Silica-secreting biota and mass extinctions: survival patterns and processes

Abstract

High survival and at least regional blooms of siliceous marine groups, with coeval retreat of calcareous organisms, is known from the Frasnian–Famennian (radiolarians, silicisponges) and end-Cretaceous (diatoms, radiolarians) mass extinctions. A strictly actualistic approach to the palaeoecology of silica-secreting biota is of limited significance, especially for silica-enriched and oligotrophic Palaeozoic epeiric seas and, in particular, during biotic turnovers. Adaptation among silica-secreting marine plankton has led toward more efficient utilization of shrinking dissolved silica resources within surface waters of Cenozoic oceans, as shown by both radiolarians and diatoms. This biosiliceous signal during the major biotic crises is mostly explained by a large-scale increase in volcano-hydrothermal activity during major plate-boundary re-arrangements, triggering global ecosystem perturbations. The cumulative stimulus favoured siliceous versus calcareous biota growth, deposition and preservation because of (1) a higher rate of input of silica and other nutrients, promoting eutrophication pulses, (2) a punctuated (or reversed) greenhouse climatic effect, episodically coupled with (3) vigorous oceanic circulation due to a variety of volcanogenic upwellings and turnovers. Siliceous communities, adapted to more eutrophic conditions, have thrived in the stressed niches. On the other hand, expanding anoxia and nutrification may lead to a stepwise loss of deep-water niches and consequent selective decline of more specialized oligotrophic radiolarians and other pelagic biota. By contrast, siliceous faunas experienced severe non-selective losses during the end-Permian mass extinction (recorded as a `chert gap'). Thus, the greatest environmental disaster of the Phanerozoic was driven by a unique set of killing factors, extremely effective for radiolarian productivity, e.g., due to the combination of a drastic volcanic winter with expanding superanoxia.