Early Transformation of Organic Matter: The Diagenetic Pathway from Organisms to Geochemical Fossils and Kerogen

Abstract

The time covering sedimentation processes and residence in the young sediment, freshly deposited, represents a very special stage in the carbon cycle. The first few meters of sediment, just below the water-sediment contact, represent the interface through which organic carbon passes from the biosphere to the geosphere. The residence time of organic compounds in this zone of the sedimentary column is long compared to the lifetime of the organisms, but very short compared to the duration of geological cycles: a 1-m section often represents 500 to 10000 years. During sedimentation processes, and later in such young sediments, organic material is subjected to alterations by varying degrees of microbial and chemical actions. As a result its composition is largely changed, and its future fate during the rest of the geological history predetermined within the framework of its subsequent temperature history.