MOLECULAR EVIDENCE OF LATE ARCHEAN ARCHAEA AND THE PRESENCE OF A SUBSURFACE HYDROTHERMAL BIOSPHERE

Molecular fossil studies of Archean sediments are highly controversial because all sediments from this time have undergone metamorphism to at least lower greenschist facies. Such thermal stress is traditionally thought to complete convert organic carbon to graphite. Additionally, the immense passage of time enables seemingly inconsequential sources of contamination to become nontrivial. As a consequence, any discussion about the paleobiological significance of a molecular fossil must be preceded by a demonstration that the molecular fossil's age is contemporaneous with that of the host-rock. The application of this simple tenet is nontrivial and often requires a multi-analytical approach. Here we apply comprehensive two-dimensional gas chromatography, scanning electron microscopy, and high pressure catalytic hydrogenation (HPCH) to resolve the composition and burial history of extractable organic matter in 2,707-2,685 million year old (Ma) metasedimentary rocks from Timmins, Ontario, Canada. These metasediments yield highly cracked and isomerized archaeal lipids and bacterial lipids that appear in conventional gas chromatograms as unresolved complex mixtures (UCMs). UCMs are largely composed of cyclic and acyclic biphytanes, C₃₆-C₃₉ derivatives of the biphytanes, and C₃₁-C₃₅ extended hopanes. Biphytane and extended hopanes are also found in HPCH products released from solvent-extracted sediments, indicating that archaea and bacteria were present in Late Archean sedimentary environments. Post-depositional, hydrothermal gold mineralization and graphite precipitation occurred prior to metamorphism (~2,665 Ma). Late Archean metamorphism significantly reduced the kerogen's adsorptive capacity and severely restricted sediment porosity, limiting the potential for post-Archean additions of organic matter to the samples. Argillites exposed to hydrothermal gold-barring fluids have disproportionately high concentrations of extractable archaeal and bacterial lipids relative to what is releasable from their respective HPCH product and what is observed for argillites deposited away from these hydrothermal settings. The addition of these lipids to the sediments likely results from a Late Archean subsurface hydrothermal biosphere of archaea and bacteria.