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

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ID: 305341
2024
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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 13 C-deplete than the composition of Phanerozoic OM: The median δ 13 C values ( ±SD) of Archean total organic carbon and kerogen were −30.5±8‰ (n=2421) and −33.7±11.3‰ (n=556; Phanerozoic OM δ 13 C ±SD = −26.7±4.6‰ with n=449 from a prior compilation). Our study also identifies a previously unrecognized bimodality within the δ 13 C 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 δ 13 C of 5–12‰. At temperatures above greenschist metamorphism, carbon atoms exchange with other reactive carbon pools, driving increases in δ 13 C 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 δ 13 C values may preserve records of Archean ecology.
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Authors Sarah S. Zeichner, Woodward W. Fischer, Noam Lotem, Kelsey Moore, Joshua E. Goldford, John M. Eiler
Journal american journal of science
Year 2024
DOI
10.2475/001c.116058
URL
https://ajsonline.org/article/116058.pdf
Keywords Keywords not found

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