Sedimentary record and archaebacteria

This chapter reviews the available ge0chemical evidence for the biogenicity of organic matter in sedimentary rocks as far as 2.7 billion years back in earth history in the light of recent findings of archaebacteria synthesizing a special closs of isoprenoid hydrocarbons and isoprenoid-glycerol ethers. The similarity of the structural distribution of isoprenoids from both the geolipio and the kerogen fractions of ancient sediments and from petroleum with that found in the lipids of archaebacteria suggests that a major amount of sedimentary organic matter has derived from the latter. Approximation of the relative contribution of chemotrophic archaebacteria as opposed to that of phototrophic bacteria and algae is difficult. However, it is apparent that a major portion of the organic matter originally assimilated by cyanobacteria (and phytoplanktion) has been reworked by archaebacteria (methanogens), at least as for back as 1 billion years and possibly as for back as 2.7 billion years ago.