Whodunit? Determining the source and eruptive characteristics of unidentified volcanic eruptions from ice and sediment core archives

Abstract

The most complete records of explosive volcanism from the last 100,000 years are preserved in polar ice cores as deposits of sulfate aerosol and microscopic ash (tephra). These records are essential for reconstructing the history of past eruptions, including their stratospheric sulfur loading, as well as understanding the climatic impact of eruptions. However, few of these deposits have been linked with volcanic sources, leading to assumptions of eruptive source latitude, plume height and stratospheric sulfur loading, which are all essential for interpreting the climate forcing potential of eruptions. A fundamental limitation preventing the correlation of ice core volcanic deposits to eruptive sources is the difficulty of geochemically characterising extremely fine-grained (<10 μm) tephra with conventional analytical methods. This thesis provides a thorough assessment and recommended workflow for analysing <10 μm tephra, increasing potential for characterising cryptotephra from distant eruptions in ice. These tephra method developments are then used alongside sulfur isotope analysis to improve source attributions of major Common Era eruptions recorded in Greenland ice during the 7th century. This allows testing of previous source latitude assumptions for eruptions associated with large climate anomalies and contributes to ongoing efforts to understand climate sensitivity to extra-tropical versus tropical eruptions. Finally, the identification of Atitlán Los Chocoyos supereruption tephra in both Greenland and Antarctic ice ~79,500 years ago suggests even volcanic stratospheric sulfate injection several times greater than any Common Era eruption does not have a long-term (greater than decadal) impact on climate. Grainsize analysis of Los Chocoyos ash in marine sediment cores provides further insights into the physical characteristics of the supereruption, suggesting mechanisms that accelerate fine ash and sulfate fallout from the atmosphere may limit stratospheric sulfur loading. By better constraining source and stratospheric sulfate estimates for both major eruptions of the Common Era and one of the largest supereruptions of the last 100,000 years, this thesis contributes to ongoing efforts to understand the history and global impacts of explosive volcanism.