Stable carbon isotope ratios and chemical properties of kerogen and extractable organic matter in pre-Phanerozoic and Phanerozoic sediments — Their interrelations and possible paleobiological significance

Abstract

Kerogen and the polar (or “humic”) fraction of benzene/methanol extractable organic matter in various pre-Phanerozoic and Phanerozoic sedimentary rocks were analyzed for stable C isotope content. Variations in the δ 13C values as functions of the geologic age and chemical properties of the organic substances were investigated. The δ 13C values of both kerogen and extractable polar organic matter in mudstones showed a tendency to decrease with increasing geologic age, possibly indicating secular decrease in atmospheric and hydrospheric CO 2. However, the secular trend of the polar extracts has a much higher statistical significance and much less scatter than the secular trend of the kerogen. This suggests that the heterogeneity and complexity of kerogen tend to mask systematic variations, whereas the polar extract, being a specific small fraction of the organic matter, varies less erratically. Bulk isotope analyses of kerogen have limited usefulness for investigation of systematic variations; the various components of the kerogen molecule should be separated by fractionation procedures and analyzed individually. The δ 13C values of all kerogen samples, regardless of rock type, age, or depositional environment, showed a highly significant negative correlation with the ratio of aliphatic to condensed aromatic components of their respective polar extracts, and δ 13C of kerogen in carbonate rocks and cherts showed a significant positive correlation with the aromaticity of the kerogen; other data indirectly suggest that carbonyl-type groups are enriched in 13C. These results demonstrate that the aliphatic components of organic matter are isotopically lighter than the associated aromatic components, suggesting that the aliphatic chains are derived from lipids of algae. Thus, fractionation of kerogen prior to isotope analysis should at least separate the C atoms of aliphatic, aromatic and carbonyl-type groups. δ 13C of kerogen in carbonate rocks and most cherts showed significant negative correlation with concentration of OH groups in polar extracts, suggesting loss of phenolic OH groups with increase in condensed-aromatic character. The observed relationships between polar extracts and kerogen confirm previously reported evidence that kerogen and its associated extractable polar substances are chemically similar and are largely derived from the same source materials. Possibly paleobiological and paleo-environmental information coded in the molecular structure of kerogen may be obtained more easily, rapidly, and economically from polar extracts. The existence of secular variation in polar extracts and a relationship between polar extracts and kerogen supports the conclusion that the polar extracts are indigenous to the rock in which they occur, and thus are valid chemical fossils. Comparisons between δ 13C values of kerogen and polar extracts from mudstones are tentatively interpreted as indicating that terrigenous organic matter, and hence land life, originated between 1.05·10 9 and 1.3·10 9 years ago as a manifestation of the early evolution of eucaryotic algae.