Continental Paleoclimate Reconstruction Through Analysis of Oxygen Isotopes in Iron Oxides

要了解气候变化对生物群演化和生态组织的影响，必须详细记录过去温度、降水、风速和各种其他特征的替代数据。虽然海洋气候记录可以从广泛的代用物中重建，但分析高度变化的大陆领域的古气候变化被证明是更加困难的。适用于极古时期的主要代用指标是基于叶片形状、动物群分布、自生矿物和化石的同位素化学分析。一种矿物的氧同位素组成是由它形成的温度和沉淀它的水的同位素组成决定的。本项目将利用土壤和地表水中的氧化铁重建与现代中高纬度地区年平均气温密切相关的大气水氧同位素组成。这项建议有四个部分。首先，我们将在现代土壤和地表水系统的各种不同气候条件下，从氧化铁相对纯净的地点测定氧化铁和大气水的氧同位素组成，以确定氧化铁是否在氧同位素平衡中与大气水形成。二是赤铁矿水分选结果与理论计算结果存在差异。如果计算正确，则赤铁矿-水分馏对温度不敏感，赤铁矿氧同位素组成仅反映大气水组成的变化。提出了一系列的合成实验来解决这一差异，并阐明了自然形成机制。最后，古土壤赤铁矿中的氧同位素将用于研究过去1亿年地球气候的两个最重要的转变，即古新世/始新世边界的急剧变暖和始新世末期逐步向较冷气候过渡。对古代气候的研究将集中在北美西部，那里有来自同一地层剖面的动物和植物变化的优秀记录，这将为同位素研究提供血液。这些研究将有助于解决当地与全球气候变化在大陆动植物进化中所扮演的角色，并将提供一个机会来详细检查变暖和变冷对生物群的影响之间的对比。

Koch 9627953 To understand the influence that climate change has on the evolution and ecological organization of the biota, detailed records of proxy data for past temperature, precipitation, wind speed, and a variety of other features are essential. While the marine climate record can be reconstructed from a wide range of proxies, analysis of paleoclimatic change in the highly variable continental realm has proven more difficult. The chief proxies that are applicable to extremely ancient periods are analyses based on leaf shape, faunal distributions, and the isotope chemistry of authigenic minerals and fossils. The oxygen isotope composition of a mineral is determined by the temperature at which it forms and the isotope composition of the water from which it precipitates. In this project, iron oxides in soils and surface waters will be used to reconstruct the oxygen isotope composition of meteoric water, which is strongly correlated to mean annual temperature in modern mid- to high-latitude regions. This proposal has four sections. First, we will determine the oxygen isotope compositions of iron oxides and meteoric waters from sites with relatively pure iron oxides under a variety of different climates in modern soils and surface water systems to determine if iron oxides are forming in oxygen isotope equilibrium with meteoric water. Second, there are discrepancies between the hematite-water fractionation as a determined by synthesis experiments and by theoretical calculation. If the calculation is correct, then the hematite-water fractionation is insensitive to temperature, and the oxygen isotope composition of hematite will reflect solely change in the composition of meteoric water. A series of synthesis experiments are proposed to resolve this discrepancy and shed light on natural formation mechanisms. Finally, oxygen isotopes in ancient soil hematites will be used to study two of the most significant transitions in the earth's climate in the last 100 million years, the dramati c warming at the Paleocene/Eocene boundary and the stepwise transition to cooler climates at the end of the Eocene. The study of ancient climates will focus on western North America, where there are excellent records of faunal and floral change from the same stratigraphic sections that will supply hematities for the isotopic study. These studies will help resolve the roles played by local versus global climate changes in the evolution of continental faunas and floras, and will provide an opportunity to examine in detail the contrast between the effects of warming versus cooling on the biota.