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

9804742范·瓦尔肯伯格位于加州洛杉矶的拉布雷亚牧场焦油坑无疑是世界上最壮观的化石矿床之一。 一个真正的lagerstatten，他们已经取得了超过400万晚更新世植物和动物化石，代表近600个物种。 虽然渗透了沥青，脊椎动物的骨骼材料从其原始状态几乎没有改变。 这些化石的年代从距今40，000年到4，000年不等，跨越了北美气候和动物群发生重大变化的时期。 然而，相对较少的时间海侵生物变化的分析已经在兰乔拉布雷亚。 这在很大程度上是由于缺乏关于其埋藏学和年代学的基本信息。 如果不了解影响保存的偏见，对化石的年龄没有信心，就很难将收集的化石用于古生态学和进化研究。 这里提出的研究将开始纠正这个问题，通过彻底的埋藏分析和强烈的放射性测年的一个子集的保存材料。 三个条件有利于在这个时候的研究。 首先，在过去15年中，放射性碳测年物理和化学的进步使得可以以更高的准确性和精确度对更小的样本进行测年。 第二，taphonohy的科学已经取得了显着的进步，现在有大量的出版的方法和案例研究，从一个框架的研究。 第三，在兰乔拉布雷亚的现代挖掘产生了47，000个标本，这些标本首次收集了详细的三维和岩性数据，所有标本都输入了数据库，可供研究。兰乔拉布雷亚脊椎动物的公认聚集模式是被包埋在粘性沥青的浅层渗漏中。 然而，河流作用和践踏也被认为发挥了重要作用。 此外，有人认为食肉动物可能是被困、垂死和死亡的食草动物吸引到这个地点的。 目前尚不清楚尸体暴露的时间以及诱捕事件发生的频率。 为了更好地了解沥青坑的埋藏历史，建议对现代发掘的91号坑中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材料的骨骼。 所有大于5 kg的哺乳动物骨骼将由经过培训的观察员检查是否存在食肉动物改造、践踏痕迹、风化、磨损、昆虫损伤和啮齿动物啃咬痕迹。 将收集每个分类群的骨骼部分代表性数据，并进行检查，以记录保存方面的偏见。 有关骨修饰和表征的数据将用于对影响沉积物的埋藏过程进行首次定量分析。 第二个要解决的主要问题是矿床内部的时间混合。 由于暴露的沥青的半流体性质，人们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91号坑的骨骼进行强烈的分子水平AMS 14C测年。 多个骨骼将从sme地层水平以及整个存款的深度，以确定在骨浓度的时间混合的程度。 建议的埋藏和年代学研究的主要意义是，它将奠定必要的基础，为未来的研究壮观的兰乔拉布雷亚收藏。

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.