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会遇到文物、动物骨骼、陶瓷材料和石器的收藏，这些东西是长期积累在分层的考古沉积物中的。居民点可能已经被占领了几十年和/或反复被重新占领了几个世纪。随着时间的推移，来自层状沉积物的考古组合的组成往往会以模式化的方式发生变化。这种变化被认为反映了人类行为的时间相关变化，因此成为一种有价值的诊断工具，如果组装组成的模式可以准确地与行为变化联系在一起的话。在这方面，长期以来，在被屠宰的动物骨骼的考古收藏中，小动物猎物和大动物猎物的不同表现一直被用来为史前狩猎策略的重建提供信息。随着时间的推移，物种代表性的变化被认为反映了在给定的觅食环境中遇到大动物与小动物的比率。大型动物数量的减少通常被认为意味着资源短缺，最大、最有利可图的猎物类型，如西南部的鹿、羚羊和大角羊，被密集猎杀，其数量因此下降。在犹他州西南部帕罗文山谷的两个连续被占领的地点，大型猎物的丰度随时间的变化很明显。中村(公元900-1050年)和埃文斯丘陵(公元1050-1175)是被称为弗里蒙特的玉米农学家占据的大型分层住宅区。埃文斯丘陵大型动物数量的减少被归因于与人类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常见解释。在对这些地点的动物群收集进行的重新分析中，乌根发现动物骨骼组合的几个方面与降雨量有显著的相关性，这些降雨量是从当地的树木年轮序列重建而来的。例如，大型动物和成年鹿与幼鹿的相对频率随着树轮宽度的变化而变化，这表明气候变化(即有效湿度)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。在这些采集中，大小动物食用的饲料中碳-13与碳-12(D13C)和氮-15与氮-14(D15N)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替代。然而，史前植物很少被保存在考古记录中。幸运的是，动物骨骼通常保存完好，这种骨骼的胶原蛋白或蛋白质成分记录了植物消耗的稳定碳(13C/12C)和氮(15N/14N)同位素比率。分析这些地点动物骨骼的稳定同位素比率将提供与产生这些组合的狩猎行为密切相关的气候变化记录。这是利用同位素技术在可控但有限的地理和时间尺度上追踪史前气候变化的第一次机会。杰克兔(Lepus cf.之所以选择它们进行分析，是因为它们在两个地点都很丰富，而且容易以具有对有效水分变化特别敏感的同位素化学的当地植物为食。在气候变化和这些遗址的动物骨骼组合的可变性之间建立明确的联系具有许多重要的意义，这与早先单纯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指标不可用的考古环境中的实用性。科尔特兰将指导Ugan进行稳定同位素分析，为Ugan提供具有众多考古学应用的方法论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.