Collaborative Research: ACACIA: Ancient Climate and the Authigenic Clay Index of Aridity

技术摘要：ACACIA解决了第四纪地质学、古气候学和湖泊地质学面临的几个关键问题。项目假设包括：(1)湖泊沉积物的全岩地球化学受自生粘土的组成和丰度的影响；2)东非湖泊流域干旱的发生主要表现为八面体Mg含量和氧同位素组成的富集；(3)八面体Al富集反映了与干旱期结束有关的淡水流域冲刷；4)环境波动反映了进动（Olduvai）和偏心（Olorgesailie）轨道影响被认为在那个时代占主导地位。ACACIA将从坦桑尼亚Olduvai盆地和肯尼亚Olorgesailie盆地的地层剖面中取样并分析亚微米自生粘土矿物和相关物质。为了开展这项工作，我们将与国际领先的实地团队合作——奥杜瓦伊峡谷的奥杜瓦伊项目（由马德里康普顿斯大学领导）和奥洛格萨莱的奥洛格萨莱钻井项目（由史密森学会和肯尼亚国家博物馆领导）。ACACIA调动了一个在东非拥有丰富实地经验的团队，以及在沉积学、x射线衍射和荧光、高分辨率透射电子显微镜和硅酸盐稳定同位素地球化学方面的分析专业知识。这些努力将提高我们对全球气候系统、热带环境对全球气候变化的响应、脊椎动物和无脊椎动物生态系统结构的变化以及人类谱系进化的理解。ACACIA将支持本科生研究，加强国际合作，并与该地区靠近坦桑尼亚Olduvai峡谷的唯一一所小学合作，为1000多名坦桑尼亚K-8学生提供教育推广和支持。摘要：ACACIA将带来现代分析方法来研究湖泊沉积物如何记录过去的环境变化。尽管科学家们以前使用工具来重建过去的环境变化，如研究化石花粉或浮游生物，但许多湖泊的沉积物堆积不包含化石或其他重建古代环境的材料。特别是在这些情况下，ACACIA将开展粘土矿物的研究，其中许多是直接从湖水中化学沉淀出来的。我们将从坦桑尼亚的Olduvai峡谷和肯尼亚的Olorgesailie盆地的露头和岩心中取样，那里有确定年代的火山灰，所以我们知道它们的年龄。从这些湖泊沉积物样本中，我们将分离出小于1/1000毫米的颗粒，分析它们的化学成分，并用电子显微镜拍照并分析它们。基于这些分析，我们将能够推断出这些古老湖泊何时含有淡水和咸水，这是东非气候湿润和干旱时期的一个强有力的指标。结合对东非环境变化的其他了解，这将为我们提供重要的线索，帮助我们了解热带环境如何对全球气候变化做出反应，非洲生态系统在过去几百万年里是如何变化的，以及早期人类进化的环境是如何随着时间而变化的。这也将使我们深入了解微观粘土颗粒如何与湖水化学相互作用，这对全球湖泊环境很重要。ACACIA将支持几名本科生和研究生，与加拿大、西班牙、坦桑尼亚和肯尼亚的机构建立国际合作，并与一所拥有1000多名学生但资源很少的坦桑尼亚小学开展外展活动。

Technical Abstract:ACACIA addresses several key issues facing Quaternary geology, paleoclimatology, and limnogeology. The project hypotheses include 1) that whole-rock geochemistry of lake sediments is influenced by the composition and abundance of authigenic clays; 2) that the onset of aridity in East African lacustrine watersheds is reflected by enrichment in octahedral Mg content and oxygen isotopic compositions; 3) that octahedral Al enrichment reflects freshwater watershed flushing associated with the termination of arid phases; and 4) that environmental fluctuations reflect precession (Olduvai) and eccentricity (Olorgesailie) orbital influences thought to dominate at those times. ACACIA will sample and analyze sub-micrometer authigenic clay minerals and associated materials from well characterized stratigraphic sections in the Olduvai (Tanzania) and Olorgesailie (Kenya) Basins. To carry out this work we will partner with leading international field teams - Proyecto Olduvai at Olduvai Gorge (led by Universidad Complutense de Madrid), and the Olorgesailie Drilling Project at Olorgesailie (led by the Smithsonian Institution and the National Museums of Kenya). ACACIA mobilizes a team with extensive field experience in East Africa, along with analytical expertise in sedimentology, X-ray diffraction and fluorescence, high-resolution transmission electron microscopy, and silicate stable isotope geochemistry. These efforts will improve our understanding of the global climate system, tropical environmental responses to global climate change, changes in vertebrate and invertebrate ecosystem structure, and the evolution of the human lineage. ACACIA will support undergraduate research, enhance international collaborations, and partner with the only elementary school in the region near Olduvai Gorge, Tanzania, providing educational outreach and support to over 1000 Tanzanian K-8 students.Non-Technical Abstract:ACACIA will bring modern analytical approaches to bear on the question of how lake sediments record environmental changes in the past. Although scientists have previously used tools to reconstruct past environmental change such as the study of fossil pollen or plankton, many lakes have sediment accumulations that contain no fossils or other materials for reconstructing the ancient environment. For those cases in particular, ACACIA will develop the study of clay minerals, many of which chemically precipitate directly from lake water. We will sample from outcrops and cores in the Olduvai Gorge, Tanzania, and the Olorgesailie Basin, Kenya, which have well-dated volcanic ashes so we know the ages. From these lake sediment samples we will separate out particles that are smaller than 1/1000th of a millimeter, analyze their chemistry, and use electron microscopes to take pictures and analyze them. Based on those analyses, we will be able to infer when these ancient lakes contained fresh versus saline water, which is a strong indicator of when climate was humid versus arid in East Africa. Combined with other knowledge of environmental change in East Africa, this will give us important clues as to how tropical environments respond to global climate change, how African ecosystems have changed over the past few million years, and how the environments of early human evolution changed through time. This will also give us great insight into how microscopic clay particles interact chemically with lake water, which is important to the environments of lakes worldwide. ACACIA will support several undergraduate and graduate students, build international collaborations with Canadian, Spanish, Tanzanian, and Kenyan institutions, and conduct outreach to a Tanzanian elementary school with over 1000 students and few resources.