Organic Molecules as Chemical Fossils — The Molecular Fossil Record

Abstract

The molecular record in the geosphere shows that carbon compounds can retain a high degree of structural integrity and stereochemical specificity over hundreds of millions of years, if they are not exposed to raised temperatures and are stored in the sedimentary column in such a way that they are not subject to oxidation or microbial attack. The signature of environments is written in sediments in the form of specific “biological marker” components, the presence of which, as “chemical fossils”, can be interpreted in palaeoenvironmental terms. The thermal history of a sediment is also reflected in the epimerisation, hydrogenation, aromatisation and carbon-carbon bond cleavage reactions observed through the study of samples of differing burial histories. Of particular interest is the possibility of relating the lipid record to past microbial activity. Thus, the archaebacteria possess unusual membrane lipids which may well prove a useful clue for microbial activity as far back as the Precambrian. There is a need for the recovery and study of unmet amorphosed sediments of all ages, including the Precambrian, so that micropalaeontologists, inorganic and organic geochemists, geologists and especially geochronologists, can work together towards a progressive characterisation, through time, of the various types of record left in the rocks. Only in this way will deep understanding of Precambrian events become possible.