Preservation and Distributions of Covalently Bound Polyaromatic Hydrocarbons in Ancient Biogenic Kerogens and Insoluble Organic Macromolecules

The likelihood of finding pristine molecular biosignatures preserved in Earth's oldest rocks or on other planetary bodies is low, and new approaches are needed to assess the origins of highly altered and recalcitrant organic matter. In this study, we aim to understand the distributions and systematics of preservation of ancient polycyclic aromatic hydrocarbons (PAHs), as both free hydrocarbons and bound within insoluble macromolecules. We report the distributions of bound PAHs generated by catalytic hydropyrolysis from ancient biogenic kerogens and from insoluble organic matter (IOM) in high-temperature carbonaceous residues from pyrobitumens and synthetic coke. For biogenic kerogens, the degree of thermal maturity exerts the primary control on the preservation and distributions of the major five-ring and six-ring PAH compounds. This holds for both Precambrian and Phanerozoic rocks, thus source variation in primary biogenic organic matter inputs does not exert the major control on bound PAH. The IOM samples, predominantly residues from hydrocarbon cracking at high temperatures, preserve a bound PAH profile significantly distinct from ancient biogenic kerogens and characterized by an absence of perylene and higher abundance of large-ring condensed PAHs. Covalently bound PAH profiles offer promise as "last resort" molecular biosignatures for aiding the astrobiological search for ancient life.