Chapter 7 Diagenesis of Organic Matter

Abstract

Geochemical data are presented in this chapter which shed some light on the type of alteration organic matter undergoes in the course of diagenesis and epigenesis. In general, the formation of fossil organic matter is accomplished by a three-step process: ( 1 ) the microbial and chemical (hydrolysis) destruction of former biochemical macromolecules in the early stage of diagenesis; (2) the condensation of metabolic and hydrolysis products, resulting in the formation of “heteropolycon-densates” (humic materials); and ( 3 ) a slow inorganic maturation of the hetero-polycondensates (e.g., loss in functional groups), with thermal degradation being the number one alteration factor. It is tentatively suggested that, given sufficient time, all fossil organic matter is eventually reduced either to aromatic condensates resembling graphite, or to light paraffinic hydrocarbons; temperature will accelerate the reactions. The occurrence of original biogenic matter such as amino acids, sugars, fatty acids, or the bases of the purines and pyrimidines in ancient rocks may serve as an indication that diagenesis and epigenesis of organic matter has not yet reached its final stage. Their presence may eventually help to outline the thermal history of the host rock. Aside from a discussion of the geologic history of biogenic matter, some ideas are expressed on the origin and diagenetic fate of abiotic organic matter that once has been synthesized on a prebiotic earth or on the meteorite parent bodies. Hydrocarbons and other organic volatiles in meteorites are assumed to have been generated from the finely disseminated organic matter. This diagenetic formation occurred rather late in the history of the meteorites, i.e., after the incident of asteroid collision and distintegration.