Integrated Stratigraphy (Radiolarians, Calcareous Nannofossils, Carbon and Strontium Isotopes) of the Sinemurian–Pliensbachian Transition at Mt. Rettenstein, Northern Calcareous Alps, Austria

A 95 m thick succession of grey siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps allowed us to study well-preserved and diverse radiolarian assemblages across the Sinemurian–Pliensbachian boundary. The distribution of 31 most characteristic radiolarian species is presented. The radiolarian data are complemented with calcareous nannofossil biostratigraphy, and carbon and strontium isotope analyses of bulk carbonate samples. Ammonites of the Jamesoni Subzone, the topmost subzone of the Jamesoni Zone, have been previously found in the upper part of the section. A negative stable carbon isotope excursion (CIE) occurs in the middle part of the section and is correlated to the supra-regional Sinemurian–Pliensbachian boundary Event. Radiolarian assemblages below the negative CIE belong to the C anutus rockfishensis – Wrangellium thurstonense and Jacus ? sandspitensis radiolarian zones. Katroma clara and several other species of the subsequent Canutus tipperi – Katroma clara Radiolarian Zone first occur at the beginning of the negative CIE. The first occurrence of the nannofossil Similiscutum cruciulus , which defines the base of the NJ4 Calcareous Nannofossil Zone, was recognized near the top of the negative CIE, 10 m above the inferred stage boundary. The strontium isotope ratio is due to diagenetic alteration of bulk micrite systematically shifted to higher values, but clearly decreases across the stage boundary. This decrease is compatible with the trend of the standard 87 Sr/ 86 Sr curve, which was established in skeletal calcite.The radiolarian zones across the Sinemurian–Pliensbachian boundary are for the first time calibrated to chronostratigraphy outside North America and, also for the first time, tied to calcareous nannofossil zones and chemostratigraphy. The results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemurian–Pliensbachian boundary in deep-marine successions, in which ammonites are missing