Post-depositional Alteration of Organic Material: Implications for Interpreting Molecular Biosignatures
Open Access
- Author:
- Fox, Allison
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 04, 2020
- Committee Members:
- Katherine Haines Freeman, Dissertation Advisor/Co-Advisor
Katherine Haines Freeman, Committee Chair/Co-Chair
Christopher Howard House, Committee Member
Julie Genevieve Cosmidis, Committee Member
Kristen Fichthorn, Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- Biosignatures
organic matter preservation
carbon isotopes - Abstract:
- Interpretations of biomarkers’ molecular and isotopic composition have been critical to understand the evolution of life and its environment on Earth. As a result, the identification of biomarkers in planetary and space environments is a top priority in the search for life beyond Earth. Both terrestrial and extraterrestrial organic materials are subject to extensive degradation and alteration after deposition, which may obscure primary molecular and isotopic signals. In order to identify biomarkers and interpret their biological- and environmental-specific characteristics, the effects of post-depositional processes in terrestrials and extraterrestrial environments must be constrained. In this dissertation, the alteration of molecular and isotopic signatures is evaluated for two post-depositional processes, degradation by exposure to ionizing radiation and preservation through organic-mineral interactions. Exposure to ionizing radiation has the capacity to completely destroy organic material or drive reactions between organic and inorganic species that lead to secondary radiolysis products. In environments that are not adequately shielded from exogenous radiation by a magnetic field or thick atmosphere, high doses of radiation will destroy organics on the planet surface, while higher energy radiation can alter and potentially destroy organic material at depth in a soil or regolith. To understand how subsurface organics are altered on Mars, I exposed meteorite-relevant macromolecular organics in fused silica or an iron mineral mixture to high-energy radiation for doses equivalent to millions of years on the surface. Organic acids were the main radiolysis product in all samples, independent of mineral matrix or organic starting material. The presence of organic acids in fused silica samples suggests that previously proposed Fenton reactions, which require a redox sensitive mineral, were not the only driver of organic acid formation. I proposed the illumination of semi-conductor surfaces by high energy radiation leads to radical species that can break down macromolecular organics. The ubiquity of organic acids as a radiolysis product in this work and others indicates that high-radiation environments do not preserve primary molecular signals. Organic material can be shielded from ionizing radiation, and other degradation processes such as microbial attack or chemical oxidation, by organic-mineral interactions. Intermolecular forces between organics and mineral surfaces control the strength of these interactions and thus influence organic material preservation in the geologic record. The isotopic properties of the organic molecule can influence the strength of intermolecular interactions, suggesting primary isotopic signals may not be preserved. I investigated the isotopic effects of two sorption interactions, n-octane sorbed to kaolinite, which is dominated by van der Waals forces, and amino acids sorbed to ice, which is dominated by hydrogen bonds. For n-octane/kaolinite sorption, I developed new computational methods that incorporated the effects of anharmonicity to predict sorption-driven H-isotope effects that were confirmed by surface-sensitive IR techniques. Although the global H-isotope fractionation was minor (< 2 ‰), we confirmed that anharmonic contributions cannot be ignored for predicting non-covalent isotope effects. In contrast, the stronger interaction involving hydrogen bonding between amino acids and ice surfaces caused significant position-specific C isotope fractionation, up to 8 ‰. The position and magnitude of C isotope fractionation was driven by the orientation of the amino acid on the surface. The results of both sorption studies presented in this dissertation indicate that mineral interactions do not influence global isotope compositions significantly, but can mask position-specific primary isotope signals.