Radiolarian palaeoecology and radiolarites: is the present the key to the past?

Radiolarian productivity pulses and related radiolarite deposition are phen omena difficult to understand from an exclusively actualistic viewpoint. Ev olutionary selection pressure among silica-secreting marine plankton, both radiolarians and diatoms, has led toward more economic usage of rapidly shr inking nutrient resources, including dissolved silica, of the photic zone i n the late Cenozoic oceans, and, in particular, a substantial modification of oceanic cycle by the diatom explosive radiation. Even if there is a prov ed link between biomineralization and dissolved silica loading among the ph ytoplankton only, the relative independence of modem siliceous planktic bio tas from the available silica pool reflects mainly their progressive physio logical specialisation during evolutionary history. Oceanic chemistry and p roductivity, as well as patterns of circulation/upwelling have changed radi cally during the Phanerozoic. Radiolarites apparently represent an 'anachro nistic' facies, as exemplified by their long-lived and ocean-wide distribut ion in palaeo-Pacific, and hitherto, highlighted actualistic models of loca lized intra-oceanic wind-driven upwelling loci are of largely questionable applicability. In addition to plate drift hypersiliceous domains and interv al are explainable mostly by a large-scale volcano-hydrothermal activity du ring major plate-boundary reconfigurations, which, in many ways, favoured s iliceous biotas acme, and their skeletal remains accumulation and preservat ion. Factors tied to rapid, voluminous submarine eruptions, such as thermal buoyant megaplumes and basin overturns, offer a viable alternative for tra ditional climatic/circulation scenarios in case of hypersiliceous high prod uctivity events irrelevant to greenhouse-to-icehouse climatic change. The e volving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering. Volcano-hydrothermal and tectonic uplift events, related mostly to extensive rifting and/or accelerated oceanic spr eading, were the endogenous driving force that created this perturbation of the exogenous system. The present biogeochemical cycle is representative o nly for the overall silica-depleted post-Eocene oceanic ecosystems, which b roadly correlates with a major expansion of diatoms groups extremely effici ent in silica removal, and closely linking the silica budget with phosphoru s and nitrogen cycles. Thus, an orthodox uniformitarian approach to biosili ceous sedimentation, based on a silica-starved vigorous ocean, is of limite d significance when applied to the pre-Neogene settings, especially in the peculiar planktic habitats of epeiric seas, as well as during biotic crises marked by strong geotectonic overprint. The major turnovers in marine sili ceous biota composition, in particular after the end-Permian radiolarite ga p, may have been coupled with discernible changes in an increasing biologic al control on the long-term oceanic silica cycling ('punctuated equilibrium '). The evolutionary turnovers have induced a stepdown decrease of dissolve d silica levels through the Phanerozoic, contemporaneously with the general secular trend of upward scaling of nutrient-related ecological processes a nd increased effectiveness of resource utilization. (C) 2000 Elsevier Scien ce B.V. All rights reserved.