Biomarker indications for microbial contribution to Recent and Late Jurassic carbonate deposits

Biomarker investigations were applied to the hydrocarbon fractions of three Recent (cyanobacterial mat, Lake Van microbialite and Lake Satonda microbialite) and two Late Jurassic carbonate samples obtained from sponge bioherms. The relative concentrations of <i>n</i>-alkanes, monomethyl alkanes, acyclic isoprenoids, steroids and hopanoids in these samples are studied and their probable biological precursors are discussed. Normal alkanes with carbon chain lenghts ranging from C₁₅ to C₃₄ and monomethyl alkanes ranging from C₁₇ to C₂₁ with a varying methyl branching pattern are found. The major hydrocarbons are low molecular (LMW) <i>n</i>-alkanes (C₁₅ - C₂₁) with a slight to strong predominance of <i>n</i>-heptadecane (C₁₇). High molecular weight (HMW) <i>n</i>-alkanes occur in low to moderate relative concentrations showing a preference of odd-numbered compounds with a maximum at C₂₉. Within the acyclic isoprenoids, pristane, phytane/phytene, pentamethyleicosane, squalane and lycopane could be identified. Polycyclic terpenoids of the sterane and/or hopane type are present in all carbonate samples. The carbon atom numbers of these compounds range from 27 to 29 and 27 to 32, respectively. These organic compounds identified can be attributed to various source organisms such as cyanobacteria, archaebacteria, algae and vascular plants. All hydrocarbon fractions of the samples are characterized by moderate to high relative concentrations of cmpounds derived from caynobacteria, signifying the role of these organisms as contributors to the Recent as well as to the Late Jurassic carbonate deposits.