Lateral variability in the quantification of calcareous nannofossils in the Upper Triassic, Austria

Sammanfattning: Calcareous nannofossils are microscopic remains of organisms that diversified considerably in and after the Late Triassic, and which may have influenced seawater chemistry and carbonate production. Coccoliths, nannoliths, and calcareous dinoflagellate cysts are important parts of many calcareous nannofossil assemblages in the Upper Triassic. This is a quantification study of an Upper Triassic limestone bed, in order to examine lateral variability in calcareous nannofossil abundance at Steinbergkogel in the Northern Calcareous Alps, Austria. Steinbergkogel was at the time of deposition a deep shelf setting in the Tethys Ocean, a hemipelagic environment as a result of salt diapirism. A scanning electron microscope was used to quantify the absolute number of calcareous nannofossils in six limestone rock samples with a 2.5 m width span between the first and last sample. Each individual specimen was counted along transects in each sample. The nannolith Prinsiosphaera triassica was the most abundant species in the nannofossil assemblage, whereas coccoliths and calcareous dinoflagellate cysts were very rare. The average quantity of calcareous nannofossils in the bed was slightly lower compared to previous studies on other nearby beds at Steinbergkogel. The lateral variations in calcareous nannofossil abundance was higher than anticipated, with a quantity difference of about 10x between two of the rock samples.