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

在美国西南部工作的考古学家经常遇到文物，动物骨骼，陶瓷材料和石器的收藏品，这些收藏品在长期的考古沉积物中积累。 住宅区可能已被占用数十年，或在几个世纪内反复被占用。 随着时间的推移，来自分层沉积物的考古组合的组成往往以模式化的方式变化。 这种变化被认为反映了人类行为的时间相关变化，因此成为一个有价值的诊断工具，如果组合组成的模式可以准确地联系到行为的变化。 在这方面，在屠宰动物骨骼的考古收藏中，小型与大型动物猎物的差异表示长期以来一直被用来为史前狩猎策略的重建提供信息。 物种代表性随时间的变化被认为反映了在给定的觅食环境中遇到大型动物与小型动物的比率。 大型动物数量的减少通常被认为是资源衰退的标志，即最大、最有利可图的猎物类型，如西南部的鹿、羚羊和大角羊，已被密集猎杀，其数量因此下降。随着时间的推移，在犹他州西南部的帕罗万山谷的两个连续被占领的地点，大型猎物丰度的变化是显而易见的。 米德村（Median Village，公元900年至1050年）和埃文斯丘（Evans Mound，公元1050年至1175年）是由被称为弗里蒙特（Fremont）的玉米农学家占据的大型分层住宅区。 埃文斯丘大型动物的减少被归因于与人口增长相关的过度狩猎，这是弗里蒙特时期（公元600-1350年）大型动物明显普遍减少的一个常见解释。 在对这些地点的动物群进行重新分析时，Ugan发现，从当地树木年轮序列重建的动物骨骼组合与降水在几个方面存在显著的相关性。 例如，大型动物和成年鹿与幼年鹿的相对频率随树木年轮宽度的变化而变化，这表明气候变化（即，有效水分）而不是过度狩猎，可能是弗里蒙特猎人捕获的大型动物数量变化的主要原因。在这些集合中的大型和小型动物消耗的饲料中的碳-13到碳-12（d13 C）和氮-15到氮-14（d15 N）的比率对有效水分的变化非常敏感，可以作为过去气候条件的代理。 然而，史前植物很少在考古记录中保存下来。 幸运的是，动物骨骼通常保存良好，这些骨骼的胶原蛋白或蛋白质成分记录了所食用植物的稳定碳（13 C/12 C）和氮（15 N/14 N）同位素比率。 分析这些遗址中动物骨骼的稳定同位素比率，将提供一个与狩猎行为密切相关的气候变化记录，这些狩猎行为产生了这些组合。 这是第一次有机会使用同位素技术在一个控制良好但有限的地理和时间尺度上跟踪史前气候变化。 长耳大野兔（Lepus cf. californicus）进行分析，因为它们在这两个地点都很丰富，并且易于以当地植物为食，这些植物的同位素化学对有效水分的变化特别敏感。 在这些遗址的动物骨骼组合中建立气候变化和变化之间的明确联系具有许多重要意义，这与早期仅呼吁过度狩猎的论点相矛盾，并证明了这种技术在其他气候代理不可用的考古环境中的实用性。 Coltrain将指导Ugan进行稳定同位素分析，为Ugan提供具有众多考古应用的方法学方面的宝贵博士后培训。

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.