THERMAL ALTERATION OF PRECAMBRIAN MICROFOSSILS: APPLICATIONS TO THE STUDY OF STABLE CARBON ISOTOPES ON THE EARLY EARTH

The search for evidence of microbial life in ancient sedimentary rocks on Earth is a subject of considerable scientific interest and continued exploration into Precambrian stromatolites, microfossils, and geochemical signatures associated with biologically produced organic matter. Optical microscopy, Raman spectroscopy, and secondary ion mass spectrometry (SIMS) can be used together to detect and characterize the preservation of kerogenous microfossils and their stable carbon isotope compositions. However, many Precambrian sedimentary rocks have been altered by increasing temperatures associated with deep burial or metamorphism, which can affect the geochemical signatures of ancient microbial remains. While these processes are relatively well explained for high-grade metasedimentary rocks containing crystalline graphite, the geochemical changes from lower temperatures (<400 °C) remain poorly constrained. To help resolve these uncertainties, we investigated the stable carbon isotope geochemistry of fossil microorganisms and kerogen preserved in cherts from ten geologic units, ranging in age from ~2,100 Ma to ~420 Ma. The combination of Raman spectroscopy and SIMS performed for individual microfossils allows for direct comparison of geothermal maturity and stable carbon isotope (δ¹³C_org) values among different fossil taxa and between rocks of varying thermal maturities, from unmetamorphosed to mid-greenschist facies. The δ¹³C_org values measured here range from approximately –32‰ to –17‰, including low values and a ~10‰ variation in the most altered samples, and these data offer significant insights into the preservation of carbon isotopes associated with microbial life in thermally mature rocks of the Precambrian Earth.