Silicification Through Time

Abstract

Silicification, which requires dissolution of original shell material and precipitation of silica, is mediated by numerous biological and environmental factors. The processes and controls on silicification, the environments and conditions under which it occurs, and the temporal and spatial distribution of silicified deposits through the Phanerozoic are reviewed. Selective dissolution of original shell material in certain taxa or lithological settings results in a taphonomic bias which impacts our understanding of paleoecology and patterns of diversity over time. The temporal pattern of silicification is mediated mainly by global ocean chemistry and climate, particularly changes in carbonate solubility (the contrast in calcite and aragonite seas). On a finer scale, silicification of shelly fossils is influenced by taxonomic factors (shell mineralogy, organic material in the soft tissues and within the shell, shell ultrastructure) and by depositional factors (from porewater geochemistry, lithology, porosity, permeability, position within a stratigraphic sequence and basin characteristics, to global ocean chemistry). Silicification is more prevalent in the Paleozoic than in younger strata. The relationship between the abundance of silicified faunas, the greenhouse/icehouse signal, and fluctuations in carbonate rock volume are complex; the observed pattern may reflect sampling of exceptionally preserved silicified faunas rather than a global signal in silicification. The influence of shifts in biodiversity, in carbonate skeletons susceptible to silicification, and sili- ceous skeletons that provide a source of silica, remains to be determined.