The emergence of pelagic calcification in the Upper Triassic and their impact on biogeochemical cycles

Calcareous nannoplankton is the most productive calcifying organism nowadays and has a tremendous influence on the climate and the seawater chemistry, as a biological pump and as a regulator of surface ocean alkalinity. Modelling studies neglected this group as a potential influencer before the Jurassic, while first quantitative paleontological studies suggested that already before the Triassic-Jurassic boundary, calcareous nannofossils reached rock-forming abundance and influenced the oceanic system. To resolve this contradiction, we investigated quantitatively the calcareous nannofossil assemblages by light and scanning electron microscope in one Romanian (~50°N) section, six Austrian (25°N) sections, one section from Turkey (palaeo-equator), six sections from Oman (20°S) and one in Australia (30°S). In parallel, geochemical analyses were performed to evaluate the impact of diagenesis on the preservation of the sediments and calcareous nannofossils. Further, isotopic measurements were performed for strontium, calcium, carbon and oxygen to better constrain the environmental conditions during the early evolution of the calcareous nannoplankton.

After the oldest occurrence of the calcareous nannofossil during the Carnian, the nannolith Prinsiosphaera triassica first appears and dominates the assemblage with a low abundance, during the Norian. It increases slightly in abundance during the lower Rhaetian, followed by a small-scale short-term increase during the middle Rhaetian and reaches rock-forming abundance only in the upper Rhaetian. The abundance of P. triassica is affected by the occurrence of a second nannolith, Eoconusphaera hallstattensis, in the lower Rhaetian. The coccolithophorids are present in low abundance, increasing slightly in the middle Rhaetian. After the first record of coccoliths in the middle Norian, the oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the upper Norian and the FOD of Crucirhabdus primulus was recorded in the lower Rhaetian. These observations suggest a rather slow temporal diversification of the first coccolithophorids with only three speciation in ~ 10 million years.

From the latest Norian to the lower Rhaetian, both the ⁸⁷Sr/⁸⁶Sr and δ⁴⁴/⁴⁰Ca record a sharp trend towards unradiogenic values. The strong correlation between the strontium and calcium isotopic systems indicates a connection through the same driving processes. In opposition, the δ¹³C_carb is quite stable around this interval. Our hypothesis explaining these observations is the dissolution of carbonate and evaporite due to a major sea-level fall around the Norian-Rhaetian boundary. On the contrary, the longer-term ⁸⁷Sr/⁸⁶Sr and δ¹³C isotopic trends are compatible with the incipient break-up of the Pangea supercontinent.

Comparing the calcareous nannofossil abundance and isotopic composition results, no clear correlations were observed. Theoretically, the increasing proportion of calcareous nannofossils in the sediments should have influenced the carbon and calcium isotopic composition. The lack of correlation suggests a limited role of the calcareous nannoplankton in global geochemical cycles during the Late Triassic and a significant contribution to shifting the major carbonate production from the shallow seas to the open marine realm only during the Jurassic.