9633351 Van Cappellen Aluminum plays a major role in the benthic regeneration of nutrient silicic acid and the preservation of biogenic opal in the sedimentary record. Based on results of a previous study of sediments from the Indian sector of the Southern Ocean, potential mechanisms by which Al intervenes in the early diagenesis of silica include: (1) primary uptake of Al by biosiliceous skeletons during biomineralization, (2) secondary uptake of Al by biosiliceous microfossils at the seafloor, (3) readjustment of the Al/Si ratio of the surface layers of biosiliceous debris during burial, and (4) simultaneous precipitation of pore water silica and aluminum. This study will test the proposed Al-Si interaction mechanisms and, in doing so, build a quantitative basis for predicting their relative importance in different marine depositional environments. Stirred flow-through reactors will be used in the experiments, because they allow the measurement of reaction kinetics under reproducible, steady-state conditions. The effect of primary Al uptake during biosilicification will be studied by measuring the steady state rates of silicic acid production from high-Al and low-Al samples of pure diatom cultures. The kinetics will be determined as a function of the degree of saturation, temperature, pH and Al content of the silica frustules. The early diagenetic interactions between silica and soluble Al will be studied by supplying dissolved Al via the input solution, or by adding a solid source of reactive Al to biogenic silica suspended in the reactor. In the latter experiments, a given amount of biogenic silica will be mixed with increasing amounts of the solid Al source. This will mimic the increased dilution of biogenic silica by detrital material in marine sediments. The experimental results will allow to examine the hypothesis that the asymptotic pore water silica level in detrital-rich sediments represents a kinetic steady state between production of silicic acid by biogenic silica dissolution and consumption by precipitation of authigenic Al-Si phases. The Al/Si stoichiometry of the authigenic precipitates will be obtained from the relative rates of Al precipitation and silica reprecipitation in the reactor experiments. Batch adsorption experiments will also be used to constrain the levels of Al sorption at which secondary Al uptake by biosiliceous surfaces switches from adsorption to the formation of precipitates. Scanning electron microscopy and solid state chemical analyses will complement the reactor-based experiments.
