Biogenic silica dissolution in sediments of the Southern Ocean. II. Kinetics

Abstract

Abstract The dissolution kinetics of biogenic silica in surface sediments collected during the ANTARES I cruise were measured in stirred flow-through reactors. The rate data exhibit a distinctly non-linear dependence on the degree of undersaturation. Near equilibrium, the rates of silica dissolution and precipitation define a single linear trend, i.e. the kinetics are symmetric about the equilibrium point. When the dissolved silica concentration drops below a critical level, however, the dissolution rate rises exponentially with increasing undersaturation. Hence, the data disagree with the linear rate law generally used to describe the dissolution kinetics of biogenic silica. It is hypothesized that the kinetic transition from the linear to the exponential regime represents the onset of localized dissolution centered on surface defects, e.g. small pores and crevices, or compositional defects. The effects of temperature and pH confirm that the critical process controlling the overall dissolution kinetics is the hydrolysis of bridging SiOSi bonds at the solid-solution interface. The rate measurements indicate that the reactivity of biogenic silica decreases substantially with depth in the sediment. The decrease in reactivity is explained by a progressive reduction of the defect density of the silica surfaces, through dissolution and reprecipitation of silica. It does not appear to result from the preferential dissolution of a more reactive fraction of biogenic debris deposited from the water column. Surface areas obtained by the N2-BET method or concentrations of extractable biogenic silica do not provide satisfactory proxies for the reactive surface area of silica in the sediments. However, a positive correlation was observed between the surface reactivity and the exchangeable Co2+ adsorption capacity of biogenic silica. Specific kinetic effects on silica dissolution of the aluminum content of the silica surfaces or organic matter coatings were not observed. Both the non-linear dissolution kinetics and the aging of the silica surfaces help restrict the dissolution of deposited biogenic silica to a narrow zone close to the water-sediment interface. The results of the flow-through experiments highlight the importance of in situ early diagenetic processes in controlling the behavior and fate of deposited biogenic silica: no evidence was found supporting a significant effect of differences in solubility or reactivity inherited from the biomineralization process in the water column.