The Carbon Isotopic Composition of Archean Kerogen and Its Resilience Through the Rock Cycle

Abstract

The Archean rock record is limited and there is minimal organic matter available to understand the origin and evolution of life on early Earth. Low carbon isotope ratios have been measured in organic and reduced carbon phases in Archean rocks and have been invoked as biosignatures. However, it can be challenging to distinguish whether these low values reflect biotic formation, abiotic reactions, or post-depositional processes. To re-address this long-standing question, we compiled a comprehensive dataset of carbon isotope ratio measurements from organic carbon phases from Archean units that were analyzed using a variety of geochemical techniques. Our compilation also includes available descriptions and measurements of the stratigraphy, mineralogy, elemental ratios, and metamorphic grade for each data point. Our statistical analyses re-enforce a result that has been noted by prior compilations, that the carbon isotopic composition of Archean organic matter (OM) is broadly more 13C-deplete than the composition of Phanerozoic OM: The median 13C values (SD) of Archean total organic carbon and kerogen were % (n=2421) and % (n=556; Phanerozoic OM 13C SD = % with n=449 from a prior compilation). Our study also identifies a previously unrecognized bimodality within the 13C values of Archean OM that is observed even with subsampling of the data to account for geographic and stratigraphic sampling bias. We describe and model the isotopic and structural changes associated with the transformation of marine Type II kerogen from formation through diagenesis, catagenesis and metagenesis, and metamorphism, as described by trends on a van Krevelin diagram. Empirically, early maturation of organic matter during diagenesis results in shifts up to a few per-mille, which can occur in either direction depending on selective preservation and degradation of compounds. Thermal cracking that occurs during catagenesis can drive increases in 13C of 5–12%. At temperatures above greenschist metamorphism, carbon atoms exchange with other reactive carbon pools, driving increases in 13C of up to 20%. Together, our analyses suggest that the most metamorphosed graphitic samples from the earliest Archean are likely signatures of alteration, while low and multimodal ranges of 13C values may preserve records of Archean ecology.