OCCURRENCE AND ORIGIN OF MONOMETHYLALKANES, DIMETHYLALKANES, AND TRIMETHYLALKANES IN MODERN AND HOLOCENE CYANOBACTERIAL MATS FROM ABU-DHABI, UNITED-ARAB-EMIRATES

n-Alkanes, highly branched isoprenoids, monomethylalkanes (MMAs), dime thylalkanes (DMAs), and trimethylalkanes (TMAs) are the most abundant components in the hydrocarbon fractions of extracts of four modern and two Holocene cyanobacterial mats (similar to 1500 and 5110 +/- 170 y sp) collected in Abu Dhabi (United Arab Emirates). The homologous fami lies of MMAs, DMAs, and TMAs were identified by comparison of mass spe ctral and relative retention time data with published data. DMAs were also identified by synthesis of authentic standards, 3,9-dimethyltrico sane, 5,9-dimethyltricosane, and 11,15-dimethylheptacosane. MMAs, DMAs , and TMAs of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures. MMAs and DMAs in the C-16-C-22 range are characterized by methyl substituents m ainly located at C-6 (or omega 6) and C-7 (or omega 7) and are derived from cyanobacteria. They are relatively abundant components in the mo dern cyanobacterial mars, but with increasing age of the mats they bec ome much less abundant. On the contrary MMAs, DMAs, and TMAs in the C- 24-C-45 range are exclusively found in the Holocene cyanobacterial mat s. Their longest chains mainly contain an odd number of carbon atoms a nd they always carry the methyl substituents at odd numbered carbon at oms. The similarity in composition of this very specific group of bran ched alkanes with that encountered in insect epicuticular waxes sugges ts that these sedimentary hydrocarbons originate from insects, which p robably grazed on the cyanobacterial mats.