Continental Paleoclimate Reconstruction Through Analysis of Oxygen Isotopes in Iron Oxides

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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.