Abstract When the input rate of readily degradable organic matter exceeds the replenishment rate of oxygen in marine sediments, anoxic environments develop. During anoxic diagenesis, the sediment system becomes a reservoir for sulfur due to reduction-oxidation reactions associated with communities of anaerobic bacteria. Concentrations of metals and organic carbon are covariant in many black muds and shales due to 1) direct chemical complexing of metal ions with hetero-atomic functional groups on thermally immature organic matter and/or 2) reduction of sulfate and metals using organic substrates as an electron donor and resulting in precipitation of metallic sulfides or elemental metals. Early diagenetic formation of metallo- and sulfuro-organic ligands plays an important role in selective preservation of specific organic compounds. Future research will reveal the extent to which such processes influence the overall preservation of organic matter in sediments and sedimentary rocks.

Abstract Biomarkers are individual organic constituents of sediments, sedimentary rocks and petroleums which derive from biological precursors. They constitute only a minor proportion of sedimentary organic matter, but their variety and structural diversity are invaluable aids to the decipherment and assessment of sediment maturity and depositional settings. The origins and sedimentary fates of biomarkers govern their occurrences, distributions and abundances which can be determined by a variety of chromatographic and spectrometric techniques. Biomarker assemblages provide a record of the environment in which they were deposited and the diagenetic processes that have subsequently influenced and modified them. Specific biomarker characteristics permit the differentiation of lacustrine and marine environments and can aid the assessment of sea surface temperatures and salinity levels. Also, biomarkers undergo systematic and sequential transformations during diagenesis and the changes in their compositions can therefore be used as measures of the thermal history of sediments. Furthermore, the temperature range of biomarker transformations is sufficient that a combination of diagnostic reactions can quantify maturity changes from the earliest stages of sedimentation through the phases of petroleum generation by the thermal breakdown of organic matter. The varied evidence of environmental and thermal history contained in the biomarkers of sedimentary rocks typically survives within the compositions of their derived petroleums, thereby enabling correlations between oils and their source rocks. Under suitable conditions, however, reservoired petroleums can be degraded by aerobic bacteria which selectively remove their biomarker components in an ordered sequence.

Abstract The thermal alteration of organic matter in buried sediments can be understood and modeled using the rate laws and the Arrhenius equation. However, modeling organic reactions at geologic temperatures and heating rates requires accurate activation energies and frequency factors. Although the mechanisms of kerogen degradation are complex and poorly known, appropriate kinetic parameters that fit both laboratory and geologic data can be derived from both open- and closed-system laboratory pyrolysis experiments. Algorithms that treat organic maturation as a set of parallel first-order reactions having a range of activation energies are widely used for nonisothermal data. This relatively simple approach is certainly not an accurate account of organic maturation, but it appears to give reasonable kinetic parameters for extrapolating to geologic conditions. While questions persist about the validity of extrapolating laboratory-derived kinetics to geologic temperatures and heating rates using the Arrhenius law, the problem can only be addressed empirically by testing the fit of the laboratory data with geologic observations.

Abstract As both researchers and educators, the authors have faced the difficult task of lecturing on the subject of organic geochemistry to an audience that is genuinely interested in but unable to keep pace with this rapidly advancing field. The technical jargon makes it difficult to become engaged with the topic of geochemistry without a major investment in background readings. This volume was written specifically for the graduate student or professional geoscientist needing a brief but reasonably comprehensive review of the potential applications of organic geochemical data to geological studies. This volume is divided into three sections. Section I, organic matter is viewed as a highly reactive constituent of soil, water column and sediment. Section II, the focus is on the molecular constituents of geological materials and their ability to record the history of changes in organic matter ranging from its biological formation, through sediment deposition and compaction, to its modification under the thermal stress of diagenesis and maturation. Section III, changes in the composition of organic matter in buried sediments are discussed in terms of chemical kinetics.