Fire and ice : understanding volcanic histories from sulfur isotopes in ice cores

Abstract

Reconstructing the history of explosive volcanic eruptions is important for understanding the frequency of eruptions and the climatic forcing of these events. Erupted volcanic SO₂ is oxidised to sulfate aerosols which scatter incoming solar radiation. Aerosols are then deposited at the poles resulting in a peak in sulfate above background concentrations. Therefore, polar ice cores provide an important record of volcanism. The mass-independent fractionation of sulfur isotopes (S-MIF) in ice cores is an indicator of sulfur exposure to ultraviolet radiation via eruption into and above the ozone layer in the stratosphere. Sulfate aerosols from large eruptions that reach the stratosphere have a longer residence time and greater climatic impact than tropospheric aerosols from smaller eruptions. In this thesis I measure sulfur isotopes in volcanic ice core sulfate to investigate the ~74ka Toba supereruption and six unidentified volcanic eruptions from 16ka to 32ka. I found large magnitude S-MIF signals (-4.75‰) for one of the Toba candidates, the timing of which is consistent with independent age estimates for Toba. This places the eruption on the transition into a cool stadial period in Greenland (GS-20). Toba can be excluded as a potential trigger for GS-20, although it could have contributed to the cooling. To help interpret the six eruptions from 16–32ka, I adapted a 1D-diffusion model to simulate the diffusion and thinning of sulfate isotopes in ice cores. I found that the ice core sulfate isotope signatures are consistent with modelled results of sulfate exposed to diffusion and thinning. Finally, I adapted a 1D-photochemical model (Atmos) to model the temporal evolution of sulfur isotope signatures in a volcanic plume. The model predicts S-MIF signatures consistent in timing and shape to those measured in the ice cores for stratospheric eruptions with S-MIF formation primarily via self-shielding due to high SO₂ column densities.