Causes and consequences of calcareous nannoplankton evolution in the Late Jurassic : implications for biogeochronology, biocalcification and ocean chemistry.

Abstract The Jurassic was a time of important changes in the ocean/continent configuration: important reorganization of oceanic and climatic condition are underlined by a most remarkable widespread shift from mostly siliceous to mainly calcareous sedimentation. The beginning of Late Jurassic was a time of exceptionally low carbonate accumulation rates, while the uppermost Jurassic is characterized by high sedimentation rates and the deposition of calcareous nannofossil oozes. During the Late Jurassic calcareous nannoplankton experienced a progressive increase in diversity, abundance and degree of calcification, culminating in the Middle Tithonian – Lower Berriasian interval. Upper Callovian – Lower Berriasian sections from the Southern Alps (Northern Italy) have been analyzed for calcareous nannofossil biostratigraphy; selected sections (Southern Alps )and the DSDP Site 534 A (Atlantic Ocean) were investigated for calcareous nannofossil relative and absolute abundances and to derive paleo-fluxes. Data were compared with litho-magnetostratigraphy, calpionellid biostratigraphy, where available, and information on the tectonic, palaeoceanographic and palaeoclimatic regime. Biostratigraphic investigations led to revise the biostratigraphic schemes available for Late Jurassic, and a new scheme is proposed for Tethyan Realm. Quantitative investigations and derived paleo-fluxes show a calcareous nannofossil increase in diversity, abundance and calcification, inducing a major change in pelagic sedimentation from predominantly siliceous (lower part of the Rosso ad Aptici) to mostly calcareous (Rosso ad Aptici – Maiolica transition and Maiolica). In particular, an impressive speciation started in the Tithonian, including the first occurrence and early diversification of nannoliths and nannoconids. The increase in abundance of coccoliths and nannoliths affected the ocean carbonate system, especially because of the high rates of some nannolith calcification. These nannoplankton evolutionary events (NCEs) occurred during times of low spreading rates, low pCO2, low Mg/Ca ratio, cool climatic conditions and relatively oligotrophic oceans. Available data suggest that calcareous phytoplankton was stimulated by environmental stability rather than perturbations. This is consistent with modern coccolithophorid distribution, showing highest diversity and abundance as well as calcification in stable oligotrophic oceanic areas. A precise stratigraphic control allows to model the Late Jurassic nannofossil speciation episode and the abundance increase of high-calcified genera (Conusphaera, Polycostella, Faviconus, Nannoconus), evaluating environmental causes and consequences of evolution. The results suggest that the Late Jurassic nannoplankton evolution was mostly controlled by the following factors: A) a decrease in pCO2 due to decreased spreading rate and/or increased weathering rate (87Sr/86Sr); B) a decrease in oceanic Mg/Ca ratio values promoting low Mg-CaCO3 and CaCO3 biomineralization (nannofossils fertilization sensu Stanley, 2006); C) cool climatic condition (Price, 1999). The Tithonian time interval provides examples of accelerated intra- and inter-generic evolutionary rates (a speciation event) during a time period of environmental stability, in absence of coeval environmental change evidences. It provides an excellent opportunity to investigate nannoplankton evolutionary behaviour, and on the basis on the achieved stratigraphic and time framework, evolutionary trends of calcareous nannoplankton were quantified: example of Philetic Gradualism, Punctuated Equilibrium and Punctuated Gradualisms as well were described.