Carbon isotopic evidence for the origin of macromolecular aliphatic structures in kerogen

Abstract

This study describes compound-specific stable carbon isotopic analyses of aliphatic hydrocarbon products from flash pyrolysis (800°C, 20 s) of kerogens representing a range of depositional environments (marine, estuarine, lacustrine) and ages (Recent to Ordovician). The carbon isotopic measurements were obtained, using isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGC-MS), for the major aliphatic pyrolysis products ( n-hydrocarbons, acyclic isoprenoids and hopanoids). While isotopic compositions of n-hydrocarbons varied substantially between samples (average values ranged from −35 to −14 0 00 ), δ-values of n-alkenes and n-alkanes generated from a given sample generally ranged over less than ±3 0 00 . This isotopic uniformity implies a common origin for n-hydrocarbons (probably from thermal dissociation of biopolymers), supporting the concept of selective preservation of highly resistant aliphatic structures in kerogens. There are notable exceptions to this general relationship, in which either mixed biopolymeric sources (e.g. Guttenberg Fm) or minor contributions from aliphatic biopolymers (e.g. Monterey) must be inferred. In several cases, average n-hydrocarbon values also closely match the corresponding total organic carbon (TOC) δ-value, indicating that the n-hydrocarbons in these samples are derived from structurally important, or at least isotopically representative, constituents of the kerogen. In other samples significant discrepancies were observed between the δ-values for these products and δ 13C TOC. In these cases the presence of additional components not yielding hydrocarbons on pyrolysis is indicated. Isotopic compositions of isoprenoid hydrocarbons varied non-systematically relative to those of n-hydrocarbons, whereas hopanoid hydrocarbons were consistently depleted in 13C relative to both TOC and n-hydrocarbons, with δ-values as low as −50 0 00 .