Advances in high spatial resolution microanalysis of volcanic glass : application to eruption source attribution in the polar ice core record

Large volcanic eruptions are a major driver of climate forcing and can produce annual-decadal scale climate variability. Reconstructing volcanic histories is essential for quantifying their role in past climate change and improving understanding of their impacts on human society. The most complete record of explosive volcanism over the past 2000 years is preserved in polar ice cores as sulfate aerosol deposits and micron-scale volcanic ash (tephra). However, most of these eruptions remain unattributed to a volcanic source, introducing substantial uncertainty in eruption timing, source latitude, terminal plume height, and stratospheric sulfur injection. Geochemical characterization of tephra co-deposited with volcanic sulfate aerosols provides the most direct means of confirming volcanic source attribution. However, ice core tephra deposits typically contain sparse glass shard populations composed of grains only 2-20 µm in diameter, placing many particles at or below the practical spatial limits of conventional geochemical methods. To address these limitations, this thesis develops high spatial resolution microanalytical methods for the geochemical characterization of extremely fine grained tephra. An optimized 1 µm/1 nA electron probe microanalysis method enables robust major element analysis, improving precision by up to sevenfold relative to existing small-beam approaches. Small-spot laser ablation inductively coupled plasma mass spectrometry methods at 5-10 µm further extend trace element characterization to finer grained cryptotephra, including shards smaller than the minimum practical spot size through controlled beam overlap analysis. These advances are integrated with glaciochemistry, triple sulfur isotope analysis, and statistical tephra correlation approaches to constrain the sources and forcing of a volcanic cluster recorded in polar ice cores at the start of the seventeenth century. Together, these findings demonstrate that high spatial resolution microanalysis can reduce long-standing uncertainties in volcanic source attribution, eruption chronology, and estimates of volcanic climate forcing preserved in the polar ice core record.