Rocks ancient, organic compounds

Efforts to evaluate ancient records prompts an exploration of the factors that influence the sources of organic matter and their alteration during and after deposition (9). Toward this objective, studies of contemporary depositional settings can provide evidence of the principal influences on the production and preservation of organic matter in the rock record. Most importantly, a detailed perspective of contem rary processes permits consideration of the varied controls that determine the survival of both total organic matter and specific organic compounds like isopentenoids. 

Molecular fossils have been successfully identified in younger Precambrian rocks and linked to certain classes of biological source material. In organic analyses of ancient sediments the cleaned, pulverized rocks are treated with organic solvents to extract a soluble fraction containing the less complex and more easily identifiable compounds. However, this fraction is more subject to contamination since it is not locked within the rock matrix. Normal alkanes have been identified in extracts of the 3 billion year old Fig Tree Shale. These alkanes have a probable biological origin in cellular lipids. The odd and even-numbered alkanes are evenly distributed, a characteristic of alkanes from ancient rocks. It is uncertain, however, whether these compounds were present at the time of deposition or derived from a later source [24]. 

Biological markers, compounds derived essentially unchanged from living organisms, are found in oilbearing rocks, petroleum and most ancient sediments. 

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