Bacterial and algal markers in sedimentary organic matter deposited under natural sulphurization conditions (Lorca Basin, Murcia, Spain)

Free lipids, sulphur-bound lipids present in macromolecular fractions and kerogen pyrolysis products have been studied in shales and early diagenetic carbonates replacing gypsum from a Messinian sequence of Lorca Basin. The high abundance of phytane, 2,3-dimethyl-5-(2,6,10-trimethylundecyl)thiophene, mid-chain C20 isoprenoid thiophenes and bithiophenes and bis-O-phytanyl and O-phytanyl-O-sesterterpanyl glycerol ethers indicates that the organic matter in all these samples was deposited under hypersaline conditions. The isopranyl glycerol ethers are essentially found in the sulphur-bound macromolecular matter which contrasts with the low concentrations of these compounds as free lipids. However, the distributions of these isopranyl glycerols is paralleled by the occurrence of free phytanic acid (shales and laminated carbonate) and 3,7,11,15,19-pentamethyleicosanoic acid (only in the laminated carbonate). The bacterial inputs are represented by 2-hydroxytetracosanoic acid, n-octadec-11-enoic acid, hopanoic acids and the distributions of iso- and anteiso-C15 and C17 homologues and minor amounts of iso-C16 and anteiso-C14. These branched fatty acids are characteristic of sulphate-reducing bacteria. The relative proportions of the iso- and anteiso-compounds in the total fatty acid distributions are correlated with the proportion of reduced sulphur in the sediments.