Isotopic studies on the palaeoecology and diagenesis of the Great Estuarine Series (Jurassic) of Scotland

Abstract

Synopsis: Over 700 carbon and oxygen isotopic analyses of fossils and carbonate rocks refine previous interpretations of the Great Estuarine Series. Many of the fossils are unaltered mineralogically and isotopically. Praemytilus and Liostrea inhabited water of variable isotopic composition and salinity; growth temperatures around 22°C are suggested for Praemytilus . Some Unio inhabited freshwater; other Unio , and Neomiodon , have normal marine δ C ¹³ and δ O ¹⁸ , which is contrary to palaeontological expectation. Fossil-matrix isotopic contrasts show the dominant influence of diagenesis on whole-rock isotopic composition. Early diagenetic carbonates have variable, usually low, δ C ¹³ (to −29), showing algal and bacterial influence. Sparry calcite cements have low δ O ¹⁸ but positive δ C ¹³ . Isotopic data confirm the solution-reprecipitation mechanism for the replacement of aragonitic shells by calcite. A stratigraphical review shows strongly variable isotopic composition of fossils from the Mytilus Shales, mainly marine values for the middle part of the Series, mainly freshwater for the Ostracod Limestones and Mottled Clays and a transgression in the Staffin Bay Beds.