Secular Distribution of Biogenic Silica through the Phanerozoic: Comparison of Silica‐Replaced Fossils and Bedded Cherts at the Series Level

The modern marine silica cycle is dominated by silica‐secreting phytoplankton, principally diatoms, but this cycle has evolved considerably during the Phanerozoic. We analyzed the temporal distribution of silica‐replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies. Trends in silica replacement of fossils match faunal radiations during the Ordovician and Siluro‐Devonian, with peaks in silica replacement in the Late Ordovician and Middle Devonian corresponding to peaks in the abundance and diversity of siliceous sponges. Sharp drops in the abundance of silica‐replaced fossils and/or bedded cherts coincide with four of the five major mass extinctions. No discernible decrease marks the extinction at the end of the Triassic. We expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling. Orogenic episodes, which may trigger continental glaciation, may also increase silica supply and further enhance siliceous deposition during these intervals. The data we have sampled provide mixed results. The Late Ordovician and Late Devonian glaciations do not correspond to peaks in bedded chert abundance, although the increase in bedded cherts relative to silica‐replaced fossils during the Carboniferous may reflect a climatic influence. The well‐known Middle Miocene circum‐Pacific chert event does correspond with a glacial interval. Lows in silica deposition should mark intervals when the ocean was stratified and/or ocean circulation was sluggish. Data from the mid‐Cretaceous and Early Triassic appear consistent with this expectation. Following the shift to a diatom‐dominated silica cycle in the Late Jurassic and Cretaceous, patterns of chert abundance through time became more volatile and more responsive to external influences on marine silica burial such as icehouse and greenhouse effects. Prior to the diatom radiation, biogenic silica burial was probably more equitably divided between radiolarians and siliceous sponges. This more cosmopolitan control of silica burial may have dampened the effects of climatic factors on silica accumulation, though better resolved data are needed to test this possibility.