Geochemical Applications of Stable Rhenium and Mercury Isotopes

Studying natural variations in the isotopic composition of oxygen-sensitive elements in ancient marine sediments is a powerful way to study the geochemical evolution of Earth’s environments in the past. My dissertation focused on two broad aspects of isotope geochemistry: 1) the development of rhenium (Re) isotopes as a paleoredox and nuclear forensics tool, and 2) the application of mercury (Hg) isotopes as a tool to trace Hg mobility in the environment and what this movement means for isotopic changes in sedimentary rocks used to study Earth’s past. Chapter 2 is the first examination of Re isotopes in sedimentary rocks that formed ~2.5 billion years ago during a period of ocean and atmospheric oxygenation prior to the Great Oxidation Event. The data show variations in Re isotope ratios coincide with evidence for changes in oceanic and atmospheric oxygenation, supporting the use of Re isotopes as a tool to track paleoredox conditions throughout Earth's history. Another application of rhenium isotopes is explored in the third chapter on nuclear forensics. Rhenium isotopes in uranium ore concentrates (UOC) from known production locations revealed more than double the range of isotope fractionation previously reported for any natural geologic samples so far. These first Re isotope ratio data indicate that Re is a promising new tool for provenance assessment of UOCs. Chapter 4 focuses on geochemical applications of Hg isotopes. Mercury isotopes in shales are a geochemical tool that can be utilized to study the prevalence of global volcanism and detect oxygen-depleted conditions in the photic zone of ancient oceans. I measured Hg isotope ratio data from a Devonian shale bed in a road cut with varying degrees of weathering that has been well characterized for variations in elemental concentrations and other isotopic ratios. I found significant variation in mass-dependent and mass-independent Hg isotope fractionation in weathered samples. Surprisingly, however, I observed both loss and gain of Hg, when only significant loss was expected based on prior weathering studies. These findings improve the understanding of Hg mobility in nature and indicate that mass-independent fractionation can be modified after deposition in surprising ways.