Taphonomy and Chronology of Rancho La Brea: A Critical Foundation for Future Research

拉布雷亚牧场的埋藏学和年代学：未来研究的重要基础

• 批准号: 9804742
• 负责人: Blaire Van Valkenburgh
• 金额: $ 15万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1998
• 资助国家: 美国
• 起止时间: 1998-08-01 至 2002-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9804742&HistoricalAwards=false
• 关键词: Taphonomy; Chronology; Rancho; La; Brea

9804742Van ValkenburghThe Rancho La Brea tar pits of Los Angeles, California are undoubtedly one of the world's most spectacular fossil deposits. A true lagerstatten, they have yielded over four millioon late Pleistocene plant and animal fossils that represnet nearly 600 species. Although permeated with asphalt, the vertebrate skeletal material is little altered from its original state. Ranging in age from 40,000 to 4,000 years before present, the fossils span a time of significant climatic and faunal change in North America. Nevertheless relatively few time-transgressive analyses of biotic change have been done at Rancho La Brea. This is largely due to a lack of basic information on its taphonomy and chronology. Without an understanding of the biases that affected preservation, and without confidence in the age of the fossils, it is very difficult to use the collection for paleoecological and evollutionary studies. The research proposed here will begin to rectify this problem by a thorough taphonomic analysisi and intense radiometric dating of a subset of the preserved material. Three conditions favor the research at this time. First, improvements in radiocarbon dating physics and chemistry over the last 15 years allow dramatically smaller samples to be dated with far better accuracy and precision. Second, the science of taphonohy has advanced significantly and there now exists a large body of published methodologies and case studies that from a framework for the research. Third, a modern excavation at Rancho La Brea has produced 47,000 specimens collected with detailed 3-dimensional and lithologic data for the first time, all entered into a database and available for study.The accepted mode of accumulation of Rancho La Brea vertebrates is entrapment in shallow seeps of sticky asphalt. However, both fluvial action and trampling have also been suggested to play significant roles. In addition, it has been argued that carnivores were likely attracted to the site by trapped, dying, and dead herbivores. It is unclear how long carcasses remained exposed and how often entrapment events occurred. To better understand the taphonomic history of the tar pits, a detailed examination of the mammal material preserved in Pit 91, the modern excavation, is proposed. All bones of mammal material preserved in Pit 91, the modern excavation, is proposed. All bones of mammal species larger than 5kg will be examined by trained observers for evidence of carnivore modification, trampling marks, weathering, abrasion, insect damage, and rodent gnaw marks. Skeletal part representation data will be collected for each taxon and examined to document biases in preservation. The data gatehred on bone modification and representation will be used to produce the first quantitative analysis of the taphonomic processes that affected the deposits. A second major question that will be addressed is that of temporal mixing within the deposits. Because of the semi-fluid nature of the exposed asphalt, it has been suggested that bones move verticlly after deposition, thus destroying any stratigraphy. To examine this issue, an intense molecular level AMS 14C dating of Pit 91 bones will be undertaken. Multiple bones will be dated from the sme stratigraphic level as well as throughout the depth of the deposit to determine the extent of temporal mixing within a bone concentration. The primary significance of the proposed taphonomic and chronological research is that it will lay the necessary groundwork for future research on the spectacular Rancho La Brea collections.

加州洛杉矶的Rancho La Brea沥青坑无疑是世界上最壮观的化石矿床之一。这是一个真正的古堡，他们已经发现了超过400万晚更新世的动植物化石，代表了近600个物种。尽管被沥青浸透，脊椎动物的骨骼材料几乎没有改变其原始状态。这些化石的年代从距今4万年到距今4000年不等，跨越了北美气候和动物发生重大变化的时期。然而，在拉布雷亚牧场进行的生物变化的时间跨度分析相对较少。这在很大程度上是由于缺乏关于其分类和年代的基本信息。如果不了解影响保存的偏见，并且对化石的年龄没有信心，就很难将这些收集物用于古生态和进化研究。这里提出的研究将开始纠正这个问题，通过彻底的埋藏学分析和强烈的放射性年代测定保存的材料的一个子集。目前有三个条件有利于这项研究。首先，在过去的15年里，放射性碳测年物理和化学技术的进步，使得更小的样本能够以更高的准确性和精度进行测年。其次，药物疗法的科学已经取得了显著的进步，现在已经有大量发表的方法和案例研究，这些方法和案例研究都来自于一个研究框架。第三，在Rancho La Brea的一次现代挖掘中，首次收集了47000个带有详细三维和岩性数据的标本，所有这些标本都进入了一个数据库，可供研究。拉布雷亚牧场脊椎动物被公认的聚集方式是被粘沥青的浅层渗漏所困住。然而，河流作用和踩踏也被认为发挥了重要作用。此外，有人认为食肉动物可能是被困住的、垂死的和死去的食草动物吸引到这个地方的。目前尚不清楚尸体暴露在环境中的时间，以及发生诱捕事件的频率。为了更好地了解沥青坑的地语学历史，建议对现代发掘的91号坑保存的哺乳动物材料进行详细检查。建议在现代发掘的91号坑中保存所有哺乳动物的骨骼材料。所有大于5公斤的哺乳动物的骨头将由训练有素的观察员检查，以寻找食肉动物改造、践踏痕迹、风化、磨损、昆虫损伤和啮齿动物咬痕的证据。将收集每个分类单元的骨骼部分表示数据，并检查以记录保存方面的偏差。关于骨修饰和表征的数据将用于对影响沉积物的地貌学过程进行第一次定量分析。要解决的第二个主要问题是沉积物内部的时间混合问题。由于暴露的沥青具有半流体的性质，有人认为骨头在沉积后会垂直移动，从而破坏了任何地层。为了研究这个问题，将对91号坑的骨头进行高强度分子水平的AMS 14C测年。将从地层水平以及沉积物的整个深度对多个骨骼进行年代测定，以确定骨骼浓度中时间混合的程度。拟议的地语学和年代学研究的主要意义在于，它将为未来对Rancho La Brea壮观藏品的研究奠定必要的基础。