Molecular Changes in Sediment Lipids as Indicators of Systematic Early Diagenesis [and Discussion]

Studies of the steroidal hydrocarbons in sediment sequences demonstrate that diagenetic changes in their molecular distributions occur systematically with burial and form a coherent continuum linking the lipid constituents of organisms to those of ancient sediments and petroleums. Such changes are initially caused by (i) microbial processes, then (ii) low-temperature reactions, which appear to be catalysed by clay minerals and, finally, (iii) thermal processes. The molecular composition at stage (iii) can depend on the proportions of the different products generated by the divergent diagenetic pathways of stages (i) and (ii). Organic molecular reactions are sufficiently sensitive that some diagenetic changes are recognizable over small (ca. 10 m) depth intervals, which should permit a system of molecular diagenetic zones to be devised. This development should enable the low-temperature thermal history of sediments to be assessed accurately. Specific diagenetic reactions within stages (ii) and (iii) also appear to reflect the sediment heat flow; thus, the reaction rates are higher in rapidly subsiding Neogene sedimentary sequences than they are in Cretaceous sediments from passive continental margins. The study of the steroidal hydrocarbons of sediment sequences recovered by the D.S.D.P. can aid the recognition and confirmation of precursor-product relations during early diagenesis. Laboratory simulations of these reactions indicate that they require acidic conditions that may be met in sediments by the action of clays. It is also apparent that single precursor compounds can give rise to more than one product as some of the diagenetic pathways are divergent. Hence, the components present in mature sediments may depend, in part, on the relative amounts of different products generated during early diagenesis. The evaluation of precursor-product relations in a series of sediment sequences demonstrates the sensitivity of molecular transormations, because differences in the extent of reactions are apparent over tens of metres and a few kelvins. A comparison of the progression of diagenetic reactions for D.S.D.P. sediments from several sites shows that a given reaction can occur (i) in sediments of different ages, (ii) at different depths and (iii) at different temperatures. Sediment sequences with similar temperature histories, however, appear to show parallel diagenetic trends. Also, precursor-product ratios may reflect the rates of heating of sediments, differ