THE ROLE OF SILICA AS A CHEMICAL SEDIMENT PRIOR TO THE ADVENT OF METAZOAN BIOMINERALIZATION (Invited Presentation)

The topology of the modern marine silica cycle is outlined by biological processes that are a relatively recent development in Earth history. The first silica biomineralizing organisms—sponges and radiolarians—evolved during Neoproterozoic and Cambrian time and biogenic silica subsequently became the largest sink of dissolved silica, drawing down its concentration in seawater to a few ppm. Prior to these biological innovations, the marine silica cycle reflected largely chemical processes for more than three billion years. The large fluxes of dissolved silica into Precambrian oceans derived from chemical weathering of the crust were balanced via varied contributions of primary precipitation of amorphous silica, diagenetic silicification of precursor sediments, co-precipitation with iron and other oxides, and precipitation of authigenic clays. Constraining the mechanisms and rates of silica polymerization and precipitation could provide useful proxies for Precambrian seawater chemistry due to inherent sensitivities to temperature, pH, concentration of dissolved silica, and salinity. Precambrian silica precipitation played a key role in the taphonomy of surface environments, including an archive of microfossils in cherts. It also played an understated role in the silicate weathering feedback. For example, authigenic clay precipitation can effectively short-circuit the silicate weathering feedback by sequestering silica and cations in clays while releasing CO₂ into solution, providing an avenue for the silica cycle to significantly affect climate and the carbon cycle. We present recent advances in constraining the fluxes and sinks of the Precambrian silica cycle using sedimentology, microscopy, and Si isotope geochemistry. Neoproterozoic silica depositional patterns, characterized by an absence of evidence of primary amorphous silica and a decreased abundance of deeper subtidal chert, are distinct from those observed in Archean and Paleoproterozoic sedimentary successions, suggesting that the silica cycle was already experiencing secular change prior to the advent of sponges. We evaluate possible explanations for these changes in depositional pattern in order to construct an improved model of the marine silica cycle just prior to the innovation of silica biomineralization.