Geochemical features of hydrocarbon distribution in the bottom sediments of Andaman Basin (the northeastern part of the Indian Ocean)

The studies described in this article centered on the composition and distr ibution of hydrocarbons in the bottom sediments from different tectonic zon es of Andaman Basin located in the northeastern part of the Indian Ocean: t he spreading center zone (rift valley), the fault zone in the deep-water pa rt of the basin, and the zone of West Andaman Fault at the southwestern fla nk of the spreading center. It was discovered that maximum amounts of hydro carbons (130-750 mug/g, 349 mug/g on average) are confined to the bottom se diments in the spreading zone. The composition of hydrocarbons in the botto m sediments of this tectonic zone is characterized by the predominance of a romatic structures over aliphatic ones, increased amounts (75%) of low-mole cular structures among alkanes (C-13-C-24, with the prevalence of even mole cules C-16, C-18, C-22), as well as by a high abundance of low-molecular is oprenoids iC(14) -iC16. In the same zone the polycyclic aromatic hydrocarbo ns (PAHs) of a typical hydrothermal association were identified: homologs o f naphthalene, phenanthrene, benzofluorene, chrysene, pyrene, and perylene, as well as individual unsubstituted structures: pyrene, perylene, benzapyr ene, and benzo(ghi)perylene. In the fault zone of the deep-water part of th e basin, the overall amount of hydrocarbons in the sediments decreases (43- 270 mug/g), and the aliphatic structures in them prevail over aromatic ones , the amount of odd n-alkanes (nC(17), nC(19), nC(27), nC(31) typical for b iogenic components increases. Concentrations of PAH in this zone also decre ase, and their composition tends to fluctuate within a wide range. Here hom ologs of diphenyl and fluorene were identified, which are typical for high- temperature volcanic processes. In some samples there is a predominance of 6-methylchrysene and perylene, typical for a sedimentary association of PAH . The distribution of hydrocarbons in the zone of the West Andaman Fault in both the spreading zone and the deep-water zone of the basin has geochemic al features.