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

Abstract

Abstract Concentrations of total hydrocarbons in sediments of the tectonically active 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. Average concentrations of total aromatic hydrocarbons ranged from 215 mg/kg in the Spreading zone to 49 mg/kg in the Deep Basin. Compound distributions were 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). Distribution of n -alkanes in this region is characterized by a carbon predominance index (CPI 13–35 ) being mainly either below or about 1,0. Substituted homologues of biphenyl, fluorene, naphthalene, phenanthrene, benzofluorenes, chrysene (6-methylchrysene), pyrene and perylene (3-methylperylene) as well as unsubstituted 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, from 27 μg/kg in the Spreading zone to 16 μg/kg in the Deep Basin. Concentrations of unsubstituted individual PAH ranged, on the average, from 25 μg/kg in Spreading zone to17 μg/kg in the Deep Basin. A composition of both aliphatic hydrocarbons and PAH in the sediments of the region studied is identical 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 Andaman Basin are mainly due to an alteration of sediment organic matter by emanation processes (hydrothermal and thermal fluxes, Earth's outgassing, petroleum show, etc.).