Molecular changes in sediment lipids as indicators of systematic early diagenesis

Abstract

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.