants with isotope chemistry particularly sensitive to changes in effective moisture. Establishing a clear connection between climate change and variability in animal bone assemblages from these sites has a number of important implications, contradicting earlier arguments that appeal solely to over-hunting and demonstrating the utility of this technique in archaeological settings where other climate proxies are not available. Coltrain will instruct Ugan in stable isotope analysis providing Ugan with valuable post-doctoral training in a methodology with numerous archaeological applications.