Primary versus diagenetic origin of Blue Lias rhythms (Dorset, UK): evidence from sulphur geochemistry

The question of a primary versus diagenetic origin for the limestone-shale rhythms of the Blue Lias has been addressed through a study of pyrite abundance and isotopic composition. Pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent, less calcareous, sediment. The abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation. The isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate, part of which was subsequently reduced back to pyrite before preservation by burial. Acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution, hence supersaturation of sulphides could be maintained. By contrast, in adjacent less calcareous sediments, carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron-rich pore solutions. Continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and, with burial below the zone of bioturbation, the alkalinity so generated caused cementation of the limestones. Diagenetic cementation would be enhanced during an hiatus in sedimentation, an event which might be related to a Milankovitch forcing mechanism, but which would not be recorded in bioturbated, less calcareous sediment, thus leaving an imperfect record. Only cyclicity in pre-diagenetic sedimentation patterns may be safely related to a Milankovitch forcing mechanism as proposed by Weedon.