Archaebacteria biosynthesize specific membrane lipids consisting of isoprenoids ether-bound to glycerol moieties (de Rosa and Gambacorta, 1988). These biological markers have been detected in sediments as well (e.g. Chappe et al., 1982) but their palaeoenvironmental significance is, until now, still unknown. In this study several sediments were analyzed for these compounds and their distribution and stable carbon isotopic compositions were determined.
High amounts of biphytanyl carbon skeletons with 0-3 cyclopentyl rings and low amounts of 2,6,10,14,18-pentamethylicosane (PME) were detected after treatment of several Arabian Sea surface sediment extracts with hydriodic acid and subsequent reduction. Using authentic standards the biphytanes with 0-2 cyclopentyl rings were unambigously identified whilst the biphytane with 3 cyclopentyl rings was identified based on mass spectral data. Since the sediments are derived from a normal marine setting it is, in this case, clear that only methanogens can be the source of these lipids. These specific structures with cyclopentyl rings have been reported to occur in Methanosarcina species (de Rosa et al., 1986). The biphytanyl compounds probably originate from a single biological source throughout the Arabian Sea because of the structural consistency and identical isotopic composition of these compounds from geographically widespread locations. The occurrence of these compounds in sediments, which are still oxygenated, suggests a significant contribution from methanogenesis in the water column, which is in complete agreement with the methane supersaturation of the Arabian Sea. This coincides with the occurrence of an intense oxygen minimum zone (OMZ) which provides an excellent environment for continued methanogenesis in the faecal pellets after excretion from the guts of zooplankton. Samples from within and below the oxygen minimum zone show large variations, up to two orders of magnitude, in concentration of the ether lipids. It is therefore likely that the methanogens continue to thrive in the anoxic sediments deposited in the OMZ. This headstart of the methanogens in the water column and subsequent excellent conditions in the sediment as far as anoxia and probably substrate type is concerned may explain the much reduced bacterial sulphate reduction. The co-occurrence of sulphate reduction and methanogenesis, or perhaps reversal in order of the "layered" reductive sedimentary zones is contrary to popular geochemical belief where sulfate reduction by definition excludes methanogenesis. We have detected similar high concentrations of methanogenic ether lipids in Holocene Black Sea sediments, in the Pliocene Lido Rossello, the Messinian Gessoso-Solfifera Formation and Miocene Monterey Formation. Depth profiles of these compounds in sediments from the Monterey Formation spanning 10 million years, show large variations (2 orders of magnitude) in ether lipid concentrations, which seem to correlate with lithology. The current data suggest that extremely high concentrations of the methanogenic ether lipids can be indicative of an intense OMZ impinging on shelf/slope sediments.
Chappe, B., Albrecht, P. & Michaelis, W., Science 217, 65-66 (1982).
de Rosa, M. et al., Biochim. Biophys. Acta 875, 487-492 (1986).
de Rosa, M. & Gambacorta, A., Prog. Lipid Res. 27, 153-175 (1988).