Experimental volcanic lightning under early Earth conditions: Implications for prebiotic synthesis and the origin of life

Springsklee, Christina (2023): Experimental volcanic lightning under early Earth conditions: Implications for prebiotic synthesis and the origin of life. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

The emergence of life is one of the most enigmatic and substantial questions in human existence. Although science has advanced progressively unraveling many potential reaction mechanisms and discovering many puzzle pieces to explain the emergence of life, it is still not fully understood nor reproducible under laboratory conditions. The emergence of life remains a mystery. The search for the emergence of life includes the search for a geologically plausible environmental setting. A plausible environmental setting must provide all necessary prerequisites like a source of energy to initiate chemical reactions, a sufficient concentration of the abiotic building blocks for life, liquid water, temperature conditions enhancing chemical reactions but not destroying organic molecules, and disequilibrium conditions. The search for plausible environmental settings for the emergence of life is a challenging task as Earth's system has changed significantly since its origin. The atmosphere has changed significantly from an an-oxic state to its current oxic conditions. Oxygen is a waste product of today’s metabolism, so free oxygen was almost nonexistent before the emergence of life. The ocean composition and oxidation state have also changed significantly since the Earth’s origin. Unfortunately, no geological rock record survived from the time most plausible for the emergence of life and the exact geological conditions of early Earth are not known today. Many plausible environmental settings have been proposed in the research concerning the emergence of life. Here, the focus is on active volcanic settings. Volcanoes were active on early Earth and represented a highly diverse environment. Surrounded by the early Earth’s ocean, volcanic eruptions were the leading source of volatiles in the primary atmosphere. In addition to the heat emitted by magma, volcanic lightning also accompanies these explosive erruptions; both of which are important energy sources for prebiotic reactions. Recent explosive volcanism is accompanied by volcanic lightning: electrical discharges in and/or from the eruption column. The electrical activity within a volcanic plume can be characterized into three categories, whereby the boundaries overlap. Close to the vent, smaller discharges, vent-discharges, and near-vent lightning are observable. Further up in the plume, a large and impressive plume lightning takes place. Various charging and discharging mechanisms contribute in different amounts to each of those three discharge categories. This thesis focuses on near-vent lightning, which is dominantly generated by triboelectrification. Triboelectrification describes the process of frictional, non-disruptive interaction between the expelled ash particles. A grain size-dependent charging is observed during this process: smaller particles tend to charge negatively, whereas larger particles obtain a positive charge due to their frictional interaction with the smaller particles. The separation of the charged particles results in discharges. As discharges are regarded as one potential prebiotic synthesis mechanism for creating the first organics, the aim of this thesis is to determine the impact of environmental conditions as the atmospheric composition on the discharge behavior in experimentally generated volcanic lightning. An experimental setup to recreate different atmospheric compositions relevant to early Earth and extra-terrestrial bodies was built, as the atmospheric composition and pressure changed during the evolution of the Earth. This setup makes it possible to explore the influencing parameters of volcanic lightning and test if volcanic lightning might have been equally efficient under those conditions as it had been in current volcanic eruptions (Chapter 2). The experimental setup needs a gas-tight and secure supply of various gases. For this thesis, the impact of CO2 and CO as components of the enveloping atmosphere was tested on volcanic lightning, and equally vigorous generation of near-vent discharges compared to experiments conducted in an atmosphere containing current air was observed. To test the impact of the transporting gas phase, two transporting gas phases (argon and nitrogen) were used in the experiments. The change in transporting gas phase significantly changes the magnitude and number of detected near-vent discharges. Those results imply that the exsolved gas phase in the volcanic plume significantly impacts near-vent discharges. The recovered ash was analyzed for organic compounds by GC/MS, but no newly formed organic compounds were detected in the samples. Also, the overall magma composition of volcanic eruptions has changed over time. Therefore, the ability of two different volcanic materials, a phonolitic pumice and a recent tholeiitic basalt, to create discharges was compared against an analog material made of synthetic soda-lime glass beads (Chapter 3). The experimental generation of discharges allowed us to apply similar eruption conditions (10 MPa). The results demonstrate that all three materials can produce discharges during an eruption. To further evaluate the parameters governing the intensity and number of discharges, experiments to investigate the influence of grain size distribution on the bimodally distributed grain size compositions were performed. Each material's coarse and fine grain size fraction was mixed and used in the decompression experiments. The results demonstrated that for the synthetic soda-lime glass beads and the tholeiitic basalt, the abundance of a very fine grain size fraction (< 10 µm) was necessary to produce discharges. The more porous phonolitic pumice needed a broader grain size distribution to generate detectable discharges. Nonetheless, the abundance of fines was crucial to generate discharges for the pumice. The analysis of the high-speed recordings of the decompressed jets suggested that the coupling of the particles to the transporting gas-phase is the main governing factor controlling the generation of charge and discharge between particles. Additionally, as clay minerals in mud are a widely used material in the prebiotic context, experiments investigating discharges in mud eruptions as a potential ignition mechanism were performed (Chapter 4). Modern mud volcanism represents a source of methane emission, a highly reactive reducing gas. Today, very explosive, destructive mud eruptions are often accompanied by spectacular flames that can sustain for centuries. As the previous experiments suggest discharges between particles as a potential energy source for reactions in reducing gas phases, it was tested if potential discharges might occur, which could potentially ignite the methane-gas emitted in those eruptions. The mud samples analyzed in this study originate from the Davis-Schrimpf location (California, USA). Careful sample preparation caused only slight changes in the grain size distribution of the sampled mud. For safety reasons, the transporting gas phase was not methane but argon. The dried samples showed intense discharges during decompression. To test the influence of grain size distribution and humidity, a fraction of the dry samples was mixed with coarser sand grains, and another fraction was exposed to controlled humid conditions. Increasing the humidity and coarse grain size fraction decreases the number and magnitude of discharges. The results demonstrate that dry natural mud samples can cause discharges, representing a potential ignition mechanism in natural mud eruptions. The experimentally obtained results emphasize the probability of volcanic lightning as a potential prebiotic synthesis mechanism on early Earth as in the experiments with an enveloping atmosphere containing CO2, N2 and CO discharges were successfully detected. The environmental conditions so far tested in the experiments permit near-vent discharges, which can react with the transporting gas phase of the plume, in the experimental case, the jet. The results obtained so far strongly encourage further investigation of active volcanic settings as a potential environment for the emergence of life.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: emergence of life, volcanic lightning
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 4. Juli 2023
• 1. Bericht­erstatter:in: Scheu, Bettina