A fundamental exploration of the interactions between minerals and life’s building blocks in deep time

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Briggs, Derek

Abstract

Complex organic matter is the most abundant form of organic carbon in the fossil record on Earth and in geological materials delivered from Space. It is unclear how complex organic matter preserved in fossils and meteorites forms in different terrestrial and extraterrestrial geological settings. The molecular make-up of decay-resistant organic polymers in fossils and meteorites has the potential to preserve biological signals and record geological conditions in deepest time. Carbonaceous fossil morphologies resembling cells, extracellular matrix and entire tissue architectures have provided pivotal insights into the evolutionary history of life. Fossil soft tissues within vertebrate hard tissues have been a focus of attention recently, but carbonaceous remains have been extracted from fossils representing all major groups of organisms. Carbonaceous preservation tends to be associated with secondary minerals, most commonly pyrite (iron sulfide) or apatite (calcium phosphate). Similar mineral affinities are observed for extraterrestrial carbonaceous matter in meteorites, in the intergranular matrix and at select mineral surfaces. Such carbonaceous particles in meteorites from the asteroid belt contain organic compounds formed in the solar nebula, predating carbonaceous remains on Earth. Complex organic matter is a challenging target for chemical analysis due to its insoluble nature and association with minerals: No comparative, large-scale analysis of patterns in its composition has been attempted previously. Thus, the origins, formation pathways, relationship with biomolecules, and stabilization mechanisms of complex organic matter on Earth and in Space are poorly understood. The search for the biological and geological significance of fossil organic matter represents a fundamental quest at the interface of geology, biology, and chemistry – and is the key aim of my dissertation. My research relies on a novel Raman microspectroscopy protocol that I developed to characterize informative patterns in the total composition of carbonaceous materials at a resolution and rate of data acquisition that was not previously possible. Chapters 1 and 2 focus on animal soft tissues: Carbonaceous fossils are composed of N-, O-, S-heterocyclic polymers. Molecular heterogeneities decrease with maturation under increased pressure and temperature. Fossil organics are distinct from those in the associated sediments, indicating endogeneity, and are, contrary to previous assumptions, abundant in oxidative settings. Experimental maturation of soft tissues reveals that biomolecule fossilization involves advanced glycoxidation and lipoxidation reactions of nucleophilic amino acid residues and sugar- and lipid-derived reactive carbonyl species, which form during early diagenesis. Statistical analysis of experimental and fossil data identifies complex organic matter as the crosslinking product of original biomolecules, and reveals the preservation of original biosignatures related to biomineralization, tissue types, metabolic performance, and phylogenetic affinity. Chapter 3 demonstrates the potential of the molecular metabolic signal in carbonaceous fossils to address evolutionary questions. Analysis of spectral data reflecting metabolic rates in modern and fossil bone reveals that the exceptional metabolism thought to be unique to modern birds evolved in the ornithodiran ancestor of dinosaurs and flying reptiles. Ancestral warm-bloodedness was reduced to secondary cold-bloodedness in all major groups of ornithischian dinosaurs. This discovery changes our understanding of extinct animal physiology and associated life styles, and highlights the novel signals preserved in fossil molecular biosignatures. Chapter 4 investigates deep-time interactions between life’s building blocks and secondary minerals. Statistical analyses of in situ Raman spectra of pyritized fossils and associated sediments reveal that biomolecules surface-template and, in part, resource the precipitation of authigenic pyrite in alkaline, anoxic settings. In acidic settings, biomolecular organo-phosphates are cleaved during early diagenesis and resource the tissue-templated precipitation of authigenic phosphate. Thus, the local chemomilieu and tissue composition of a fossilizing organism promote the precipitation of authigenic minerals, and the preservation of soft tissue morphologies in deep time. Diagenetically mineralized complex organic matter records environmental conditions. Chapter 5 uses molecular biological and geological signatures to identify the precursors of ancient terrestrial and extraterrestrial complex organic matter. Statistical analyses of a combined in situ Raman spectroscopic data set reveal shared processes involved in the formation of complex organic matter in organismal fossils and associated sediments on Earth, and meteorites from Mars and the asteroid belt: a subset of shared precursor molecules undergo identical mineral-catalyzed crosslinking in asteroid and planetary bodies (including Earth). By analyzing compositional data of an unprecedented sample of experimentally matured organismal tissues, fossils, sediments, and meteorites, this thesis provides the first quantitative insights into the scope of organic diagenesis on Earth and in Space. The biological and environmental proxies identified have already begun to advance our understanding of the evolutionary history of life and the geological history of our solar system.

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