The Proterozoic Eon extends from 2500 to 550 Ma, from the close of the Archean to the beginning of the Cambrian. It includes three principal geochronologic divisions: Lower or Proterozoic I (2500 to 1600 Ma), Middle or Proterozoic II (1600 to 900 Ma), and Upper or Proterozoic III (900 to 550 Ma). These definitions are consistent with previous usage (Schopf 1983a) and with recommendations of the Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences (Plumb and James 1986). Although some criticism has been voiced at the use of absolute ages rather than stratigraphic or paleontologic events for subdividing Precambrian time (Cloud 1987), we find that the lack of well-developed, widespread, narrowly constrained, isochronous Precambrian biostratigraphic markers, equivalent to Phanerozoic faunal successions, presents an as yet insurmountable barrier to the establishment of globally useful Precambrian biostratigraphic subdivisions.
Systematic treatment of the geological evolution of the Proterozoic earth and similar long-term or large-scale aspects of Proterozoic history is complicated at present by our incomplete knowledge of existing Proterozoic rocks, selective preservation/obliteration of certain types of terranes, and uncertain geochronology and correlation. Many Proterozoic sequences remain essentially unstudied, especially in parts of central and northern Africa, South America, and Asia, and their stratigraphies, ages, and tectonic settings are unresolved. Information from these sequences is essential to evaluation of global patterns of Proterozoic geologic evolution, sediment recycling, and tectonics.
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.
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