The composition and the source of hydrocarbons in sediments taken from thetectonically active Andaman Backarc Basin, Indian ocean

Concentrations of total hydrocarbons in sediments of the tectonically activ e part of the Andaman Basin ranged from 130-720 (with an average of 349) mg /kg in the most active Spreading zone (the Rift Valley) to 43-180 (with an average of 113) mg/kg in the less active Fault zone of the Deep Basin. Aver age concentrations of total aromatic hydrocarbons ranged from 215 mg/kg in the Spreading zone to 49 mg/kg in the Deep Basin. Compound distributions we re dominated by short-chain n-alkanes n-C-13-n-C-24 (on the average, 77% in the Spreading zone and 64% in the Deep Basin, correspondingly). Distributi on of n-alkanes in this region is characterized by a carbon predominance in dex (CP13-35) being mainly either below or about 1,0. Substituted homologue s of biphenyl, fluorene, naphthalene, phenanthrene, benzofluorenes, chrysen e (6-methylchrysene), pyrene and perylene (3-methylperylene) as well as uns ubstituted individual polycyclic aromatic hydrocarbon (PAH): pyrene, benzo( a)pyrene, perylene, benzo(g, h, i)perylene, were identified in all samples. Concentrations of substituted homologues of PAH ranged, on the average, fr om 27 mug/kg in the Spreading zone to 16 mug/kg in the Deep Basin. Concentr ations of unsubstituted individual PAH ranged, on the average, from 25 mug/ kg in Spreading zone to 17 mug/kg in the Deep Basin. A composition of both aliphatic hydrocarbons and PAH in the sediments of the region studied is id entical to the composition of non-biological compounds: petroleum products or hydrothermal organic matter. Anthropogenic sources in region studied are of minor importance. From the results obtained, it may be deduced that the hydrocarbons in the sediments of the tectonically active part of the Andam an Basin are mainly due to an alteration of sediment organic matter by eman ation processes (hydrothermal and thermal fluxes, Earth's outgassing, petro leum show, etc.). (C) 2001 Elsevier Science B.V. All rights reserved.