Development of early calcareous nannoplankton in the late Triassic (Northern Calcareous Alps, Austria)

Abstract

Calcareous nannofossils are considered to be the most productive calcifying organisms, but the impact of their emergence on the chemistry of the ocean in the Late Triassic is not understood yet. Evolutionary details of this bio-event are missing due to the scarcity of well-preserved outcrops worldwide. The current study focuses on the calcareous nannofossil assemblage of the middle Norian to lower Rhaetian at Sommeraukogel and Steinbergkogel in the Northern Calcareous Alps (Austria). The sediments were deposited on a topographic high on a deeper shelf at a palaeolatitude around 20–30°N. The steps in the emergence of pelagic calcifiers are documented by a scanning electron microscope (SEM) study. The First Occurrence (FO) of coccolith, not identified at a species level, was recorded in the middle Norian (Alaunian 3). The oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the late Norian (Sevatian) and were followed by the FO of Crucirhabdus primulus in the early Rhaetian. These observations suggest a rather slow temporal evolution of the first coccolithophorids, with millions of years from the ancestor C. minutus to C. primulus, and in between the evolution of the new genus A. koessenensis. Diagenetic overprinting of the sedimentary succession has affected the preservation degree of the calcareous nannofossils but not the trend of their quantity in the studied samples, as proven by petrographic studies and trace element signatures of the calcareous sediments. This supports our quantitative estimates of the CaCO3 volume- and palaeo-fluxes due to the export productivity of the calcareous nannofossils, Prinsiosphaera triassica. Our results show the dominance and the increase in abundance of Prinsiosphaera triassica slightly above the Norian/Rhaetian boundary. However, calcareous nannofossils did not reach rock-forming abundances at this time and thus did not significantly influence the geochemical composition of the Western Tethys Ocean.