Hydrocarbons of the hypersaline basins of Shark Bay, Western Australia

Abstract

Shark Bay is an 8000 sq. km shallow marine embayment on the western coast of Western Australia. The waters exhibit a well established salinity gradient from oceanic to hypersaline. The chemical assemblage of sediment hydrocarbons along the salinity gradient may be classified into two distinct chemogeographic types. Firstly, oceanic sediments contain n-alkanes and a suite of highly branched and branched/cyclic C 25 alkenes. Hypersaline sediments are characterised by a high relative abundance of a C 25H 50 alkene together with an analogous C 20H 40 alkene and its parent C 20H 42 alkene (2,6,10-trimethyl-7-(3-methylbutyl)-dodecane). A pair of alkanes C 21H 42 and C 22H 44 increase in concentration and relative abundance with depth. The hydrocarbons of the hypersaline basins are found in only trace amounts in oceanic sediments. These chemical signals are overlain by further input indicative of the immediate biotic community.