Dissolution of biogenic silica: Solubility, reactivity and the role of aluminum

The processes causing spatial and temporal variability of the reactivity, solubility, and surface chemistry of biogenic silica are poorly understood. This represents a major obstacle to the development of mechanistic models of the silica cycle in the world's ocean. In this thesis, the effects of aluminum and detrital material on the dissolution of biogenic silica are investigated. Dissolution experiments, using biosiliceous oozes and diatom cultures grown in media with different dissolved aluminum levels, demonstrate that Al(III) incorporation in the silica lattice reduces the solubility and reactivity of biogenic silica. Batch experiments with mixtures of biogenic silica and detrital phases (kaolinite and basalt) reveal an inverse relationship between the build-up of silicic acid in solution and the detrital to opal ratios. This trend is similar to the one observed in natural sediments between the asymptotic silicic add pore water concentrations and the ratio of detrital material to biogenic opal. Results from flow-through reactor experiments indicate that release of soluble Al(III) from detrital minerals may further enhance the retention of silica in sediments through the precipitation of authigenic alumino-silicates. Overall, the study demonstrates that Al(III)-mediated interactions between biogenic silica and detrital matter play a crucial role in the marine silica cycle.