The emergence of pelagic calcification and its influence on seawater chemistry in the Upper Triassic

Sammanfattning: After the early biomineralisation event in the early Paleozoic (Cambrian –Ordovician; 541 – 443.8 Ma), the Late Triassic (237 – 201.3 Ma) represents another key period of biomineralisation with the onset of the Mesozoic plankton radiation. In the Carnian (237 – 227 Ma), the first calcifying dinoflagellates are recorded followed by the nannoliths in the middle Norian (Alaunian) and the coccolithophorids dated until this study at the Norian/Rhaetian boundary. Those marine primary producers are influencing the concentration of oceanic and atmospheric CO2 via their photosynthesis and calcification processes. Timing and quantification of their early evolution would help to understand their impact on the ocean chemistry during the Late Triassic. The Austrian Northern Calcareous Alps, Romanian North Dobrogea and North West Australia record several good potential sediments from the Upper Triassic. This project aims to investigate Norian to Rhaetian outcrops from widespread localities around the poorly studied Paleo-Tethys as well as the Neo-Tethys Ocean in the Upper Triassic. The calcareous nannofossils contents were observed by light and scanning electron microscope and quantified (if present) in six Austrian and Romanian sections (25°N), one section from Turkey (palaeo-equator), and six sections from Oman (20°S). In parallel, geochemical analyses were performed first with trace elements concentration to evaluate the impact of diagenesis on the preservation of the sediments and calcareous nannofossils but also to trace changes in weathering rate during the Late Triassic. Second, isotopic measurements were performed for strontium, calcium and carbon to better constrain the environmental conditions during the early evolution of the calcareous nannofossils.The first coccoliths, not identified at a species level, occurred in the middle Norian (Alaunian 3). The oldest coccolith species identified is Crucirhabdus minutus, observed in the upper Norian (Sevatian), followed by Archaeozygodiscus koessenensis. Crucirhabdus primulus first occurred in the lower Rhaetian. These occurrences suggest C. minutus as the ancestor of the coccolithophorids and a slow temporal evolution. C. primulus occurred ~ 4.2 million years after the ancestor C. minutus, and the evolution of the new genus A. koessenensis takes around 0.35 Myr. The incertae sedis, Prinsiosphaera triassica dominates the assemblage throughout the Upper Triassic. Its abundance increases slightly in the lower and middle Rhaetian and reaches its maximum abundance in the upper Rhaetian. Detailed microscopic investigations detected two different significant structures of the conical Rhaetian forms belonging to Eoconusphaera. Based on this, a new species, E. hallstattensis was described and E. zlambachensis was emended. Those two species represent new biostratigraphic markers for the Rhaetian with short and specific occurrence intervals. Eoconusphaera hallstattensis is constrained to the upper Paracochloceras suessi Zone (lower Rhaetian) and disappeared during the lower Vandaites stuerzenbaumi Zone (middle Rhaetian), wherein it is progressively replaced by the second Eoconusphaeraceae species E. zlambachensis. Throughout the Rhaetian, P. triassica is affected by biological and environmental stress conditions. First, with the occurrence of Eoconusphaeraceae introducing competition between the P. triassica and the Eoconusphaeraceae and, second palaeo-environmental changes alter its calcification potential, leading to a size decrease from the lower Rhaetian. These two species, P. triassica and E. zlambachensis were observed for the first time in Romanian sections (North Dobrogea) located in the Palaeo-Tethys Ocean during the Late Triassic and are common (if present). During the Upper Triassic, the calcareous nannofossils are observed in both hemispheres and show increasing abundance through the Rhaetian and are described as rock-forming during the upper Rhaetian. The comparison between quantification data and calcium isotope measurements does not show any evidence for a significant influence of those calcifiers on the geochemical composition of the Western Neo-Tethys Ocean.