Abstract:

Molecular composition of extractable and detectable organic matter in hypothermal product from 49.6°E hydrothermal vent field were analyzed by GC and GC-MS. Combined with the analysis of biomarkers and individual isotopic composition, we discussed the composition, origin and genetic implication of hydrocarbons. N-alkanes(3.4373.962 μg/g) are the main hydrocarbon of sulfide chimney, with low L/H ratio (<1) and slight odd carbon number predominance in >C22 alkanes (CPI=1.1401.209); NAR is close to 0. Sulfide chimney has a variety of biomarkers (Sq, IS40, cyclohexylalkanes) ; 22S/(R+S) of C31 hopanes achieved 0.77; 17β(H) and 21β(H)hopanes were absent. The fatty acid of sulfide chimney is comprised of short chain saturated fatty acid and iso/antiiso fatty acid, and lacks monounsaturated fatty acids. Isoalkanes(2.094 μg/g)are the main hydrocarbon of hydrothermal altered rock; n-alkanes are characterized by more low carbon (L/H=1.33) and even carbon number predominance (CPI=0.377); monounsaturated fatty acids are the most abundant fatty acid. These results suggested that the autochthonous organisms were the major source of hydrocarbons in 49.6°E hydrothermal area, and that the temperature and chemical condition of hydrothermal fluid constrained the distribution of hydrothermal community and the composition of hydrocarbon of hydrothermal product. The molecular evidence for the abiogenic synthesis of organic compounds was rare, however, the coexistence of methanogens and sulfate reducing bacteria indicated that H2 were rich in hydrothermal fluids of 49.6°E vent and the abiogenic synthesis approach could not be neglected.

Key words:  ultraslow-spreading ridge, hydrothermal vent, hydrocarbon, biomarkers