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

位于加利福尼亚州洛杉矶的兰乔拉布雷亚沥青坑无疑是世界上最壮观的化石矿藏之一。作为一个真正的滞后统计，他们已经产生了400多万年的晚更新世动植物化石，代表了近600个物种。尽管渗入了沥青，这种脊椎动物的骨骼材料与最初的状态几乎没有什么变化。这些化石的年龄从现在的4万年到4000年不等，跨越了北美显著的气候和动物群变化时期。然而，对牧场拉布雷亚的生物变化进行的时间超前分析相对较少。这在很大程度上是由于缺乏关于其攻丝学和年代学的基本信息。如果不了解影响保存的偏见，对化石的年代没有信心，就很难利用这些藏品进行古生态和进化研究。这里提出的研究将通过对保存下来的材料的子集进行彻底的土工学分析和高强度辐射测年来纠正这个问题。目前有三个条件有利于研究。首先，在过去的15年里，放射性碳测年物理和化学的改进使得能够以更高的准确度和精确度来测定小得多的样品。其次，发音科学有了很大的进步，现在已经有了大量已发表的方法论和案例研究，这些方法和案例研究是从研究框架中得出的。第三，兰乔拉布雷亚的现代发掘首次收集了4.7万个标本，收集了详细的三维和岩性数据，所有标本都进入了数据库，可供研究。公认的牧场拉布雷亚脊椎动物的积累模式是被困在粘性沥青的浅渗漏中。然而，河流作用和践踏也被认为发挥了重要作用。此外，有人认为，食肉动物很可能是被困住的、垂死的和死亡的草食动物吸引到该地点的。目前尚不清楚身体暴露了多长时间，以及诱捕事件发生的频率。为了更好地了解焦油坑的挖掘历史，建议对保存在第91号坑的哺乳动物材料进行详细的检查，这是现代挖掘。建议在第91号坑中保存所有哺乳动物的骨骼，这是现代的挖掘。所有超过5公斤的哺乳动物骨骼都将由训练有素的观察员进行检查，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，并进行检查以记录保存时的偏差。关于骨骼修饰和表示的数据门将用于对影响沉积物的挖掘过程进行第一次定量分析。将解决的第二个主要问题是矿床内的时间混合问题。由于裸露的沥青具有半流体性质，有人认为骨骼在沉积后垂直移动，从而破坏了任何地层。为了研究这个问题，将对第91号坑的骨骼进行高分子水平的AMS 14C测年。将从SME地层水平以及整个矿床深度确定多块骨骼的年代，以确定在骨骼集中范围内的临时混合程度。拟议的墓葬年代学和年代学研究的主要意义在于，它将为未来对壮观的牧场牧场收藏品的研究奠定必要的基础。

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.