The 40-m-thick Pliensbachian–Aalenian succession in Kermanshah (Kermanshah Basin; West Iran) includes three main radiolarian-bearing deposits: sponge spicule-radiolarian limestones, cherts, and pyroclastic deposits. Sedimentological and ichnological analyses of these facies reflect the different mechanisms of nutrient supply and proliferation of radiolarians. Pliensbachian p.p.—early Toarcian sponge spicule-radiolarian limestones with low proportions of radiolarians and scarce bioturbation (Chondrites) reflect low radiolarian productivity and accumulation in oxygen-deficient bottom waters. Their deposition was linked to the Pliensbachian—early Toarcian long-term warming, when the greenhouse-warm climate mode led to weak thermohaline circulation, stratified waters, and a well-developed pycnocline. However, a higher proportion of radiolarians is found in intercalated discrete pyroclastic deposits and in limestone beds located on top of coarse-grained internal wave deposits. These pulses of radiolarian productivity are related to an input of nutrients to surface waters due to volcanic eruptions and currents generated by internal waves propagating along the pycnocline. In contrast, middle Toarcian–Aalenian cherts with abundant radiolarians and ubiquitous bioturbation (Thalassinoides) were deposited in oxygenated bottom waters. High radiolarian productivity was related to the long-term middle Toarcian–Aalenian cooling trend and the greenhouse-cool climate. Biosiliceous production was linked to upwelling currents transporting cold deep waters rich in nutrients, which were driven by west-trending monsoon winds.

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