Mesozoic radiolarites - accumulation as a function of sea surface fertility on Tethyan margins and in ocean basins

Radiolarites, commonly comprising chert-shale couplets, feature prominently in the Mesozoic stratigraphy of the Neotethyan realm. The origin and significance of these deposits, especially with regard to understanding palaeoproductivity in that region, remains controversial. This study examines transport processes and accumulation rates of radiolarians/radiolarites, as well as the chronology of radiolarite sedimentation based on revised data from the Southern Alps and new data from the Balagne Nappes, Alpine Corsica. Results suggest that the spatio-temporal distribution of carbonate and silica on Tethyan margins was governed by extensive lateral transport of radiolarian tests from topographic highs to basins. Palaeoclimatic, rather than palaeotectonic, changes seem to have triggered changes in surface fertility which, in turn, brought about major facies changes observed in Tethyan marginal basins: Regional platform demise possibly due to eutrophication and onset of radiolarite deposition in the Early Bajocian can be correlated with a positive shift in the carbon isotope curve, whereas platform recovery and the end of radiolarite accumulation correlates with a gradual decline in d13C-values. The existence of a vigorous trans-Pangaean, equatorial current system that could have produced regional upwelling is questioned because of evidence for Middle Jurassic to Early Cretaceous oligotrophic conditions in the Proto-Caribbean and the Central Atlantic. The Caribbean River Plume Model is proposed as a possible alternative to upwelling in the Jurassic Western Tethys. Dissolved nutrients may have been dispersed in low salinity lids that originated from river plumes of rivers from tropical Africa and warm-temperate Eurasia. Vast areas of the pre-Late Cretaceous world ocean must have accumulated radiolarites at low rates (<4 g cm-2 10-3years) represented by the condensed, long-lived (>50 Myr) radiolarite sections observed in many Circumpacific terranes. Radiolarian chert and claystone could be the normal pelagic sediment during Ordovician to Early Cretaceous time in far-offshore, open oceanic settings, where dilution by terrigenous material and periplatform shallow-water carbonates was absent.