Secular change in the Precambrian silica cycle: Insights from chert petrology

Chert deposits preserve a record of secular change in the oceanic silica cycle. The evolutionary radiation of silica-secreting organisms resulted in a transition from abiological silica deposition, characteristic of the Archean and Proterozoic eons, to the predominantly biologically controlled silica deposition of the Phanerozoic. Comparative petrography of Phanerozoic and Precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the Paleoproterozoic era (ca. 1.8 Ga). In Neoproterozoic and Mesoproterozoic strata, early diagenetic chertification is largely restricted to peritidal environments. These early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures. The cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation. Some of the Paleoproterozoic cherts associated with iron formations, however, are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities, and are not associated with unsilicified carbonate deposits of similar depositional composition. Cherts in some Paleoproterozoic iron formations (e.g., the Gunflint Formation, northwestern Lake Superior region) are composed of silica types similar to those in Phanerozoic sinters (e.g., the Devonian Rhynie and Windyfield cherts, Scotland). Such cherts may provide evidence that basinal, and perhaps global, oceanic silica concentrations were higher during the Paleoproterozoic era than at later times.