PhD defence: Bettina D. Livermore

Bettina D. Livermore defends her thesis,

Tracking Uranium Isotopic Variations in Deep Time: From Solar System Formation to the Evolution of Complex Life on Earth

Supervisors:
Professor James N. Connelly, Centre for Star and Planet Formation, IGN
Professor Martin Bizzarro, Centre for Star and Planet Formation, IGN

Assessment Committee:
Senior Research Scientist Dr. Daniel Condon, British Geologycal Survey
Associate Professor Morten B. Andersen, Cardiff University
Professor MSO Michael Storey (chair), SNM

Summary:
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.
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 sufficient 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 different U isotopic compositions through time as the oxygen levels rose and fell resulting in different 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 first 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 oxy- gen 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 sea-water. These results support the use of U isotopes as a proxy for oxygen changes throughout Earth’s history.

The thesis is available from the PhD administration office, 04.1.413