Are the n-alkane carbon isotopic profiles of South Australian coastal asphaltites indicative of their extent of weathering?

Abstract

Asphaltic bitumen (asphaltite) found on the ocean beaches of South Australia is considered to be a product of an adjacent, but as yet unlocated, submarine oil seep. Four large asphaltites recently stranded on three different beaches were sub-sampled (n = 5–6). Samples were analysed to determine if their n-alkane δ13C profiles displayed systematic variation along a transect from the centre to the outer rim of the specimen. Previously reported 13C-enrichment in n-alkanes isolated from the outer portions of similar archival asphaltites was attributed to weathering. With one notable exception, these new specimens exhibited no unidirectional enrichment from interior (fresh) to exterior (weathered), although in every instance n-alkanes in the intermediate and outermost sub-samples were for the most part isotopically heavier than the same homologues in the central interior of the specimen. To be a viable measure of the extent of weathering in a population of coastal asphaltites, and hence also their relative exposure time in the ocean, this analytical protocol requires the specimens to have remained physically intact during their transit from the parent seep to shore. Whether this requirement has been met is impossible to determine, thereby limiting the effectiveness of the method to those specimens in which there is a consistent pattern of internal variation in their n-alkane carbon isotope profiles.