CARBON ISOTOPIC EVIDENCE FOR THE ORIGIN OF MACROMOLECULAR ALIPHATIC STRUCTURES IN KEROGEN

This study describes compound-specific stable carbon isotopic analyses of aliphatic hydrocarbon products from flash pyrolysis (800-degrees-C , 20 s) of kerogens representing a range of depositional environments (marine, estuarine, lacustrine) and ages (Recent to Ordovician). The c arbon isotopic measurements were obtained, using isotopo-ratio-monitor ing gas chromatography-mass spectrometry (irmGC-MS), for the major ali phatic pyrolysis products (n-hydrocarbons, acyclic isoprenoids and hop anoids). While isotopic compositions of n-hydrocarbons varied substant ially between samples (average values ranged from -35 to -14 parts per thousand), delta-values of n-alkenes and n-alkanes generated from a g iven sample generally ranged over less than +/-3 parts per thousand. T his isotopic uniformity implies a common origin for n-hydrocarbons (pr obably from thermal dissociation of biopolymers), supporting the conce pt 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 m inor 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) delta-value, indica ting that the n-hydrocarbons in these samples are derived from structu rally important, or at least isotopically representative, constituents of the kerogen. In other samples significant discrepancies were obser ved between the delta-values for these products and deltaC-13(TOC). In these caws the presence of additional components not yielding hydroca rbons on pyrolysis is indicated. Isotopic compositions of isoprenoid h ydrocarbons varied non-systematically relative to those of n-hydrocarb ons, whereas hopanoid hydrocarbons were consistently depleted in C-13 relative to both TOC and n-hydrocarbons, with delta-values as low as - 50 parts per thousand.