The Evolution of the Modern Silica Cycle Revisited

Abstract:

Diatoms control the marine silica cycle and make up some 40% of primary productivity in the modern ocean. Their rise to dominance is believed to have drawn down surface water silica concentrations by two orders of magnitude, displaced the ecological and geochemical prominence of other silicifiers, and shifted the geographic locus of silica deposition. Diatoms' expansion also impacted the marine carbon cycle and global climate through their role in organic carbon burial and the effect of silica concentration on reverse weathering. Although recognized as one of the largest shifts in Cenozoic marine nutrient cycling, it is unclear when this transition occurred. We use a diagenetic model to investigate the effect of sedimentation rate and temperature--two factors that collated deep sea drilling data show have changed substantially over the past 65 m.y.--on the burial efficiency of biogenic silica. We find that taphonomic biases significantly overprint primary signals from the Cenozoic fossil record of diatoms and radiolarians, on which the classical Cenozoic silica cycle narrative is based. Our results partially reconcile discordant timelines produced by the fossil record and Si isotope data, and add to a growing body of evidence suggesting that the evolution of the modern ocean silica cycle occurred prior to 20 Ma and perhaps before 40 Ma, earlier than traditionally thought.