Isomerisation, rearrangement and aromatisation of steroids in distinguishing early stages of diagenesis

Abstract

The distributions of proposed reactants and products of stages of early diagenesis of steroid hydrocarbons have been studied by GC and GC-MS in a variety of downhole sequences of DSDP cores of various ages, which include the San Miguel Gap, Walvis Ridge, Gulf of California and Falkland Plateau. The compounds comprise aliphatic and aromatic hydrocarbons such as diasterenes, monoaromatic anthrasteroids and spirosterenes. From the study of the progress of proposed reactions and of presumed product/reactant relationships a pattern emerges that extractable lipid distributions can provide a means of distinguishing levels of early diagenesis which cannot be recognised readily from other methods. A number of diagentic trends are evident such as progressive isomerisation at C 20 in diasterenes, which occurs at different relative rates and temperature ranges in sediment sequences with different heat fluxes. Such studies allow investigation of the changes in biological marker compounds in the part of the continuum between changes during sediment deposition and the advent of catagenesis.