Climate-changing volcanic eruptions from magma source to ice core archives: a sulfur isotope perspective
Explosive volcanic eruptions loft prodigious quantities of ash, gas and aerosol into the Earth’s atmosphere and can have severe impacts on human health, the environment and the global economy. A detailed record of past volcanic events is critical for improving societal resilience to future eruptions, and of all the surface archives available to Earth Scientists it is the polar ice cores which preserve the finest time-resolved record of past volcanic events.
The key challenge with the ice core record of volcanism is that most analytical techniques fail to provide detailed information about the eruptive style (plume height) and source location (parameters that are essential for evaluating climate impacts). One new and promising tool for fingerprinting plume height and source location is sulfur isotope analysis of ice core sulfate deposits (Figure 1). These analyses can be used to confirm whether the sulfate was formed in the stratosphere (e.g. Baroni et al., 2007) and provide information on the spatial separation between stratospheric and tropospheric aerosol clouds (which is strongly linked to source location, Burke et al. 2019). However, though the technique has immense potential there is only a qualitative understanding of the atmospheric chemical processes that generate these unique isotope signals because no previous studies have carefully assessed plume isotope evolution from the eruptive source to the ice sheet deposition (Gautier et al., 2018).
This project will focus on carefully calibrating and validating these methods for well-known contemporary eruptions, and will compare and contrast the sulfur isotope signatures of two of the largest eruptions in recent history: the 1875 Askja (Öskjuvatn Caldera) eruption (Iceland), and the cataclysmic 1815 Plinian eruption of Tambora (Indonesia). These events will provide a fascinating contrast; both caused prolonged Earth surface cooling and had significant global impacts (Sigl et al. 2015), but their magma composition, eruption style and location is fundamentally different. The Askja eruption had significant regional impacts, while the Tambora eruption caused prolonged Earth surface cooling and had global impacts (Sigl et al. 2015).
The main objectives of this project are:
1) Characterise the sulfur isotope values of the magma source. To understand how atmospheric chemical processing of the plume affects its isotope composition, it is important to constrain the initial S isotope value for these eruptions. To achieve this, we will undertake detailed petrography and microanalysis of volcanic eruption deposits to constrain S speciation, concentration and isotopes in pre-eruptive melts, S-bearing minerals (apatite) and use this to reconstruct S isotopes in co-existing gas phase (e.g. McKibben et al. 1996).
2) Create high temporal resolution records of plume fallout using ice core records. To fully understand the time-evolving plume chemistry, ice core records will be analysed for quadruple sulfur isotopes using new MC-ICP-MS methods (under active development at St Andrews, Burke et al. 2019). These measurements will provide isotope fingerprints for the key eruptions (at bi-monthly resolution) and will be used to evaluate the proportions of tropospheric to stratospheric sulfate deposited through time (Martin, 2018).
3) Calibrate and validate models of the isotopic evolution of volcanic plumes. Having generated detailed isotopic constraints on the plume sources (Objective 1) and high temporal resolution records of their evolution and fall out (Objective 2), the final goal of the project is to use isotope modelling to link these measurements and quantify isotope fraction in an evolving volcanic plume. We will use existing photochemical models (e.g., ATMOS, Claire et al. 2014) to simulate plume dispersal and track the redistribution of S isotopes as a consequence of both photochemical oxidation and distillation processes.
The overarching goal of the project is to unite the time resolved model predictions with the new source and ice core isotope measurements. Evaluating model findings in this way will provide a powerful demonstration that our understanding of the processes that generate and preserve volcanic sulfate isotope signals for these key eruptions is sound.
We expect the project to be of wide significance to volcanology and ice core science communities. The project will yield fascinating insights into the magmatic processes that took place at Askja and Tambora and initiated these massive volcanic eruptions. It will also be one of the first attempts to quantify the atmospheric processes that modify the erupted S isotope signal and will provide an invaluable template for future investigations of unknown eruption ice core signals.
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Fig. 1: Schematic illustrating how volcanic eruptions are preserved in ice core archives.,Fig. 2: The Öskjuvatn Caldera lake (background) formed during the 1875 Askja eruption (A) and the 1815 eruption of Tambora (B) had profound environmental and societal impacts and will be the targets of this study (images by NASA and Henrik Thorburn, CC BY 3.0).