Silica-replaced oolites, bedded shelf cherts and Paleozoic changes in the silica cycle

An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation

“…Secular changes in the loci of Phanerozoic chert deposition refl ect the evolution and ecology of silicifying organisms Schubert et al, 1997;Kidder and Erwin, 2001;Kidder and Mumma, 2003). In modern ocean basins, diatoms quantitatively control the removal of silica, maintaining silica concentrations in the oceans (surface water generally <2 μM, deep water ~100 μM) far below saturation with respect to amorphous silica (~1000 μM).…”

“…Such shallow water cherts are common in Precambrian carbonates of all ages, largely disappearing only with the radiations of sponges and radiolarians in the early Paleozoic ( Fig. 1) Kidder and Mumma, 2003). Offshore subtidal cherts are less common in the Proterozoic, and infrequently preserve microfossils (Maliva et al, , 2005.…”

Biogenic chert and the Ordovician silica cycle

“…The record of silica-replaced oolites also supports retreat of cherty facies at or near the Lower-Middle Ordovician boundary (Kidder and Mumma, 2003). The key assumption for the Kidder and Mumma (2003) study was that carbonate oolites today are a lithology that is extremely unlikely to be replaced by silica because the concentration of dissolved silica is so low in modern shallow waters.…”

“…The key assumption for the Kidder and Mumma (2003) study was that carbonate oolites today are a lithology that is extremely unlikely to be replaced by silica because the concentration of dissolved silica is so low in modern shallow waters. The Maliva et al (1989) thinking would suggest that high levels of dissolved silica in Neoproterozoic-to-Silurian waters could allow silica to replace carbonate oolites during early diagenesis so that silicified oolites should indirectly record the presence of available shallow-water silica.…”

“…The silica replacement of the oolites must have happened during early diagenesis and the oolites must be in situ. Kidder and Mumma (2003) took great care to eliminate silica-replaced oolites from their abundance plots if those oolites had been transported and/or were replaced by latediagenetic silica. The Ordovician oolite data reported herein have not been similarly scrutinized for diagenesis and transport history.…”