SI ISOTOPIC ANALYSES OF ORDOVICIAN-SILURIAN SPONGE SPICULES AND RADIOLARIANS INDICATE SILICA-DEPLETED PALEOZOIC OCEANS

The beginning of the Paleozoic Era marked a major transition from marine silica deposition via abiotic pathways to a system dominated by biogenic silica production, dissolution, and sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition was marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms and patterns of silicification of radiolarian tests, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ³⁰Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms: the magnitude of Si isotope fractionation during spiculogenesis is sensitive to dissolved silica concentration, while the magnitudes of Si isotope fractionations associated with silica precipitated by diatoms or radiolarians are not. We present 446 ion microprobe analyses of δ³⁰Si and δ¹⁸O values of sponge spicules and radiolarians from Ordovician-Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges displayed small Si isotope fractionations relative to coeval radiolarians, inconsistent with the high dissolved silica concentrations postulated for Paleozoic seawater. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we concluded that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that Paleozoic silica-biomineralizing organisms controlled the marine silica cycle just as strongly as diatoms do today.