Effects of irradiation from uranium decay on extractable organic matter in the Alum Shales of Sweden

Abstract

Extractable organic matter in the Alum Shales of Sweden is believed to have been altered by irradiation from the natural decay of uranium. The amount of extractable bitumen is inversely proportional to uranium concentration suggesting conversion of extractable bitumen by irradiation. The ratio of aromatic to saturated hy hydrocarbons increased with increasing uranium concentration, presumably because aromatic hydrocarbons are more stable in the presence of radiationthan saturated hydrocarbons. Bitumen from samples with high uranium concentrations tends to be relatively enriched in aromatic hydrocarbons and contains a larger percentage of polars, asphaltenes and resins. Acyclic isoprenoid hydrocarbons are absent in extracts and kerogen pyrolysates of samples with high uranium concentrations. They therefore appear more susceptible to irradiation induced alteration than normal alkanes, which are present in almost all extracts and kerogen pyrolysates. Parent/alkylated polyaomatic hydrocarbon (PAH) ratios are highest in shallow samples with high uranium concentrations. Extracts from samples low in uranium concentration contain C 20, C 21, C 26, C 27 and C 28 triaromatic steroids, whereas extracts from samples of high uranium concentration contain only C 20 and C 21. Alkyl side chain cleavage by irradiation is the mechanism presumed responsible. Ratios of pyrene/phenanthrene and pyrene/chrysene are highest whereas ratios of 1+9/2+3 methylphenanthrenes are lowest in shallow samples with high uranium concentrations.