The origin of pelagic calcification and its influence on seawater chemistry

Impacts of climate change on marine biology and the potential feedback mechanisms, positive or negative, are considered to be among the greatest unknowns in our understanding of future climate evolutions. A key question is how the uptake of increased atmospheric CO2 into the ocean affects the ocean ecosystem. Coccolithophorids are considered to be the most productive calcifying organism on earth and therefore to play an important role in the marine carbon cycle, as biological pump and mixer of oceanic alkalinity, a change in their calcification rate will directly affect air/sea CO2 exchange. Laboratory and field experiments on calcifying organisms indicate a range of responses to acidification, including both decreased and increased calcification. The geological records can complement modern observational data by providing records on longer time scales, integrating short- term physiological and longer-term evolutionary responses to elevated pCO2. One particular point of interest is the emergence of the calcifying plankton in the marine ecosystem shifting the major carbonate production from the shallow seas to the open marine realm. This paradigm change in sea- water chemistry, predicted by models, is called the Mid-Mesozoic Seawater Revolution. Recent works have constrained this major evolution to the Upper Triassic. This time slice sees the first occurrence of the coccolitophorids, an explosion of the other calcareous nannoplankton and a reefal bloom, but at the same time an important extinction among higher nektonic organisms. It is followed by the end-Triassic mass-extinction, where these organisms went almost extinct and their second expansion during the early Jurassic. In a short interval we can thus observe two times the effect of the emergence of calcifying plankton and the effect of their disappearance on the seawater chemistry. This project is designed to better understand the timing of the emergence of planktonic calcification and its influence on seawater geochemistry. To reach these goals, a multi-disciplinary and multi- proxies approach is developed. The project combines paleontological studies of the calcareous nannoplankton, geochemistry on traditional (Ccarb, Corg) and non-traditional isotope system (Ca, Mg, Sr) and climate models. To better understand as well paleo-latitude as bathymetric effects, analyses will be conducted on two shallow-to-depth transects in the Tethys, in the Northern Calcareous Alps of Austria (palaeo 20N) and in Oman (Palaeo 20S), as on some sections at palaeo-equator (today Turkey). The final goal of this project is to better constrain the effect of the emergence of the modern biological pump on the atmospheric pCO2 and thus better estimate the impact of the calcifying plankton on the climate.

Calcareous nannoplankton represents the most productive group of calcifying organisms nowadays with tremendous influence on climate and seawater chemistry. Foreseen changes in their calcification rate due to anthropogenic climate changes will directly affect global air-sea CO2 exchange. The aim of this study was to verify the hypothesis that the development of the calcareous nannoplankton had a significant influence on the seawater chemistry at an early stage of its evolution (Late Triassic). This is of significant interest for the understanding of the effect of rising anthropogenic CO2 on the calcifying nannoplankton and on ocean acidification and will better constrain models of the past oceans. Therefore, this project investigated quantitatively calcareous nannofossil assemblages in Austrian sections in the middle Norian and Rhaetian (Late Triassic). Parallel to these investigations, isotopic proxy measurements (Strontium, calcium, carbon, oxygen), Hg concentration, elemental and mineralogical determinations were carried out. The geochemical analyses targeted the Late Triassic time interval (~ 218-199 millions years ago (Ma)) in sections in Austria, Turkey and Oman. By comparing the calcareous nannofossil abundances with calcium and carbon isotopic composition data, no clear correlations were observed. Therefore, regarding the initial question of this study it can be said that their importance in global geochemical cycles remained limited in the Late Triassic. The micropaleontological investigations significantly improved the knowledge about the early evolution of calcareous nannofossils. The record of the oldest coccolith was dated back to the middle Norian (~ 215 Ma). Our observations suggest a rather slow temporal diversification of the first coccolithophorids. While the isotopic proxies revealed important paleoenvironmental changes. The strontium and calcium isotopic results highlighted the possible role of carbonate and evaporite dissolution on the chemical composition of the seawater due to a major sea-level fall at the Norian-Rhaetian boundary (~208 Ma). Longer-term strontium and carbon isotopic trends of the middle and late Rhaetian (~204-201 Ma) are compatible with the incipient break-up of the Pangea supercontinent. The emplacement of the Central Atlantic Magmatic Province (201 Ma) left its imprint on all the investigated isotopic systems. Preceding this event, no evidence was found for another significant volcanic event during the investigated time interval.