Biogeochemistry encompasses the study of chemical fossils. It includes and draws on knowledge of the biochemical activities of contemporary organisms in modern sedimentary environments, including their roles in the biogeochemical cycling and isotopic fractionation of important elements such as carbon, oxygen, sulfur, and nitrogen, and their production of taxonomically distinctive organic compounds. This Section deals with the chemical entities preserved in the Proterozoic sedimentary record that may carry information about the biology and evolution of early life.

Chemical fossils can be discerned at the atomic level, in the occurrence of anomalous concentrations of a particular element or an isotope; at a molecular level, in the structure and stereochemistry of hydrocarbons derived from membrane lipids or pigments; and at a macromolecular level by way of the preservation of detailed chemical structures in kerogen and morphologically distinct microfossils. Paleobiochemical information is encoded in the nucleic acids of extant organisms and in their comparative biochemistry; this topic is treated in Chapter 9. Here we examine and discuss the occurrence of isotopic and molecular fossils. A considerable and consistent body of information derived, in part, from techniques developed during exploration for petroleum and minerals is now available. Rapid expansion of this knowledge is presently taking place, particularly with regard to chemical processes in early preservation of organic matter, structures of kerogen, isotopic composition of individual biomarkers, and global secular variations in organic and inorganic isotopic abundances.