Fate of calcareous nannofossils during the Rhaetian (Late Triassic): evidence from the Northern Calcareous Alps, Austria

Calcareous nannofossils evolved in the global ocean from the Carnian (early Late Triassic) and have contributed to the accumulation of biogenic calcium carbonate in marine sediments since then. Bio-diversification and bio-productivity became more significant in the Rhaetian (Late Triassic), representing an important period to understand the dynamics of calcareous nannofossil evolution. The calcareous nannofossil content of the Zlambach Formation, Northern Calcareous Alps, Austria, was qualitatively and quantitatively investigated using both scanning electron and light microscopy. The nannolith, Prinsiosphaera triassica triassica, dominates the assemblage in most samples and increases slightly in abundance in the lower Rhaetian, followed by a small-scale short-term increase during the middle Rhaetian and reaches rock-forming abundance in the upper Rhaetian. A systematic size decrease is recorded from the lower Rhaetian onwards, possibly due to changes in the palaeo-environment. The abundance of P. triassica triassica is slightly affected by the occurrence of Eoconusphaera hallstattensis and strongly by E. zlambachensis. E. hallstattensis is constrained to a relatively short interval, from the upper Paracochloceras suessi Zone (lower Rhaetian) and disappeared during the lower Vandaites stuerzenbaumi Zone (middle Rhaetian), after the occurrence of a second Eoconusphaeraceae species: E. zlambachensis. The last occurrence of E. hallstattensis comes along with the subspecies Prinsiosphaera triassica crenulata showing characteristic parallel-oriented calcite lamellae. Those three species are suggested as good biostratigraphical markers for the Upper Triassic. The coccolithophorids are present in low abundance, increasing slightly in the middle Rhaetian. After the first record of coccoliths in the middle Norian (Alaunian), the oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the upper Norian (Sevatian) and the first occurrence of Crucirhabdus primulus was recorded in the lower Rhaetian. These observations suggest a rather slow temporal diversification of the first coccolithophorids.. Zlambach, Prinsiosphaera triassica, Crucirhabdus, Archaeozygodiscus, Eoconusphaera

Automated summary

Calcareous nannofossils evolved in the global ocean from the Carnian and have contributed to the accumulation of biogenic calcium carbonate in marine sediments since then. The Rhaetian period is crucial for understanding calcareous nannofossil evolution, with significant changes in bio-diversification and bio-productivity. A study conducted in the Northern Calcareous Alps, Austria, examined the qualitative and quantitative characteristics of the calcareous nannofossil content of the Zlambach Formation. The dominant species in most samples was Prinsiosphaera triassica triassica, which increased in abundance during the lower and middle Rhaetian, eventually reaching rock-forming abundance in the upper Rhaetian. A systematic size decrease was observed, possibly indicating changes in the paleo-environment. The occurrence of other species such as Eoconusphaera hallstattensis and E. zlambachensis also influenced the abundance of P. triassica triassica, with E. hallstattensis disappearing after the appearance of E. zlambachensis. Coccolithophorids were present in low abundance, showing a slow diversification over time.