Three series of non-isoprenoidal dialkyl glycerol diethers in cold-seep carbonate crests

Abstract

Three novel series of non-isoprenoidal dialkyl glycerol diethers with an inferred sn-1,2 stereochemistry were tentatively identified in carbonate crusts precipitated from methane-rich bottom-waters and pore-waters associated with Mediterranean mud volcanoes. In the first series of diethers, an 11,12-methylenehexadecyl moiety is ether-bound at the sn-2 position of the glycerol group, and in diethers from the second series, an anteiso pentadecyl group is ether-bound at the sn-2 position. In the third and minor series a pentadecyl moiety is ether-bound at the sn-2 position. In all series, ether-bound C14–C17 alkyl units (either n-alkyl, branched alkyl, alkylcyclohexyl or methylenealkyl) occur at the sn-1 position. The two major series are also distinguished by their 13C values, which differ by 15–30‰ at the sites studied. Only the minor series has been previously reported in sediments. Members of the second and third series are similar to diethers containing C16–C18 iso, anteiso, and n-alkyl units present in the thermophilic bacteria Thermodesulfobacterium commune, Ammonifex degensii and Aquifex pyrophilus. The first series is unique, in that diethers possessing alkyl chains that contain a cyclopropane ring have never been reported.