TY - BOOK
T1 - Tracking Uranium Isotopic Variations in Deep Time: From Solar System Formation to the Evolution of Complex Life on Earth
AU - Livermore, Bettina Drøscher
PY - 2018
Y1 - 2018
N2 - Understanding the origin and evolution of the Earth emerges from a fundamental need to comprehend our own place in the natural world. The Earth has undergone many changes from the cooling and separation of a core, mantle and crust to the development of oceans, the atmosphere and ultimately complex lifeforms. By studying rocks and minerals we can find new pieces of the puzzle to continually improve our understanding of the processes at play. For many years, the uranium (U) to lead (Pb) decay system has been of interest for its use as chronometer and isotope tracer but variations of U isotopic compositions has only recently received attention. This reflects advances in mass spectrometry that uncovered natural isotopic variability of U from its previously assumed invariant value of 137.88. This created both problems for geochronology and opened entirely new and promising fields for studying processes that caused this newly-discovered fractionation. This thesis examines the technical challenges of measuring U isotopes to the necessary high precision as well as U isotope variability as related to the U-Pb geochronometer and its use in tracking of the oceans oxygenation through time. A number of studies have successfully analyzed the U isotopic ratio of samples each containing more than 100 ng U. More complicated, however, is the analysis of samples containing 10 ng U or less, which is commonly required when sample sizes are limited. This technical challenge originates in the very low abundance of the isotope 235U, which only constitutes approximately 0.72% of the total U abundance. This low abundance enhances the risk of direct interferences leading to analytical artifacts in the reported 238U/235U ratio. As such, matrix elements and organics that interfere on mass 235 that are not suƥciently removed from the sample can lead to spuriously low 238U/235U ratios. This thesis reviews the procedures involved in reproducibly analyzing U isotopes of both large (+100 ng) and small (<20 ng) samples. The U isotopic ratio constitutes a critical parameter in age determination of rocks and minerals using the long-lived U-Pb geochronometer. The two U isotopes decay to two isotopes of Pb (238U to 206Pb and 235U to 207Pb) providing a means to obtain an absolute age of appropriate samples. However, the age calculation depends on knowledge of the 238U/235U ratio of each sample. Since most samples are too small to accurately measure this ratio for individual samples, a ratio and uncertainty must be assigned based on the analyses of larger samples with suƥcient U available. This thesis presents a ratio of 137.817 0.031 based on the analyses 28 zircon populations and several other U-bearing minerals. This value is in full agreement with the only previously published value by Hiess et al. (2012) but with a smaller uncertainty. Collectively, these two studies provide increased confidence that the ages and assigned uncertainties of future U-Pb studies based on a fixed 238U/235U ratio are robust. Uranium occurs in nature mainly as either insoluble U+4 or highly-soluble U+6 depending on the local redox conditions. Due to differences in nuclear volumes of 235U and 238U, there is a measurable preference for 238U to be oxidized to the U+6 state over 235U. . This means that the oceans would have diơerent U isotopic compositions through time as the oxygen levels rose and fell resulting in diơerent redox conditions. Using carbonate rocks as proxy records of the U isotopic composition of ancient oceans, several previous studies have attempted to evaluate the rise and fall of oxygen in critical periods of geologic time. This work relies on the assumption that carbonate rocks have the capacity to faithfully record and preserve the correct U isotopic composition of the ancient oceans in which they were deposited. This thesis tests the Ƥrst part of this assumption by analyzing a variety of modern carbonate shells from different locations in oceans of the northern hemisphere. We show that the 238U/235U ratio of the ocean is directly recorded in corals and brachiopods living in the ocean today. This validates the method of exploiting U isotopes to track changes in the oxygen composition of the Earth’s atmosphere and oceans in the past. Brachiopods are of special interest as they are composed of low Mg calcite that is stable over geological timescales. We present the first analyses of modern brachiopods and show that they faithfully record the U isotopic composition of the ambient seawater. These results support the use of U isotopes as a proxy for oxygen changes throughout Earth’s history.
AB - Understanding the origin and evolution of the Earth emerges from a fundamental need to comprehend our own place in the natural world. The Earth has undergone many changes from the cooling and separation of a core, mantle and crust to the development of oceans, the atmosphere and ultimately complex lifeforms. By studying rocks and minerals we can find new pieces of the puzzle to continually improve our understanding of the processes at play. For many years, the uranium (U) to lead (Pb) decay system has been of interest for its use as chronometer and isotope tracer but variations of U isotopic compositions has only recently received attention. This reflects advances in mass spectrometry that uncovered natural isotopic variability of U from its previously assumed invariant value of 137.88. This created both problems for geochronology and opened entirely new and promising fields for studying processes that caused this newly-discovered fractionation. This thesis examines the technical challenges of measuring U isotopes to the necessary high precision as well as U isotope variability as related to the U-Pb geochronometer and its use in tracking of the oceans oxygenation through time. A number of studies have successfully analyzed the U isotopic ratio of samples each containing more than 100 ng U. More complicated, however, is the analysis of samples containing 10 ng U or less, which is commonly required when sample sizes are limited. This technical challenge originates in the very low abundance of the isotope 235U, which only constitutes approximately 0.72% of the total U abundance. This low abundance enhances the risk of direct interferences leading to analytical artifacts in the reported 238U/235U ratio. As such, matrix elements and organics that interfere on mass 235 that are not suƥciently removed from the sample can lead to spuriously low 238U/235U ratios. This thesis reviews the procedures involved in reproducibly analyzing U isotopes of both large (+100 ng) and small (<20 ng) samples. The U isotopic ratio constitutes a critical parameter in age determination of rocks and minerals using the long-lived U-Pb geochronometer. The two U isotopes decay to two isotopes of Pb (238U to 206Pb and 235U to 207Pb) providing a means to obtain an absolute age of appropriate samples. However, the age calculation depends on knowledge of the 238U/235U ratio of each sample. Since most samples are too small to accurately measure this ratio for individual samples, a ratio and uncertainty must be assigned based on the analyses of larger samples with suƥcient U available. This thesis presents a ratio of 137.817 0.031 based on the analyses 28 zircon populations and several other U-bearing minerals. This value is in full agreement with the only previously published value by Hiess et al. (2012) but with a smaller uncertainty. Collectively, these two studies provide increased confidence that the ages and assigned uncertainties of future U-Pb studies based on a fixed 238U/235U ratio are robust. Uranium occurs in nature mainly as either insoluble U+4 or highly-soluble U+6 depending on the local redox conditions. Due to differences in nuclear volumes of 235U and 238U, there is a measurable preference for 238U to be oxidized to the U+6 state over 235U. . This means that the oceans would have diơerent U isotopic compositions through time as the oxygen levels rose and fell resulting in diơerent redox conditions. Using carbonate rocks as proxy records of the U isotopic composition of ancient oceans, several previous studies have attempted to evaluate the rise and fall of oxygen in critical periods of geologic time. This work relies on the assumption that carbonate rocks have the capacity to faithfully record and preserve the correct U isotopic composition of the ancient oceans in which they were deposited. This thesis tests the Ƥrst part of this assumption by analyzing a variety of modern carbonate shells from different locations in oceans of the northern hemisphere. We show that the 238U/235U ratio of the ocean is directly recorded in corals and brachiopods living in the ocean today. This validates the method of exploiting U isotopes to track changes in the oxygen composition of the Earth’s atmosphere and oceans in the past. Brachiopods are of special interest as they are composed of low Mg calcite that is stable over geological timescales. We present the first analyses of modern brachiopods and show that they faithfully record the U isotopic composition of the ambient seawater. These results support the use of U isotopes as a proxy for oxygen changes throughout Earth’s history.
UR - https://rex.kb.dk/primo-explore/fulldisplay?docid=KGL01012004251&context=L&vid=NUI&search_scope=KGL&tab=default_tab&lang=da_DK
M3 - Ph.D. thesis
BT - Tracking Uranium Isotopic Variations in Deep Time: From Solar System Formation to the Evolution of Complex Life on Earth
PB - Natural History Museum of Denmark, Faculty of Science, University of Copenhagen
ER -