Taking the Pulse of Life: Intramolecular and Clumped Isotopic Perspectives on the Origins and Evolution of Hydrocarbons in Geological and Prebiotic Systems

Citation

Dong, Guannan (2024) Taking the Pulse of Life: Intramolecular and Clumped Isotopic Perspectives on the Origins and Evolution of Hydrocarbons in Geological and Prebiotic Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yw0g-s893. https://resolver.caltech.edu/CaltechTHESIS:06012024-213407041

Abstract

Life’s origins and fate are tightly intertwined. All life as we know it is composed of proteins, carbohydrates, lipids, and nucleic acids. These biomolecules are synthesized today by cellular machinery made of the same components, leaving open questions about the origins of life and prebiotic chemistry. After death, organic remnants are buried in sediments, undergoing microbial reworking, consolidation, and transformation into kerogen. As temperature and pressure increase with depth, kerogen matures, releasing oil and gas before ultimately transforming into graphite. The question remains: can we decipher the traces of life (and non-life) from somewhat altered organic matter from the past or on other planets? This thesis explores the origins and evolution of one of the most fundamental classes of compounds—hydrocarbons—across geological and prebiotic settings through novel applications of intramolecular and clumped isotope analysis.

Chapter 2 delves into the evolution of isotopic signatures in methane, the simplest hydrocarbon, during the maturation process. By studying the clumped isotope effects of thermogenic methane formation through pyrolysis experiments, this chapter challenges previously held assumptions about abiotic and microbial signatures. The findings offer new opportunities to constrain the thermal maturation of sedimentary organic matter and have implications for the search for extraterrestrial life.

To facilitate high-precision measurements of hydrocarbon isotopic structures, Chapter 3 presents hardware and software developments enabling automated, high-throughput analysis using Orbitrap mass spectrometry. Chapter 4 then introduces a novel method coupling gas chromatography and Orbitrap MS to simultaneously measure intramolecular ¹³C and ²H distributions in n-alkanes, validating the technique for forensic fingerprinting and natural sample characterization.

Turning to the impact of thermal maturation, Chapter 5 examines how n-alkane intramolecular isotope patterns evolve through pyrolysis experiments. Kinetic isotope effects control residual n-alkane isotopic compositions, with minimal alteration to intramolecular distributions under the studied conditions, suggesting preservation of primitive signatures.

Chapter 6 brings together these analytical developments to compare intramolecular isotope compositions of n-nonadecane from sedimentary, abiotic, and biological sources. Distinctive isotopic fingerprints are established for each source, with implications for interpreting organic matter histories and detecting potential signatures of extraterrestrial life.

Collectively, this thesis expands the "molecular detective" toolkit for tracing hydrocarbon origins across diverse environments, from deep petroleum systems to potential prebiotic reaction pathways. The findings illuminate key processes governing isotopic biosignatures and their preservation through geological time.

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